Hot-Dip Galvanizing Kettle Cases: Kettle & Fixture Protection

A hot-dip galvanizing kettle case protects a family of parts that are simultaneously heavy, hot, corrosive and brittle. A welded steel kettle is a heavy low-carbon steel structure with walls commonly 30 to 70 mm thick, and poor lifting or poor blocking immediately produces distortion and flatness error. Ceramic kettle blocks and kettle support bricks are brittle: a single point contact can start a crack. Jigs, racks, sink rolls and roll shafts must hold their geometry while repeatedly carrying thermal load. And every part that will meet zinc at 445 to 460 C must be preheated and dried beforehand. The core conclusion is that a galvanizing kettle case must be compartmentalized under four logics: continuous beam-type load bearing for thick-wall kettle sections against distortion, independent flexible cells for ceramic parts against impact, profile location for jigs and roll shafts against bending, and a delivery file that carries preheat, drying and dross-cleaning requirements. Shipping on a plain pallet blocked with a couple of timber offcuts typically ends in kettle flatness out of tolerance and uneven heat transfer, cracked ceramic blocks, or water carried into the zinc bath and a violent splash that injures people.

The site contradictions in a kettle project differ from ordinary equipment parts. A kettle is progressively consumed by the reaction between molten zinc and iron, wall thickness thins year by year, and service life is measured empirically in roughly one to three years, so kettle replacement and spare stocking are routine events at a galvanizing plant rather than occasional ones. That means kettle parts travel often, both as new sections and as used parts returned for repair. A single kettle section can weigh several tonnes, and if it is lifted from only two points or laid on a discontinuous set of timber blocks, it arrives with residual bending; once set in the furnace the distance between kettle wall and heat source is no longer uniform, local overheating sharply shortens kettle life. Ceramic kettles are more extreme: high-alumina and corundum blocks have almost no plasticity at room temperature, so point-contact stress during handling is enough to open a through crack, and the crack then propagates rapidly under the thermal shock of 450 C zinc. Fixture problems centre on geometry, because once a jig main beam or arm bends, workpiece position and dwell time in the zinc lose accuracy and coating thickness and appearance vary with them. Most dangerous of all is water, because any moisture left on or inside a kettle part, jig or roll shaft flashes instantly on contact with zinc and throws molten metal outward. This article gives executable packing parameters, insert compartment plans, preheat and drying targets, and acceptance criteria organized by hot-dip galvanizing line part family, for use by galvanizing plant equipment departments, galvanizing equipment builders and spare-part stores when selecting, accepting and transferring parts.

Table of Contents

• Transport Environment and Case-Selection Boundary
• Steel Kettle: Anti-Distortion and Anti-Corrosion for Thick-Wall Welded Bodies
• Brittle No-Impact Handling for Ceramic Kettles and Support Bricks
• Preserving the Geometry of Jigs and Racks
• Precision Protection for Sink Rolls, Stabilizer Rolls and Roll Shafts
• Preheat and Drying Requirements before Entering the Kettle
• Cleaning of Dross, Coating Residue and Flux
• Heavy Lifting, Lifting Points and Saddle Design
• Packing Kettle Support Bricks and Structural Supports
• Insert Compartmentalization, Weight Distribution and Moisture Control
• Sealing, Pressure Equalization and Rain Protection on Site
• Labeling, Inspection and Unpacking Acceptance
• Coating Quality and Transport Test Basis
• FAQ
• Conclusion and Related Reading

Transport Environment and Case-Selection Boundary

Drawing a clear line between what goes into a case and what does not is the step where kettle projects most often drift. A complete large steel kettle, commonly 6 to 12 m long, a furnace shell already bricked around the kettle, and an integrated kettle assembly carrying insulation and a combustion system all exceed the load and lift capacity of a standard protective case; they move on dedicated steel frames and saddle cars with multi-point lashing, and forcing them into a case drags both the lashing system and the case into risk. What genuinely needs a dedicated case is six part families: kettle section plates and bottoms, kettle wall sections including weld-bevel parts; ceramic kettle blocks and shaped brick; kettle support bricks, support blocks and load beams; jigs, racks, baskets and clamping fixtures; sink rolls, stabilizer rolls, their shafts, bearing housings and arms; and complete sets of temperature and level instruments, thermowell tubes and fasteners. These six families share the trait that their value concentrates in geometric accuracy or surface integrity, they must be managed as a set, and one out-of-tolerance part drags down coating uniformity across the whole galvanizing line.

Case-selection principles compress into three rules. The first is to compartmentalize by load path rather than by weight: a thick-wall kettle section must transfer load through a continuous beam-type support into the case floor structure, while a jig or roll shaft mainly needs bending protection, and the two cannot be handled by the same support method. The second is to isolate by brittleness and cleanliness: ceramic parts mixed with steel parts mean scale and rust shed by steel become the origin of pressure marks on ceramic faces, while parts still carrying dross mixed with cleaned parts carry residue where it does not belong. The third is to set the moisture protection grade by cleaning and drying state: parts already cleaned and dried and ready for the kettle must be actively protected against moisture with recorded humidity, while uncleaned used parts must be cleaned before packing, otherwise hygroscopic salts remain inside the case for the whole storage period.

Steel Kettle: Anti-Distortion and Anti-Corrosion for Thick-Wall Welded Bodies

The welded steel kettle is the mainstream hot-dip galvanizing structure, usually low-carbon or low-alloy plate with walls commonly 30 to 70 mm thick (typical values), formed from a bottom, long side walls and end walls welded together. That thickness exists to resist the loss of high-temperature strength at 450 C and the dissolution of iron by molten zinc; it is not there to resist bending. Thick wall does not mean high stiffness, and a wall plate 6 to 12 m long supported only at its two ends will sag visibly at mid-span, leaving residual bow that is very difficult to correct after it is set in place.

Kettle distortion has systemic consequences. Flatness and perpendicularity error between kettle wall and heat source destroys the uniformity of the gap between them, so the locally overheated zone runs hotter, iron dissolves into the zinc faster there, and that zone becomes the first place to thin and fail. Flatness error in the kettle bottom lets dross and ash collect in the low spots, complicating skimming and accelerating local corrosion. Weld bevels are more delicate still: burrs and impact damage on a bevel face produce lack of fusion and slag inclusions when the site welds the sections, and every kettle weld is a weak point for corrosion and thermal stress. On corrosion, the outer kettle wall forms scale at temperature, and the exposed surface under spalled scale rusts rapidly in a humid environment; outer-wall rust does not touch the zinc directly, but it degrades heat transfer between the combustion gas and the wall and becomes a corrosion origin.

Five executable measures follow. First, kettle sections and wall plates always use continuous beam-type supports: the support beam runs the full length and lands on the case floor structure, and the support spacing is calculated from plate thickness and span so self-weight deflection stays within an acceptable range. Blocking with two timber offcuts at the ends only is prohibited. Second, only surface contact is allowed between a support face and the outer wall, with a soft non-metallic liner to prevent scratching; the liner must not absorb moisture or bleed oil. Third, weld bevel faces get guard plates or guard strips, and bevel parts are separated by dividers rather than laid face to face, so bevels cannot strike each other. Lifting points must never be placed on a bevel edge. Fourth, the kettle body needs both axial and lateral restraint inside the case: bent parts and kettle sections lifted in a free state will shift under transit impact, so fit stops at both ends with an elastic layer to absorb the shock. Fifth, the outer wall gets temporary rust protection before dispatch, and the treatment must be compatible with the site welding process, because residual rust-preventive coating near a bevel contaminates the weld. Bevel edges should be masked off, and this belongs in the packing work instruction.

Brittle No-Impact Handling for Ceramic Kettles and Support Bricks

Ceramic kettles, including high-alumina, corundum and quartz assembled structures, can deliver longer life and cleaner zinc than steel kettles under some process conditions, but their transport protection logic is the exact opposite of a steel kettle: a steel kettle fears bending, a ceramic kettle fears impact. Ceramic materials have high compressive strength and low tensile strength, with almost no elastic deformation at room temperature, so any loading that concentrates surface tensile stress, whether point contact, a pressed-in edge or a local hammer blow, can start a crack at the surface. The crack may not propagate immediately under load, but when the block meets 450 C zinc, thermal stress from the shock drives it through, and cascading failure across multiple assembled blocks usually begins there.

A second failure path is the accumulation of hidden damage during transport and storage. If assembled blocks micro-move against one another inside a case, the surfaces develop fine abrasion marks and a powdered layer, and that layer becomes a non-compact interlayer at bricking time and a preferential path for zinc penetration and attack. Shaped and corner bricks have edge stress concentrations; a chipped corner forces the site to fill with mortar, and because mortar and brick have different thermal expansion behaviour, a new crack forms under repeated thermal cycling. Kettle support bricks carry the weight of the whole kettle and its molten zinc, and once they take up moisture their high-temperature strength drops noticeably, producing crushing and settlement at the load-bearing positions after long service.

Five executable measures follow. First, ceramic parts always get their own flexible cells, with soft liners between cell wall and part on a surface-contact basis. Point or line contact is prohibited, as is gripping an edge with a claw. Second, adjacent ceramic parts are separated completely by a hard divider rather than simply by a soft pad, because a soft pad still permits relative displacement and rubbing under vibration. Third, edges and corners get formed guards, and guard material must be non-metallic; bare metal guards are not acceptable. Fourth, support and structural bricks are stored on single-layer trays and never stacked. The batch receives active moisture protection with humidity indicator cards, and if bricks arrive visibly wet they should be dried before bricking rather than used directly. Fifth, handling must not use a steel hammer or a pry bar directly on ceramic parts. Move them with soft blocks and by hand, and state this explicitly at the unloading site, because the step between unpacking and bricking is precisely where ceramic parts most often fail.

Preserving the Geometry of Jigs and Racks

Jigs, racks, baskets and clamping fixtures are the most numerous and most frequently replaced parts on a hot-dip galvanizing line, and their value lies almost entirely in geometric form. The main beam, arms and workpiece support points together determine a workpiece's position, attitude and dwell time in the zinc, and coating thickness and appearance follow from those three. Once the main beam bows, workpieces in the same batch do not reach the same depth: shallower workpieces heat more slowly and receive less effective zinc contact time, appearing as thin or missed coating, while deeper ones may approach the bottom dross zone and pick up dross particles. Twisted arms make workpieces touch each other, and the contacting faces cannot exchange zinc properly, producing dull dead-coated areas and zinc nodules that are hard to remove in later finishing.

The mechanism is straightforward. A jig carrying load at 450 C creeps gradually, and that deformation is a designed-in progressive process; extra deformation during transport and storage is not part of that allowance. Bundled stacking, single-point suspension, using a jig as a support for other parts, and running wire rope directly across a main beam all introduce bowing and twisting. Welds and bends on a jig are stress concentration points, and repeated transit impact initiates cracks there. In addition, repeated zinc immersion builds a thick zinc-iron alloy layer on jig surfaces, and that layer spalls in a brittle manner under impact; the spall is both a hard contaminant inside the case and a hazard when it becomes airborne, so a jig returned for repair must be cleaned before packing.

Five executable measures follow. First, jigs sit in profile saddles that carry the straight section of the main beam with several support points along the span, so self-weight and transit impact are shared. Supporting only at the two ends, or using an arm as a support point, is prohibited. Second, bundled stacking is strictly prohibited; if several pieces must share a cell, separate them with hard dividers at least two-thirds the jig height. Third, lift with a wide sling cradling both ends of the main beam. Running wire rope or chain directly across a main beam or a bend is prohibited, as is placing a lifting point near a weld. Fourth, a jig returned for repair is cleaned of zinc before packing, then given temporary rust protection and dried, so residue and moisture do not enter the case together. Fifth, jigs are used in matched sets, so they should be numbered and packed by set; shipping mixed sets leads the site to pair jigs of different wear levels, and workpieces in one batch then see different thermal conditions.

Precision Protection for Sink Rolls, Stabilizer Rolls and Roll Shafts

Sink rolls and stabilizer rolls on a continuous galvanizing line operate submerged in the kettle, and roll-face condition, body roundness and journal accuracy together determine strip stability and coating uniformity. The protection difficulty lies in the combination of long, slender and heavy: a roll body is typically a cylinder with a high length-to-diameter ratio, its self-weight is significant while its stiffness is moderate, the journals at each end are precision-machined faces with tight tolerances and surface finish requirements, and the bearing housings and arms are assembly references. Any loss of accuracy in transit shows up on site as strip wandering, transverse coating variation or increased vibration.

Damage mechanisms fall into three groups. The first is roll body bending and sagging: with the two journals as the only supports, mid-span sag on a long roll converts into permanent bow under transit impact, and roll roundness goes out of tolerance with it. The second is surface damage to roll face and journals: after long service in zinc, the roll face carries an alloy layer that spalls brittlely when struck, and the resulting pits become collection points for dross; a scratched journal damages the fit with the bearing housing, producing excess clearance and grease leakage. The third is damage to bearing housings and seals: housings in the kettle area usually use a high-temperature design with seals, and seals fear heat, compression set and dust ingress in transit.

Five executable measures follow. First, roll bodies use V-shaped or large-radius curved saddles with at least three support points along the body length so mid-span sag stays small; saddle inner arcs carry a soft, oil-resistant liner that does not bleed plasticizer. Second, the roll face gets a peelable low-tack protective film chosen for no adhesive residue, and each journal gets a protective sleeve with a soft inner face; a metal sleeve directly on a journal is prohibited. Third, bearing housings and arms go into their own cells, are never placed under other parts, and never carry roll weight through the housing. Fourth, seals and grease are packed by size in separate containers, seals stored away from light and heat, and grease must not contact the packing liner directly, since liner material can swell in oil. Fifth, lift with a wide sling cradling both ends of the roll body; using a journal or bearing housing as a lifting point is prohibited, as is letting the roll body slide across the sling. Where a project also involves transferring other rotating parts, the support-point layout described for hoist drum and shaft support in winch parts cases is a useful reference, but galvanizing rolls additionally need protection against brittle spalling of the roll-face alloy layer, which raises the protection grade.

Preheat and Drying Requirements before Entering the Kettle

One of the most dangerous operating risks in hot-dip galvanizing is a zinc splash. When a part carries moisture on its surface or inside a cavity, contact with zinc at 445 to 460 C flashes that water to steam that expands violently, throwing molten zinc and ash outward. The result ranges from lost zinc and coating defects to serious burns. Every part that will meet or approach the zinc, which for kettle bodies means jigs, racks, baskets, sink rolls, roll shafts, thermowell tubes and tooling fixtures, must therefore be preheated and dried first. This is fundamentally a delivery-file item, but it must be made explicit in the packing scheme: whatever the case does about moisture, the site must match with drying.

Four executable measures follow. First, define the preheat band and hold time: use 120 to 150 C (empirical) as the preheat band and set hold time from part weight and section thickness. The goal is to let moisture escape from the surface and from internal cavities, not merely to make the surface look dry. Second, focus on cavities and blind holes: tubular jig structures, roll-shaft keyways and bolt blind holes all trap water, so blow them dry with compressed air before packing and re-check on arrival, using a borescope where necessary. Third, set a defined waiting limit and a second confirmation step between drying and entering the kettle, so a part left open in wet weather is not put straight into the zinc. Fourth, case moisture control and site drying must work together: if the case carries desiccant and humidity indicator cards, read and record the cards first and use that to set the drying duration, extending it when the cards show significant moisture uptake rather than running a fixed time.

Cleaning of Dross, Coating Residue and Flux

Parts removed from service or replaced carry three kinds of residue, and all must be dealt with before packing or they will persist inside the case and cause new damage. The first is dross and zinc ash: dross is a precipitated zinc-iron intermetallic phase, hard and sharply angular, and if left in a case it becomes the hard particle that scratches kettle inner walls and roll faces; zinc ash is mainly zinc oxide, and once it takes up moisture it turns alkaline and accelerates rust on steel outer walls. The second is coating and alloy layer residue: the zinc-iron alloy layer on jigs and roll faces is brittle, and fragments spalling under transit impact contaminate other parts and create a handling hazard at unpacking. The third is flux residue: zinc ammonium chloride flux is strongly hygroscopic, and residue inside a case keeps absorbing water, forming a corrosive liquid film on steel surfaces and contaminating the insert as well.

Four executable measures follow. First, complete mechanical cleaning before packing: remove dross from kettle inner and outer walls with non-metallic scrapers. Chiselling the wall with a steel chisel is prohibited even on thick wall steel, because a chisel mark is also a stress concentration point. Second, clean the alloy layer from jigs and roll faces by a controlled process, then apply immediate temporary protection, so the surface does not rust in the gap between cleaning and packing. Third, parts with flux residue must be washed and dried before packing; if this cannot be done immediately, give them their own sealed cell so their hygroscopic behaviour does not affect the rest of the case. Fourth, cleaned parts and uncleaned used parts must never ship in the same case, and this is the most commonly overlooked rule: one mixed load is enough to recontaminate a freshly cleaned kettle wall with salts, wasting the entire cleaning effort. When customizing inserts for such repair parts, JUNZHIJIA normally distinguishes a clean cell from a to-be-cleaned cell in the insert structure and marks the cleaning state on the outside of the case, so the site does not mix the two back together.

Heavy Lifting, Lifting Points and Saddle Design

Kettle sections, ceramic assemblies, jig sets and rolls range from several hundred kilograms to several tonnes each. After packing, the load path must run completely from the workpiece into the liner, into the case floor, then into the pallet and the transport vehicle, and the most commonly overlooked element is dynamic amplification: load peaks from uneven pavement reach 1.5 to 3 times the static load (empirical), and a large long part such as a kettle section also amplifies impact through sway inside the case. If the liner is designed for static load only, transit produces liner compaction, part displacement and even a cracked case floor.

Five executable measures follow. First, the case floor carries load beams, or a load-bearing pallet carries the load, so heavy parts transfer into the vehicle directly rather than through the case walls and corner fittings. Second, saddle contact area is set by contact stress: the allowable contact stress for soft material against a kettle outer wall is commonly taken as 2 to 5 MPa (empirical) and lower for ceramic contact, from which contact area and support width are back-calculated and the calculation kept in the design file. Third, lashing angle stays between 30 and 45 degrees, since vertical downward tightening provides only friction restraint and slips once the liner compacts; lashing must also never cross a weld bevel or a ceramic edge. Fourth, lifting points and the centre of gravity are marked on the outside of the case; for eccentric loading, add counterweight or adjust cell positions so the case does not tilt during lifting. Fifth, a long heavy part must not be suspended from a single support or allowed to sag at mid-span; set the number of support points from the span, adding an intermediate support beam inside the case where needed. Where a project also covers other heavy handling equipment, the lifting point and saddle practice for lift mechanism cases is a useful reference, but kettle parts are special in that they carry both a high-temperature service history and ambient transport loads, so the calculation should superimpose the two.

Packing Kettle Support Bricks and Structural Supports

The support system holding the kettle in the furnace consists of support bricks, structural bricks and load beams, which transfer the weight of the kettle and its molten zinc into the furnace foundation. Support bricks are usually high-alumina or clay refractory brick with tight dimensional requirements, because the contact condition between kettle bottom and brick determines how evenly the bottom is heated, and poor contact produces local overheating at the one place on a kettle that is hardest to repair. Structural bricks also carry concentrated load, and once they absorb moisture their high-temperature compressive strength drops noticeably, producing crushing and settlement at bearing positions in service.

Support brick damage follows three patterns: chipped edges and corners, strength loss from moisture, and dimensional deviation. Chipping happens mainly during handling and stacking; moisture comes from the storage environment; dimensional deviation is partly manufacturing tolerance and partly deformation under load during packing, since refractory brick, although brittle, can still take a slight plastic set and edge impression under long-term compression, enough to affect the fit at bricking.

Four executable measures follow. First, support and structural bricks are stored on single-layer trays and never stacked. Where multiple layers are unavoidable, use a continuous divider between layers and align upper and lower support points vertically. Second, apply active moisture protection across the batch with low-shedding desiccant and humidity indicator cards; if bricks arrive visibly wet, dry them before bricking rather than using them directly. Third, the bearing face of a brick, the face contacting the kettle bottom, faces up or to the side and is never used as a supporting surface under other parts. Fourth, matching load beams and metal supports go into their own cells, completely separated from refractory brick, so metal rust does not contaminate brick faces. Where a project also covers other refractory parts of the furnace, the moisture-control practice for industrial kiln cases and refractory linings is a useful reference. Both share the principle that refractory parts fear moisture more than impact, but kettle support bricks have tighter dimensional requirements, because their deviation translates directly into uneven heating of the kettle bottom.

Insert Compartmentalization, Weight Distribution and Moisture Control

The insert of a galvanizing kettle parts case must satisfy four functions at once: a continuous bearing surface for heavy parts, flexible location for brittle parts, separation of metal parts from ceramic and dross-bearing parts, and a controllable-humidity closed environment for parts already cleaned and dried. These functions pull against one another, since bearing needs stiffness, flexible location needs compliance, separation needs a hard divider and moisture control needs enclosure, so the insert structure has to allocate space and material among them. Insert materials differ widely in hardness, resilience and moisture behaviour, and the selection logic can follow the discussion of load-bearing and energy-absorbing layer division in cushion liner zoning design, then be re-verified against the part families of the specific project.

Five compartmentalization criteria follow. First, separate bearing from cushioning: heavy parts transfer load first to a rigid support layer, which then spreads it through an energy-absorbing layer, rather than relying on one block of soft insert to bear load directly. Second, hard dividers take priority: ceramic versus steel parts, cleaned versus uncleaned parts, and support brick versus metal supports are all separated by hard dividers with a soft strip on top forming a weak seal. Third, humidity zoning: jigs and rolls already dried and awaiting the kettle, plus support brick cells, get active moisture control with humidity indicator cards, while uncleaned used parts get their own sealed cell. Fourth, weight distribution: heavy parts sit near the geometric centre of the case and near the load beams, while light and brittle parts go around the perimeter, avoiding a centre-of-gravity shift that destabilizes lifting. Fifth, serviceability: desiccant and humidity cards should be reachable from outside or from the top without opening every cell.

Executable moisture targets can be given. During storage, relative humidity inside a case for general kettle spares should be kept below 60 percent, and support brick cells plus dried jig and roll cells below 40 percent (empirical). These are design control targets rather than performance guarantees: actual behaviour depends on storage duration, the climate of the transport corridor and the desiccant replacement interval. Where a project involves very cold or very humid regions, cross-check the requirement against the temperature and humidity control practice in extreme temperature cases and assess the condensation risk from day-night temperature swings.

Sealing, Pressure Equalization and Rain Protection on Site

A kettle parts case sees significant pressure and temperature variation in transit. In an ocean container the daily temperature swing can reach tens of degrees, and the pressure difference between inside and outside flexes and relaxes the case seal repeatedly. If the case seals well but has no pressure equalization, the differential at the moment of opening stirs up dust inside and can make the lid hard to open. On the other hand, galvanizing kettle projects are usually unpacked and handled outdoors or in a semi-open shed, so rain protection requirements are higher than for general equipment parts: a kettle outer wall soaked by rain for a long period corrodes noticeably faster, and for jigs and rolls already cleaned and awaiting the kettle, rain destroys the dried state outright.

Four executable measures follow. First, choose the seal by use case: for repeated opening and long outdoor storage, the seal should use a material with good weather resistance and low compression set. The design logic can follow the discussion of seal compression and service life in hinge, latch and seal structures for protective cases, but a kettle case must additionally consider chemical compatibility between the seal material and any alkaline zinc ash residue inside the case. Second, fit a pressure equalization valve as the project requires so the differential during transport and opening stays controlled; valve selection and mounting position should avoid orientations where zinc ash and dust could block it, as discussed in pressure equalization valve selection and layout. Third, fit a removable divider between the case interior and the pallet so used parts awaiting cleaning are separated from cleaned parts immediately on opening, following the zoning approach in removable divider systems. Fourth, for outdoor storage fit a rain cover and keep the case floor off the ground, preventing rain from entering along the floor seam and pooling beneath.

Labeling, Inspection and Unpacking Acceptance

Packing labels and unpacking acceptance are the last step that lands all the earlier design on site. Outer labels should follow GB/T 191 and GB/T 13384, marking fragile items, this way up, keep dry, no rolling, and stacking limits, along with case number, gross weight, centre of gravity and lifting points. Where several cases ship together, each should be marked with its part family and galvanizing line station number, so line A's jigs are not fitted to line B. For repair parts, the cleaning state and whether drying has been completed must also be marked, and this is exactly where the site most often errs.

Unpacking acceptance should follow a checklist by part family: kettle sections and wall plates for flatness, bevel faces, and any transit bruising or rust; ceramic blocks inspected piece by piece for chipped edges and cracks, with a light tap tone check where necessary and never a direct steel hammer blow; jigs for main beam straightness, arm deformation and weld cracks; rolls for body roundness, roll-face scratches and journal fit surface damage; support brick for chipped edges and moisture, spot-checked with a portable moisture meter; dried jigs and rolls checked against the humidity card reading before deciding on a second drying pass; instruments for cracked thermowells and intact terminals. Any nonconformance should be recorded and notified to the supplier rather than absorbed on site.

On test basis, the purchase specification should separate the purpose of three families. The ISTA and GB/T 4857 series evaluate a packed case under vibration, impact and stacking in transport. ASTM D4169 designs test sequences by distribution cycle, as described in designing transport test sequences by distribution cycle. GB/T 10125 (neutral salt spray) provides a comparative ranking of corrosion resistance for case hardware and liner contact combinations. It should be stated clearly that salt spray testing gives a ranking between schemes, cannot be used to project field life directly, and does not support any claim of certification. Likewise, MIL-STD-810H clauses are used in this field only as environmental test method reference, for example to define the conditions of vibration and temperature-humidity cycling, and do not mean the product holds a military certification. Acceptance of the zinc coating itself follows the relevant requirements of GB/T 13912 and is performed on site by the galvanizer, independent of the packing scheme, and the two should be kept distinct in commercial discussion. On kettle parts case projects, JUNZHIJIA normally supplies seals, plugs, desiccant and humidity cards by part family, and can provide inspection documents for case dimensions, insert structure and hardware. Insert plans and transport test recommendations are issued against drawing dimensions, part weight distribution and transport route, with OEM/ODM and volume supply support.

FAQ

Q: The steel kettle wall is tens of millimetres thick, so why worry about distortion in transit?

A: Because wall thickness and stiffness are not the same thing. The 30 to 70 mm wall (typical values) is chosen to resist the loss of high-temperature strength at 450 C and the dissolution of iron by molten zinc; it is a material strength provision, not a stiffness provision against bending. A kettle wall plate 6 to 12 m long, even at 50 mm thickness, will sag at mid-span under its own weight if it is supported only at the two ends, and dynamic loads under transit impact amplify that deflection further, leaving residual bow after it is set down. The consequences are systemic: flatness and perpendicularity error between wall and heat source destroys the uniformity of the gap, so the locally overheated zone runs hotter, iron dissolves into the zinc faster there, and that zone becomes the first place to thin and fail, noticeably shortening kettle life. The correct method is a full-length continuous beam support, with support spacing calculated from plate thickness and span so self-weight deflection stays within an acceptable range, and with the support beams landing on the case floor structure. The kettle also needs axial and lateral restraint to prevent shifting inside the case. Two-point or single-point lifting is prohibited, as is placing a lifting point on a weld bevel edge.

Q: Why can't ceramic kettle blocks be shipped in the same case as steel parts?

A: For three reasons covering mechanical damage, chemical contamination and electrochemical effects. Mechanically, ceramic material has low tensile strength and almost no elastic deformation, so any loading that concentrates surface tensile stress starts a crack at the surface, and a steel edge under vibration is the strongest point-load source available; scale and rust shed by steel also press marks into ceramic faces. Chemically, rust and metal debris produced by steel parts contaminate the bedding faces of ceramic parts, and at bricking those particles become a non-compact interlayer that serves as a preferential path for zinc penetration and attack. Electrochemically, ceramic itself does not conduct, but the interface between ceramic and steel can still form a locally wet environment under humid conditions, accelerating corrosion of the adjacent steel, and the volume expansion of the corrosion product then presses back on the ceramic. The correct approach is therefore to give ceramic parts their own flexible cells, allow only surface contact, separate adjacent ceramic parts completely with hard dividers, fit non-metallic edge guards, and prohibit direct prying with a steel hammer or bar during handling. Support and structural bricks additionally need moisture protection, because their high-temperature compressive strength drops noticeably once they absorb water.

Q: Why does jig deformation affect coating quality?

A: Because jig geometry directly determines workpiece position, attitude and dwell time in the zinc, and coating thickness and appearance follow from those three. Once the main beam bows, workpieces in the same batch do not reach the same immersion depth: shallower workpieces heat more slowly and receive less effective zinc contact time, appearing as thin or missed coating, while deeper ones may approach the bottom dross zone and pick up dross particles. Twisted arms make workpieces touch each other, and the contacting faces cannot exchange zinc properly, producing dull dead-coated areas and zinc nodules that are very hard to remove in later finishing. Deformation also changes the load capacity and stress state of the jig, making bends and welds stress concentration points that initiate cracks and eventually fracture under repeated thermal cycling. Extra deformation during transport and storage is not part of the high-temperature creep allowance, so it must be prevented: jigs sit in profile saddles carrying the straight section of the main beam with several support points along the span; bundled stacking is prohibited; lifting uses a wide sling cradling both ends of the main beam rather than wire rope across the beam or a bend; and a jig returned for repair is cleaned of zinc before packing and shipped numbered by set.

Q: Why must parts be preheated and dried before entering the zinc kettle, and how exactly should it be done?

A: Because molten zinc sits at 445 to 460 C, and any residual moisture flashes instantly on contact, expanding violently and throwing molten zinc and ash outward. This is what the industry calls a zinc splash, and it ranges from lost zinc and coating defects to serious burns. Every part that will enter or approach the zinc, including jigs, racks, baskets, sink rolls, roll shafts, thermowell tubes and tooling fixtures, must therefore be preheated and dried first. The executable sequence is as follows. Preheat in a 120 to 150 C band (empirical), with hold time set from part weight and section thickness, so moisture escapes from the surface and from internal cavities rather than merely from the visible surface. Then address cavities and blind holes specifically: tubular jig structures, roll-shaft keyways and bolt blind holes all trap water, so blow them dry with compressed air before packing and confirm with a borescope on arrival. Then set a defined waiting limit and a second confirmation step between drying and entering the kettle, so a part left open in wet weather is not put straight in. Where the case carries desiccant and humidity indicator cards, read and record the cards first and use them to set the drying duration.

Q: Where do sink rolls and roll shafts most often go wrong in transit?

A: In three places: the roll body mid-span, the journal fit surfaces, and the bearing housings and seals. Mid-span problems come from the support method: rolls typically have a high length-to-diameter ratio, and if the two journals are the only supports, mid-span sag on a long roll converts into permanent bow under transit impact, so roll roundness goes out of tolerance and the site sees strip wandering or transverse coating variation. Journal problems are surface damage: a scratch destroys the fit with the bearing housing, producing excess clearance and grease leakage. Bearing housing and seal problems are compression set and dust ingress: housings in the kettle area usually use a high-temperature design with seals, and seals fear heat, compression set and transit dust. The protection method: use V-shaped or large-radius curved saddles with at least three support points along the body length and a soft oil-resistant liner on the saddle arc; apply a peelable low-tack film to the roll face, chosen for no adhesive residue; give each journal a guard sleeve with a soft inner face, never a metal sleeve directly on the journal; put bearing housings and arms into their own cells so roll weight never passes through the housing; and lift with a wide sling cradling both ends of the body, never using a journal or housing as a lifting point.

Q: Why must dross and flux be cleaned off repair parts before packing?

A: Because all three residue types keep causing new damage inside the case. First, dross and zinc ash: dross is a precipitated zinc-iron intermetallic phase, hard and sharply angular, and if left in a case it becomes the hard particle that scratches kettle inner walls and roll faces; zinc ash is mainly zinc oxide and turns alkaline once it takes up moisture, accelerating rust on steel outer walls. Second, coating and alloy fragments: the zinc-iron alloy layer on jigs and roll faces is brittle, and fragments spalling under transit impact contaminate other parts and become a handling hazard at unpacking, so a returned jig must be cleaned before packing. Third, flux residue: zinc ammonium chloride flux is strongly hygroscopic, and residue keeps absorbing water inside the case, forming a corrosive liquid film on steel surfaces and contaminating the insert material. Cleaning notes: remove dross from kettle inner and outer walls with non-metallic scrapers, never chiselling the wall with a steel chisel, because a chisel mark is itself a stress concentration point; apply immediate temporary protection after cleaning the alloy layer from jigs and roll faces so the surface does not rust; and wash and dry parts carrying flux residue before packing. The single most important rule is that cleaned parts and uncleaned used parts must never ship in the same case, because one mixed load recontaminates a freshly cleaned kettle wall with salts.

Q: Why are kettle support bricks so critical to kettle life, and what does packing need to address?

A: Because the support bricks determine the contact condition between kettle bottom and heat source, and the kettle bottom is the hardest part of the kettle to repair. If vertical or chipped bricks leave gaps between brick and kettle bottom, local heat transfer at the bottom is blocked, that zone of the wall runs hotter, iron dissolves into the zinc faster, and it becomes the first place to thin. Once the bottom thins to its limit it cannot be repaired on site and the whole kettle must be replaced. Support brick damage follows three patterns: chipped edges and corners from handling and stacking, strength loss from moisture in the storage environment, and edge impression plus slight deformation from long-term compression during packing. Packing measures: store support and structural bricks on single-layer trays and never stack, using continuous dividers with vertically aligned support points where multiple layers are unavoidable; apply active moisture protection across the batch with low-shedding desiccant and humidity indicator cards, drying bricks before bricking if they arrive visibly wet; orient the brick bearing face up or to the side and never use it as a supporting surface under other parts; and put matching load beams and metal supports into separate cells, completely apart from refractory brick, so metal rust cannot contaminate brick faces. The point is to control dimensional accuracy and moisture state at the same time.

Q: Which acceptance items are most often missed at unpacking?

A: Three categories of invisible items are most often missed. The first is cleaning and drying state: whether repair parts still carry dross and flux, and whether dried jigs and rolls have stayed dry, cannot be judged without a close look, so read the humidity indicator card readings first and then decide whether a second drying pass is needed. The second is hidden ceramic damage: chipped edges are visible at unpacking, but abrasion marks and internal microcracks from transit vibration are not, so inspect piece by piece with a light tap tone check, never a direct steel hammer blow, and never release a part just because it looks intact. The third is minor damage to fit surfaces: burrs on kettle weld bevel faces, scratches on roll journals and impressions on bearing housing fits are all sub-millimetre defects, yet they amplify directly into quality problems at welding and assembly, so inspect them under magnification.

Conclusion and Related Reading

A galvanizing kettle parts case comes down to four rules: thick-wall kettle sections on continuous beam supports, ceramic parts in independent flexible cells, jigs and roll shafts located by profile against bending, and cleaning plus preheat and drying written into the delivery file.

Related Reading

• Industrial Kiln Cases: Refractory Lining
• Pressure Vessel Component Cases: Shell & Head
• Winch Parts Cases: Drum & Shaft Support