Clad Welding Definition, Processes, and Practical Guide

A refinery maintenance crew once faced a $2 million replacement for a corroded pressure vessel shell — until an engineer pointed out that only the inner 3 mm actually touched the corrosive product. The fix was not a new vessel but a corrosion-resistant layer welded onto the inside of the old steel. That instinct — protect the surface, not the whole part — is the entire idea behind clad welding.

What Is Clad Welding? Definition, Scope, and Why It Matters

A refinery maintenance crew was once quoted roughly $2 million to replace a corroded pressure vessel shell. The engineer who inspected it found that only the innermost 3 mm had actually met the corrosive product; the steel behind it was still sound. The fix was not a new vessel, it was a corrosion-resistant layer welded onto the old shell’s interior. That instinct, protect the surface rather than the whole part, is the entire idea behind clad welding, and it explains why this question shows up everywhere from pressure vessel shops to maintenance planning desks.

A Working Definition of Clad Welding

Clad welding is the deposition of one or more layers of corrosion-resistant or otherwise specially qualified weld metal onto a less expensive base metal, fused to it, so the finished face takes the service environment while the underlying material carries the strength.

Three words in that definition do the heavy lifting. Fused means the overlay forms a true metallurgical bond with the base metal rather than sitting on it like paint or a shrink-fitted sleeve, which is what allows a clad surface to handle pressure, thermal cycling and mechanical loads. Layers means cladding is almost never a single pass; it is built up, and the first pass is inevitably mixed with the base metal. Face means only the surface that touches the product is alloyed, so the bulk of the part stays carbon steel, low-alloy steel or another economical material.

The Vocabulary You Will See Used Interchangeably

Specs, catalogs and trade articles mix these terms constantly, so it helps to anchor them before the rest of the guide builds on them:

• Surfacing, the umbrella term. AWS A3.0, Standard Welding Terms and Definitions defines surfacing as the application, by welding, brazing or thermal spraying, of a layer or layers of material to a surface to obtain desired properties or dimensions, as opposed to making a joint.
• Weld cladding or weld overlay, the surfacing variant aimed at the surface’s chemistry rather than its geometry. The ASM Handbook defines weld cladding as the application of a relatively thick layer, about 3 mm or 1/8 in. or more, of weld metal to provide a corrosion-resistant surface.
• Corrosion-resistant weld metal overlay, the code language used in ASME BPVC Section IX, which treats “cladding” as a non-standard term for surfacing and sets separate qualification rules for corrosion-resistant overlay under QW-214 and hard-facing overlay under QW-216.
• Base metal and overlay, the structural substrate versus the deposited alloy that faces the service.

Hardfacing, buildup and buttering belong to the same surfacing family but answer different questions, and the next section separates them properly.

Where Clad Welding Sits in Fabrication

Cladding resolves a conflict between two uneconomic extremes: a vessel built entirely from solid corrosion-resistant alloy, which is costly, heavy and sometimes mechanically unforgiving, and bare carbon steel, which would not survive the service. Typical clad components include refinery and petrochemical pressure vessels, reactor internals, heat exchanger channels and tubesheets, digesters, stainless-lined piping, and repaired valve bodies and pump housings. In each case the steel supplies strength, toughness, weldability and low cost, while a few millimetres of overlay supply the corrosion or wear resistance.

Two broad routes reach that end state: fusion weld overlay applied during fabrication, and clad plate manufactured before fabrication starts. Alloy choice follows the corrosive medium, as Alloy 20 Welding: Procedures, Filler Metals, and Best Practices illustrates for sulfuric acid service, while the low-alloy side of the joint is governed by consumable selection like those discussed in Alloy Steel Welding Electrodes: Selection, Use & Safety. Weld overlay in particular bonds metallurgically, and overlay thickness is generally greater than 3 mm according to a DOE survey of cladded component design.

Why People Search for the Clad Welding Definition

Almost nobody looks this up out of idle curiosity. Most readers are facing one of four practical situations: a corrosion failure that an engineer has proposed fixing with overlay instead of replacement; a specification or purchase order containing the words “clad” or “weld overlay” that needs correct interpretation; a welder or inspector preparing for a performance qualification where the coupon will be overlay-welded and bend-tested; or a designer deciding between clad plate, a loose lining and field-applied overlay.

A solid definition is what lets you ask the right follow-up questions: which process deposits this alloy cleanly, how much dilution is acceptable, which standard qualifies the procedure, and how many layers the chemistry requires. Those are exactly the questions the rest of this guide answers.

Cladding, Hardfacing, Buildup, and Buttering: Knowing the Difference

Ask ten welders what “cladding” means and you will get ten different answers. Some will describe a stainless steel lining inside a pressure vessel, others will talk about hardfacing a bucket tooth, and a few will mention a buttering layer on a dissimilar metal joint. The confusion is understandable: all four operations belong to the same family, surfacing, the general term for adding weld metal to a surface rather than joining two parts. But they solve completely different problems, and picking the wrong one can cost a shop days of rework.

The authoritative definitions come from the American Welding Society, whose glossary is the vocabulary behind every welding procedure specification in the industry. As set out in the AWS surfacing terminology reference, these variations are classified by intent: what the added metal is supposed to do for the part. That intent is the whole key.

The Surfacing Family at a Glance

Cladding (weld overlay) is a surfacing variation in which material is deposited or applied, usually to improve corrosion or heat resistance. The goal is chemical: a thin, continuous layer of a costly, resistant alloy shields a cheaper structural base metal from its environment. A carbon steel vessel wall overlaid with 309L/316L stainless is the classic example.

Hardfacing is a surfacing variation in which surfacing material is deposited to reduce wear, abrasion, impact, erosion, galling, or cavitation. The goal is mechanical, not chemical. Hardfacing alloys are deliberately hard and often brittle, and the coating is typically thinner than a corrosion-resistant weld cladding. You will see it on excavator teeth, crusher rolls, and farm equipment.

Buildup restores a component to required dimensions. A worn shaft journal or a gouged machine way is built back up with weld metal until it can be machined to its original size. Buildup alloys are generally not designed to resist wear; they exist to return a worn part to its geometry, often using a filler that machines easily or matches the base metal.

Buttering adds one or more layers to a joint face for a metallurgical purpose rather than a protective one. When you must join carbon steel to stainless, or steel to another dissimilar metal, a buttering layer acts as a transition zone that dilutes and grades the chemistry across the interface. It is a joint preparation technique, not a surface protection technique.

Side-by-Side Comparison

Feature	Cladding / Weld Overlay	Hardfacing	Buildup	Buttering
Primary purpose	Corrosion or heat resistance	Wear, abrasion, impact resistance	Restore original dimensions	Transition chemistry at a joint
Typical alloys	309L, 316L, nickel alloys, titanium (mill products only)	Cobalt, iron-based, chromium carbide	Matches base metal or machinable filler	Filler compatible with both sides of joint
Deposit thickness	3 mm and up, often multi-layer	Usually thin, single or few layers	Can be very thick	One or more layers on joint face only
Critical control	Dilution of base metal into overlay	Hardness and cooling rate	Machinability, cracking risk	Composition gradient across interface
Governing codes	ASME BPVC Section IX, Section VIII	Manufacturer specs, AWS wear surfacing guidance	Repair procedures, manufacturer specs	ASME Section IX, procedure qualification

How to Tell Which One Your Job Needs

Start with a single question: what is failing, and why?

• Is the part losing material to a corrosive or high-temperature environment? That is cladding. The base metal is structurally sound but chemically vulnerable, so it needs a resistant surface layer. This is the world of pressure vessels, chemical piping, and refinery equipment, and it is what this article means by clad welding throughout.
• Is the part losing material to mechanical wear? That is hardfacing. A plow blade worn thin by soil abrasion does not need stainless steel; it needs a hard, abrasion-resistant alloy, often applied by SMAW or flux cored processes in the field.
• Is the part the wrong size but the surface itself is fine? That is buildup. See any machine repair shop, and a related example is alloy wheel repair, where worn or damaged material is filled back in and finished to restore geometry.
• Are you joining two dissimilar metals and worried about the interface? That is buttering. The layer is deposited on one or both joint faces, then the joint is completed, so the finished weld transitions gradually in composition instead of creating a hard, brittle boundary.

Real jobs often mix these operations. A clad plate joint might use a buttering layer on the clad side before welding to a solid alloy pipe. A rebuilt shaft might receive hardfacing on its wearing surface after buildup returns it to size. The practical skill is naming each step correctly, because the qualification requirements differ. Weld overlay cladding must be qualified under the surfacing rules of ASME BPVC Section IX, with dilution checks and chemistry verification, while a simple buildup or hardfacing job may need little more than a sound procedure and a visual inspection.

Confusing them also creates paperwork trouble. A procedure qualified for corrosion-resistant overlay does not automatically cover hardfacing, even with the same process and filler, because the essential variables and acceptance criteria differ. That distinction matters as shops take on work across industries, from chemical processing and mining equipment to marine fabrication such as marine welding applications.

Once you have identified the operation, the next decision is how to get that resistant layer onto the part: through weld overlay itself, or through an integral clad plate made by rolling or explosion bonding. That comparison is the subject of the next section.

Weld Overlay Cladding vs. Clad Plate: Two Routes to a Clad Surface

Once a part needs a corrosion-resistant surface rather than a solid alloy body, how does that surface actually get there? Two routes exist, and picking the wrong one costs money, schedule, and sometimes the whole component. One builds the protective layer by depositing molten filler metal onto the base steel, which is what weld overlay cladding actually is. The other buys the layer already attached, metallurgically bonded during plate manufacture. Both give you a clad surface, but they behave differently in service, in the shop, and under inspection.

Weld Overlay Cladding: Fusion Built, Layer by Layer

Weld overlay, sometimes called corrosion-resistant overlay (CRO) or weld surfacing, is the fusion process most readers mean by “clad welding.” An arc melts filler metal and a thin skin of base metal together, and the solidified deposit becomes the new working surface. The defining characteristic is dilution: because the base steel melts into the first pass, the deposit is a mixture of filler and parent, not filler alone. So overlay almost always runs in multiple layers, with later passes diluting the first instead of the steel.

The route’s great advantage is flexibility. Overlay follows complex inside geometry, applies in field conditions, and builds to whatever thickness the service demands. It is also the only realistic way to clad irregular shapes, nozzles, and weld repair areas. Our guide to Alloy 20 welding procedures covers a typical corrosion alloy overlay scenario in detail.

Integral Clad Plate: Bonded Before You Ever Strike an Arc

Integral cladding means the corrosion-resistant lining was metallurgically joined to the backing steel during plate manufacture, before it reached your fabricator. Two processes dominate:

• Hot roll bonding. A stainless or nickel alloy layer is stacked with carbon steel, heated in a furnace, and rolled under enormous pressure until the two metals fuse across the interface. It is economical in flat, high-volume production and produces plates that weld and form like a single sheet.
• Explosion bonding. A detonation wave drives the cladding plate into the backing plate at high velocity, forcing a true solid-state bond. Because nothing melts, there is zero dilution, and the corrosion-resistant alloy keeps its full nominal composition right at the interface. That makes explosion cladding the preferred route for metallurgically tricky pairs, such as titanium or zirconium on steel, and for very thick or very thin combinations.

Studies of the process consistently confirm that the impact-clad interface carries a characteristic wavy morphology and a narrow zone of plastic deformation rather than a cast fusion zone, which is why it retains near-baseline corrosion performance (Materials Today: Proceedings, explosion cladding studies).

Loose Linings and Back Cladding: The Supporting Cast

Two related terms cause confusion, so they are worth pinning down. A loose lining is a corrosion-resistant sheet held inside a vessel by fillet welds and plug welds at seams and attachments. It is not bonded to the shell across its whole area, so it relies on the process fluid never finding its way behind it. It is cheap and easy to replace, but it is not clad in the metallurgical sense.

Back cladding solves a different problem. When you weld a joint through an integrally clad plate, the root pass exposes bare base steel on the back side. Back cladding is overlaying that exposed root so the corrosive side of the joint is protected again. It is standard practice on clad equipment, but it is typically not applicable for reactive metals such as titanium, zirconium, and tantalum, so those systems rely on explosion-bonded plate and carefully designed joint details instead.

Choosing Between the Routes

Factor	Weld overlay	Integral clad plate	Loose lining
Dilution	Present, controlled with multiple layers	Zero (explosion bond); minimal (roll bond)	None at the sheet face
Geometry	Complex, field-appliable	Flat plate, formed shapes	Simple shells and heads
Thickness range	Flexible, 1 mm upward	Supplier-defined limits	Fixed sheet gauges
Cost profile	Higher labor, lower material	Higher material, lower field labor	Lowest installed cost
Inspection	Bond and chemistry testing per layer	Bond testing at mill	Minimal, but leaks behind

In practice, flat shell courses, heads, and plates in volume favor integral clad plate because the bond is already proven and your field work shrinks to welding seams. Irregular internals, nozzles, repairs, and field retrofits favor weld overlay, which goes where plate cannot. Where the alloy is reactive or the corrosion margin is razor thin, explosion bonding wins on dilution control alone.

One caveat applies to both routes: any subsequent welding on the clad side must respect the dilution and heat-input rules of the qualified procedure. Clad plate is not a license to weld casually, and the qualification requirements that govern it are covered next.

Welding Processes Used for Clad Welding and How to Choose One

Choosing the right process for clad welding decides whether you get a cost effective, low dilution overlay or one ruined by iron pickup and early corrosion. Because cladding geometry varies so widely, no single method fits every job. Weld overlay cladding therefore relies on a handful of proven processes, each chosen on deposition rate, dilution control, position capability, and typical use case.

Manual and Semi-Automatic Processes for Precision Work

For small areas, tight clearances, or repairs, manual and semi-automatic routes give welders maximum control over bead placement, which keeps excess iron out of the cladding alloy.

• Shielded Metal Arc Welding (SMAW): Stick welding is one of the most versatile options for clad welding repairs, using surfacing electrodes formulated for corrosion or wear resistance. Deposition rates trail automated methods, but portability makes it invaluable in the field. Our Advantages of Stick Welding guide explains why SMAW remains so prevalent.
• Gas Tungsten Arc Welding (GTAW) Cold Wire: Known for clean, precisely controlled heat input, cold-wire GTAW feeds filler into the puddle without preheat, delivering extremely low dilution and a refined bead at a much lower deposition rate.
• GTAW Hot Wire: The workhorse of precision cladding, hot-wire GTAW resistively preheats the filler before it enters the puddle, raising deposition considerably, especially with twin-wire setups. It suits valve bores and nozzles where consistency matters. Under ASME BPVC Section IX overlay qualification, heat input and layer control are critical. Most notably, GTAW hot-wire cladding typically shows about 15% dilution per layer, so two layers are usually required to reach under 5% iron in the deposit.
• Pulsed Gas Metal Arc Welding (Pulsed GMAW): A pulsing current transfers droplets in a controlled way, cutting heat input versus conventional spray transfer and helping dilution control. Being semi-automatic, it handles complex contours at faster travel speeds than GTAW.
• Flux Cored Arc Welding (FCAW): FCAW suits select overlay work thanks to high deposition rates and tolerance to outdoor conditions. Filler selection must match the slag system to the cladding alloy to avoid contamination.

Automated Processes For Large Surface Areas

Vessels, piping spools, and broad flat surfaces call for automation, which delivers efficiency while keeping deposit chemistry consistent.

• Submerged Arc Welding (SAW) Wire Cladding: Widely accepted, weld overlay cladding is most commonly performed using submerged arc welding, and for good reason: SAW offers some of the highest deposition rates of any arc process, with granular flux shielding the puddle from air and limiting spatter. The key limitation is that submerged arc welding can be used only in the flat position, which limits its use on complex geometries. Parameters are built around qualified procedures, as outlined in AWS welding procedure guidance for pressure containing equipment.
• Submerged Arc Strip Cladding: Swapping round wire for a wide, flat strip electrode broadens each bead, boosting coverage with strong dilution control when paired with the right flux. A staple for large diameter vessels needing a uniform overlay.
• Electroslag Strip Cladding: SAW and electroslag welding are most economical for large areas. A molten slag bath melts both strip and base metal for very high deposition, though the setup keeps it on flat, horizontal surfaces. Fusion process research in Journal of Manufacturing Processes research underscores its efficiency on heavy walls.

High Energy Density Processes For Specialized Applications

Niche jobs demanding ultra low dilution, tiny heat affected zones, or exotic alloys reach for high energy density processes.

• Plasma Cladding: Plasma transferred arc (PTA) cladding constricts the arc through a nozzle into a concentrated heat source, giving precise dilution control often better than conventional GTAW. Ideal for valve seats, seal surfaces, and small diameter parts.
• Laser Cladding: A focused laser yields the lowest dilution of many fusion processes with an exceptionally small heat affected zone. The upfront investment is higher, but critical components justify it. Rapid solidification behavior appears in metallurgical work such as Materials Today: Proceedings. For more on these systems, see our Alpha Laser Welding guide.

How To Choose The Right Process

The job drives the choice. Use this comparison of the deciding factors:

Process	Deposition Rate	Dilution Control	Position Capability	Typical Use Cases
SMAW	Low	Moderate	All Positions	Field repairs and small areas
GTAW (Cold Wire)	Very Low	Excellent	All Positions	Root passes or ultra critical surfaces
GTAW (Hot Wire / Twin Wire)	Medium	Very Good	All Positions	Valve bores, nozzles, and precision parts
Pulsed GMAW	Medium – High	Good	Most Positions	Complex contours and medium sized overlays
FCAW	High	Moderate	Most Positions	Outdoor work with proper alloy selection
SAW (Wire/Strip)	Very High	Good	Flat Only	Large vessels and shell courses
Electroslag Strip Cladding	Extremely High	Good	Flat Only	Heavy wall, large surface area cladding
Plasma / Laser Cladding	Low – Medium	Excellent	Varies by Setup	Wear surfaces, seals, and exotic alloys

Dilution: The Technical Heart of Clad Welding

For any corrosion-resistant overlay, chemistry is everything. The factor that will make or break that chemistry is dilution. Simply put, dilution is the mixture of melted base metal and added filler metal within the deposited weld metal. Instead of a pure layer of your chosen alloy sitting on top of carbon steel, a portion of the base metal is melted and blended in, changing the final composition of that clad layer.

This change isn’t guesswork, it’s measurable. Dilution % = x / (x + y) × 100, where x is the amount of base metal melted, and y is the amount of filler metal added. For example, if a single pass melts 1.0 mm² of base metal and deposits 3.0 mm² of filler metal, the dilution would be 1.0 / (1.0 + 3.0) × 100 = 25%. That means a quarter of that bead is no longer the corrosion-resistant alloy you specified, it’s the base metal.

Why Too Much Dilution Is Detrimental

The more base metal that gets pulled in, the more the cladding drifts away from its designed chemistry. For austenitic stainless or nickel-based overlays, this iron pickup is especially harmful. Excessive dilution can cause brittle phases at the interface, hot cracking, and loss of pitting corrosion resistance through uncontrolled delta-ferrite levels. These aren’t surface blemishes, they directly attack the very reason the vessel or pipe was clad to begin with.

These risks are taken seriously enough that they are considered throughout the qualification process under frameworks such as ASME BPVC Section IX, where overlay procedure qualifications must prove the chemistry and soundness of the deposited layer.

Controlling Dilution Through Process Selection

Heat input is the driving force behind how much base metal melts, so process choice is one of the most powerful tools to limit it. Some processes spread a wide, shallow puddle with low penetration, while others dig deep into the joint and pull in far more of the parent metal.

This difference can be seen in practice. In a published study on super-austenitic stainless cladding, hot-wire GTAW reduced dilution from about 26.7% to 19.8% compared with cold-wire GTAW. By preheating the filler wire and feeding it more efficiently into the leading edge of the puddle, less arc energy is wasted on melting extra base metal, resulting in a much tighter control of chemistry. Other low-dilution options, such as strip cladding, can do this even further, though at the cost of position flexibility.

Mitigating Dilution With Multiple Layers

Even with the best process, the first pass of any arc-based overlay will dilute heavily into the steel. This is why multiple weld clad layers are used to mitigate dilution. The first layer acts as a buffer, blending into both the base metal and the filler. The second layer then dilutes into that mixed buffer instead of directly into the carbon steel, and the third can dilute down even more until it is much closer to the virgin filler metal chemistry.

A documented example of this is the use of three layers of ERNi-1 (about 6 mm total) needed for the top layer to reach the filler metal chemistry. This is not wasted weld metal, it’s an insurance policy for corrosion resistance. For some aggressive services, a chemistry check between layers is just as important as the final visual inspection.

The Bottom Line

Every decision, from amperage and travel speed, all the way down to how many layers are laid, is in service of controlling x and y in that equation. Ignore dilution, and the most expensive nickel alloy becomes, chemically, far less than advertised. Control it, and a cost-effective carbon steel shell can gain the protection of a solid alloy. With that in mind, it’s vital to also consider the safety side of laying these dense alloy deposits, keeping in line with proper ventilation practices found within OSHA’s welding and cutting guidance. At its core, mastering dilution is mastering the entire purpose of clad welding itself.

Codes, Standards, and Qualification for Clad Welding

An overlay can look flawless and still fail the paperwork. The bead is smooth, the color is right, the spool label says 316L. What a code inspector actually wants to know is whether the metal that got deposited, after mixing with the steel underneath it, still matches the chemistry the procedure promised. That single question is why clad welding sits inside a fairly tight framework of procedure qualification, welder qualification, and documented acceptance criteria.

ASME Section IX: Qualifying the Procedure

For ASME construction work, ASME BPVC Section IX governs welding procedure and performance qualification. Three provisions do most of the heavy lifting for overlay work:

• QW-214 defines procedure qualification for corrosion-resistant weld metal overlay. The procedure qualification record has to prove the deposited weld metal meets its required composition after dilution, not merely that the coupon survived bending.
• QW-216 works alongside it, setting what the overlay qualification must demonstrate and record, including weld-metal chemistry requirements and the layer build-up needed to reach them.
• Table QW-453 lists the essential variables for overlay. Change the process, the filler metal or strip classification, the number of weld-metal layers, or the deposited chemistry outside the qualified range, and that PQR and WPS no longer cover the job.

In practice this means a WPS reading “corrosion-resistant overlay, 316L, carbon max 0.03%” carries chemistry requirements that must be verified by analysis of the deposited weld metal. If that analysis is missing or out of range, good bead appearance cannot rescue it.

Welder Performance Qualification: The Cladding Coupon

Here is the trap that catches fabricators. A welder qualified on a groove weld is not automatically qualified to deposit corrosion-resistant overlay when the WPS carries chemistry requirements. Under QW-381, Section IX expects a separate performance demonstration on a cladding test coupon, because overlay technique — travel speed, weave width, pass overlap — governs dilution, and dilution governs whether the chemistry lands where the procedure says it should.

• Plate coupons are at least 6 in. × 6 in. (150 mm × 150 mm), with a cladding bead at least 2.5 in. (38 mm) wide. Pipe coupons receive a contiguous circumferential overlay.
• The test consists of transverse side bend tests: two for plate coupons, and four for pipe welded in the 5G or 6G positions.
• Acceptance permits no open discontinuity exceeding 1/16 in. (1.5 mm) in the cladding, and none exceeding 1/8 in. (3 mm) along the weld interface.

The two limits differ for a reason: the overlay is the corrosion barrier, so it is held tighter than the bond line beneath it. Side bends are chosen because they expose the two defects overlay qualification really hunts — lack of fusion between beads or passes, and cracking at the interface.

Section VIII: Clad and Lined Vessel Construction

If Section IX answers “is this procedure and welder qualified,” Section VIII answers “can this vessel be built with a lining.” Division 1 and Division 2 both accept clad and lined construction, and both treat the corrosion-resistant layer as a barrier rather than a load-bearing part of the design. Pressure-part thickness, impact testing, and postweld heat treatment rules are set by the base metal. Designers normally do not credit the overlay toward required thickness: the steel carries the pressure and the alloy carries the corrosion.

PWHT is where the two codes meet awkwardly. Carbon steel vessels frequently require it, while austenitic overlays are sensitive to it. When a PWHT cycle is applied, the procedure must show the overlay still meets its chemistry and corrosion requirements afterward. The same caution applies to low-alloy bases such as 8630 or 4130, where filler selection and preheat get more specific, as covered in our guide to alloy steel welding electrodes. Inspection, NDE, and the Manufacturer’s Data Report still apply to the finished vessel. The cladding exempts nothing; it adds a surface that must also be examined and documented.

AWS A3.0 and API 582: Shared Language, Shared Chemistry

Two documents clear up most specification confusion. AWS A3.0, Standard Welding Terms and Definitions, fixes the vocabulary — weld overlay, buttering, buildup, hardfacing, and surfacing are distinct operations, and a specification that uses them loosely will get a welder depositing the wrong thing. AWS also publishes technical guidance on examining and evaluating the deposited result.

API 582 handles the chemistry side. It provides weld-metal composition limits, dilution guidance, and process recommendations for the oil and gas industry, including limits on impurity elements such as phosphorus, sulfur, silicon, and niobium that promote solidification cracking in highly diluted first layers. It is a guideline rather than a mandatory code — until a project specification or an API product standard references it, which is exactly what normally happens.

The metallurgy behind those limits is well established in sources such as ASM International’s published joining and welding material: the first pass laid on bare steel is a mixture of two metals, and every element the base contributes pushes the deposit toward the cracking-susceptible range.

Keeping Qualifications Alive on the Floor

Codes care about continuity as much as competence. Keep the PQR available, the current revision of the WPS at the station, and records showing the welder has kept welding, because a long interruption can trigger requalification. Since much field cladding and repair is still deposited with stick electrode, the habits that make stick welding productive — consistent arc length, clean and correctly stored consumables, controlled amperage — translate directly into overlay quality and clean qualification records.

Get four things right and the rest of the job follows: a qualified procedure with verified chemistry, a welder qualified on the actual cladding coupon, an acceptance standard everyone agreed to up front, and documentation that survives the audit.

Materials Used in Clad Welding: Filler Metals, Strip Electrodes, and Base Metals

Every clad weld is a pairing decision. In almost all pressure vessel and process equipment work, the cladding material is an austenitic stainless steel or a nickel-base alloy, and it is fused onto a thick, inexpensive substrate of carbon steel or low-alloy steel. The expensive alloy supplies corrosion resistance; the steel supplies strength, stiffness, and cost control. Choose either half badly and the whole overlay fails, so material selection deserves as much attention as process selection.

The Cladding Alloys: Stainless, Nickel, Cobalt, and Iron-Based

Four families cover most cladding work:

• Austenitic stainless steels. Type 309L is the workhorse first-layer or backup electrode because its higher chromium and nickel content compensates for iron picked up from the base metal. The finish layer is usually 316L or 308L, which delivers the final corrosion-resistant chemistry once dilution is no longer a factor. Stabilized grades such as 347 appear where sensitization during service or heat treatment is a concern.
• Nickel-base alloys. ERNiCr-3 (Alloy 82 type) handles moderate corrosion and dissimilar-metal transitions, while ERNiCrMo-3 (Alloy 625 type) is the standard for chloride pitting, seawater, and sour service. ERNiCrMo-4 steps in for more aggressive reducing acids.
• Cobalt-based alloys. Stellite-type overlays are chosen for wear, galling, and hot hardness rather than bulk corrosion resistance — valve seats and trim are the classic targets.
• Iron-based hardfacing alloys. Chromium-carbide and martensitic irons cover abrasion duty at far lower cost, though they bring little of the chemical resistance that defines clad welding proper. The ASM International surfacing reference lays out where each family belongs.

Filler Metal Forms: Stick, Coiled Wire, and Strip

Cladding consumables come in three delivery forms: covered electrodes for SMAW, coiled wire for GTAW (cold and hot wire), GMAW, and FCAW, and flat strip electrodes for submerged arc and electroslag cladding. The AWS filler metal guidance covers the classification system behind all three.

Strip electrodes deserve special attention because strip cladding with submerged arc or electroslag is the most economical route for very large areas such as vessel shells and heads. Typical strips run roughly 25 to 60 mm wide and 0.4 to 1.0 mm thick, with common sizes like 30 × 0.5 mm and 50 × 0.5 mm. Wide strip lays down a broad, shallow bead with low dilution, minimal overlap, and far fewer passes than wire, but it is effectively limited to flat and horizontal positions.

Base Metals: Carbon Steel, 8630/4130, and Duplex

Carbon steel remains the dominant substrate: rolled plate such as SA-516 Grade 70 for vessel shells and heads, plus pipe and forgings for nozzles. In offshore equipment, however, the base is frequently a low-alloy steel. AISI 8630 and AISI 4130 forgings and castings are typical for gate valves, choke manifolds, and separators, where high strength and hardenability matter. Welding these requires matched alloy steel welding electrodes and careful control of preheat, interpass temperature, and post-weld heat treatment, because the heat-affected zone can harden and crack before the overlay even goes on. Duplex and super-duplex steels appear as base metals in clad plate for seawater duty, where the metallurgy is compatible on both sides of the bond.

Choosing the Alloy by Service Environment

Selection logic always starts with the process fluid, not the equipment drawing.

Service environment	Typical overlay	Selection driver
Dilute chemicals, moderate corrosion	309L first layer + 316L finish	Low cost, forgiving of dilution
Chlorides, seawater, pitting	ERNiCrMo-3 (625 type)	Pitting resistance, iron tolerance
Sour service (H₂S, chlorides)	625-type nickel alloy, multi-pass	Hardness limits and iron content
Sulfuric acid pickling	Alloy 20 type	Acid resistance at reasonable cost
High-temperature wear, galling	Cobalt or iron-based hardfacing	Hardness over corrosion resistance

The broader pattern is consistent: the base metal carries the load, the overlay carries the chemistry, and the filler metal is chosen so the diluted mixture in between still performs. Get the combination right and the vessel behaves like a solid alloy part at a fraction of the cost.

Benefits of Clad Welding — and Where It Falls Short

Clad welding is often presented as a straightforward win: put the expensive, corrosion-resistant alloy only where it is needed and save the rest of the budget. In practice it is a trade. Understanding what you gain and what you give up is what separates an overlay that lasts thirty years in service from one that disbands during the first hydrotest. Both sides, honestly, so you can judge whether weld overlay is the right route before you commit a procedure qualification to it.

The Economic Case: Corrosion Is a Cost Problem, Not Just a Materials Problem

It is hard to overstate how expensive corrosion is. The NACE International IMPACT study estimated global corrosion costs at roughly US$2.5 trillion per year, about 3.4% of global GDP, and found that between 15% and 35% of that cost is potentially avoidable through better materials selection and corrosion management (NACE IMPACT report). For the United States specifically, the FHWA/NACE study commissioned by Congress put direct corrosion costs at approximately $276 billion per year, around 3.1% of GDP (FHWA/NACE corrosion cost study).

Clad welding attacks that cost directly. A solid 316L or Alloy 825 pressure vessel shell can cost many times more than a carbon steel shell of the same dimensions. Weld overlay lets you buy a structurally adequate carbon or low-alloy steel body, then deposit a 3 to 5 mm corrosion-resistant layer on the wetted surface. That routinely cuts material cost by 50% or more versus a solid alloy part while delivering the same corrosion performance at the product face. The same logic applies to fasteners, flanges, and pipe spools: the steel carries the load, the overlay carries the chemistry.

Engineering and Fabrication Advantages

Beyond raw cost, clad construction offers design flexibility a solid alloy part cannot match. The substrate can be welded, bolted, or machined using familiar carbon steel procedures, so attachments, support lugs, and nozzles go on the back side of the clad where they never contact the corrosive medium. Welders on the job already know the consumables; a shop that runs stick and flux-cored work daily can pick up procedures like those in our alloy steel welding electrodes guide without a full retraining program.

Overlay is also repairable in the field in ways clad plate is not. A worn or locally corroded area can be ground out, re-buttered, and re-deposited on site, while hot roll bonded or explosion bonded clad plate offers no such luxury once fabrication has started. And because the deposited layer is metallurgically bonded rather than mechanically attached, there is no liner seam to leak behind, which is why overlay remains the standard answer for vessel internals, reactor internals, and heat exchanger channel sheets.

Benefit	What It Means on the Floor
Material cost reduction	CRA layer over cheap structural steel; big savings on vessels and piping
Design freedom	Supports and attachments welded to the steel side, out of the corrosive path
Repairability	Local re-overlay during a turnaround; clad plate is not repairable that way
Metallurgical bond	No backing strip, no liner gap, no crevice for chloride ingress
Geometry flexibility	Irregular shapes and nozzle bores that plate cannot reach

The Limitations You Have to Design Around

• Dilution is always present. Any fusion process melts some base metal into the deposit, pulling iron and carbon into the corrosion-resistant layer and reducing alloy content at the interface. It is controllable but never zero on a first pass, which is why multi-layer deposits and careful process selection are non-negotiable.
• Position capability is limited. High-rate processes such as submerged arc strip cladding and electroslag strip cladding are essentially flat-position work. If your component cannot be rotated into a downhand position, deposition rate drops sharply and the cost advantage erodes.
• Post-weld heat treatment is risky. Prolonged or repeated PWHT on austenitic overlays can precipitate carbides and promote sigma phase formation, damaging corrosion resistance, and high-chromium nickel alloy overlays are similarly sensitive. That constrains how many PWHT cycles a clad component can survive and can force changes to the fabrication sequence.
• Inspection burden is heavier. A clad surface needs fusion verification, dilution checks via chemistry sampling, and often ferrite measurement on stainless overlays. Ultrasonic examination of the bond line adds cost and time that a plain carbon steel part never sees.
• Some alloys simply will not overlay. Titanium, zirconium, and tantalum cannot be reliably deposited by arc welding onto carbon steel. The arc oxidizes them, dilution is uncontrollable, and the resulting layer is metallurgically unsound. Those applications must use integral clad plate or loose lined construction instead.

The Practical Takeaway

The honest summary is that clad welding wins when the corrosion problem is real, the component is large or geometrically complex, and the shop can rotate the part for downhand deposition. It loses when the alloy is reactive, when the geometry forbids position control, or when repeated heat treatment cycles are unavoidable. Run those three checks against your design before you commit to the cost model, and you will choose the right route the first time.

Common Clad Welding Defects, Mistakes, and How to Fix Them

Most clad welding failures are designed in before the arc is struck. A welder who chases a defect with more filler metal, without understanding why it formed, usually makes things worse: another dilution spike, another heat cycle, another risk of cracking. This is the floor-side field guide. For each common defect you get the symptom, the root cause, the prevention, and the corrective action a welder or QC inspector can apply on the job.

Lack of Fusion: The Defect That Killed Short-Circuit GMAW

Lack of fusion shows up as cold laps between stringer beads or a cold root at the bond line, where the filler wetted the surface but never truly fused. The classic cause is too little energy. Short-circuiting GMA transfer was largely abandoned for Alloy 625 overlay work because of its tendency to produce lack-of-fusion defects from very low heat input. It looks attractive because it is spatter-free and position-friendly, but it does not dig into the substrate fast enough for consistent metallurgical bonding across long overlay runs.

Prevention is a process decision made at qualification time: use pulsed GMA, hot-wire GTAW, or submerged arc strip cladding; keep travel speed slow enough for full wetting; and grind or wire-brush between passes to kill oxide films. Correction means removal, not re-passing: grind or gouge back to sound metal, feather the edges, and re-weld under a controlled procedure.

Hot Cracking

Hot cracking in the overlay or at the bond line usually traces to chemistry. Excessive dilution drags base-metal carbon, sulfur, and phosphorus into the first pass, and a fully austenitic deposit with no ferrite is far more crack-sensitive. Research on dissimilar and clad weld metallurgy repeatedly confirms that dilution-driven segregation and compositional imbalance are the dominant cracking mechanisms (Materials Today: Proceedings). The fix is layered: deposit a low-dilution buttering layer first, keep each pass thin, verify finished-surface chemistry with XRF or OES, and stay inside the AWS and AWS Guide to Welding Fabrication composition limits for the alloy family.

High Dilution and Iron Contamination

High dilution is the silent killer of corrosion resistance. The overlay looks fine on the surface, yet its chromium and molybdenum have been diluted below the threshold that forms a stable passive film, so the part fails early in service. Process selection is the main defense. Spray transfer GMAW’s deep penetration is undesirable for low-dilution overlay welding, which is why pulsed GMA became the preferred GMAW mode: pulsed spray gives a stable, directional spray with far shallower penetration per unit of deposited metal. Submerged arc and electroslag strip cladding achieve similar results through flux chemistry and bead geometry rather than amperage control.

On the floor, iron contamination is measured, not guessed: XRF readings on the final layer typically need to stay in the low single digits for stainless and nickel overlays. When readings run high, add another layer or grind and re-overlay. Do not try to blend it out with a single hot pass, which only pushes more iron forward.

Porosity

Porosity comes from trapped gas, and the source is almost always preparation or consumables: wet flux, unbaked electrodes, oily plate, residual paint, or compromised shielding gas on open-arc processes. Strip cladding adds its own variables, since flux coverage must stay intact across the arc. Prevention is routine: bake flux and electrodes per the manufacturer’s data, degrease the substrate, check torch liners, gas lines, and flow rates, and watch wind in open shops. Porous metal is removed and replaced; there is no legitimate way to fix porosity by re-melting.

Disbondment

Disbondment is the most dangerous clad defect because it is often invisible until the lining lifts. Hydrogen introduced during welding or post-weld heat treatment migrates to the fusion line, where a hard, brittle martensitic layer can form in carbon steel, and the overlay separates along that plane. The industry response is built into ASME BPVC practice: joint types and procedures for clad components must be selected to minimize brittle weld composition from mixing base and clad alloys, as set out in the Section VIII Special Requirements. Buttering the joint with the clad alloy, controlling hydrogen, and specifying slow cooling through critical temperature ranges all reduce risk.

Distortion

Long overlay runs on thin shells pull the geometry out of tolerance. Balance the heat: weld alternating passes on opposite sides, use skip sequences, fixture the work, and keep interpass temperature in range. Fixing distortion after the fact often means straightening, which is a mechanical stress event on a corrosion-resistant layer.

Repair Over Rework: Know the Cost of the Second Try

The mistake most crews make is treating repair as free. It is not. Repeated post-weld heat treatment cycles can ruin base material properties, so parts that cannot be repaired many times demand low-defect-rate processes such as hot-wire GTAW. A welded overlay repair on a test coupon also triggers additional ASME Section IX testing requirements, and any gouging operation carries its own hazards, including carbon arc fumes and electrical exposure, as covered by OSHA welding and cutting guidance. For removal of defective overlay, Air Arc Welding and Carbon Arc Gouging covers safe setup and technique; for filler selection in aggressive media, see our Alloy 20 Welding procedures guide.

The discipline is simple: qualify the procedure properly, inspect early, and repair once. A clad part that needs three repairs is usually cheaper to re-fabricate than to keep heating and hoping.

Practical Checklist: Planning a Clad Welding Job From Start to Finish

Most clad welding failures happen weeks before the arc is struck: wrong alloy, unqualified procedure, overlay welded across a joint that was never radiographed. Clad welding — fusion-bonded deposition of a corrosion- or wear-resistant alloy onto a stronger, cheaper load-bearing base metal — punishes shortcuts. Here is the workflow.

Step 1: Define the Service Environment Before Choosing Anything

Record what the surface will face: medium, concentration, temperature, pH, flow velocity, pressure, and whether abrasion or thermal cycling is present. That page drives the alloy family, the required barrier thickness, and the acceptable dilution limit. If attack is abrasive rather than corrosive, reclassify: that is hardfacing, not a clad specification. If your base metal is a low-alloy steel such as 8630 or 4130, confirm its preheat, hydrogen control, and temper-bead rules early, because those constraints shape the weld sequence.

Step 2: Choose the Cladding Route

Pick the route: weld overlay, mill-bonded clad plate, explosion-bonded plate, loose mechanical lining, or back-cladding a completed joint. Field work, complex geometry, small runs, and repair favor weld overlay; flat, high-volume plate favors clad plate. Loose linings win where disbondment must stay detectable, and back-cladding seals a corrosion-resistant groove weld from the process side.

Step 3: Select the Welding Process

Match the process to position, thickness, and production rate. Flat, high-volume work goes to submerged-arc strip or wire cladding and electroslag strip cladding. Field roots and out-of-position work lean toward SMAW, pulsed GMAW, or hot-wire GTAW. Thin barriers and heat-sensitive parts suit plasma-transferred arc or laser cladding. Vertical or overhead overlay tightens the filler and technique choices; the all position welding rod guide covers what changes in those positions.

Route / Process	Deposition rate	Dilution control	Typical use
SMAW	Low	Moderate	Field, out-of-position, small areas
Hot-wire GTAW	Medium	Excellent	Root layers, tight chemistry control
Pulsed GMAW	Medium	Good	Positional overlay, thinner sections
SAW strip / electroslag strip	High	Very good (low with electroslag)	Flat plate, vessels, large areas
Plasma / laser	Low to medium	Excellent	Thin barriers, low heat input

Step 4: Write and Qualify the PQR and WPS

Weld overlay procedure qualification follows ASME BPVC Section IX, with weld overlay treated as its own weldment type under Table QW-453. For corrosion-resistant overlay, chemical analysis is taken across the deposited weld metal and fusion zone per QW-214.1, where dilution is proven. Delta ferrite determination is not required as part of overlay qualification. Put the base metal, filler classification, preheat, interpass, heat input window, layer count, and chemistry acceptance in the WPS itself. Welder qualification uses an overlay coupon with guided bend testing, so build those fixtures before production needs them.

Step 5: Plan the Layers and the Chemistry Checks

Plan the layers before the first pass. A single layer almost always carries too much diluted iron, so most specifications demand at least two, with the final layer run at the lowest practical dilution. Take the chemistry coupon after the final layer, not the first. The economics hinge on meeting required chemistry at the highest practical deposition rate in the fewest layers, not raw speed. If the overlay will see sensitizing temperatures, decide up front how delta ferrite or chemistry will be measured on the finished surface.

Step 6: Sequence PWHT and Inspection in the Right Order

Clad materials must be post-weld heat treated when required by the base material, with the cycle based on base material thickness — but verify that the overlay alloy tolerates that temperature and hold time. Order matters: inspection of clad components must occur before weld overlay is applied over the underlying welds, because once the overlay is on, the structural weld beneath is hidden from radiography and surface NDT. For shop safety and fume control, follow OSHA’s welding, cutting, and brazing guidance.

Step 7: Document, Hand Over, and Maintain

Hand over a complete package: PQR and WPS, welder qualification records, layer and heat input logs, chemistry results, NDT reports, and PWHT charts. In service, schedule corrosion monitoring and UT thickness checks at a defined interval. For nickel-bearing filler in aggressive service, alloy-selection references like the Alloy 20 welding guide help align filler chemistry with the environment.

Conclusion: The Definition, Restated as a Decision Framework

Clad welding is the fusion-bonded application of a corrosion- or wear-resistant alloy onto a base metal chosen for strength and cost. Every planning question follows from it: define the environment, choose the route, select the process, qualify the procedure, plan the layers, sequence inspection and heat treatment, document everything. Do that, and the $2 million replacement stays a story you tell, not an invoice you pay.

Frequently Asked Questions

What is the simple definition of clad welding?

Clad welding is the deposition of one or more layers of weld metal onto a base metal surface to give that surface a property the base metal lacks — most often corrosion resistance, sometimes wear resistance. It creates a metallurgical bond rather than a mechanically attached lining.

Is clad welding the same as hardfacing?

No. Cladding targets corrosion or heat resistance and is usually 3 mm or thicker, while hardfacing deposits a thinner layer intended to resist wear, abrasion, impact, erosion, galling, or cavitation. Both are surfacing processes, but they solve different problems.

Which welding process is best for cladding?

There is no single best process. Submerged arc and electroslag strip cladding are most economical for large flat areas, hot-wire GTAW is preferred for precision bores and nozzles because of its very low defect rate, and pulsed GMAW suits intermediate work. Selection depends on geometry, required dilution, and position.

Why is dilution a problem in clad welding?

Dilution is mixing of melted base metal into the deposited layer, which pulls chromium, nickel, and molybdenum away from their specified levels and can promote hot cracking or brittle interface phases. Fabricators control it with low-penetration processes, higher travel speeds, and multiple layers.

How many layers of weld overlay are typically required?

Most corrosion-resistant overlays use two layers — a first pass that may be heavily diluted and a second finish pass that meets the specified chemistry. Some applications need three layers; for example, roughly 6 mm of ERNi-1 over stainless steel was required for the surface to reach filler metal composition.

Do welders need separate qualification for cladding?

Under ASME BPVC Section IX, a welder must qualify on a cladding test coupon (QW-381) to deposit corrosion-resistant weld metal overlays when the WPS carries chemistry requirements, and that qualification does not extend to groove welds. Qualification uses transverse side bend specimens from a plate or pipe coupon.

When should you use clad plate instead of weld overlay?

Explosion-bonded or roll-bonded clad plate is preferred for large flat shells and heads because it gives zero dilution in a single bonding operation, while weld overlay is the practical choice for nozzles, bores, complex internals, repairs, and small areas where plate waste and cutting costs dominate.

Can any metal be weld-clad onto steel?

No. Fusion welding cannot produce a sound arc-deposited overlay of reactive metals such as titanium, zirconium, or tantalum on carbon steel. Those materials are supplied as explosion-bonded clad plate, or as mechanically attached linings, rather than weld overlay.

How is clad weld quality verified?

Qualification relies on chemical analysis of the deposited layer plus mechanical side bend tests per ASME Section IX, while production work is checked with visual and liquid penetrant examination against acceptance limits such as no open discontinuity over 1.5 mm in the cladding. Ultrasonic testing is used for bonded clad plate.