Cladding Welding Process: Complete Guide to Weld Overlay

A refinery lab discovered that a carbon steel reactor costing a fraction of a solid alloy vessel had been safely containing corrosive chemicals for decades — because its inner surface was protected by a few millimetres of stainless steel welded onto cheap steel. That thin weld-deposited skin is cladding, and understanding how it is made is the difference between a vessel that lasts and one that fails inspection.

What Is the Cladding Welding Process?

Cladding welding is a surfacing technique in which a corrosion-resistant alloy is welded onto the surface of a cheaper, tougher base metal so the part gets the best of both materials. In practice, it produces a continuous metallurgically bonded skin, typically a few millimetres thick, that stands between the service environment and the structural steel underneath. Pressure vessels, storage tanks, pipework, chemical reactors and power-generation components all rely on it, because a carbon steel shell with a stainless or nickel-alloy interior can handle acids, chlorides and high-temperature media at a small fraction of the cost of a solid alloy fabrication, while still being welded, formed and inspected like ordinary steel.

The Standard Definitions Behind the Term

Welding standards are precise about this, and using their language keeps procedures, qualifications and inspections aligned. AWS A3.0, the standard welding terminology document, defines surfacing as the application of material by welding, brazing or thermal spraying to obtain desired surface properties or dimensions, as opposed to making a joint. That single sentence matters: cladding is not a joining process at all. No two members are being connected. Material is being built onto one member to change what its surface does.

Within that surfacing family, AWS defines cladding as a surfacing variation in which surfacing material is deposited usually to improve corrosion or heat resistance. The AWS Welding Handbook chapter on clad and dissimilar metals expands on how these overlays behave metallurgically, how they are qualified, and how they are examined once service begins.

ASM International puts a dimensional figure on the concept, defining weld cladding as a relatively thick layer, about 3 mm (1/8 in) minimum, of weld metal applied to a carbon or low-alloy steel base to provide a corrosion-resistant surface. That minimum is one practical way to tell cladding apart from the thinner cosmetic or lightly protective coatings people sometimes confuse with it. You can see the full framing in the ASM International reference preview on surfacing and cladding.

Cladding Is Not Buildup, Buttering or Hardfacing

All four belong to the surfacing family, but they answer different questions, and mixing them up leads directly to wrong filler selection and failed service:

• Cladding asks: how do I make this surface resist corrosion or heat? It uses stainless, duplex or nickel alloys and is judged on chemistry and corrosion performance.
• Buildup asks: how do I restore a worn or undersized dimension? The goal is geometry and often mechanical strength, not corrosion resistance.
• Buttering asks: how do I create a metallurgically compatible transition layer before joining dissimilar metals? It is a preparatory layer for a joint, not a finished service surface.
• Hardfacing asks: how do I resist abrasion, impact or wear? It uses high-hardness alloys such as iron-carbide or cobalt-based types and is judged on hardness and wear life.

A repair job illustrates the difference. Restoring a worn shaft is buildup; applying a chromium-carbide layer to a crusher tooth is hardfacing; laying a soft nickel-rich layer before welding stainless to cast iron is buttering; lining a reactor with 316L is cladding. The same welder, arc and equipment can do all four, yet each demands a different filler, procedure and acceptance test. This is why filler selection for alloy work deserves its own careful reading, whether the job involves alloy steel welding electrodes or a fully qualified Alloy 20 welding procedure.

What the Process Actually Produces, and Why Industries Pay For It

The metallurgical goal is deliberately simple: keep the cheap metal doing the structural work while a thin, expensive alloy handles the chemistry. The bond between them is a true fusion bond, so the overlay cannot be peeled or blistered like a paint or lining, and it survives thermal cycling, pressure and fabrication that would destroy an adhesive liner.

Industries choose it for three reasons. First, cost: a vessel made entirely from a nickel alloy can be many times the price of a clad one. Second, performance: the overlay provides corrosion resistance that plain steel cannot approach, while the steel shell provides toughness and weldability the alloy alone may not offer in the same section. Third, inspectability: because the overlay is weld-deposited metal, its chemistry, bonding and surface condition can be checked ultrasonically and by penetration testing before the component ever enters service.

In short, the cladding welding process produces a bonded, corrosion-resistant skin on an economical substrate, qualified by procedure and verified by examination. Everything that follows in this guide, from process choice and dilution control to codes and defect repair, exists to make sure that skin performs exactly as designed.

Cladding vs. Hardfacing, Buildup and Buttering

Ask ten welders what “surfacing” means and you will get ten different answers, because the term covers at least four jobs that look similar under the arc but are engineered for completely different results. The AWS Practical Reference Guide for Hardfacing sorts these into four families: surface cladding, buildup, buttering and hardfacing. Knowing which family you are in decides your filler metal, your heat input, your layer count and the way the finished weld is accepted. Get that call wrong and you can end up with a corrosion layer that cracks, a wear surface that spalls, or a joint that no inspector will sign off.

The Four Surfacing Families at a Glance

Family	Purpose	Typical alloys	Thickness	Acceptance criteria	Common parts
Cladding (corrosion-resistant weld overlay)	Protect the base metal from a corrosive service environment	309L/316L stainless, duplex, Alloy 625, 825, C-276, titanium	Roughly 3–6 mm, usually two layers	Dilution and final chemistry (ferrite, corrosion tests), bond UT, penetrant on the cap	Pressure vessel内 surfaces, pipe bores, reactor liners, flare stacks
Buildup	Restore lost dimensions so a part can be machined back to size	Matching or slightly strengthened carbon and alloy steel	Can be tens of millimetres	Dimensions after machining, absence of lack of fusion, sometimes hardness	Worn shafts, bearing journals, mill rolls, pump sleeves
Buttering	Create a metallurgically compatible layer so two dissimilar metals can be joined	Nickel fillers (ENiCrFe-3, ERNiCrMo-3), 309L	A single layer or two, typically 3–5 mm	Dilution of the first pass, chemistry of the buttered face, soundness before the joint is completed	Low-alloy to stainless nozzles, turbine-to-pipe transitions, bimetal transition joints
Hardfacing	Reduce wear from abrasion, impact, erosion, galling or cavitation	Iron-carbon-chromium, cobalt-based (Stellite), nickel-carbide, tungsten carbide composites	Applied thinner than a weld clad, often a single layer of a few millimetres	Hardness, bond integrity, pattern coverage and thickness, not chemistry	Excavator teeth, crusher hammers, mixing paddles, valve seats, drill augers

Two rows deserve extra attention. Hardfacing is a wear-control measure: the deposit is hard, often brittle, and deliberately applied in a thin layer because thick, highly alloyed hard layers are prone to cracking under impact. Cladding is a chemistry measure: the deposit must hold a corrosion-resistant composition after mixing with the base metal, which is why it is applied in at least two layers and why its chemistry, not its hardness, is what gets tested.

Why Mixing Them Up Causes Real Failures

The failure pattern is almost always the same: a filler metal chosen for the wrong job. A hardfacing rod run onto a vessel inner wall will give you a surface that is hard but contains the chromium in the wrong form, and it will corrode. A soft 316L overlay applied to a pump shaft rated for abrasion will wear away in weeks. A buttering pass used as a corrosion barrier may pass a visual check while its first pass, heavily diluted by the base metal, sits far below the required chromium and nickel levels.

Filler selection is where these errors start, and it is the one decision that cannot be reversed after the metal has solidified. Written procedures and consumable datasheets matter more than habit here, which is why resources such as Alloy Steel Welding Electrodes: Selection, Use & Safety and Alloy 20 Welding: Procedures, Filler Metals, and Best Practices are worth reading before the arc is struck rather than after a rejected coupon. The AWS Welding Handbook chapter on clad and dissimilar metals is the authoritative cross-reference when you need to confirm which family a given joint belongs to.

Cladding Strength Is Not Credited in Design

One rule surprises newcomers every time: for both cladding and hardfacing, the strength of the deposit is not considered in the design of the component, as ASM International notes in its surfacing guidance. In practice this means a vessel designed from carbon steel is calculated as carbon steel. The stainless or nickel overlay contributes corrosion resistance and nothing else to your allowable stress. It follows that the clad layer cannot be used to justify a thinner wall, and it follows just as sharply that if the overlay is damaged or locally removed, the pressure-boundary calculation does not change. Design and inspection both treat the substrate as the only structural material, which is why bond integrity and overlay thickness are checked so aggressively during fabrication.

How Weld Cladding Works: Process Steps from Prep to Pass

Knowing what cladding is and knowing how to put it down are two different things. A corrosion-resistant weld overlay is a controlled metallurgical operation, not a bead squeezed onto cheap steel. Sloppy cleaning leads to porosity, an unqualified procedure leads to rejected chemistry, and a careless final pass undoes everything earlier layers achieved. Here is the workflow, from raw plate to finished clad.

Step 1: Select and Clean the Base Metal

Cladding starts with the substrate. Carbon steel and low-alloy steel are the usual candidates because they carry the structural load at low cost. Before any arc is struck, the surface must be free of mill scale, oil, grease, rust, paint and moisture; shops grind or blast the area, then wipe it with a residue-free solvent. Cutting fluids and layout dye are classic hidden contaminants that survive visual inspection and turn up later as porosity or lack of fusion.

Step 2: Qualify the Procedure and the Welder

Commercial cladding is not a “welder’s-discretion” job. The procedure specification (WPS) and its procedure qualification record (PQR) must be qualified to ASME BPVC Section IX, which carries dedicated essential variables for corrosion-resistant weld overlay: QW-253.1 for SMAW, QW-255.1 for GMAW and QW-256.1 for GTAW. They govern maximum permitted dilution, minimum layers, and changes in filler or shielding gas. QW-381 separately governs performance qualification for overlay work, so the operator must be qualified for the overlay route, not just a groove weld on the same material.

Once qualified, the WPS becomes a recording document: cladding requires detailed parameter logging so clad quality stays repeatable shift to shift, operator to operator.

Parameter recorded	Why it matters
Preheat and interpass temperature	Controls moisture, hydrogen and thermal buildup that drives dilution up
Current, voltage, travel speed	Determine heat input and therefore bead penetration into the base metal
Bead overlap and layer count	Guarantees full coverage and keeps the final layer uncontaminated
Shielding and backing gas	Prevents oxidation and loss of chromium on hot, uncovered bead surfaces

Step 3: Preheat and Control Heat Input

Preheat is set by the substrate, not the overlay alloy. Low-alloy and carbon steels with higher carbon equivalent need more preheat to avoid hydrogen-induced cracking, and low-hydrogen consumables must be baked and handled accordingly; electrode selection rules are covered in Alloy Steel Welding Electrodes: Selection, Use & Safety. Heat input stays as low as the procedure allows, because every extra joule melts more base metal and dilutes the chromium the overlay exists to deliver.

Step 4: Bead Sequencing, Interpass and Layers

Overlay beads run as stringers, not wide weaves, each overlapping the previous enough to eliminate unfused valleys between crowns. Interpass temperature is checked with a contact thermometer or thermal crayon and held below the WPS limit, usually a few hundred degrees Celsius. If the part gets too hot, the operator stops and lets it cool rather than chase production.

Virtually every corrosion-resistant overlay is at least two layers. The first, sometimes called the buttering layer, is deliberately allowed to mix with the base metal, and a compatible first-layer filler such as 309L absorbs that dilution. The second then runs over relatively clean first-layer chemistry, so its final composition lands where the design intends. Section 5 covers the dilution arithmetic.

Step 5: Post-Weld Heat Treatment and Finishing

Shop-applied overlays are generally recommended to receive post-weld heat treatment to relieve residual stress in the thick substrate. Field boiler work is the common exception, and it often qualifies for a PWHT exemption under ASME Section I Table PW-39. The trade-off deserves care: holding stainless and nickel-based overlays in certain temperature ranges can precipitate detrimental phases and cost corrosion resistance, so PWHT temperature and time must be set in the procedure.

Finally, the clad is ground or machined to the specified finish, then inspected. Dye penetrant checks the overlay for cracking and lack of fusion, hardness checks confirm the heat-affected zone behaved, and chemistry verification, typically XRF or OES, confirms chromium and nickel content. Rejected beads are removed by grinding or air-carbon arc gouging and rewelded under a written repair procedure, covered further in Air Arc Welding and Carbon Arc Gouging.

Done in this order, the steps yield a clad with predictable final chemistry, a sound metallurgical bond and a surface ready for service inspection. Skip one and the defect surfaces later, usually somewhere far more expensive to fix.

Welding Processes Used for Cladding

There is no single “cladding machine.” The job decides the process. Weld cladding is usually performed using submerged arc welding, with flux-cored arc, plasma arc and electroslag processes also fully capable of producing sound, code-compliant claddings. What separates them is deposition rate, dilution, position flexibility and part geometry. Cladding the inner wall of a pressure vessel demands a completely different choice than repairing a 60 mm shaft journal, and picking the wrong one is how projects end up with warped plates and rejected bonds.

Submerged Arc Welding and Strip Cladding

Submerged arc welding (SAW) is the long-standing workhorse of the cladding world. The arc sits under a blanket of granular flux, so there is no visible arc, no spatter and very low fume release. SAW runs with either standard filler wire or, far more commonly for cladding, a flat consumable strip typically 30 to 90 mm wide.

Strip cladding is where SAW earns its reputation. A wide strip produces a wide bead, meaning fewer passes, less overlap and a dilution profile distributed evenly across the weld width instead of spiking at the bead centre. For very large areas, strip welding with submerged arc or electroslag techniques is the most economical route available, which is why vessel shells, tank bottoms and large pipe bores are almost always clad this way. The main limitation is geometric: gravity-fed flux and a fluid weld pool confine SAW to the flat position, so it cannot handle field overlay work without turning the entire component. For the consumable side, see our guide to alloy steel welding electrodes. The governing practice documents and qualification requirements welders get audited against are covered in the AWS weld overlay guide and the AWS Welding Handbook chapter on clad and dissimilar metals.

Electroslag Strip Cladding

Electroslag strip cladding looks superficially like SAW strip cladding, but the physics differ. A conductive flux is added at startup, current flows through the molten slag, and the resistance of that slag melts the strip. There is no arc at all, so travel speeds are high, spatter is essentially zero and deposition efficiency is excellent.

The practical payoff is single-pass thickness. Electroslag commonly deposits a full 3 to 5 mm layer in one pass, so a single-layer overlay can often be finished where SAW would need multiple passes. It is a favourite of clad plate mills and shops building nuclear reactor vessels and heavy-wall petrochemical equipment, though like SAW it is essentially a flat-position, large-surface process.

Flux-Cored, GMAW, GTAW and Hot-Wire GTAW

Flux-cored arc welding (FCAW) trades some of SAW’s raw deposition rate for portability and position freedom. Flux-cored and metal-cored wires can run vertically and overhead, making FCAW the natural choice for field overlay, vertical seam cladding and internal repairs on installed equipment. If you work out of position routinely, our all position welding rod guide covers the trade-offs.

GMAW offers the widest process flexibility: all positions, easy automation and controllable heat input through short-circuit, pulsed and spray modes. Dilution runs higher than SAW strip or plasma under equivalent conditions, so GMAW suits patch repairs, irregular geometries and build-up work rather than large single-layer corrosion barriers.

GTAW produces the cleanest, most controllable deposit of the conventional processes, and at low heat input it achieves the lowest dilution of any arc wire process. The drawback is speed. Hot-wire GTAW solves it by preheating the filler wire with an auxiliary current before it enters the pool, multiplying deposition rates while keeping dilution low. Hot-wire GTAW has become the standard for automated bore overlay, nuclear component cladding and buttering passes on dissimilar metal joints, an area where EPRI guidance is frequently referenced.

Plasma Arc and Laser Cladding

Plasma arc cladding (PTA) uses a constricted arc to deliver a concentrated, high-energy-density heat source. It accepts either powder or wire feed, produces a narrow deep bead, and routinely holds dilution in the low teens or single digits while maintaining good travel speed. PTA suits automated rotary work on valve seats, pump shafts and sealing faces where a precise, low-dilution layer matters more than raw coverage rate.

Laser cladding, sometimes called direct metal deposition, is the precision end of the spectrum. A laser beam melts the fed powder or wire together with only a thin skin of the substrate, so dilution is often held below five percent and the heat-affected zone is very small. That makes laser cladding the go-to method for precision repair of shafts, rollers and moulds and for building graded transitions between dissimilar alloys. The trade-offs are speed, equipment cost and careful fume control, as covered in our alpha laser welding guide. Broader process-selection data appears in the ASM International Metals Handbook preview.

Process	Consumable	Deposition rate	Dilution	Best suited to
SAW strip cladding	Strip 30-90 mm	Very high	Moderate	Large flat vessel and tank surfaces
Electroslag strip	Strip	Very high, thick single pass	Low to moderate	Heavy single-layer overlays, clad plate mills
FCAW	Flux-cored wire	High	Moderate to high	Field, vertical and overhead overlay
GMAW	Solid or metal-cored wire	High	Moderate to high	Patch repair, irregular geometry
GTAW / hot-wire GTAW	Solid wire	Low to high (hot wire)	Very low	Nuclear overlay, bore cladding, buttering
Plasma arc (PTA)	Powder or wire	Medium	Low	Rotary parts, valve seats, sealing faces
Laser cladding	Powder or wire	Low to medium	Very low	Shafts, rollers, moulds, precision rebuild

The process sets the ceiling on cladding quality. Choose for dilution control and geometry first, deposition rate second, and cost last, because a cheap process that forces an extra layer or fails bond inspection is never actually cheap.

Dilution Control: The Heart of Cladding Quality

If you remember one thing about weld overlay, make it this: dilution decides everything. Dilution is the amount of base metal that melts into the weld pool and mixes with your corrosion-resistant filler. A little mixing is unavoidable in fusion cladding. Too much, and the chemistry you paid for never lands on the surface of the part.

Take a 316L overlay filler carrying roughly 18% chromium and 12% nickel as deposited from the wire. Blend that pool 40% with plain carbon steel and you have stripped out about two-fifths of the chromium that was supposed to make the surface stainless. The bead still looks silvery and sound, and it passes a visual check. A pitting test or a corrosion coupon tells a very different story.

What Dilution Actually Does to the Chemistry

Dilution is usually expressed as a percentage: the cross-sectional area of base metal melted into the bead, divided by the total cross-sectional area of the bead. Practically, welders control it without doing math on the plate. They know that deep penetration brings dilution up, shallow penetration brings it down, and every parameter that changes penetration changes dilution with it. The trend is consistent across processes and alloys:

• Higher current and voltage increase penetration and raise dilution.
• Faster travel speed reduces heat input per unit length and generally lowers dilution.
• Bead width, weave width and electrode position change how much base metal is pulled into the pool.
• Process selection is the biggest lever of all, because processes differ enormously in inherent dilution.

That last point is why the process section of this guide matters. Manual GTAW can run at very low dilution with careful technique, while a single-pass submerged arc bead on carbon steel can easily exceed 30 to 40% dilution. Electroslag strip cladding and hot-wire GTAW were developed precisely to break the old trade-off between deposition rate and dilution.

Why Two Layers Is Standard Practice

The economics of stainless steel weld cladding depend on achieving the required chemistry at the highest practical deposition rate in a minimum number of layers, according to ASM International’s guidance on corrosion-resistant overlays. That single sentence is the whole design problem in miniature: deposit fast, deposit thin, but still land on the right chemistry.

In practice, most fabricators solve it with a two-layer overlay. The first layer, sometimes called the buttering or root layer, accepts the dilution. It is allowed to fall short of the specified chemistry because it will be covered. The second layer, deposited with stringer beads at controlled heat input, provides the specified surface composition and the corrosion performance the customer specified. Some specifications even permit a slightly over-alloyed filler for the first layer so that a single-layer deposit can still meet minimum chromium levels after dilution.

Bead Geometry and Overlap

Dilution is not uniform across a bead. The centre carries more base-metal mixing than the edges, which is why overlapping matters. A common rule is to overlap each bead by about half its width, so the lightly diluted edge of the next bead covers the heavily diluted centre of the previous one. Run the overlap too small and you leave diluted stripes running the length of the overlay. Run it too wide and you waste expensive filler while building thickness that adds nothing to corrosion life.

Stringer beads outperform wide weaves for overlay work. A long arc length or an aggressive weave widens and deepens the pool, pulling in more base metal. This is the same heat-input discipline you would apply when managing alloy steel electrodes on critical joints: slow, controlled and consistent beats fast and improvised.

Verifying the Final Chemistry

You cannot judge dilution by eye. Verification is where cladding quality gets confirmed, and it normally involves one or more of these checks:

1. Chemical analysis of the deposited overlay by X-ray fluorescence or optical emission spectroscopy on a machined surface, ideally on a test plate qualified alongside the production weld.
2. Ferrite measurement with a magnetic gauge, since ferrite content in stainless overlays shifts with dilution and signals when chemistry has drifted.
3. Macro-section and dilution measurement on procedure qualification coupons, recording bead geometry and penetration for the WPS.
4. Corrosion testing, such as ASTM A262 practice tests for stainless overlays or ASTM G48 pitting tests, when the specification calls for it.

All of this belongs in the procedure qualification record, referenced by the WPS under ASME and AWS clad construction practice. Once the procedure is locked, production weldors follow it without improvising parameters.

Get dilution control right, and the overlay behaves exactly as the datasheet promises. Get it wrong, and no amount of downstream inspection or repair will restore the corrosion life that was lost in the first pass.

Cladding Methods Beyond Fusion Welding

Walk into a pressure-vessel shop and clad material arrives by truck as flat plate, not laid down bead by bead. Weld overlay gets the attention, but it is only one route to a clad surface, and rarely the cheapest or most dimensionally stable. The AWS Welding Handbook chapter on clad and dissimilar metals lists the principal cladding techniques as hot roll bonding, cold roll bonding, explosive bonding, centrifugal casting, brazing and weld overlaying. All do the same thing — put a corrosion-resistant alloy metallurgically onto a cheaper backing — but the physics, cost profile and inspection rules differ sharply.

Roll Bonding: The Route Behind Most Clad Plate

Hot roll bonding accounts for more than 90% of clad plate production worldwide, according to ASM International’s cladding overview. Two clean plates are stacked, heated in a rolling mill and squeezed hard enough that the surfaces break through their oxides and fuse into a continuous metallic bond across the whole interface. The result is plate you can shear, form and weld, with an atomic rather than mechanical bond.

Cold roll bonding does the same job at or near room temperature. It produces thinner sheet and strip with less distortion and no heat-tinted zone, but it cannot build the heavy sections hot rolling reaches. Thickness, alloy pairing and the final part form decide which product you get.

Method	How the bond forms	Typical output
Hot roll bonding	Heat plus mill pressure	Heavy clad plate, vessel shells
Cold roll bonding	Room-temperature pressure	Thin sheet and strip
Explosive cladding	High-velocity impact	Plate, transition joints, dissimilar pairs
Centrifugal casting	Molten alloy spun against steel	Liners, pipe, cylinders
Brazing	Filler wets both surfaces	Plate-fin assemblies, thin overlays

Clad plate is bought against published standards, so know which applies before you order. ASTM A265 covers nickel and nickel-base alloy-clad steel plate; ASTM A263 and ASTM A264 cover corrosion-resistant chromium steel clad plate. Chemistry, bonding and testing requirements differ between them, and specifying the wrong one is a common cause of receiving-inspection rejections.

Explosive Cladding for Difficult Pairings

Explosive cladding drives one plate into another with a detonation wave. The impact jets surface contamination away, and the two metals bond in a characteristic wavy interface at such speed that the bulk material never gets hot: no heat-affected zone, minimal distortion, and the ability to join combinations fusion welding cannot touch — copper to stainless, titanium to carbon steel, aluminium to steel.

Cost is the deciding factor. Explosion cladding became cost-effective over solid alloy construction from roughly 10-12 mm of wall thickness upward, which is why heavy clad heads, transition spools and repair patches are made this way while thin linings are not.

Centrifugal Casting, Brazing, Thermal Spray and HIP

Centrifugal casting spins molten corrosion-resistant alloy against a steel mould or mandrel so solidification builds a uniform bimetallic wall. It is the standard route for lined pipe, bimetallic cylinders and wear sleeves, and it avoids the bead sequencing problems of lining a bore by hand.

Brazing fills the gap between parts with a filler metal that melts while the base metals do not. It suits thin cladding, flat assemblies and plate-fin heat exchangers that a full fusion bond would distort; flux selection is central to wetting, as covered in aluminum flux welding. Thermal spraying builds a coating rather than a true metallurgical bond — fine for wear and repair on non-code parts, but generally not acceptable as pressure-boundary cladding. Hot isostatic pressing bonds clad layers by diffusion under heat and gas pressure for near-net shapes that cannot be rolled.

Choosing Bonded Clad Plate Over Field Overlay

A fabricator picks bonded plate when geometry allows. Flat shells, large panels and long runs are cheaper to buy as clad than to overlay bead by bead, and mill-bonded plate gives uniform thickness, consistently low dilution and a single heat-treatment cycle instead of dozens. Distortion is far lower, and bond quality is demonstrated at the mill rather than proven on your floor.

Field overlay still wins at seams, nozzle welds, small patches, in-situ repairs and any part too large to move. Those welds use matching filler to a qualified procedure, and nickel-alloy seam work follows established practice such as Alloy 20 welding procedures. Thin sheets and heat-exchanger panels of bonded aluminium-clad plate are joined with the techniques in aluminum MIG welding.

Materials and Filler Metals for Cladding

The success of a weld overlay relies heavily on one decision: selecting the right filler metal for the service conditions it’s meant to resist. While dilution control is what keeps that alloy chemically sound, material selection is what determines whether the clad surface will withstand years of attack, or fail far ahead of its intended life.

Choosing Alloys Based on Service Chemistry

No single alloy is universal for cladding. The corrosive agents present in a vessel or piping system will always be the driving factor behind the choice of deposited metal. The most common selections used in industry include:

• Austenitic stainless steels (309L, 316L): Often used as a first layer or for general corrosion resistance. Type 309 and 312 stainless steel are common cladding fillers due to their tolerance to dilution when overlaying carbon or low-alloy steels.
• Duplex stainless steels: Used when both chloride resistance and higher strength are required, particularly in offshore and chemical processing environments.
• Nickel alloys (ERNiCrMo-3 / Alloy 625, ERNiCrMo-10 / Alloy 622, Alloy 825, Alloy C-276): These are go-to choices for severe sour service, high temperature cycling, or strong acids. For those looking to expand into highly specialized nickel-chromium-molybdenum overlays, resources such as Alloy 20 Welding: Procedures, Filler Metals, and Best Practices can help illustrate just how niche these alloys can become.
• Titanium alloys: Primarily seen for highly oxidizing acids or chloride-rich environments where stainless and even some nickel alloys fall short. These are more common when utilizing roll-bonded clad plate rather than large field overlays.

In fact, AWS guidance on clad and dissimilar metals notes that clad materials for roll-bonded plate can include ferritic, martensitic and austenitic stainless steels, nickel alloys and titanium alloys. The choice between bonded plate and a deposited overlay will often come down to geometry, accessibility, and project budget.

Strip vs Wire Consumables

The form of filler metal can be just as impactful as its chemistry. Two forms dominate the majority of production cladding:

• Strip consumables: Most associated with submerged arc strip cladding and electroslag strip cladding. Strips allow for wide, uniform beads, extremely high deposition rates, and a much easier path towards low dilution. This is ideal for large, flat surfaces such as pressure vessel shells.
• Wire consumables: Used for processes such as GMAW, GTAW, hot-wire GTAW, flux-cored, and plasma-based methods. Wires grant far more freedom for complex curves, nozzles, and intricate repairs. For heavily restricted areas, this flexibility is invaluable.

Just as welders must be selective with consumables when working with more exotic base materials, such as those covered within Aerospace Welding: Processes, Standards, and Careers Guide or marine applications like Aluminum Boat Welding: The Complete Guide for Strong Hull Joints, cladding demands the same level of consumable scrutiny. Even with steels, the principles seen in Alloy Steel Welding Electrodes: Selection, Use & Safety still apply: the electrode or strip must be fit for the joint, not just the metal.

Matching vs Over-Alloyed Fillers

With dissimilar metal combinations being the norm, fabricators must decide whether to match the minimum required corrosion alloy, or over-alloy.

Common cladding fillers include Type 309 and 312 stainless steel, as well as nickel alloys ERNiCrMo-3 (Alloy 625) and ERNiCrMo-10 (Alloy 622). For lower-alloy steel substrates, austenitics are often sufficient, however, their thermal expansion mismatch can be punishing. In fact, EPRI reports that 309 stainless overlays can fail by dissimilar-metal-weld type cracking where thermal expansion differs strongly from low-alloy tubing.

For this reason, nickel-based alloys are more expensive to install, but resist cracking and wastage in severe cycling service. When working with these fume-intensive nickel and chromium bearing alloys, strict adherence to controls outlined by OSHA’s hexavalent chromium guidance is non-negotiable.

Ultimately, filler selection is a balancing act. The goal isn’t to use the most exotic alloy available, it’s to use the least expensive alloy that will remain stable against the exact chemistry, temperature, and cycling that the component will see in service.

Codes, Standards and Inspection of Clad Components

A weld overlay is only as good as the documents and inspection behind it. In a refinery or a nuclear plant, “it looks corrosion-resistant” is not an acceptance criterion. Fabricator and inspector work from a defined set of documents that say which chemistry is acceptable, how much dilution is tolerable, who is qualified to run the process, and how the finished bond gets verified. Mapping that framework before you quote a clad job saves rework, rejected lots and failed audits.

ASME BPVC Section VIII: Clad Pressure Vessels

Most clad process equipment is built to the ASME Boiler and Pressure Vessel Code, Section VIII, and cladding is addressed in both Division 1 and Division 2. For corrosion-resistant weld overlay, the code does not judge the vessel by the strength of the overlay. It judges it by chemistry and bond integrity: the deposited weld metal must meet the specified corrosion-resistant composition after dilution, and the fusion boundary must be sound and continuous.

In practice, that means the Welding Procedure Specification (WPS) has to demonstrate, through a qualification record, that the chosen process, number of layers, heat input and bead overlap produce the required final chemistry on the actual base metal. Division 2 (Alternative Rules) imposes stricter design and examination obligations than Division 1, so clad vessels destined for high-intensity service typically carry more extensive NDT plans.

Section IX, Section I and NB-23: Who Is Qualified, and What Counts as Repair

Procedure and welder qualification for overlay work falls under ASME BPVC Section IX. Qualification is essential-variables driven: base metal group, filler classification, process, preheat, heat input range and the number of layers all matter. A procedure qualified for a 309L buttering pass is not automatically valid for a final 316L corrosion layer or for a nickel alloy overlay.

Boiler work sits under ASME Section I, and the National Board’s NB-23 (ANSI/NB-23) is the document most repair organizations actually work from. The detail worth committing to memory: NB-23 treats corrosion-resistant weld overlay application as a Repair, not a routine fabrication step. That classification brings specific requirements for documentation and examination and, in many cases, National Board acceptance. If your shop applies overlay inside a boiler or a registered repair, the NB-23 path governs how it is recorded, a discipline familiar from work covered in Alamo Welding & Boiler Works: Services & Boiler Expertise.

ASME Section III covers nuclear components. In pressurized water reactors (PWRs), the reactor pressure vessel itself is carbon or low-alloy steel, while internals are clad in stainless steel to resist the borated coolant, which can be aggressively corrosive to carbon steel. Overlay in nuclear service carries Section III quality requirements, enhanced records, and often mandatory chemical and hardness survey testing of every overlay layer.

Sour-service and pipeline equipment adds another layer. API 6D requires weld overlay qualification to ASME BPVC Section IX or ISO 15614-7, with hardness surveys performed to NACE MR-0175/ISO 15156, because a hardened heat-affected zone in H₂S service is a hydrogen cracking risk. Alloys with their own procedural sensitivities, including Alloy 20, are covered in Alloy 20 Welding: Procedures, Filler Metals, and Best Practices.

The AWS Welding Handbook ties the theory together, particularly Chapter 4: Clad and Dissimilar Metals Welding and Cutting, which covers dissimilar-metal joints, overlay metallurgy and process selection, with ASM International reference material as a useful supplement on cladding metallurgy.

Inspection: How a Clad Layer Is Actually Verified

Inspection of clad components combines several complementary methods:

• Ultrasonic testing (UT) bond examination — the primary volumetric check. Pulse-echo UT maps the fusion boundary and finds unbonds, disbonding and lack of fusion along the interface. It is routinely specified for clad plate and overlay in pressure-vessel work.
• Dye penetrant testing (PT/DP) — surface-sensitive; it catches porosity, hot cracks and lack of fusion that break the corrosion barrier at the finished surface. Mandatory after machining or grinding of the final layer.
• Chemistry verification — core drilling or chemical analysis of the final layer confirms chromium, nickel and molybdenum levels after dilution, sometimes supported by ferrite measurements on stainless overlays.
• Hardness surveys — traverse readings across the overlay, fusion line and heat-affected zone detect untempered martensite or sensitized zones, particularly for sour service under MR-0175.
• Radiography and replication — used selectively; RT is less common for thin overlay than UT, while replication supports nuclear component life assessment.

Handled correctly, the standards framework is not bureaucratic overhead. It is the mechanism that turns a few millimetres of deposited metal into a corrosion barrier a plant will trust for thirty years. Plan the codes around the service, qualify the procedure honestly, and inspect the bond and the chemistry: that is what separates a compliant clad component from an expensive failure. Safety obligations around overlay fume and consumables are covered separately by OSHA guidance on hexavalent chromium and OSHA welding safety publications.

Common Cladding Defects, Mistakes and How to Fix Them

Cladding rarely fails because someone chose the wrong alloy on paper. It fails because of a variable that slipped during the run: too much arc energy, a cold tie-in, a skipped bond scan, a repair made with whatever wire was on the truck. The good news is that the same handful of mistakes show up over and over, and each one has a recognizable signature and a practical fix.

High Dilution and Low Chromium

This is the silent killer. If travel speed is too high, current too high, or bead overlap too small, the arc digs into the carbon steel and drags iron into the deposit. Chromium and nickel get diluted below the level the alloy needs for passivity, and the part passes visual inspection while failing in corrosion service. The fix is procedural, not cosmetic: verify chemistry on the final layer, not the first, use a second or third layer where the dilution calculation demands it, and confirm the result with portable XRF or an ingot analysis from the procedure qualification. If the required corrosion performance is borderline, over-alloying the first layer with a 309L-type filler is far cheaper than a rejected vessel.

Lack of Fusion at the Bond Line

Undercut at the bead edges, insufficient heat input on the stringer start, or stopping a bead on top of cold slag all produce a bond-line void that ultrasonic testing will find months later. Prevention is unglamorous: grind tie-ins clean, keep interpass temperature in the WPS range, never restart on top of unmelted slag, and verify the first layer’s fusion with bond UT before covering it with the second. Gouging out a suspect area is acceptable only with the right process and a clean, bright groove, which is where carbon arc gouging technique and practice earn their keep.

Hot Cracking and Solidification Problems

Fully austenitic and some nickel-rich deposits are vulnerable to solidification cracking, especially when restraint is high or the chemistry drifts into a risky range with too little ferrite or excessive trace elements. AWS Welding Handbook coverage of clad and dissimilar metals highlights this as a recurring qualification issue. Nickel alloy 625 overlays, for instance, have shown circumferential cracking problems in extreme service, and the metallurgy behind it is worth understanding: materials that solidify through a eutectic reaction produce a large molten pool that solidifies almost instantly, which makes out-of-position overlay difficult or even impossible. Where that is the case, restrict the work to flat position, sequence beads to reduce restraint, and hold heat input inside the qualified window. Position-specific rod choice matters too, so a solid all position welding rod guide is a useful cross-check before anyone assumes a consumable will behave the same overhead as it does flat.

Disbonding and Interface Cracking

The most expensive failures happen at the interface. EPRI has documented repeat boiler tube failures at overlay interfaces where 309 overlays experienced dissimilar-metal-weld type cracking under thermal cycling, a pattern serious enough that it now drives repair philosophy for waterwall overlays. The countermeasures are specific:

• Evaluate hydrogen damage from poor water chemistry before applying waterwall overlays, because overlaying a hydrogen-embrittled tube simply hides the problem.
• Control heat input and preheat to keep the heat-affected zone ductile enough for cyclic service.
• Where cycling is severe, consider a buttering layer or a more compliant filler instead of a rigid, highly restrained single-pass bond.
• Never weld over a tube that has already thinned from hydrogen blistering or creep.

Service history matters as much as the WPS here; EPRI’s published guidance on overlay interface cracking is the reference to read before signing off a boiler repair.

Distortion, Wrong Filler and Skipped NDT

Distortion comes from unbalanced bead sequencing and excessive net heat. Alternate sides, use back-step or skip patterns, and keep a running check with a straightedge rather than discovering the warp at fit-up. Wrong filler is usually a procurement shortcut: substituting a generic 308L for a specified 316L, or an unqualified nickel filler, destroys the corrosion margin the design assumed, so substitutions need a written equivalency review against the service chemistry. The same discipline applies to alloy-grade welding procedures and filler metals where chloride or acid exposure drives selection.

Skipping bond NDT is the mistake with the highest deferred cost. A clad surface looks perfect while a lack-of-fusion channel sits underneath it. Hydrostatic testing will not find it; ultrasonic bond testing will.

Improper Repair of Worn Overlays

Worn or damaged overlays are often ground and re-welded casually, stacking dilution on top of dilution until the final layer is mostly base metal. Strip the failed layer back to sound, uncontaminated material, confirm remaining thickness by UT, then rebuild to the original layer count rather than resurfacing in a single pass. Repairs on service-worn parts also require a hardness and chemistry check, since a hardened, work-damaged substrate changes preheat requirements.

Run this sequence on every cladding job and most defect reports disappear before they are written: qualify the procedure, verify final-layer chemistry, scan the bond after the first layer, sequence for balance, and inspect before the part leaves the stand.

Safety, Health and Practical Tips for Cladding Work

Cladding work comes with a unique set of hazards. Unlike many fabrication jobs, the risks are mostly chronic. Long hours in tight spaces, overhead positions, and exposure to metal fumes mean that prevention, not reaction, is the difference between a safe overlay job and a hazardous one.

Hexavalent Chromium: The Hazard You Cannot See

When chromium in stainless or duplex consumables is exposed to the heat of an arc, it can form hexavalent chromium, or Cr(VI). These ultrafine particles are nearly invisible, yet carry some of the greatest risks of the trade. OSHA’s hexavalent chromium standard guidance sets the permissible exposure limit at 5 µg/m³ as an 8-hour time-weighted average, with an action level of 2.5 µg/m³. This limit came into effect after OSHA’s final rule, published February 28, 2006, dropped the previous level from roughly 52 µg/m³ down to 5 µg/m³.

As outlined in OSHA’s welding fume fact sheet FS-3647, Cr(VI) can damage the eyes, skin, nose, throat, and lungs, and is known to cause cancer. Nickel compounds found in many overlay fumes can cause respiratory sensitization. In addition, arc UV can cause photokeratitis and skin irritation, a risk that is heightened by reflections off freshly ground clad surfaces.

Noise, Fire and Ergonomic Strain

Fume isn’t the only concern for cladding crews. Equipment such as strip cladding heads, flux recovery vacuums, grinding, and carbon-arc gouging can routinely exceed 85 dBA, which is the action level that triggers a hearing conservation program. Jobs inside vessels also force welders into prolonged overhead postures, leading to strain on the neck, shoulders, and back. With hot work often happening near solvents or other combustibles, strict fire watch discipline is vital, a topic that is expanded on in our air acetylene welding safety basics.

The Hierarchy of Controls for Overlay Work

For any cladding job, controls should be applied in the following order:

• Elimination or substitution: If service conditions allow, consider a lower-chromium alloy or a process, such as electroslag strip cladding in a hooded station, that produces less fume.
• Engineering controls: Use local exhaust ventilation with the pickup as close to the arc as possible, hoods for strip heads, or downdraft tables for smaller parts. For work involving carcinogens, OSHA recommends filtering and exhausting air to non-occupied locations.
• Recirculation rules: Never return shop air to an occupied space unless it has passed through filtration of appropriate efficiency, typically HEPA for metal particulate. Dilution ventilation alone cannot reliably control Cr(VI) levels inside a vessel.
• Administrative controls: Rotate workers out of the hot zone, schedule welding away from occupied work areas, and maintain current exposure monitoring.
• PPE and respiratory protection: Use the correct filter shade, leathers, gloves, hearing protection, and fit-tested, NIOSH-approved respirators anytime engineering controls cannot prove exposure is below the limit.

Medical surveillance is also required in general industry. Any worker exposed above the action level for Cr(VI) for 30 or more days per year must be enrolled in a program under OSHA’s Cr(VI) standard. This includes a medical and work history, a physical examination of the skin and respiratory tract, a chest X-ray, pulmonary function testing, periodic exposure assessments, and a written report to the employee.

Cladding Job Planning Checklist

1. Define the environment: Confirm service chemistry, such as chlorides, CO₂, H₂S, acids, and operating temperature, before selecting filler metal.
2. Qualify the procedure: Qualify the WPS and PQR to ASME Section IX, with dilution trials to prove the first layer meets the required minimum chemistry.
3. Plan the weld: Set bead sequence, layer count, heat input, interpass temperature, PWHT, and distortion tolerances before striking the arc.
4. Prepare for safety and supply: Size, test, and document ventilation. Confirm exposure monitoring, gas storage, and hot-work arrangements. The basics found in our AllGas welding supply safety guide apply directly to bottle handling in the bay.
5. Set inspection criteria: Define requirements up front, such as 100% ultrasonic bond examination, dye penetrant on the finished surface, and hardness or chemistry checks.
6. Clean and prep: Write cleaning and surface-finish requirements into the traveler, as residual oil and mill scale are among the most common causes of porosity and lack of fusion.
7. Plan for repairs: Have a defined repair strategy for defects and worn overlays before welding begins.
8. Stay compliant: Schedule medical surveillance and respirator fit testing for anyone exposed above the action level for 30 or more days per year.
9. Brief the crew: Cover the specific fume, UV, noise, and ergonomic risks unique to the long hours of overlay work.

Frequently Asked Questions

What is the cladding welding process?

It is a surfacing process that fusion-welds a corrosion-resistant or heat-resistant alloy layer onto a lower-cost base metal, usually carbon or low-alloy steel. The deposit, typically 3 mm or thicker, protects the substrate while the steel carries the structural load.

What is the difference between cladding and hardfacing?

Cladding aims mainly at corrosion and heat resistance, while hardfacing deposits a hard layer to resist wear, abrasion and impact. The alloys, thicknesses and acceptance criteria differ, so selecting the wrong surfacing type causes premature failure.

Which welding process is best for cladding?

Submerged arc welding with strip electrodes is the standard for large flat areas because of its high deposition rate and economy. GTAW, GMAW, plasma arc and laser cladding are preferred where dilution control, out-of-position work or precision matters more than raw speed.

Why is dilution important in weld cladding?

Dilution is the mixing of base metal into the weld deposit, which lowers chromium and nickel content and reduces corrosion resistance. Controlling dilution through process choice, bead geometry and multiple layers ensures the final surface meets the specified alloy chemistry.

How thick should a weld cladding layer be?

Weld cladding generally denotes a layer of about 3 mm (1/8 in) or more, applied in one or two passes. Exact thickness depends on code requirements, corrosion allowance, dilution and the fabrication method, with roll-bonded clad plate typically using 1.5 to 10 mm of cladding alloy.

What codes govern cladding welding?

ASME BPVC Section VIII covers cladded pressure vessels, Section IX qualifies overlay procedures and welders, and NB-23 and ASME Section I govern boiler overlay repairs. API 6D, ASTM A265 and AWS Welding Handbook guidance are also commonly referenced.

Is cladding welding hazardous to welders?

Yes. Welding stainless or high-chromium alloys generates hexavalent chromium fume, a carcinogen regulated by OSHA at a 5 µg/m³ 8-hour PEL. Local exhaust ventilation, respiratory protection where controls are insufficient, and medical surveillance are required controls.

When should I choose clad plate instead of weld overlay?

Buy roll-bonded or explosion-clad plate when large flat surfaces, consistent clad thickness and low dilution are needed in a mill environment. Weld overlay is usually better for field work, complex geometries, repairs and small production runs where mill lead times are impractical.