# Kunwu Kettle -- full site text Batch hot-dip galvanizing kettles (zinc kettles / zinc pots). Source: https://kunwukettle.com/ | Last updated: 2026-09-27 This file is the complete substantive text of the site in markdown, provided so that a language model does not have to parse the HTML to quote it accurately. Technical figures are cited to clause level; the standard referenced throughout is BG/T 1501-2015, the Chinese industry standard for steel hot-dip galvanizing kettles. Where a figure is a specification we contract to rather than a measured result, the text says so. --- ## Galvanizing kettles built to survive the heat input, not just the pressure test. URL: https://kunwukettle.com/ Aluminium-killed low-silicon plate. Electroslag welded. Warm formed. From 2 t to 85 t, up to 16.5 m. Every structural weld ultrasonically tested — and your inspector is welcome to witness it. **Q: Delivered to date** 8,000+ tof kettles **Q: Per kettle** 2 – 85 tup to 16.5 m long **Q: Structural welds** UTtested before dispatch **Q: Weld metal silicon** ≤ 0.03 %specified, Grade 1 ## Your kettle is a countdown Every galvanizing kettle is a consumable. Zinc dissolves iron, and from the day you fire it the wall is getting thinner. A well-built kettle in a well-run furnace loses **1.0 – 2.5 mm of wall per year**. A badly built one, or a good one fired too fast on day one, can be through the wall in months. So the question is never “is this kettle good”. The question is: **how many years of production does this kettle buy me, and can the supplier prove it before I pay?** Most of what decides the answer is fixed before the kettle ever sees zinc — in the chemistry of the plate, in how the corners were formed, in the silicon content of the weld metal, and in the heat-up curve you follow on the first firing. Those four things are what this site is about. ## What actually decides kettle life - **Silicon, not strength** -- Silicon accelerates the dissolution of iron in molten zinc. Stainless and heat-resisting steels resist zinc worse than plain low-silicon structural steel — the opposite of what most people expect. - **The weld, not just the plate** -- Weld metal can carry thirty times the silicon of the parent plate depending on the consumable. That is why kettles go through at the seam while the wall is still thick. - **Stress you build in, or don't** -- All plate deformation done warm at 850–900 °C. Electroslag welding lays the full wall thickness in one pass, so heat input is uniform through the wall. **And then the first 48 hours.** More kettles are shortened by a bad commissioning than by bad manufacturing. Room temperature to 300 °C at **≤ 2 °C/h**. A **24-hour hold at 300–330 °C**. Wall inside-to-outside differential kept under 50 °C throughout. We supply the curve with every kettle, and we will walk your team through it. Read the commissioning guide → ## What ships with the kettle Five documents, with every unit. - **Mill test certificate — EN 10204 Type 3.1** -- Signed by an inspection representative independent of production at the mill. - **Welding consumable certificates** -- Including the low-silicon flux used to hold weld metal silicon to specification. - **Manufacturing inspection reports** -- Dimensional check against drawing, fit-up records, weld visual, coating thickness. - **NDT reports** -- Plate inspection records and ultrasonic testing of every structural weld. - **Commissioning heat-up curve** -- The controlled firing schedule for your first heat. Most suppliers leave this out. It is also the one that decides whether the other four mattered. See the full documentation package → ## Common questions **Q: How long does a galvanizing kettle last?** Between three and ten years, depending far more on the furnace and the operating discipline than on the kettle itself. Under the operating rules in BG/T 1501-2015 — bath at 445–460 °C, inner wall below 480 °C, hourly throughput at 1/30 to 1/40 of the zinc volume — the corrosion rate stays at 1.0–2.5 mm per year. On a 50 mm wall, that is a long life. Break those rules and no kettle survives. **Q: What steel do you use?** Aluminium-killed, low-silicon hot-rolled plate, specified by chemistry rather than by grade name. There is no equivalent EN or ASTM grade designation for kettle steel. Our specification is Si ≤ 0.03 %, P ≤ 0.02 %, Al ≤ 0.045 %, with the combined Si + P + Al held to ≤ 0.045 %. **Q: What sizes can you build?** From 1.0 × 0.6 × 1.2 m up to 16.5 × 3.6 × 3.2 m, and from 2 t to 85 t per unit. Wall thickness 40, 50 or 60 mm. **Q: Do you test the welds?** Every structural weld, ultrasonically, to GB/T 11345-2023 — which is identical (IDT) to ISO 17640:2018. Plate is ultrasonically inspected plate-by-plate before cutting. Reports are issued in English. **Q: Who is responsible if something goes wrong?** Kunwu Kettle. We take full responsibility for the entire process — steel sourcing, welding, NDT, documentation and delivery. The kettles are built at our works in Hebei, China, which is open for your audit and pre-shipment inspection. You deal with one party from sizing through to the first heat, and that party is accountable for all of it. **Q: Can we audit the plant before we order?** Yes. Factory visits and pre-shipment inspection are normal in this business and the works is open for both. Tell us when you are in the region, or send your own inspector — we will arrange access and provide the inspection and test plan in advance so your inspector knows what to look at. ## Tell us the workpiece. We will size the bath. We do not sell kettles from a catalogue. Send us the largest piece you need to dip and the throughput you need to hold, and we will come back with a sized kettle, a wall thickness, a zinc volume and a lead time.Request a quoteOr work through the sizing guide --- ## Galvanizing kettles — geometries, sizes and wall thickness URL: https://kunwukettle.com/products/ Four standard configurations plus custom work. The choice between them is driven mostly by bath depth and by how you handle dross. **Specification range** | | Metric | Imperial | |---|---|---| | Length | 1.0 – 16.5 m | 3.3 – 54 ft | | Width | 0.6 – 3.6 m | 2.0 – 11.8 ft | | Depth | 1.2 – 3.2 m | 3.9 – 10.5 ft | | Wall thickness | 40 / 50 / 60 mm | 1.6 / 2.0 / 2.4 in | | Weight per kettle | 2 – 85 t | 2.2 – 94 short tons | | Corner radius | 125 – 200 mm | 4.9 – 7.9 in | | Operating temperature | 445 – 460 °C | 833 – 860 °F | 40 / 50 / 60 mm covers the great majority of baths. If your design calls for a heavier wall, ask — we will tell you what the works can roll and form for your dimensions rather than quote a number that does not apply to your kettle. ## Choosing the geometry _Figure: Transverse section through the wall-to-base junction — the one place the two geometries differ. Both have radiused wall-to-wall corners; those are seen in plan, not here. The 2 m rule is BG/T 1501-2015 Table 4, not a preference._ **Geometry selection — BG/T 1501-2015 Table 4** | Geometry | Where walls meet the base | Advantage | Trade-off | Use it when | |---|---|---|---|---| | Full rounded corner | Radiused at every junction | Strongest weld geometry; bottom dross flows with no dead corner | Higher build cost | Bath deeper than 2 m | | Rounded corner, flat bottom | Radiused between walls, square at the base | Corners do not trap dross; corner zinc less prone to local overheating | Dross can settle in the base corners | Bath 2 m and under | Single or double flange on either geometry, depending on how your furnace is built and how the kettle is supported. This is not a preference — it is a rule in BG/T 1501-2015 Table 4. Below 2 m depth, a flat bottom is the right answer and a full radius is money you do not need to spend. Above 2 m, the hydrostatic load makes the full radius worth it. ## The standard configurations - **Full rounded corner · double flange** -- Deep baths, heaviest duty. Widest bearing surface and the stiffest top edge. - **Full rounded corner · single flange** -- Deep baths with a simpler furnace interface. - **Rounded corner flat bottom · double flange** -- Shallow baths, higher throughput. - **Rounded corner flat bottom · single flange** -- Shallow baths and compact furnaces. - **Custom and oversized** -- Up to 16.5 m (54 ft) and 85 t. Tell us your largest workpiece. _Figure: Flange edge and the wall-to-flange weld. Plate thickness is visible in section at the cut edge._ ## What every kettle gets, regardless of configuration - **Aluminium-killed low-silicon plate, Si ≤ 0.03 %** -- Ultrasonically inspected plate-by-plate to GB/T 2970 Class III before cutting. - **All forming done warm, 850 – 900 °C** -- Cold bending locks residual stress into a vessel that will run at 450 °C for years. - **Electroslag welding, ISO 4063 process 72** -- Full wall thickness in a single pass. No inter-pass interfaces. - **Weld metal specified to Si ≤ 0.03 %** -- Grade 1 of BG/T 1501-2015 Table 10. Weld silicon decides where a kettle goes through first, so it is written into the order and evidenced by the welding consumable certificates that ship with the kettle. - **Ultrasonic testing of structural welds before dispatch** -- GB/T 11345-2023 — identical (IDT) to ISO 17640:2018. Coverage and acceptance level are agreed with you and recorded in the inspection and test plan. Reports in English. - **No cruciform plate joints** -- Parallel welds kept at least 0.5 m apart, per BG/T 1501-2015 §5.2.4. - **Full documentation package** -- Including the commissioning heat-up curve for your first firing. Blast cleaned and painted in house. **Surface preparation grade, coating system and dry film thickness are specified per order** — send us your standard (most European buyers work to ISO 8501-1 for cleanliness and ISO 12944 for the coating) and it goes into the inspection and test plan rather than being left to works practice. ## What we do not do We do not build complete galvanizing lines. If you are planning a new plant from scratch, you need a line integrator, not us. What we do is the kettle — the one part of the line you will replace several times over the life of the plant, and the part that decides how much of your production time you actually get. Around it we supply what touches the bath: zinc pumps, dross removal, molten zinc storage, covers, dross shovels cut to your kettle’s corner radius, thermocouple wells. Tell us what you need around the bath. Anything we cannot build ourselves we will say so about rather than quote it and sub-contract quietly. ## Not sure which geometry? Send the largest piece you dip and the throughput you need to hold. We will tell you which geometry, what depth, and how much zinc.Request a quoteSizing guide --- ## How we build a galvanizing kettle URL: https://kunwukettle.com/technology/ Four things are fixed before the kettle ever sees zinc, and none of them can be corrected afterwards. These pages are about those four. Most kettle suppliers describe their product. We would rather describe the decisions, because the decisions are what you are actually buying — and because they are checkable. Plate verification → cutting and bevelling → warm forming (850–900 °C) → fit-up → electroslag welding → stress relief → NDT → blast cleaning → painting → marking → despatch - **Steel specification** -- Why silicon matters more than strength, and why there is no equivalent EN or ASTM grade for kettle plate. - **Electroslag welding** -- ISO 4063 process 72. Full wall thickness in a single pass — and an honest note about what the process costs you. - **Warm forming** -- All plate deformation at 850–900 °C. Cold bending locks stress into a vessel that will run at 450 °C for years. - **Quality control and NDT** -- GB/T 11345-2023 is identical to ISO 17640:2018. Here is the full inspection and test plan, and the standards mapped onto references your inspectors know. - **Documentation package** -- Five documents with every kettle. The fifth is the reason the other four matter. ## Have a technical question? If you are not buying yet and just want an answer — about kettle life, weld silicon, commissioning or sizing — email us. We will answer it.Ask an engineer --- ## Sizing a galvanizing kettle URL: https://kunwukettle.com/sizing-guide/ Three inputs decide almost everything. Work through them here, or send them to us and we will do it. _Figure: Two rules do most of the work. Sizing to the workpiece alone is the most common way to end up with a kettle that cannot hold temperature at the throughput it was bought for. — BG/T 1501-2015 §4.2.2_ ## Step 1 — Length and width, from your largest workpiece Take the largest piece you need to dip. Add clearance to lower it in, withdraw it, and skim the surface with the work still in the bath. That sets the minimum internal length and width. **Do not size to your average piece.** You will be asked to dip the exception, and a kettle is not something you can extend later. ## Step 2 — Depth, from width Depth is normally **1.3 to 2 times the width**. — BG/T 1501-2015 §4.2.2 b) Going deeper, within that range, is usually the better decision: - More zinc means more thermal mass, so bath temperature swings less when you charge - More wall area means lower heat input per m², which directly extends kettle life - Deeper baths let dross settle well below the work, so it is not stirred back onto the coating ## Step 3 — Zinc volume, from throughput Zinc volume should be **30 – 40 × your maximum hourly throughput**, including jigs. For wire galvanizing, 20 × is acceptable. — BG/T 1501-2015 §4.2.2 c) **This is the rule that gets broken most often, and it is the one that kills kettles.** Under-size the bath relative to throughput and the only way to hold temperature is to raise heat input — which pushes the inner wall towards 480 °C, and above 480 °C the corrosion rate climbs steeply, peaking between 490 and 510 °C. ## Step 4 — Wall thickness | Wall | Use for | |---|---| | 40 mm | Small kettles | | 50 mm | The normal choice | | 60 mm | Very long kettles, or depth over 3 m | Above 1.8 m depth, add one adjustable external support band. Above 2.5 m, two. Support points at 1.5–2.0 m intervals, with expansion clearance along the length. ## The operating envelope you are sizing for | Parameter | Value | |---|---| | Bath temperature | 445 – 460 °C | | Inner wall temperature | Below 480 °C — 490–510 °C is the peak corrosion band | | Wall-to-bath temperature difference | 30 – 50 °C normally; 15 – 20 °C during steady production | | Furnace chamber temperature | Below 700 °C on oil or producer gas; below 650 °C on natural gas or electric | | Permitted heat input at 445 °C | 31 kW/(m²·h) | | Permitted heat input at 460 °C | 24 kW/(m²·h) | | Resulting corrosion rate | 1.0 – 2.5 mm/year | Note the direction of that relationship. **A lower bath temperature permits a higher heat input.** If your bath is well sized, running cooler lets you run harder. ## Once you have the numbers Take them to the product range to see which geometry fits your depth, and to the steel specification for what the wall is actually made of. If you are replacing an existing kettle rather than building new, replacement planning is the page you want. Source: BG/T 1501-2015 — §4.2, §4.3, Appendix A.1.5, A.2.3, A.2.4, Table A.1. ## Send us three numbers Largest workpiece (L × W × H), required throughput in t/h, and how your furnace is heated. That is enough for a sized kettle, a wall thickness, a zinc volume, a weight and a lead time.Request a sized quote --- ## Delivered kettles URL: https://kunwukettle.com/projects/ Kettles from our works are in service across eight countries. Detailed project records are being compiled. Kettles built at our manufacturing facility are in service in **Malaysia, Turkey, Iran, Russia, Kazakhstan, Pakistan, India and Saudi Arabia**. **No kettle of ours has, so far, gone into the European Union.** We would rather say that plainly than imply a track record we do not have. What it means practically: the metallurgy, the welding and the fabrication are proven over years and across eight countries. What is new is the European documentation layer — EN 10204 3.1 certificates, CBAM emissions data, standards mapped onto ISO references your inspectors already use, and technical response in your time zone. **That layer is the part we do.** ## Why there is no gallery on this page Most suppliers in this category publish a wall of kettle photographs with no dimensions, no dates and no way to tell whether the kettle in the picture was theirs. It looks like evidence and it is not. We would rather give you something checkable. **Tell us the market and the size you are considering, and we will come back with comparable units we have built — year, dimensions, wall thickness, weight and geometry** — and, where the customer permits it, a contact you can call. Customer names are not published without permission, and we will not publish yours either. That cuts both ways: it is why this page is thin, and it is why your kettle will not appear on it next year. ## In the meantime If a reference list is not available yet, the next best thing is verifiable detail. Our inspection and test plan shows exactly what is measured and to which standard, the documentation package shows what you receive with the kettle, and the works page shows where it is built. ## Want a reference for your market? Tell us which country and what size you are considering, and we will tell you what comparable units we have built and where.Ask us --- ## Standards, certificates and what we do not claim URL: https://kunwukettle.com/compliance/ A short page about what we hold, what we do not, and why we would rather tell you than let you assume. ## What we build to **BG/T 1501-2015 — Steel Hot-Dip Galvanizing Kettles.** The Chinese industry standard for this specific product, issued by the China Surface Engineering Association. It covers plate chemistry, geometry selection, wall thickness, forming, welding, heat treatment, weld quality classes, dimensional tolerances, NDT acceptance, and the delivery documentation set. We are not aware of any equivalent European or American standard written specifically for galvanizing kettles. Where you need to map our testing onto standards your inspectors know, the table on the Quality Control page does that. ## What we do not claim We do not hold CE marking for a bare kettle, and we will explain why rather than imply otherwise. - **Pressure Equipment Directive 2014/68/EU** -- Applies above 0.5 bar. A galvanizing kettle is an atmospheric vessel — normally out of scope. - **Machinery Directive** -- A bare kettle is not machinery. Supplied as part of a complete galvanizing line, the line may be in scope. - **EN 1090 / Construction Products Regulation** -- Applies to structural steelwork for construction. A kettle is industrial equipment, normally out of scope. If your project scope brings the supply within one of these, tell us and we will address it properly rather than produce a certificate that does not mean what it appears to mean. Where a certificate exists, we will send the current copy on request — and where one does not, this page will say so rather than leave you to assume. If your procurement process requires a specific certification we do not hold, tell us early: some are obtainable and some are not, and you should know which before you spend time on a quotation. ## Documents you receive Mill certificate to EN 10204 Type 3.1 · welding consumable certificates · manufacturing inspection reports · NDT reports · commissioning heat-up curve. Full documentation package → ## Importing into the EU A galvanizing kettle classifies under **CN 7309** and is a CBAM good. If you are importing one into the EU, that affects you. Read the CBAM guidance → --- ## Installation, commissioning and replacement planning URL: https://kunwukettle.com/service/ A kettle is not finished when it leaves the works. Most of what decides its life happens in the week after it arrives. ## Commissioning is where kettles are won or lost We supply the heat-up curve with every kettle, and we will go through it with your team before the first firing at no charge — by call, by video, or in writing, whichever suits your engineers. Attendance on site during commissioning can be arranged and is quoted per project, because the cost depends entirely on where you are and how long the firing takes. Ask when you enquire rather than after the kettle ships. What we will tell you, whether you ask or not: - **Charge with pure zinc, above 99.99 %** -- No lead in the base of a new kettle. The wall has no alloy layer yet and is at its most vulnerable to intergranular attack. - **Stack ingots tight against the wall — but leave a 100 mm gap in the middle** -- The kettle is side-heated, so contact matters. But zinc's coefficient of expansion is about three times iron's, and a tightly packed charge will push outward as it heats. - **Cover the bath during melt-down** -- Limits heat loss from the top while the kettle is being heated from the sides. - **Hold 24 hours at 300 – 330 °C** -- This is not padding in the schedule. It is where the kettle releases the stress from its own manufacture. Full commissioning guide → ## Running the kettle so it lasts | | | |---|---| | Bath temperature | 445 – 460 °C | | Inner wall | Below 480 °C. 490–510 °C is the peak corrosion band | | Hourly throughput | 1/30 to 1/40 of zinc volume, including jigs. Run it level | | Temperature logging | Every 30 minutes, recorded | | Thermocouples | Type E or K, in protective sheaths, calibrated on a schedule | | Over-temperature alarm | Required | | Zinc leak alarm | Required. Escaping zinc shorts the detector, triggers the alarm and shuts down heating | | Emergency zinc pit | Where site conditions allow — 25–30 % of total zinc volume, conical for recovery | Follow this and the corrosion rate holds at **1.0–2.5 mm/year**. Chase output by raising heat input and it does not. ## Planning a replacement instead of reacting to a leak Kettle replacement is a full plant shutdown. Planned, it is a scheduled outage. Unplanned, it is a zinc spill and an unknown number of weeks. Measure wall thickness ultrasonically on a schedule, plot the trend, and you will know roughly a year in advance. That is enough time to specify, build and ship a replacement into a window you choose. **Tell us your current kettle’s dimensions, age and latest thickness readings, and we will tell you what a replacement looks like — even if you are not ready to buy yet.** Bear in mind that a proper commissioning alone takes about a week. Work backwards from the outage window you want, not forwards from when you notice a problem. ## Send your thickness readings Current kettle dimensions, age, and your latest ultrasonic wall measurements. We will come back with a replacement specification and a realistic timeline.Talk to us about replacement planning --- ## Who you are buying from URL: https://kunwukettle.com/about/ We would rather be clear about this than have you work it out later. _Figure: The fabrication hall. Photographs on this site are of the actual works — no renderings, no stock images._ ## What we are **We build galvanizing kettles at our works in Hebei, China**, and we answer for the whole of it — steel sourcing, welding, NDT, documentation and delivery. The works is **open for your audit and pre-shipment inspection**, and we would rather you came. That matters more than it sounds. A galvanizing kettle is a piece of capital equipment that will sit in your furnace for years. What goes wrong is rarely a single defect — it is a specification that did not match the furnace, documentation that did not satisfy an inspector, a shipment that could not be lifted off the truck, or a commissioning that nobody supervised. Every one of those sits between disciplines, which is exactly where responsibility usually gets dropped. We do not hand you off. The plate specification, the welding procedure, the test reports, the export documentation and the heat-up curve are all ours to answer for — there is nobody behind us to point at. **You deal with one party, from sizing through to the first heat — and you can go and look at the works before you commit.** ## The manufacturing facility **Works capability** | | | |---|---| | Location | Hebei province, northern China | | Site area | 10,000 m² (108,000 sq ft) | | Overhead crane | 32 t gantry over the main bay | | Plate preparation | CNC cutting and bevelling | | Forming | Hydraulic press, warm forming at 850 – 900 °C | | Welding | Consumable-nozzle electroslag welding · submerged arc · manual metal arc | | Surface | Shot blasting and painting, in house | | Single-unit capacity | 150 t | | Delivered to date | 8,000+ tonnes of kettles | | Kettle range | 2 – 85 t · up to 16.5 m · 40/50/60 mm wall | **Why the electroslag welder matters more than the site area.** Most fabricators that can weld 50 mm plate do it in multiple passes. A consumable-nozzle electroslag machine is a specific piece of capital equipment for a specific job — laying the full wall thickness in one pass — and it is the same process the most expensive kettle builders in Europe use. It is the reason we can make the claims on our Technology pages. See the facility → ## Where these kettles already run Kettles from our works are in service in **Malaysia, Turkey, Iran, Russia, Kazakhstan, Pakistan, India and Saudi Arabia**. **No kettle of ours has, so far, gone into the European Union.** We would rather say that plainly than imply a track record we do not have. What it means practically: the metallurgy, the welding and the fabrication are proven over years and across eight countries. What is new is the European documentation layer — EN 10204 3.1 certificates, CBAM emissions data, standards mapped onto ISO references your inspectors already use, and technical response in your time zone. **That layer is what is new here, not the kettles.** It is why our specification pages carry more verifiable detail than suppliers with longer European histories: we would rather be judged on the metallurgy and the welding, which are proven across eight countries, than on a European reference list we do not yet have. ## How we work - **We quote from your workpiece, not from a catalogue** -- Send the largest piece you dip and the throughput you hold; we size the bath. - **We publish the numbers** -- Chemistry limits, weld silicon, NDT standards, tolerance classes. If a number is on this site, it is in the contract. - **We do not claim certifications we do not hold** -- See Compliance for what we do and do not have. - **We quote EXW or FOB** -- Import duties, safeguard measures and CBAM are handled by your customs representative, who knows your position better than we do. We supply whatever documentation they need. ## Kunwu Kettle and Kunwu Heavy **Kunwu Kettle and Kunwu Heavy are brands of the same company.** Kunwu Kettle builds galvanizing kettles and what touches the bath. Kunwu Heavy supplies heavy weldments and large machined parts — thick-plate fabrication with post-weld CNC machining. Two brands, two dedicated works: kettles are built at the kettle works described on this site; Kunwu Heavy fabrications are machined at its own plant. Same company, and the same habit of publishing the numbers. If your enquiry is a fabrication rather than a kettle, that is the right place to send it. Its machined fabrications have shipped into the European Union — Germany and Sweden — which is that line's track record, not the kettles'. Kettle deliveries are listed above, and no kettle of ours has gone to the EU yet. --- ## Technical resources URL: https://kunwukettle.com/resources/ Written for people who run galvanizing plants, not for search engines. Every claim is referenced to a standard clause. - **The weld is the weakest link** -- Weld metal often carries several times the silicon of the parent plate — and dissolves faster. Why kettles fail at the seam while the wall is still thick. - **What actually determines kettle life** -- Suppliers quote 3 to 10 years. The spread is not about the kettle — it is about heat input density, bath temperature and the first 48 hours. With the numbers. - **The 24-hour hold at 330 °C** -- The first firing decides more about kettle life than the manufacturing did. The full heat-up schedule, the loading rules, and why the hold is not optional. ## Also worth reading - Why silicon matters more than strength — and why there is no equivalent EN or ASTM grade for kettle plate - Sizing a kettle — depth from width, zinc volume from throughput - CBAM for EU buyers — CN 7309 is in scope, and one kettle can cross the 50-tonne threshold on its own - Standards mapping — GB/T 11345-2023 is an identical adoption of ISO 17640:2018 --- ## Tell us the workpiece. We will size the bath. URL: https://kunwukettle.com/contact/ The fastest route to a useful answer is three numbers: your largest workpiece, your required throughput, and how your furnace is heated. Name *Company *Email *Country *So we can tell you what applies on duties and CBAM.Largest workpiece, L × W × H *This one field decides most of the quotation.Required throughput, t/hAnything elseSend enquiry We reply with a sized specification, a price and a lead time — not a brochure. If you want to work out the sizing yourself first, the sizing guide walks through it, and the product range has the full specification tables. ## Technical questions If you are not buying yet and just want a technical answer — about kettle life, weld silicon, commissioning, sizing or CBAM — email us. We will answer it. engineering@kunwukettle.com That address reaches the people who size kettles, not a shared inbox. Write in English or German; we will answer in the same language. ## Why there is no phone number on this page Because the useful answer to almost every question in this business needs a drawing, a dimension or a thickness reading in front of both parties. A call where we take your numbers down verbally and quote from memory helps neither of us. Send the enquiry by email with whatever you have — a sketch, a photograph of the existing kettle, the furnace opening, a line from your maintenance log. If a call is then the right next step, we will set one up with the drawings already open. **US enquiries:** quoted EXW or FOB. Import duties, including Section 232, are handled by your customs broker. **EU enquiries:** see our CBAM guidance before comparing landed costs. --- ## Full rounded corner galvanizing kettle — double flange URL: https://kunwukettle.com/products/full-rounded-corner-double-flange/ For baths deeper than 2 m, where hydrostatic load on the wall-to-base joint is highest and bottom dross has to be able to move. _Figure: A double flange top edge on a full-length shell. The two flanges give the furnace a wider bearing surface and stiffen the least supported part of the vessel._ ## Why a full radius below 2 m of depth At depth, two things go wrong with a square wall-to-base joint. The weld sits exactly where hydrostatic pressure is highest, and the internal corner becomes a dead zone where bottom dross accumulates and forms an insulating layer — which drives local overheating on the outside of that same corner. Radiusing the junction removes both. There is no weld at the point of highest load, and dross has nowhere to sit still. The internal bend radius is **125–200 mm**, formed warm at a radius of at least 2.5× plate thickness, with wall thinning held to **under 5 %** of plate thickness. _Figure: Transverse section through the wall-to-base junction — the one place the two geometries differ. Both have radiused wall-to-wall corners; those are seen in plan, not here. The 2 m rule is BG/T 1501-2015 Table 4, not a preference._ ## Why double flange Two flanges give the furnace a wider bearing surface and a stiffer top edge, which matters on long kettles where the top edge is the least supported part of the vessel. If your furnace is built for a single flange, we build single. It is not a quality difference, it is an interface difference. ## Specification | | Metric | Imperial | |---|---|---| | Length | up to 16.5 m | up to 54 ft | | Width | up to 3.6 m | up to 11.8 ft | | Depth | over 2.0 m, up to 3.2 m | over 6.6 ft, up to 10.5 ft | | Wall thickness | 50 / 60 mm | 2.0 / 2.4 in | | Corner radius | 125 – 200 mm | 4.9 – 7.9 in | | Weight | up to 85 t | up to 94 short tons | | Design standard | BG/T 1501-2015 | | **Support note.** Above 1.8 m depth the kettle needs one adjustable external support band; above 2.5 m, two. Support points at 1.5–2.0 m intervals, and expansion clearance along the length. Any part of the support inside the furnace hot zone must be heat-resisting steel. We specify this with the kettle. ## Sizing this kettle for your plant Three inputs decide it. - **Your largest workpiece** -- Sets minimum length and width, with clearance to dip, withdraw and skim. - **Your required throughput** -- Zinc volume should be **30–40×** your maximum hourly throughput including jigs, so the bath temperature holds. - **Depth** -- Normally **1.3–2× the width**. Going deeper increases thermal mass, reduces temperature swing, lowers heat input density per m² of wall, and helps dross settle away from the work. ## What every kettle gets - **Aluminium-killed low-silicon plate, Si ≤ 0.03 %** -- Ultrasonically inspected plate-by-plate to GB/T 2970 Class III before cutting. - **All forming done warm, 850 – 900 °C** -- Cold bending locks residual stress into a vessel that will run at 450 °C for years. - **Electroslag welding, ISO 4063 process 72** -- Full wall thickness in a single pass. No inter-pass interfaces. - **Weld metal specified to Si ≤ 0.03 %** -- Grade 1 of BG/T 1501-2015 Table 10. Weld silicon decides where a kettle goes through first, so it is written into the order and evidenced by the welding consumable certificates that ship with the kettle. - **Ultrasonic testing of structural welds before dispatch** -- GB/T 11345-2023 — identical (IDT) to ISO 17640:2018. Coverage and acceptance level are agreed with you and recorded in the inspection and test plan. Reports in English. - **No cruciform plate joints** -- Parallel welds kept at least 0.5 m apart, per BG/T 1501-2015 §5.2.4. - **Full documentation package** -- Including the commissioning heat-up curve for your first firing. ## Questions **Q: Can you build this deeper than 3 m?** Yes, up to 3.2 m. Above 3 m depth we would normally specify 60 mm wall rather than 50 mm. **Q: How long is the lead time?** It runs from confirmed drawings, not from your enquiry, and it depends on size and on what is already on the shop floor. We quote a date with the price, and we would rather quote a date we can hold than the shortest one that wins the enquiry. If you have a fixed outage window, tell us the date you need the kettle on site and we will tell you whether it is achievable before you place the order. **Q: How is a kettle this size shipped?** Anything over about 12 m will not fit a standard container and moves on flat rack or as breakbulk. We arrange it — and we will tell you what your site needs to lift it off the truck before you order, not after. **Q: Can you match our existing furnace?** Yes. Send the furnace opening dimensions, the support arrangement and how it is heated. Where a replacement has to drop into an existing furnace, the interface matters more than the catalogue. ## Send three numbers, get a sized quote Largest workpiece, required throughput, and how your furnace is heated. That is enough for us to come back with a sized kettle, a wall thickness, a zinc volume, a weight and a lead time.Request a quoteSizing guide --- ## Full rounded corner zinc kettle — single flange URL: https://kunwukettle.com/products/full-rounded-corner-single-flange/ The same radiused geometry for deep baths, with a simpler furnace interface. _Figure: The radiused corner, formed warm and then welded. The section drawing below shows why the radius goes where it does; this is what it looks like in steel._ ## Why a full radius below 2 m of depth At depth, two things go wrong with a square wall-to-base joint. The weld sits exactly where hydrostatic pressure is highest, and the internal corner becomes a dead zone where bottom dross accumulates and forms an insulating layer — which drives local overheating on the outside of that same corner. Radiusing the junction removes both. There is no weld at the point of highest load, and dross has nowhere to sit still. The internal bend radius is **125–200 mm**, formed warm at a radius of at least 2.5× plate thickness, with wall thinning held to **under 5 %** of plate thickness. _Figure: Transverse section through the wall-to-base junction — the one place the two geometries differ. Both have radiused wall-to-wall corners; those are seen in plan, not here. The 2 m rule is BG/T 1501-2015 Table 4, not a preference._ ## Why single flange A single flange is the simpler furnace interface and the lighter top edge. On shorter kettles, and where the furnace structure already provides the bearing, the second flange adds weight and cost without adding service life. On long or deep kettles we will usually recommend double. Tell us your furnace opening and we will say which fits. ## Specification | | Metric | Imperial | |---|---|---| | Length | up to 16.5 m | up to 54 ft | | Width | up to 3.6 m | up to 11.8 ft | | Depth | over 2.0 m, up to 3.2 m | over 6.6 ft, up to 10.5 ft | | Wall thickness | 50 / 60 mm | 2.0 / 2.4 in | | Corner radius | 125 – 200 mm | 4.9 – 7.9 in | | Weight | up to 85 t | up to 94 short tons | | Design standard | BG/T 1501-2015 | | **Support note.** Above 1.8 m depth the kettle needs one adjustable external support band; above 2.5 m, two. Support points at 1.5–2.0 m intervals, and expansion clearance along the length. Any part of the support inside the furnace hot zone must be heat-resisting steel. We specify this with the kettle. ## Sizing this kettle for your plant Three inputs decide it. - **Your largest workpiece** -- Sets minimum length and width, with clearance to dip, withdraw and skim. - **Your required throughput** -- Zinc volume should be **30–40×** your maximum hourly throughput including jigs, so the bath temperature holds. - **Depth** -- Normally **1.3–2× the width**. Going deeper increases thermal mass, reduces temperature swing, lowers heat input density per m² of wall, and helps dross settle away from the work. ## What every kettle gets - **Aluminium-killed low-silicon plate, Si ≤ 0.03 %** -- Ultrasonically inspected plate-by-plate to GB/T 2970 Class III before cutting. - **All forming done warm, 850 – 900 °C** -- Cold bending locks residual stress into a vessel that will run at 450 °C for years. - **Electroslag welding, ISO 4063 process 72** -- Full wall thickness in a single pass. No inter-pass interfaces. - **Weld metal specified to Si ≤ 0.03 %** -- Grade 1 of BG/T 1501-2015 Table 10. Weld silicon decides where a kettle goes through first, so it is written into the order and evidenced by the welding consumable certificates that ship with the kettle. - **Ultrasonic testing of structural welds before dispatch** -- GB/T 11345-2023 — identical (IDT) to ISO 17640:2018. Coverage and acceptance level are agreed with you and recorded in the inspection and test plan. Reports in English. - **No cruciform plate joints** -- Parallel welds kept at least 0.5 m apart, per BG/T 1501-2015 §5.2.4. - **Full documentation package** -- Including the commissioning heat-up curve for your first firing. ## Questions **Q: When would you recommend double instead?** On long kettles — roughly over 10 m — or where the furnace does not already provide a stiff bearing along the top edge. **Q: How long is the lead time?** It runs from confirmed drawings, not from your enquiry, and it depends on size and on what is already on the shop floor. We quote a date with the price, and we would rather quote a date we can hold than the shortest one that wins the enquiry. If you have a fixed outage window, tell us the date you need the kettle on site and we will tell you whether it is achievable before you place the order. **Q: How is a kettle this size shipped?** Anything over about 12 m will not fit a standard container and moves on flat rack or as breakbulk. We arrange it — and we will tell you what your site needs to lift it off the truck before you order, not after. **Q: Can you match our existing furnace?** Yes. Send the furnace opening dimensions, the support arrangement and how it is heated. Where a replacement has to drop into an existing furnace, the interface matters more than the catalogue. ## Send three numbers, get a sized quote Largest workpiece, required throughput, and how your furnace is heated. That is enough for us to come back with a sized kettle, a wall thickness, a zinc volume, a weight and a lead time.Request a quoteSizing guide --- ## Rounded corner flat bottom kettle — double flange URL: https://kunwukettle.com/products/rounded-corner-flat-bottom-double-flange/ For baths of 2 m and under, where the flat base is the economical answer and the corner radii still do the work that matters. _Figure: Flange plate and radiused inner corners on flat bottom shells. The wall-to-wall corners are radiused on both geometries; the flat base is what distinguishes this one._ ## Why a flat bottom is the right answer under 2 m Below 2 m of depth the hydrostatic load on the wall-to-base joint is modest, and the full radius stops earning its cost. What matters more at shallow depth is that the corners do not become dross traps and that corner zinc does not overheat. Radiusing the wall-to-wall junctions handles both, while the flat base keeps the build simpler and the price lower. BG/T 1501-2015 Table 4 draws the line at 2 m, and so do we. Bottom dross does settle in the base corners on this geometry — plan your skimming and dross removal accordingly. We can supply a dross shovel cut to your kettle’s corner radius. _Figure: Transverse section through the wall-to-base junction — the one place the two geometries differ. Both have radiused wall-to-wall corners; those are seen in plan, not here. The 2 m rule is BG/T 1501-2015 Table 4, not a preference._ ## Why double flange Two flanges give the furnace a wider bearing surface and a stiffer top edge, which matters on long kettles where the top edge is the least supported part of the vessel. If your furnace is built for a single flange, we build single. It is not a quality difference, it is an interface difference. ## Specification | | Metric | Imperial | |---|---|---| | Length | up to 16.5 m | up to 54 ft | | Width | up to 3.6 m | up to 11.8 ft | | Depth | up to 2.0 m | up to 6.6 ft | | Wall thickness | 40 / 50 mm | 1.6 / 2.0 in | | Corner radius | 125 – 200 mm | 4.9 – 7.9 in | | Weight | 2 – 45 t | 2.2 – 50 short tons | | Design standard | BG/T 1501-2015 | | **Support note.** Above 1.8 m depth the kettle needs one adjustable external support band; above 2.5 m, two. Support points at 1.5–2.0 m intervals, and expansion clearance along the length. Any part of the support inside the furnace hot zone must be heat-resisting steel. We specify this with the kettle. ## Sizing this kettle for your plant Three inputs decide it. - **Your largest workpiece** -- Sets minimum length and width, with clearance to dip, withdraw and skim. - **Your required throughput** -- Zinc volume should be **30–40×** your maximum hourly throughput including jigs, so the bath temperature holds. - **Depth** -- Normally **1.3–2× the width**. Going deeper increases thermal mass, reduces temperature swing, lowers heat input density per m² of wall, and helps dross settle away from the work. ## What every kettle gets - **Aluminium-killed low-silicon plate, Si ≤ 0.03 %** -- Ultrasonically inspected plate-by-plate to GB/T 2970 Class III before cutting. - **All forming done warm, 850 – 900 °C** -- Cold bending locks residual stress into a vessel that will run at 450 °C for years. - **Electroslag welding, ISO 4063 process 72** -- Full wall thickness in a single pass. No inter-pass interfaces. - **Weld metal specified to Si ≤ 0.03 %** -- Grade 1 of BG/T 1501-2015 Table 10. Weld silicon decides where a kettle goes through first, so it is written into the order and evidenced by the welding consumable certificates that ship with the kettle. - **Ultrasonic testing of structural welds before dispatch** -- GB/T 11345-2023 — identical (IDT) to ISO 17640:2018. Coverage and acceptance level are agreed with you and recorded in the inspection and test plan. Reports in English. - **No cruciform plate joints** -- Parallel welds kept at least 0.5 m apart, per BG/T 1501-2015 §5.2.4. - **Full documentation package** -- Including the commissioning heat-up curve for your first firing. ## Questions **Q: Will dross build up in the base corners?** Some will. That is the trade-off for the flat base, and it is manageable with normal skimming practice. If your bath runs deeper than 2 m, take the full radius instead. **Q: How long is the lead time?** It runs from confirmed drawings, not from your enquiry, and it depends on size and on what is already on the shop floor. We quote a date with the price, and we would rather quote a date we can hold than the shortest one that wins the enquiry. If you have a fixed outage window, tell us the date you need the kettle on site and we will tell you whether it is achievable before you place the order. **Q: How is a kettle this size shipped?** Anything over about 12 m will not fit a standard container and moves on flat rack or as breakbulk. We arrange it — and we will tell you what your site needs to lift it off the truck before you order, not after. **Q: Can you match our existing furnace?** Yes. Send the furnace opening dimensions, the support arrangement and how it is heated. Where a replacement has to drop into an existing furnace, the interface matters more than the catalogue. ## Send three numbers, get a sized quote Largest workpiece, required throughput, and how your furnace is heated. That is enough for us to come back with a sized kettle, a wall thickness, a zinc volume, a weight and a lead time.Request a quoteSizing guide --- ## Rounded corner flat bottom kettle — single flange URL: https://kunwukettle.com/products/rounded-corner-flat-bottom-single-flange/ The most economical of the four. Shallow baths, compact furnaces, straightforward interface. _Figure: Single flange flat bottom shells with lifting lugs. Wall thickness is visible at the flange edge, and the radiused inner corners run the full depth._ ## Why a flat bottom is the right answer under 2 m Below 2 m of depth the hydrostatic load on the wall-to-base joint is modest, and the full radius stops earning its cost. What matters more at shallow depth is that the corners do not become dross traps and that corner zinc does not overheat. Radiusing the wall-to-wall junctions handles both, while the flat base keeps the build simpler and the price lower. BG/T 1501-2015 Table 4 draws the line at 2 m, and so do we. Bottom dross does settle in the base corners on this geometry — plan your skimming and dross removal accordingly. We can supply a dross shovel cut to your kettle’s corner radius. _Figure: Transverse section through the wall-to-base junction — the one place the two geometries differ. Both have radiused wall-to-wall corners; those are seen in plan, not here. The 2 m rule is BG/T 1501-2015 Table 4, not a preference._ ## Why single flange A single flange is the simpler furnace interface and the lighter top edge. On shorter kettles, and where the furnace structure already provides the bearing, the second flange adds weight and cost without adding service life. On long or deep kettles we will usually recommend double. Tell us your furnace opening and we will say which fits. ## Specification | | Metric | Imperial | |---|---|---| | Length | up to 16.5 m | up to 54 ft | | Width | up to 3.6 m | up to 11.8 ft | | Depth | up to 2.0 m | up to 6.6 ft | | Wall thickness | 40 / 50 mm | 1.6 / 2.0 in | | Corner radius | 125 – 200 mm | 4.9 – 7.9 in | | Weight | 2 – 45 t | 2.2 – 50 short tons | | Design standard | BG/T 1501-2015 | | **Support note.** Above 1.8 m depth the kettle needs one adjustable external support band; above 2.5 m, two. Support points at 1.5–2.0 m intervals, and expansion clearance along the length. Any part of the support inside the furnace hot zone must be heat-resisting steel. We specify this with the kettle. ## Sizing this kettle for your plant Three inputs decide it. - **Your largest workpiece** -- Sets minimum length and width, with clearance to dip, withdraw and skim. - **Your required throughput** -- Zinc volume should be **30–40×** your maximum hourly throughput including jigs, so the bath temperature holds. - **Depth** -- Normally **1.3–2× the width**. Going deeper increases thermal mass, reduces temperature swing, lowers heat input density per m² of wall, and helps dross settle away from the work. ## What every kettle gets - **Aluminium-killed low-silicon plate, Si ≤ 0.03 %** -- Ultrasonically inspected plate-by-plate to GB/T 2970 Class III before cutting. - **All forming done warm, 850 – 900 °C** -- Cold bending locks residual stress into a vessel that will run at 450 °C for years. - **Electroslag welding, ISO 4063 process 72** -- Full wall thickness in a single pass. No inter-pass interfaces. - **Weld metal specified to Si ≤ 0.03 %** -- Grade 1 of BG/T 1501-2015 Table 10. Weld silicon decides where a kettle goes through first, so it is written into the order and evidenced by the welding consumable certificates that ship with the kettle. - **Ultrasonic testing of structural welds before dispatch** -- GB/T 11345-2023 — identical (IDT) to ISO 17640:2018. Coverage and acceptance level are agreed with you and recorded in the inspection and test plan. Reports in English. - **No cruciform plate joints** -- Parallel welds kept at least 0.5 m apart, per BG/T 1501-2015 §5.2.4. - **Full documentation package** -- Including the commissioning heat-up curve for your first firing. ## Questions **Q: Is this a lower-quality kettle?** No. Same plate chemistry, same welding process, same weld silicon limit, same NDT. The geometry and flange arrangement are chosen to suit the furnace and the bath depth, not to set a quality tier. **Q: How long is the lead time?** It runs from confirmed drawings, not from your enquiry, and it depends on size and on what is already on the shop floor. We quote a date with the price, and we would rather quote a date we can hold than the shortest one that wins the enquiry. If you have a fixed outage window, tell us the date you need the kettle on site and we will tell you whether it is achievable before you place the order. **Q: How is a kettle this size shipped?** Anything over about 12 m will not fit a standard container and moves on flat rack or as breakbulk. We arrange it — and we will tell you what your site needs to lift it off the truck before you order, not after. **Q: Can you match our existing furnace?** Yes. Send the furnace opening dimensions, the support arrangement and how it is heated. Where a replacement has to drop into an existing furnace, the interface matters more than the catalogue. ## Send three numbers, get a sized quote Largest workpiece, required throughput, and how your furnace is heated. That is enough for us to come back with a sized kettle, a wall thickness, a zinc volume, a weight and a lead time.Request a quoteSizing guide --- ## Custom and oversized galvanizing kettles URL: https://kunwukettle.com/products/custom-oversized-kettles/ Up to 16.5 m (54 ft) long and 85 t. If your workpiece does not fit a standard bath, this is the conversation. _Figure: Kettle shells in the fabrication bay. The 32 t gantry runs the length of the shop._ ## Where the limits actually are **Build envelope** | | Metric | Imperial | |---|---|---| | Maximum length | 16.5 m | 54 ft | | Maximum width | 3.6 m | 11.8 ft | | Maximum depth | 3.2 m | 10.5 ft | | Maximum weight | 85 t | 94 short tons | | Minimum size | 1.0 × 0.6 × 1.2 m | 3.3 × 2.0 × 3.9 ft | | Single-unit works capacity | 150 t | 165 short tons | For reference, the average kettle in North America is about 40 ft long, with a number of plants running 50–60 ft. Our 16.5 m covers 54 ft — the average and most of the large end. ## Shipping something this size A standard 40 ft container is about 12.0 m inside. Anything longer than that moves on **flat rack** or as **breakbulk**, and the inland leg from the works to the port needs an oversize permit. We will tell you before you order what your site needs to receive it: the lift capacity, the approach, and the laydown area. A kettle that arrives at a plant that cannot unload it is a very expensive problem. On arrival, if the kettle will wait before it goes into the furnace: coat it against rust, block it clear of the ground with **one end higher than the other**, and cover it. **Under no circumstances allow water to collect inside the kettle.** Store away from acids and alkalis. **What we will tell you before you order, not after.** The lift capacity your site needs, the approach the trailer requires, and where the kettle can be set down. On an oversize unit these are not logistics details to be sorted out later — they decide whether the delivery works at all. Send us your site access and unloading capability with the enquiry. If your plant cannot receive the kettle you are asking for, that is a conversation worth having at quotation stage. ## What we need from you - **Largest workpiece, L × W × H** -- Plus how it is jigged. This sets the minimum internal dimensions. - **Required throughput in t/h** -- Including jigs. Zinc volume is 30–40× this figure. - **Heating type and furnace opening** -- Gas, oil or electric, and the existing furnace dimensions if this is a replacement. - **Site access** -- Lift capacity available, approach road, and whether the kettle can be craned directly into the furnace. ## Before you specify an oversized kettle Bigger is not automatically better. Zinc volume should be 30–40× your hourly throughput — oversize the bath relative to what you actually run and you are paying to hold zinc molten that you do not need. The sizing guide works through it. If a standard configuration does fit, the four standard geometries are quicker and cheaper to build. ## Send the workpiece drawing We will come back with a sized kettle, a weight, a shipping method and a lead time — not a brochure.Request a quote --- ## Kettle steel: why silicon matters more than strength URL: https://kunwukettle.com/technology/steel-specification/ The most common mistake in specifying a galvanizing kettle is asking for a stronger or more corrosion-resistant steel. For molten zinc service, that is backwards. ## Molten zinc dissolves iron, and silicon speeds it up Zinc attack on a kettle wall is not oxidation and it is not conventional corrosion. Iron dissolves into the molten zinc and forms an Fe-Zn alloy layer at the interface. Whether that layer protects the wall or not depends on the chemistry of the steel. With low silicon, the alloy layer is dense and adherent, and it slows further attack. Raise the silicon and the layer becomes non-adherent — it sloughs off, exposes fresh iron, and the wall keeps dissolving. Phosphorus and aluminium do the same thing. This is why stainless steels and heat-resisting steels — which outperform plain carbon steel almost everywhere else — resist molten zinc **worse** than a plain low-silicon structural steel does. ## Our specification There is no equivalent EN or ASTM grade designation for galvanizing kettle plate. **Kettle steel is specified by chemistry, not by grade name.** Anyone quoting you an “equivalent” pressure-vessel grade is quoting a steel with roughly ten times the silicon a kettle should have. **Kettle plate chemistry** | Element | Our specification | Why | |---|---|---| | Si | ≤ 0.03 % | The single largest accelerator of zinc attack on the kettle wall | | P | ≤ 0.02 % | Same mechanism as silicon | | Al (Als) | ≤ 0.045 % | Same mechanism | | Si + P + Al | ≤ 0.045 % combined | The controlling limit per BG/T 1501-2015 §3.2 | | C | 0.05 – 0.11 % | Kept up, not down — carbon holds wall strength against hydrostatic load at 450 °C | | Mn | 0.30 – 0.50 % | Same reason | | S | ≤ 0.015 % | Cleanliness | | Residuals | Ni ≤ 0.3 %, Cr ≤ 0.1 %, Cu ≤ 0.20 %, N ≤ 0.008 % | | These are the limits we order plate to. The evidence is the mill certificate that ships with the kettle — heat number, full analysis, signed independently of production. Do not take a chemistry table on any supplier's website as proof, including this one; take the certificate. **Mechanical properties:** tensile strength ≥ 325 MPa, elongation after fracture ≥ 33 %, 180° bend test passed. **Steelmaking:** aluminium-killed, hot-rolled. Converter or electric arc melting with secondary refining, and heavy reduction during rolling to close out centreline defects. **A note on very low carbon.** It is tempting to read “low silicon, low carbon” and ask for the lowest carbon available. BG/T 1501-2015 explicitly advises against plate below 0.05 % C — the wall needs that strength to resist the static pressure of the zinc at temperature without bulging. ## Verified twice, not once The mill inspects every plate ultrasonically to GB/T 2970, Class III or better. We then re-verify on receipt: chemistry to GB/T 222, mechanical properties to GB/T 228.1, and ultrasonic re-check. Plate that fails re-verification is not used in a kettle shell. It may be used for flanges or external supports, and it is specially marked if it is. Mill certificates are supplied to **EN 10204 Type 3.1** — signed by an inspection representative independent of production at the steel mill. **Type 3.2**, countersigned by a third party or by your own inspector, is available where your specification requires it. **How traceability actually works.** The heat number on the mill certificate is transferred onto every piece cut from that plate, and carried through cutting, forming and fit-up (BG/T 1501-2015 §5.2.1). At the end you can point at a seam on your kettle and trace it back to a heat and an analysis. That is what the paperwork is for. A certificate that cannot be tied to a specific plate in a specific kettle is a document, not evidence. ## Questions **Q: What is XG08?** It is the Chinese designation commonly used for galvanizing kettle plate. It is useful shorthand in China and meaningless in Europe, which is why we quote the chemistry instead. What matters is the numbers in the table above and the mill certificate that proves them. **Q: Isn't a thicker wall better than a purer steel?** They solve different problems. Thickness buys you years of sacrificial material and resistance to bulging. Chemistry decides how fast those years are consumed. A 60 mm wall in a high-silicon steel can have a shorter life than a 50 mm wall in a low-silicon one. **Q: Can you supply to a customer-specified chemistry?** Yes, within what the mill will roll. Send your specification and we will tell you whether it is available, what it costs and what the lead time on the plate is. Source: BG/T 1501-2015, _Steel Hot-Dip Galvanizing Kettles_, China Surface Engineering Association — §3, Tables 1, 2, 3. ## The weld is a different steel Plate chemistry is half the story. Weld metal can carry thirty times the silicon of the parent plate depending on the consumable — and that is where kettles usually go through first.Read: the weld is the weakest linkHow we weld --- ## Electroslag welding — full wall thickness in a single pass URL: https://kunwukettle.com/technology/electroslag-welding/ ISO 4063 process 72. It is the process the most expensive kettle builders in the world use, and it is used for a specific reason. _Figure: Both fill the same joint. The difference is how many times the surrounding plate is heated and cooled, and how many interfaces are left inside the weld. On a kettle wall that difference is residual stress you either build in or you do not._ ## What it does differently Conventional multi-pass welding builds a thick joint in layers. Every layer is a separate thermal cycle, every interface between layers is a potential discontinuity, and the heat input into the surrounding plate varies from root to cap. Electroslag welding fills the whole joint in **one continuous pass**, vertically, with the arc replaced by a resistively heated molten slag pool. On a 50 mm kettle wall that means one weld, one thermal cycle, no inter-pass interfaces, and heat input that is uniform through the full thickness. **Process window** | Parameter | Value | |---|---| | Process designation | ISO 4063 process 72 | | Voltage | 40 – 50 V | | Current | 300 – 1200 A | | Single-electrode thickness range | 25 – 75 mm | | Our kettle wall range | 40 / 50 / 60 mm | The range above is the process window, not our settings. Actual voltage, current and travel speed come from the qualified welding procedure for your plate thickness and joint, and the procedure and welder qualification records are supplied with the order. ## Combined with warm forming, and with joint design ESW on its own is not the point. The point is total residual stress in a vessel that will spend its life at 450 °C. - **All plate deformation done warm, 850–900 °C** -- Never cold. - **Full thickness welded in one pass** -- No layered thermal cycling. - **Joint layout that avoids the worst locations** -- No cruciform plate joints anywhere; parallel welds at least 0.5 m apart; and on full rounded corner kettles, no weld at the wall-to-base junction at all. - **Double-bevel joints welded alternately from both sides** -- Balances heat input and limits distortion. _Figure: A vertical butt weld on a kettle wall, full height of the plate._ _Figure: Corner weld and base plate. The radius is formed warm, then welded — not folded cold and filled._ ## What we control instead of impact toughness We should be straight about a known characteristic of the process. **Electroslag welding produces a coarse, columnar grain structure**, because the weld pool cools slowly. In structural steelwork, that costs low-temperature impact toughness, and the textbook remedy is a normalising heat treatment. A galvanizing kettle is a different service case. | | Structural steelwork | Galvanizing kettle | |---|---|---| | Service temperature | Ambient or below | Constant ~450 °C | | Loading | Dynamic, impact, fatigue | Hydrostatic and thermal, no impact | | Is toughness the design driver? | Yes | No | | Actual failure mode | Brittle fracture | Zinc dissolution and thermal stress cracking | That is why BG/T 1501-2015 Table 10 specifies **weld silicon content** and **NDT acceptance class** for kettle welds — and specifies no impact energy requirement at all. So the variables we control are the ones that decide kettle life: - **Weld metal Si ≤ 0.03 %**, using low-silicon flux — Grade 1 of BG/T 1501-2015 Table 10 - **100 % ultrasonic testing** of structural welds to GB/T 11345-2023 (identical to ISO 17640:2018) - Back-gouging and grinding of the reverse side before the second pass, with penetrant testing where required **Why weld silicon gets its own specification.** Weld metal is not the same steel as the plate. Submerged arc welding with standard flux can deposit metal with silicon several times the plate limit. When that happens, the weld dissolves faster than the plate around it — and the kettle goes through at the seam while the wall is still thick. Nobody else publishes their weld silicon limit. It is the number we would want to see if we were buying. The full explanation is here. ## Stress relief Full stress-relief annealing at **600 – 650 °C**, heating and cooling at **≤ 50 °C/h**, held 2 – 4 hours, furnace-cooled below 300 °C. **Whether your kettle is annealed at the works is specified on the order.** Ask for it and we will price it; occupying a furnace for the better part of a day has a real cost, and we would rather show you that cost than bury it or quietly skip the step. Where a kettle ships without full annealing, the stress relief is completed during commissioning — which is precisely what the 24-hour hold at 300–330 °C in the heat-up curve is for. We supply that curve with every kettle. Source: BG/T 1501-2015 — §5.2.4, §5.5, §5.6, Tables 5, 6, 10. ISO 4063 welding process designations. ## See the inspection, not just the certificate Every structural weld is ultrasonically tested, and your own inspector is welcome to witness it before the kettle ships.Quality control and NDTArrange an inspection --- ## Warm forming — the stress you do not build in URL: https://kunwukettle.com/technology/warm-forming/ A kettle spends its life at 450 °C under hydrostatic load. Residual stress from manufacturing does not stay put under those conditions. **How you check this on any supplier, including us.** Warm forming leaves evidence and cold forming leaves different evidence. Ask for the forming temperature record, ask what the pyrometer read, and look at the corner: warm-formed plate keeps an even surface through the bend, while heavily cold-worked plate shows its strain at the outer radius. It is also visible in the numbers. Cold forming needs a bend radius of at least 3× plate thickness; warm forming gets to 2.5×. A supplier quoting you a tight corner radius on a thick wall is telling you something about their process whether they mean to or not. ## Why cold bending is the wrong answer here Cold forming a 50 mm plate to a 150 mm radius puts a large amount of locked-in strain into the corner — exactly where the kettle will later carry the highest hydrostatic load and the highest thermal gradient. Heat it first and the same shape is achieved with a fraction of the residual stress. This is not a refinement; it is the difference between a corner that stays where you put it and one that moves in service. **Forming parameters** | Parameter | Value | Source | |---|---|---| | Plate heating temperature | 850 – 900 °C | BG/T 1501-2015 §5.3.1 | | Temperature monitoring | Infrared pyrometer | §5.3.1 | | Bend radius, warm forming | ≥ 2.5 × plate thickness | §5.3.3 | | Bend radius, cold forming | ≥ 3 × plate thickness | §5.3.3 | | Maximum wall thinning | 5 % of plate thickness | §5.3.3 | | Internal corner radius | 125 – 200 mm | §4.2.2 d) | This is the same principle the most expensive kettle builders in Europe work to. It is not a secret and it is not proprietary — it is just slower and more expensive than cold bending, so not everyone does it. ## What happens before and after the press - **Plate and die surfaces cleaned before pressing** -- Debris pressed into a plate face becomes a defect that no later process removes. - **Scale and oxidation removed from bevels after forming** -- Before assembly, back to bright metal — otherwise it ends up in the weld. - **Edges trimmed or flame-straightened after forming** -- Any bevel touched in the process is re-prepared. - **Assembly platform flatness ≤ 1 mm/m²** -- A kettle assembled on an uneven platform is built out of shape from the start. ## Where forming meets welding Warm forming and electroslag welding are two halves of the same objective: put as little residual stress into the vessel as possible before it ever sees zinc. The corner radii those two processes produce are also what decides which geometry suits your bath depth. Source: BG/T 1501-2015 — §4.2.2, §5.3, §5.4. --- ## Inspection and testing — and what the standard numbers actually mean URL: https://kunwukettle.com/technology/quality-control/ Chinese standard numbers are not a discount. On the tests that matter for a kettle, the Chinese standard and the ISO standard are the same document. ## Standards, translated **Standards mapping** | What | We test to | Which is | US equivalent | |---|---|---|---| | Weld ultrasonic testing | GB/T 11345-2023 | Identical (IDT) to ISO 17640:2018 | ASME BPVC V Art. 5 | | Weld UT acceptance | GB/T 29712 | Modified from ISO 11666 | — | | Plate ultrasonic testing | GB/T 2970 | Comparable to EN 10160 | ASTM A578 | | Penetrant testing | JB/T 4730.5 | Comparable to ISO 3452 | ASME BPVC V Art. 6 | | Weld quality classes | BG/T 1501-2015 Table 7 | — | — | | Radiographic testing (optional) | GB/T 3323 | Comparable to ISO 17636 | ASME BPVC V Art. 2 | For reference: ISO 11666 acceptance levels 2 and 3 correspond to quality levels B and C of ISO 5817. Reports are issued in English. This mapping exists because “we test to GB/T 11345” means nothing to an inspector in Rotterdam, while “identical to ISO 17640:2018” means exactly what it says. Same test, same document, different number on the cover. ## Inspection and test plan | Stage | Method | Standard | |---|---|---| | Incoming plate | Ultrasonic, plate by plate | GB/T 2970 Class III | | Incoming plate | Chemical re-verification | GB/T 222 | | Incoming plate | Mechanical re-verification | GB/T 228.1 | | Cutting | Material tracking — contract, heat number, grade and size carried onto every piece | BG/T 1501-2015 §5.2.1 | | Forming | Warm forming temperature by infrared pyrometer; thinning ≤ 5 % | §5.3 | | Fit-up | Platform flatness ≤ 1 mm/m²; local gap ≤ 5 mm over less than 20 % of seam length | §5.4 | | Weld preparation | Back-gouge, grind, penetrant test where required | §5.5.3 | | Structural welds | Ultrasonic testing | GB/T 11345-2023 (= ISO 17640:2018) | | Weld metal | Silicon content | Si ≤ 0.03 % — Table 10, Grade 1 | | Dimensional | Tolerance class B | Table 8 | | Coating | Visual and dry film thickness | §6.3 | **Coverage is agreed with you before we start, and written into the inspection and test plan.** Full-length testing of every structural weld and partial coverage are both legitimate specifications — what is not legitimate is leaving it vague and letting each side assume. Tell us what your inspector expects, or ask us what we propose, and it goes in the ITP where it can be checked. ## Product grades BG/T 1501-2015 defines two finished-product grades. **Which one applies to your kettle is written into the order** — the difference is not cosmetic, and it is worth knowing which one you are buying. | | Grade 1 | Grade 2 | |---|---|---| | Plate | Si ≤ 0.02 %, P ≤ 0.015 %, S ≤ 0.010 % | Si ≤ 0.03 %, P ≤ 0.025 %, S ≤ 0.020 % | | Plate UT | GB/T 2970 Class III | GB/T 2970 Class IV | | Weld class | I, II | III | | Weld metal Si | ≤ 0.03 % | ≤ 0.12 % | | Dimensional tolerance | Class B | Class C | | Geometric tolerance | Class E | Class F | ## Witness it yourself Certificates are easy to produce. An inspection you watched is not. The plant is open for pre-shipment inspection. Send your own inspector, or appoint a third-party agency you already use, and we will schedule the ultrasonic testing so it can be witnessed — and send you the inspection and test plan in advance, so your inspector arrives knowing exactly what to look at. That is a stronger check than anything we could send you after the fact, and it is the one we would want if we were buying. What comes out of all this testing goes to you as paper: see the documentation package. For why the weld silicon line in the table above matters more than it looks, see the weld is the weakest link. Source: BG/T 1501-2015 — §5, §6, Tables 7, 8, 10. GB/T 11345-2023 is an identical adoption (IDT) of ISO 17640:2018. --- ## What ships with every kettle URL: https://kunwukettle.com/technology/documentation-package/ Five documents. The fifth one is the reason the other four matter. - **Mill test certificate — EN 10204 Type 3.1** -- Issued by the steel mill and signed by an inspection representative **independent of production**. Covers heat number, full chemical analysis (C / Si / Mn / P / S / Als), mechanical properties and ultrasonic inspection result. **Type 3.2**, countersigned by a third party or by your own inspector, is available where your specification requires it — tell us before we place the plate order, because it cannot be added retrospectively without re-sampling. - **Welding consumable certificates** -- Including the low-silicon flux used to hold weld metal silicon to specification. - **Manufacturing inspection reports** -- Dimensional check against drawing, fit-up records, weld visual inspection, coating thickness. - **NDT reports** -- Plate ultrasonic inspection records and ultrasonic testing of every structural weld, to GB/T 11345-2023 (identical to ISO 17640:2018). Issued in English. - **Commissioning heat-up curve** -- The controlled firing schedule for your first heat, specific to your kettle's dimensions and wall thickness. ## Why the heat-up curve is in the package A kettle that is fired too fast on day one will not reach its design life, no matter how well it was built. BG/T 1501-2015 Appendix A sets out why. On first firing, the wall has no Fe-Zn alloy layer yet, so the risk of intergranular attack and cracking is at its highest of the kettle’s whole life. At the same time the vessel is releasing manufacturing and welding stress, and the refractory is still giving up moisture. _Figure: The schedule in BG/T 1501-2015 Appendix A.2.2, plotted to scale. Ambient to 300 °C at 2 °C/h is about six days on its own. The flat section is not slack in the programme — it is where the kettle releases the stress from its own manufacture._ **The curve that manages all of that** | Stage | Rate or hold | |---|---| | Ambient → 300 °C | ≤ 2 °C/h | | 100 – 120 °C | Hold 6 – 8 h — drives moisture out of new refractory | | 300 – 330 °C | Hold 24 h — this is where kettle distortion and residual stress release | | 300 → 420 °C | ≤ 5 °C/h | | Throughout | Wall inside-to-outside differential < 50 °C; wall-to-base differential < 100 °C | Plus the loading rules that go with it: charge with **pure zinc ingot above 99.99 %**, no lead in the base, ingots stacked tight against the wall because the kettle is side-heated, and a **100 mm gap left in the middle of the stack** — zinc expands roughly three times as much as iron, and a tightly packed charge will push on the wall as it heats. Most of this is not in the sales conversation, and it should be. It is the difference between a kettle that lasts eight years and one that lasts eighteen months. Full commissioning guide → ## Marking Every kettle is marked on both flanks with product name, dimensions, manufacturer, date of manufacture, the standard it was built to, and works serial number (BG/T 1501-2015 §8.1). If you need additional marking for your asset register, tell us before painting. The tests behind reports 3 and 4 are set out in full on the inspection and test plan, and what we do and do not certify is on standards and compliance. Source: BG/T 1501-2015 — §6.5, §8.1, Appendix A.2.1–A.2.2. EN 10204 inspection document types. --- ## CBAM and imported galvanizing kettles URL: https://kunwukettle.com/compliance/cbam-guidance/ A galvanizing kettle is a CBAM good. If you are importing one into the EU, this affects you, and it is worth understanding before you compare quotations. This page is general guidance, **not legal or customs advice**. Confirm your own position with your customs representative. ## Kettles are in scope Galvanizing kettles classify under **CN 7309** — reservoirs, tanks and vats of iron or steel, capacity over 300 litres, not fitted with mechanical or thermal equipment. CN 7309 sits in Chapter 73 and is within the scope of CBAM Regulation (EU) 2023/956, Annex I. ## The 50-tonne threshold matters more here than in most sectors An importer whose total annual imports of CBAM goods stay **under 50 tonnes** is exempt from CBAM obligations entirely. A single large galvanizing kettle can weigh 20 to 85 tonnes. **One kettle can take you over the threshold on its own.** If you are over it, the obligations fall on you as the importer (or on your indirect customs representative): authorised CBAM declarant status, a verified declaration of embedded emissions, and surrender of CBAM certificates priced against the EU ETS carbon price. Worth checking before you assume the burden applies. If your kettle is small, and your total annual CBAM-good imports also stay under 50 tonnes, you have no CBAM obligation at all. ## Why the emissions data your supplier gives you changes your cost For iron and steel, only **direct emissions** are counted in the definitive period. Two calculation methods are accepted: a calculation-based method using activity data and laboratory-derived emission factors, or a mass balance method. Data must be at **installation level**. Here is the part that affects your invoice. If your supplier cannot give you verified actual emissions data, you fall back on **EU default values** — and default values are deliberately set above typical actual performance. Using verified actual data instead can materially reduce your certificate cost. So when you compare two quotations for the same kettle, the landed cost difference is not only the price. It is the price **plus** the CBAM certificates, and those depend on data your supplier either has or does not have. Kettle weight is what drives your exposure, so it is worth sizing before you model the cost — the sizing guide gets you to a weight, and the product range lists weights by configuration. For what else we do and do not claim on the regulatory side, see standards and compliance. ## Emissions data We are working with our plate mill and our works to prepare installation-level embedded emissions data to the CBAM methodology, so that you can declare actual values rather than fall back on EU default values. If you are evaluating a kettle purchase now and need to model your CBAM exposure, **tell us and we will tell you exactly what we can and cannot supply today.** We would rather give you an honest answer than a certificate that does not hold up at declaration. Source: Regulation (EU) 2023/956 (CBAM), Annex I. Sector guidance for iron and steel. Figures current at time of writing; confirm with your customs representative. ## Modelling your landed cost? Tell us the kettle size you are considering and your country of import, and we will tell you what we can supply toward your declaration.Ask us --- ## The works URL: https://kunwukettle.com/about/manufacturing-facility/ 10,000 m² in Hebei province, building one thing: heavy welded steel vessels for molten metal service. _Figure: The main fabrication bay. The gantry is rated 32 t._ ## Process line Plate verification → CNC cutting and bevelling → warm forming (850–900 °C) → fit-up → electroslag welding → stress relief → NDT → shot blasting → painting → marking → despatch **Equipment** | Stage | Equipment | |---|---| | Cutting | CNC plate cutting and bevelling | | Forming | Hydraulic press, warm forming at 850 – 900 °C | | Welding | Consumable-nozzle electroslag welding (ESW), submerged arc, manual metal arc | | Heat treatment | Stress-relief furnace, 600 – 650 °C | | NDT | Ultrasonic testing of plate and welds | | Surface | Shot blast and paint, in house | | Lifting | 32 t overhead gantry over the main bay | **A kettle can weigh more than the crane that built it.** The gantry handles shells through fabrication; the heaviest finished units are moved, turned and loaded with lifting arrangements planned for that specific kettle. This matters at your end too. Before you order, we will tell you what lift capacity and what access your site needs to receive the kettle — a unit that arrives at a plant which cannot unload it is a very expensive problem, and an avoidable one. ## Why blast and paint being in house matters It is the least glamorous line in the table and it decides more than it looks. A kettle that leaves the works to be blasted somewhere else has spent time uncoated, usually outdoors, and arrives at the coating stage with exactly the surface condition you do not want under paint. In house, the gap between the last weld and the first coat is measured in hours. Surface preparation grade and coating system are specified on your order — see the product pages. Every photograph on this site is of the actual works. We do not use renderings, and we do not use stock images of somebody else’s factory. Where we do not have a photograph yet, this site says so rather than filling the gap. ## What happens at each stage The equipment list above is only half the story. What each stage is for — and what it costs if it is skipped — is set out on the steel specification, warm forming, electroslag welding and quality control. ## Come and see it Factory visits are normal in this business and we welcome them. Tell us when you are in the region.Arrange a visit --- ## The weld is the weakest link: silicon in galvanizing kettle welds URL: https://kunwukettle.com/weld-silicon-galvanizing-kettle/ Most conversations about kettle life are conversations about plate. They are the right questions. They are just incomplete. Which steel, how low the silicon, how thick the wall. Those are the right questions, and they are incomplete — because a kettle is not made of plate. It is made of plate **and weld metal**, and weld metal is a different steel with a different chemistry, deposited by a different process, from a different consumable. When a kettle goes through at a seam while the surrounding wall is still comfortably thick, this is usually why. ## Why silicon attacks a kettle Molten zinc dissolves iron. At the interface, iron and zinc form an Fe-Zn alloy layer, and the fate of the kettle depends on what that layer is like. In a low-silicon steel, the alloy layer is dense and adherent. It sits on the wall and slows further attack. Raise the silicon and the layer becomes loose and non-adherent — it detaches, exposes fresh iron underneath, and dissolution continues at close to its initial rate. Phosphorus and aluminium do the same thing by the same mechanism. This is why BG/T 1501-2015 specifies plate at **Si ≤ 0.03 %** in its baseline requirement (Table 1), tightens it to **Si ≤ 0.01 %** in its recommended specification (Table 2), and adds a combined limit: **Si + P + Al ≤ 0.045 %**. It is also why stainless and heat-resisting steels — which beat plain carbon steel almost everywhere else — resist molten zinc **worse**. The standard says so directly in §3: alloying and impurity elements mostly increase the corrosion rate of steel in a zinc bath. ## Now apply the same logic to the weld If silicon in the plate accelerates attack, silicon in the weld metal does exactly the same thing. And weld metal silicon is not automatically low. It depends entirely on the welding process and consumable. **BG/T 1501-2015 Tables 5 and 6 — typical deposited chemistry** | Process | Consumable | Typical deposited Si | Note | |---|---|---|---| | Manual metal arc | E4303 (J422) | 0.13 % | Slow, but silicon is manageable | | Manual metal arc | CJ427A | 0.04 % | Low silicon | | Semi-automatic gas shielded | ER50-6 + CO₂ | 0.86 % | Very high silicon. The standard restricts this to flange work | | Submerged arc | H08A + HJ431 flux | High | The standard flags this explicitly | | Submerged arc | H08A + low-silicon kettle flux | ≤ 0.03 % | The one you want | Look at the ER50-6 line. **0.86 % silicon** — roughly **thirty times** the 0.03 % limit specified for the plate. A kettle built from carefully specified low-silicon plate and then welded with a high-silicon consumable has a set of seams that will dissolve far faster than the wall around them. The plate certificate will be perfect. The kettle will still go through at the seam. ## What the standard requires BG/T 1501-2015 Table 10 grades finished kettles on six criteria. Two of them are about welds. | | Grade 1 | Grade 2 | |---|---|---| | Weld quality class | I, II | III | | Weld metal silicon | ≤ 0.03 % | ≤ 0.12 % | Grade 1 holds weld metal to the same silicon limit as the plate. Grade 2 allows four times as much. Note also what the standard does **not** require of a kettle weld: there is no impact energy requirement anywhere in Table 10. A galvanizing kettle runs at a constant 445–460 °C under hydrostatic load. It is not an impact-loaded structure, and low-temperature toughness is not what decides its life. Silicon and internal soundness are. ## How the process choices follow Two consequences fall out of this. **First, the flux matters as much as the wire.** The difference between the last two rows of the table above is not the electrode — both are H08A. It is the flux. Standard HJ431 puts silicon into the deposit; a low-silicon kettle flux holds it under 0.03 %. Same wire, same machine, order-of-magnitude difference in the number that decides kettle life. **Second, gas shielded welding gets restricted to where it cannot hurt you.** BG/T 1501-2015 Table 5 permits ER50-6 with CO₂ specifically for flange work — the top edge, above the bath, where the weld never sees molten zinc. Below the zinc line, it does not belong. The standard’s default arrangement (§5.5.3) is manual metal arc on the inner wall and submerged arc on the outer wall — with the note that if a low-silicon flux is used, submerged arc can be used on the inner wall too. ## The questions worth asking your supplier - **What is your weld metal silicon limit, and how do you verify it?** If the answer is a number, ask to see it on a report. If the answer is a description of their quality culture, that is not an answer. - **Which flux, on the inner wall?** Standard submerged arc flux below the zinc line is a red flag. - **Are you building to Grade 1 or Grade 2 of Table 10?** The difference is 0.03 % versus 0.12 % on the seams. - **What is the NDT standard and acceptance level for the structural welds?** Silicon governs how fast a sound weld dissolves. It says nothing about a weld that had a defect in it to begin with. ## Where we stand We build to Grade 1: weld metal **Si ≤ 0.03 %**, using low-silicon flux. Every structural weld is ultrasonically tested to **GB/T 11345-2023**, which is identical (IDT) to **ISO 17640:2018**, with acceptance to GB/T 29712. We publish this because we would want to see it if we were buying, and because as far as we can tell nobody else does. Source: BG/T 1501-2015, _Steel Hot-Dip Galvanizing Kettles_, China Surface Engineering Association — §3.2, §5.5.3, Tables 1, 2, 5, 6, 10. ## How we weld Electroslag welding, ISO 4063 process 72, with low-silicon flux and 100 % ultrasonic testing of every structural weld.Electroslag weldingInspection and test plan --- ## What actually determines galvanizing kettle life URL: https://kunwukettle.com/galvanizing-kettle-life/ Ask five kettle suppliers how long their kettles last and you will get five answers between three and ten years. The spread is real, and it is not mostly about the kettle. ## The number that matters: corrosion rate BG/T 1501-2015, Appendix A.2.3.4, states it plainly: follow the operating rules and **the kettle corrosion rate can be held to 1.0–2.5 mm per year**. On a 50 mm wall, that is a straightforward calculation. What the standard is really saying is that kettle life is a **controlled variable**, not a property of the kettle — provided the kettle was built right in the first place. So there are two questions, not one. Was the kettle built so that a good corrosion rate is achievable? And is the furnace and the operation keeping it there? Most disappointing kettle lives are failures of the second question blamed on the first. ## Question 1 — what the kettle has to get right Four things are fixed before the kettle ships and cannot be corrected afterwards. **Plate chemistry.** Si ≤ 0.03 %, and Si + P + Al ≤ 0.045 % combined. Silicon, phosphorus and aluminium make the Fe-Zn alloy layer non-adherent, so it sloughs off and attack continues instead of slowing. Carbon, counter-intuitively, should _not_ be minimised — the standard advises against plate below 0.05 % C, because the wall needs strength to resist the static pressure of zinc at temperature. **Weld metal chemistry.** Weld silicon can be thirty times plate silicon depending on the consumable. A kettle with low-silicon plate and high-silicon seams fails at the seams. Covered separately here. **Geometry.** Above 2 m depth the standard calls for a full rounded corner, because a square wall-to-base weld sits exactly where hydrostatic pressure is highest, and the internal corner traps dross that then insulates and drives local overheating. **Residual stress.** All forming done warm at 850–900 °C rather than cold. No cruciform plate joints. Parallel welds at least 0.5 m apart. Double-bevel joints welded alternately from both sides. ## Question 2 — what the furnace and the operator have to get right This is where the three-to-ten-year spread actually comes from. **Inner wall temperature is the master variable.** | Temperature | What happens | |---|---| | 445 – 460 °C | Normal bath operating range | | Below 480 °C | Inner wall safe zone | | Above 480 °C | Corrosion accelerates sharply | | 490 – 510 °C | Peak corrosion band | Inner wall temperature is hard to measure directly. The standard’s practical proxy: the difference between bath temperature and outer wall temperature normally runs 30–50 °C, tightening to 15–20 °C during steady balanced production. **Heat input density is what pushes the wall temperature up.** **Appendix A.2.4, Table A.1** | Bath temperature | Permitted heat input | Critical hourly throughput incl. jigs | |---|---|---| | 445 °C | 31 kW/(m²·h) | 1/30 of zinc volume | | 460 °C | 24 kW/(m²·h) | 1/40 of zinc volume | Note the direction of that relationship. **A lower bath temperature permits a higher heat input.** If your bath is well sized, running cooler lets you run harder. **Furnace chamber temperature.** Below 700 °C on oil or producer gas. Below 650 °C on natural gas or electric heating. **Throughput discipline.** Hourly throughput at 1/30 to 1/40 of zinc volume, and — the standard is explicit about this — **run it level**. Chasing output by raising heat input inevitably shortens kettle life. Overloading is called out by name. **Logging.** Temperatures recorded every 30 minutes. Not for bureaucracy: without a record you cannot see the trend that is quietly killing the kettle. ## The first 48 hours are worth more than the first year On first firing, the wall has no Fe-Zn alloy layer at all. This is the moment of highest risk of intergranular attack and cracking in the kettle’s entire life. At the same time the vessel is releasing manufacturing stress and the refractory is giving up moisture. **Appendix A.2.2** | Stage | Requirement | |---|---| | Ambient → 300 °C | ≤ 2 °C/h | | 100 – 120 °C | Hold 6 – 8 h — drives moisture out of new refractory | | 300 – 330 °C | Hold 24 h — distortion and residual stress release | | 300 → 420 °C | ≤ 5 °C/h | | Throughout | Wall inside-to-outside differential < 50 °C; wall-to-base < 100 °C | And the loading rules that go with it: charge with pure zinc above **99.99 %**, no lead in the base, ingots stacked tight against the wall (the kettle is side-heated), and a **100 mm gap left in the middle of the stack** — zinc expands about three times as much as iron, and a tight charge will push on the wall as it heats. A kettle rushed through this schedule will not reach its design life regardless of how it was built. This is the single largest avoidable loss in the industry, and it costs nothing to avoid. Full commissioning guide → ## Design decisions that buy you years **Go deeper.** Depth is normally 1.3–2 × width. Deeper means more thermal mass and less temperature swing when you charge; more wall area, so lower heat input per m²; and dross settling well below the work. **Size the zinc volume to throughput, not to the workpiece.** Zinc volume should be **30–40 × maximum hourly throughput including jigs** (20 × for wire). Under-size it and the only way to hold temperature is more heat input — straight into the danger band. **Protect the kettle from the burners.** Baffle or heat shield at the nozzles; insulation from 100 mm above the dross line downwards, so the wall does not take direct radiant heat where it is most vulnerable. **Support it properly.** Above 1.8 m depth, one adjustable support band; above 2.5 m, two. Support points at 1.5–2.0 m intervals. Adjustable, because the gap changes with temperature. Heat-resisting steel for anything inside the hot zone. Expansion clearance along the length. **Fit a zinc leak alarm.** Escaping zinc shorts the detector, triggers the alarm and shuts down heating. Where site conditions allow, an emergency zinc pit at 25–30 % of total volume, conical for recovery. ## So how long does a kettle last? If the kettle was built to the chemistry and geometry above, and the furnace holds the inner wall below 480 °C at a heat input inside Table A.1 with level throughput and a proper commissioning — the corrosion rate stays at **1.0–2.5 mm/year**, and you can work out your own answer from your wall thickness. If any of those are missing, the number the supplier quoted you was never going to be the number you got. ## What to ask before you buy - What is the plate silicon limit, and the **weld metal** silicon limit? - Which finished-product grade, Grade 1 or Grade 2? - What NDT, to which standard, at what coverage? - **Will you supply the commissioning heat-up curve?** - What heat input density does your sizing assume? That last one is the tell. A supplier who has sized your kettle against your throughput has done the engineering. A supplier who has quoted you a box has not. Source: BG/T 1501-2015 — §3, §4.2, §5, Appendix A.1.2, A.1.5, A.1.6, A.2.1–A.2.4, Table A.1. ## Work out your own number Send your current kettle's dimensions, age and latest ultrasonic wall readings. We will tell you where you are on the curve.Replacement planningSizing guide --- ## The 24-hour hold at 330 °C URL: https://kunwukettle.com/commissioning-a-new-galvanizing-kettle/ A new kettle arrives, the plant has been down for the changeover, and everyone wants zinc in it. The pressure to fire it fast is the single most expensive pressure in this industry. ## Why the first firing is the most dangerous moment in the kettle’s life Three things are true simultaneously on day one, and never again. **The wall has no Fe-Zn alloy layer.** In service, a thin adherent alloy layer forms on the inner wall and slows further attack. On first firing there is bare iron in contact with molten zinc. BG/T 1501-2015 Appendix A.2.1.1 is explicit: the risk of **intergranular attack initiating cracks** is at its highest here. **The kettle is full of manufacturing stress.** Cutting, forming and welding all leave residual stress. If the kettle did not receive a full stress-relief anneal, that stress is still in it — and it will come out during the first heat, one way or another. Controlled, it relaxes. Uncontrolled, it distorts the vessel. **The refractory is wet.** New furnace linings hold moisture. Heat it too fast and that moisture turns to steam inside the lining. ## The schedule _Figure: The schedule in BG/T 1501-2015 Appendix A.2.2, plotted to scale. Ambient to 300 °C at 2 °C/h is about six days on its own. The flat section is not slack in the programme — it is where the kettle releases the stress from its own manufacture._ | Stage | Rate or hold | Why | |---|---|---| | Ambient → 300 °C | ≤ 2 °C/h | Limits thermal gradient through the wall | | 100 – 120 °C | Hold 6 – 8 h | Drives moisture out of new refractory | | 300 – 330 °C | Hold 24 h | Kettle distortion and residual stress release happen here | | 300 → 420 °C | ≤ 5 °C/h | | | ~419.5 °C | Zinc melts — natural hold | Top up ingot to working level | | → working temperature | Per process requirement | | Two limits apply throughout, and they are the ones to instrument for: - **Wall inside-to-outside differential < 50 °C** - **Wall-to-base differential < 100 °C** From ambient to 300 °C at 2 °C/h is about **six days**. Plus the 24-hour hold. Plus the climb to 420 °C. A proper commissioning is a week-long operation, and every hour of it is buying years at the other end. ## Why the 24-hour hold is the part not to cut Everything else in the schedule is a rate limit — go slower and you are safe. The 300–330 °C hold is different: it is a **soak**, and its value is entirely in its duration. This is the temperature band where the steel is hot enough for residual stress to relax, but the kettle is not yet carrying the load of molten zinc. Twenty-four hours here is the kettle settling into the shape it will keep. Cut it to eight hours and the stress does not disappear. It comes out later — under the hydrostatic load of a full bath, at working temperature, as distortion. For kettles that did **not** receive a full stress-relief anneal at the works, this hold is not just beneficial, it is the substitute for it. BG/T 1501-2015 §5.6.6 requires the manufacturer to supply a heat treatment curve in that case. **If your supplier did not send one, ask why.** ## Loading the kettle **Pure zinc only, above 99.99 %. No lead in the base.** With bare iron on the inner wall, purity matters more on the first charge than it ever will again. **Stack tight against the wall.** The kettle is side-heated. Heat transfers into the charge through wall contact — ingots stacked in the middle of an empty kettle heat slowly and unevenly. **Leave a 100 mm gap in the middle of the stack.** This is the rule that gets missed. Zinc’s coefficient of linear expansion is about three times that of iron. A tightly packed charge with no relief expands against the kettle wall as it heats, and pushes outward on a vessel that is simultaneously trying to release its own stress. Timber battens can steady the stack — they carbonise and float off once the zinc melts. **Cover the bath during melt-down.** A lid, or rock wool blanket on steel supports, to limit heat loss from the top while the kettle is being heated from the sides. ## Instrumentation before you start | | | |---|---| | Thermocouples | Type E or K, in protective sheaths of a thermally conductive, zinc-resistant material | | Bath measurement | At both ends of the kettle | | Chamber measurement | Multiple points along the kettle length, including at the flue inlet | | Outer wall | Additional points — the practical proxy for inner wall temperature | | Calibration | Thermocouples and instruments verified on a schedule | | Over-temperature alarm | Fitted and tested | | Zinc leak alarm | Fitted and tested before firing | | Logging | Every 30 minutes, recorded | ## And before any of that: what the kettle sat on while it waited If the kettle was delivered ahead of the furnace being ready: coat it against rust; block it clear of the ground, **one end higher than the other**; cover it; **never allow water to collect inside**; do not store with acids or alkalis. Before installation, clean off loose rust and scale mechanically — without roughening the surface. Lifting lugs welded to the top edge for installation get cut off afterwards. ## The base and the supports The foundation is reinforced concrete with a refractory insulating layer, level to **≤ 1.5 ‰**. The pit needs waterproofing or a sealed steel outer trough — a leak into a wet pit is a different order of incident than a leak into a dry one. Furnace wall and kettle top edge both insulated; outer surface not above 55 °C. Above 1.8 m depth (2.2 m for electric heating), one support band; above 2.5 m, two. Support points at 1.5–2.0 m. **Adjustable**, so the gap stays even as things move. Heat-resisting steel for anything in the hot zone. Expansion clearance along the length. ## The economics A week of controlled commissioning against a kettle that reaches its design life instead of half of it. On a kettle costing several hundred thousand dollars, plus the plant downtime of an unplanned changeover, the arithmetic is not close. We supply the curve with every kettle, sized to that kettle’s dimensions and wall thickness, and we will go through it with your team before you light the burners. Source: BG/T 1501-2015 — §5.6, §6.5, Appendix A.1.2–A.1.7, A.2.1, A.2.2. ## We supply the curve Sized to your kettle's dimensions and wall thickness, and we will walk your team through it before the first firing.The documentation packageService and support ---