Weld Overlay and Clad Construction: CRA Cladding, Dilution and Qualification

Quick Answer

Clad construction puts a thin layer of corrosion-resistant alloy on a thick carbon or low-alloy steel backing, giving the corrosion performance of the expensive alloy at a fraction of the solid-alloy cost — typically 30-60% of solid CRA price for the same component. There are two routes. Weld overlay deposits the alloy by welding, usually 3-6 mm finished thickness, and is the only practical method for nozzles, flange faces, tubesheets, and local repair. Clad plate is produced in the mill — roll bonding (hot roll bonding of a sandwich) for large flat areas and most pressure vessel shells, explosion bonding for alloys that cannot be roll bonded and for tubesheets and transition joints, and weld overlay clad plate where a mill-applied overlay suits the geometry. The controlling variable in weld overlay is dilution: molten backing steel mixes into the first deposited layer, raising its iron content and destroying its corrosion resistance. For Inconel 625 overlay, specifications typically require iron content below 5% (often 10% maximum, 5% preferred) at 2 mm below the finished surface, and this is achieved by depositing two layers, by using high-deposition low-penetration processes such as ESW or hot-wire TIG, and by qualifying the procedure with chemical analysis of the as-deposited surface — not by assuming the filler's nominal composition.

A sour gas processing project in the Caspian region ordered six separator vessels with Inconel 625 weld overlay on all wetted internal surfaces. The specification called for iron content below 5% at the finished surface. The fabricator qualified a single-layer submerged arc overlay procedure and produced test coupons showing 7.8% iron.

The fabricator argued, with some justification, that 7.8% was close, that the standard allowed 10% in some readings, and that the schedule was tight. The client's metallurgist held the line and required a two-layer procedure.

Two of the six vessels had already been overlaid single-layer. They were reworked: the overlay was ground out and re-deposited in two layers. The rework cost about 480,000 USD and eleven weeks.

Four years later a different operator in the same basin had a single-layer 625 overlay vessel with iron content in the 8-10% range. It developed chloride pitting through the overlay at the heat-affected weld bands within thirty months, and then rapid attack of the carbon steel underneath. The vessel was scrapped.

Iron dilution is not a documentation requirement. It is the difference between a 625 surface and a slightly nickel-rich stainless surface that is not what anybody specified.

Why Clad Instead of Solid

Consider a 2,500 mm diameter, 40 mm thick pressure vessel shell in sour service requiring Inconel 625 corrosion resistance.

Solid 625: approximately 40 mm of a material costing in the region of 35-50 USD/kg.

Clad: 37 mm of A516 Gr 70 at roughly 1.2-1.8 USD/kg plus 3 mm of 625.

The alloy content falls by more than 90%, and total material cost typically lands at 30-60% of the solid alternative depending on alloy, thickness and quantity. On a large vessel this is routinely a seven-figure difference.

Clad construction also gives:

• Backing steel strength and toughness — carbon and low-alloy steels have excellent toughness and well-understood design codes, and in some cases better mechanical properties than the CRA
• Thinner CRA than solid construction would require, because the backing carries the pressure
• Weldability and inspectability of a familiar structural material for the bulk of the component

The costs are:

• Fabrication complexity. Every weld, every nozzle, every attachment has to maintain the clad barrier.
• A metallurgical interface that must bond properly, stay bonded through fabrication and service, and survive PWHT.
• Qualification burden. Procedures must be qualified for dilution and for bond integrity, not just for mechanical properties.
• Disbonding risk in hydrogen service, discussed below.

Clad is the correct answer for large components in aggressive service. Solid CRA remains correct for small components, where the fabrication complexity of cladding costs more than the alloy it saves, and for thin-wall items like small-bore piping and instrument tubing.

Weld Overlay

Processes

SAW (submerged arc welding), strip cladding — a 30-120 mm wide strip electrode under flux. Very high deposition rate, 15-35 kg/h, excellent surface finish, well suited to large flat and cylindrical areas. Significant penetration means dilution must be managed, usually with two layers.

ESW (electroslag welding), strip cladding — the same strip geometry but the arc is replaced by a resistive molten slag pool. Penetration is much shallower than SAW, so first-layer dilution is markedly lower — iron content in the first layer typically 8-12% against 15-25% for SAW. Deposition rates reach 20-40 kg/h. The preferred process for large-area high-integrity overlay, and the route by which some specifications permit single-layer 625.

GMAW / FCAW (MIG, flux cored) — wire processes, moderate deposition, flexible and positionally capable. Pulsed GMAW with controlled transfer reduces penetration. Used for nozzles, smaller areas and positional work.

GTAW (TIG), including hot-wire TIG — lowest heat input and lowest dilution, best control and best surface quality, slowest. Hot-wire TIG preheats the filler resistively before it enters the pool, raising deposition several times over conventional TIG while keeping dilution low. The standard for small bores, nozzle IDs, flange faces and anywhere dilution margin is tight.

PTA (plasma transferred arc) — powder or wire fed into a plasma arc. Very low dilution, excellent for hardfacing and for thin precise overlays on valve seats and sealing faces.

Laser cladding — extremely low dilution and minimal heat-affected zone, thin deposits, high precision, high equipment cost. Growing in repair and in precision applications.

Dilution — the governing variable

When the arc melts the filler, it also melts some of the backing steel. The two mix in the weld pool, so the deposited chemistry is not the filler chemistry — it is a blend.

For Inconel 625 (Ni-22Cr-9Mo-3.5Nb) on carbon steel:

Why it matters: the corrosion resistance of 625 comes from its chromium, molybdenum and niobium in a nickel matrix. Dilution with iron does not simply add iron; it reduces the effective Cr and Mo concentration and changes the solidification behaviour, promoting segregation and secondary phases that become preferential corrosion sites.

Typical specification requirements:

• Iron content below 5% (preferred) or 10% (maximum) measured at 2 mm below the finished surface for Inconel 625 overlay
• Minimum finished overlay thickness after machining, typically 3 mm, measured at the thinnest point
• Chemical analysis of the as-deposited surface as part of procedure qualification, not reliance on filler certificates
• Two-layer deposition unless a single-layer procedure is qualified and proven to meet the Fe limit — which in practice usually means ESW or hot-wire GTAW

Measuring at 2 mm below the surface is deliberate: it is the depth that will be the surface after machining and after any allowance for service loss, so it is the chemistry that will actually face the process fluid.

Common overlay alloys

The 309L buffer layer is standard practice for stainless overlay. 309L has deliberately high Cr and Ni so that even after 20-30% iron dilution, the first layer remains fully austenitic with adequate alloy content. The second layer of 316L is then deposited on 309L rather than on carbon steel, so its dilution is with an already-alloyed layer and the finished surface achieves genuine 316L chemistry. Depositing 316L directly on carbon steel in a single layer produces a diluted, partly martensitic deposit that cracks and corrodes.

PWHT and the nickel-alloy problem

Carbon steel pressure vessels usually require post-weld heat treatment, typically 580-620°C. Nickel alloy overlays respond to PWHT in ways that need checking:

• Sensitisation and secondary phase precipitation in some alloys. 625 can precipitate carbides and intermetallic phases in the 650-760°C range; standard PWHT at 580-620°C is generally acceptable but multiple PWHT cycles and higher temperatures need evaluation.
• Sigma and chi phase formation in highly alloyed deposits
• Carbon migration from the backing steel into the overlay across the fusion line, forming a carbon-depleted band in the steel and a carburised band in the overlay
• Duplex overlays are particularly sensitive — the ferrite-austenite balance and the risk of intermetallic precipitation make PWHT of duplex overlay a specialist matter

Where multiple PWHT cycles are expected — because of repair, or weld-after-PWHT work — the cumulative time at temperature must be included in the qualification.

Hydrogen-induced disbonding

In high-pressure, high-temperature hydrogen service — hydrotreaters, hydrocrackers, hydrogen reformers — hydrogen dissolves in the steel at operating temperature. On cooldown, hydrogen solubility falls and it concentrates at the clad-to-base interface, where the different alloys have different hydrogen solubility and diffusivity. Pressure builds at the interface and the overlay disbonds.

Mitigations:

• Controlled cooldown rates on shutdown, typically limited to 20-30°C/h below about 200°C, specified by the designer
• Overlay alloy and procedure selection — some deposits and interface structures resist disbonding better than others, and this is a qualification requirement in hydrogen service
• Disbonding testing to the applicable standard as part of procedure qualification
• Austenitic overlay rather than roll-bonded clad in some hydrogen service, because the weld interface can be made less susceptible than a roll-bond interface

This is not a theoretical risk; disbonded cladding in hydroprocessing reactors is a documented and expensive failure mode, and the cooldown rate restriction becomes a permanent operating constraint.

Clad Plate

Roll bonding

A sandwich is assembled — backing steel, a separating medium at the edges, CRA layer — and hot rolled. The bond is metallurgical, formed by pressure and temperature with the oxide broken up by deformation. Produced in large plates, then formed and welded like ordinary plate.

• Bond strength: typically above 200 MPa shear, exceeding code requirements
• Thickness: CRA from about 1.5 mm up, total plate to 100 mm and beyond
• CRA thickness ratio: usually 5-20% of total
• Alloys: stainless steels, nickel alloys, titanium (with care), copper alloys
• Quality: excellent bond uniformity, verified by ultrasonic examination over 100% of the area
• Cost: the most economical route for large areas
• Uses: pressure vessel shells and heads, tanks, columns, heat exchanger shells, piping

Roll bonding is the default for large flat and cylindrical areas, and the main route for clad pressure vessel plate under ASTM A263 (chromium stainless clad), A264 (chromium-nickel stainless clad) and A265 (nickel and nickel-alloy clad).

Explosion bonding

A controlled explosive charge accelerates the CRA plate onto the backing plate at high velocity. The collision point generates a jet that strips oxides from both surfaces and the metals bond in the solid state with a characteristic wavy interface.

• Bond strength: very high, often exceeding the weaker parent metal
• Alloys: can bond combinations impossible by roll bonding — titanium to steel, zirconium to steel, tantalum, aluminium to steel, copper alloys
• No bulk heating, so parent metal properties and tempers are preserved
• Thickness: very thick claddings possible
• Size: limited by press and explosive facility; smaller maximum plate size than roll bonding
• Cost: higher per square metre than roll bonding
• Uses: tubesheets, transition joints, heat exchanger components, titanium and zirconium clad vessels, repair plates, bimetallic transition pieces

Explosion bonding is the answer when the alloy combination cannot be roll bonded, when the parent temper must be preserved, or when a very thick cladding is needed.

Weld overlay clad plate

A mill or fabricator applies weld overlay to flat plate before forming. Combines overlay's alloy flexibility with plate handling convenience, and allows alloys and thicknesses not available as roll-bonded stock.

Clad pipe

Metallurgically clad pipe (CRA-lined by overlay or bonded) — the CRA is metallurgically bonded to the carbon steel. Handles any temperature and any differential pressure, and the bond cannot separate under vacuum or depressurisation.

Mechanically lined pipe (MLP) — a CRA liner is expanded hydraulically or thermally into the carbon steel outer pipe, held by residual contact stress. Much cheaper than metallurgical cladding. Limitations are real: the liner can collapse under external pressure or rapid depressurisation, temperature cycling can relax the fit, and the weld-end transition requires a carefully engineered overlay. Widely used in offshore flowlines with appropriate design limits.

Fabrication Rules

Cladding is only as good as the fabrication that preserves it.

Welding clad plate uses a defined sequence:

1. Prepare the joint from the backing side with the clad layer back-stripped at the weld area — typically 8-15 mm back from the joint, so the carbon steel weld is not contaminated with CRA
2. Weld the backing steel with a matching carbon steel procedure
3. Clean back to sound metal on the clad side
4. Deposit a buffer layer of a transition alloy — 309L under stainless, or a nickel alloy such as ERNiCrMo-3 under 625
5. Deposit the CRA capping layer to achieve full chemistry at the surface
6. NDE and surface examination

Welding the carbon steel joint with CRA filler, or allowing CRA dilution into the carbon steel weld, creates hard, crack-prone martensitic zones at the fusion line. The sequence exists to keep the two metallurgies apart until the transition is made deliberately.

Forming. Clad plate can be cold and hot formed, but the clad layer must be on the correct side of the neutral axis for the bend, bend radii must respect the CRA's formability, and hot forming temperature must be compatible with both metals — hot forming stainless-clad steel above the sensitisation range needs post-form solution treatment or a stabilised grade.

Flange faces. Flange gasket faces in clad vessels are usually weld overlaid and then machined, because a roll-bonded clad layer thinned by machining may not leave adequate CRA thickness at the gasket seating surface. The overlay must be thick enough that the finished machined face still carries the specified minimum, which usually means depositing 6-8 mm to finish at 3 mm.

Nozzles. Nozzle bores, nozzle-to-shell welds and reinforcing pads are where clad integrity is most often lost. Standard practice is weld overlay of the nozzle bore and the internal corner, with a qualified procedure and full examination. Hot-wire GTAW is the usual process for small bores.

Attachments. Internal attachments — support rings, trays, baffles, ladder clips — should be CRA or CRA-clad and welded to the overlay with a qualified procedure, never welded directly through the cladding to the backing steel.

Surface finish. Grinding and blending of the overlay matters in corrosion service: a rough or lapped surface traps deposits and initiates pitting. Specify surface finish, typically a maximum Ra, and require blending of all stop-starts and crater sites.

Inspection and Testing

Visual examination of the full clad surface, with specific attention to stop-starts, craters, arc strikes and lack of fusion at bead toes.

Dye penetrant (PT) on the finished clad surface. Magnetic particle is not applicable to austenitic and nickel overlays.

Ultrasonic examination for bond integrity — the primary test for roll-bonded and explosion-bonded clad, usually 100% of the clad area, with acceptance criteria on the size and number of unbonded areas. Also used on weld overlay to detect lack of bond and interface defects.

Thickness measurement of the finished clad or overlay layer, typically by ultrasonic thickness gauge or by eddy current, at a specified grid density, with the minimum rather than average value governing.

Chemical analysis of the as-deposited surface by portable XRF or by sampling and laboratory analysis, to verify iron content and the key alloying elements against the specification. For procedure qualification, laboratory analysis of a prepared coupon at the specified depth. XRF on the finished component is a verification tool, not a substitute for qualification analysis — it is surface-sensitive and its accuracy for light elements is limited.

Ferrite measurement on duplex and some stainless overlays.

Corrosion testing where specified: ASTM G48 Method A pitting test for duplex and super austenitic surfaces, ASTM A262 practices for sensitisation, and NACE TM0177 or TM0284 for sour service qualification.

Bend and shear tests on clad plate and procedure qualification coupons to demonstrate bond strength and ductility.

Hydrogen disbonding test where the service requires it.

Common Specification Mistakes

1. Single-layer overlay where the iron limit requires two layers. The Caspian example: 480,000 USD of rework, and in the second case a scrapped vessel after thirty months.

   Prevention: Specify maximum iron content at a stated depth below the finished surface, require chemical analysis in procedure qualification, and accept single-layer only where a qualified procedure demonstrates compliance.

2. Specifying the filler alloy and assuming the deposit chemistry. The deposit is a dilution blend, not the filler.

   Prevention: Specify the required as-deposited chemistry at depth, not just the filler classification.

3. 316L overlay deposited directly on carbon steel with no 309L buffer. The diluted first layer is low in alloy and partly martensitic.

   Prevention: Specify a 309L or 309LMo buffer layer under stainless overlay, or a nickel-alloy buffer under nickel overlay.

4. No minimum overlay thickness at the thinnest point after machining. An average thickness is met while local areas are below specification.

   Prevention: Specify minimum thickness at the thinnest point, state the measurement grid, and require that machining allowance be added to the as-welded thickness.

5. PWHT specified without evaluating the overlay alloy's response. Secondary phase precipitation, carbon migration and duplex phase imbalance all occur.

   Prevention: Include the full PWHT cycle, including any repair cycles, in procedure qualification, and verify corrosion and mechanical properties after the cumulative heat treatment.

6. Mechanically lined pipe specified for service with rapid depressurisation or external pressure. The liner collapses.

   Prevention: Use metallurgically clad pipe where the service includes vacuum, rapid depressurisation, or temperature cycling beyond the liner's qualified envelope.

7. No hydrogen disbonding consideration in hydroprocessing service. Cladding disbonds on cooldown.

   Prevention: Specify disbonding testing in qualification and establish a controlled cooldown rate, typically 20-30°C/h below 200°C, as an operating requirement.

8. Flange faces machined from roll-bonded cladding. Insufficient CRA remains at the gasket seating surface.

   Prevention: Weld overlay flange faces with enough deposited thickness that the finished machined surface carries the specified minimum.

9. Internal attachments welded through the cladding to the backing steel. A direct corrosion path to the carbon steel at every attachment.

   Prevention: Specify CRA or CRA-clad attachments welded to the overlay with a qualified procedure, and examine the attachment welds.

10. No ultrasonic bond examination on clad plate. Unbonded areas are invisible until they blister during forming or PWHT.

    Prevention: Require 100% ultrasonic examination of clad plate with stated acceptance criteria for unbonded area size and distribution.

11. Rough overlay surface accepted in corrosion service. Deposits lodge in surface irregularities and initiate pitting.

    Prevention: Specify a maximum surface roughness and require blending of all stop-starts and craters.

12. Arc strikes on the clad surface left unaddressed. Each is a localised unalloyed, hardened spot that corrodes preferentially.

    Prevention: Prohibit arc strikes on clad surfaces, require removal by grinding with thickness verification afterwards, and PT the repaired area.

Supply from Kasko Makine

Kasko Demir Çelik Makine supplies clad materials and clad fabrication:

Clad plate

• Roll-bonded clad plate to ASTM A263, A264 and A265
• Explosion-bonded clad plate for titanium, zirconium, tantalum, copper alloy and aluminium combinations
• Weld overlay clad plate
• Backing steels: A516 Gr 60/70, A387 Gr 11/12/22, A537, A533, carbon steel to EN 10028
• Clad layers: 304L, 316L, 317L, 321, 347, duplex 2205, super duplex, 904L, Alloy 20, Inconel 600/625/825, Alloy 276, Monel 400, titanium Gr 1/2, zirconium, copper and copper-nickel
• Ultrasonic bond examination to 100% of area with documented acceptance criteria

Clad and lined pipe

• Metallurgically clad pipe with CRA overlay
• Mechanically lined pipe with engineered weld-end transitions
• Clad fittings, elbows, tees and reducers
• Clad flanges with overlaid gasket faces
• Transition joints and bimetallic connectors

Weld overlay services and consumables

• ESW and SAW strip cladding for large areas
• Hot-wire GTAW and pulsed GMAW for nozzles, bores and flange faces
• PTA and laser cladding for precision and hardfacing work
• Strip electrodes in 309L, 316L, 347, 2209, ERNiCrMo-3 and ERNiCrMo-4, in 30 to 120 mm widths
• Welding wire and flux-cored consumables in matching alloys
• Hardfacing consumables: cobalt-based Stellite grades, nickel-based and tungsten carbide composites

Clad fabrication

• Clad pressure vessels, reactors, separators and columns to ASME Section VIII Div 1 and Div 2
• Clad tanks and process vessels
• Clad heat exchanger shells, channels and tubesheets
• Overlaid nozzles, manways and internals
• CRA internals and attachments

Associated supply

• Solid CRA plate, pipe, flanges and fittings where cladding is not economical
• Gaskets, stud bolts and fasteners in matching and compatible alloys
• Carbon steel and low-alloy plate for backing

Engineering support

Send the service conditions — fluid composition including H₂S and CO₂ partial pressures, chlorides, pH, temperature, and hydrogen partial pressure where applicable — with component drawings and the governing code. We will recommend the cladding route (roll bonded, explosion bonded or weld overlay), the alloy, the required thickness, the dilution and iron-content specification, and the inspection scope. For sour service we will state the NACE MR0175 / ISO 15156 basis for the selection, and for hydroprocessing we will include the disbonding and cooldown requirements.

Certification

Material certificates to EN 10204 3.1 and 3.2, clad plate bond shear and bend test reports, ultrasonic bond examination reports with mapping, as-deposited chemical analysis reports at specified depth, WPS and PQR documentation to ASME Section IX or ISO 15614 with dilution qualification, welder qualifications to ISO 9606, PT and UT reports, ferrite measurement records, corrosion test reports to ASTM G48 and A262 and to NACE TM0177 or TM0284 where specified, PWHT charts, and ASME U-stamp or PED documentation for completed vessels.

Logistics

Clad plate typically 8-16 weeks from mill depending on alloy and quantity. Explosion-bonded plate and tubesheets 10-18 weeks. Weld overlay consumables generally 3-6 weeks. Clad fabricated vessels 20-40 weeks depending on size, alloy and inspection requirements. Shipping from Istanbul by road to Europe, the Caucasus and Iraq, and by sea to Gulf, African, Central Asian and Southeast Asian destinations.

Send your service conditions and component drawings and we will return a cladding specification with inspection scope within five working days. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Materials and Fabrication Series

• WPS and PQR: Welding Procedure Qualification
• Stainless Steel Plate: 304, 316, 321
• Carbon Steel Plate: A516 and A36 Grades

Frequently Asked Questions

Q: What is weld overlay cladding?
A: Weld overlay cladding deposits a layer of corrosion-resistant alloy onto a carbon or low-alloy steel component by welding, usually to a finished thickness of 3 to 6 mm. It gives the corrosion performance of the expensive alloy on a cheap, strong backing, and it is the only practical method for nozzle bores, flange faces, tubesheets and local repairs where clad plate cannot be used.

Q: Why does iron content matter in Inconel 625 weld overlay?
A: The arc melts some of the backing steel along with the filler, so the deposit is a dilution blend rather than pure 625. Iron dilution reduces the effective chromium and molybdenum concentration and promotes segregation, which creates preferential corrosion sites. Specifications typically require iron below 5% preferred, or 10% maximum, measured 2 mm below the finished surface, which usually means two-layer deposition or a low-penetration process.

Q: What is the difference between roll-bonded and explosion-bonded clad plate?
A: Roll bonding hot rolls a sandwich of backing steel and corrosion-resistant alloy, producing large plates economically with excellent bond uniformity, and is the standard route for pressure vessel shells. Explosion bonding uses a controlled explosive charge to bond the plates in the solid state, which allows combinations impossible by rolling — titanium, zirconium and tantalum to steel — preserves parent metal temper, and suits tubesheets and transition joints.

Q: Why is a 309L buffer layer used under stainless overlay?
A: 309L has deliberately high chromium and nickel so that even after 20 to 30% iron dilution from the carbon steel, the first layer remains fully austenitic with adequate alloy content. The 316L capping layer is then deposited onto 309L rather than onto carbon steel, so its dilution is with already-alloyed material. Depositing 316L directly on carbon steel in a single layer gives a low-alloy, partly martensitic deposit that cracks and corrodes.

Q: How much cheaper is clad construction than solid alloy?
A: Clad construction typically costs 30 to 60% of the solid alloy equivalent, because the corrosion-resistant layer is only 3 to 6 mm or 5 to 20% of the plate thickness while the bulk is carbon steel at a fraction of the price per kilogram. On a large pressure vessel in nickel alloy service this is routinely a seven-figure saving, offset by added fabrication complexity and qualification requirements.

Q: What is hydrogen-induced disbonding of cladding?
A: In high-pressure, high-temperature hydrogen service such as hydrotreaters and hydrocrackers, hydrogen dissolves in the steel at operating temperature and concentrates at the clad-to-base interface on cooldown as its solubility falls. Pressure builds at the interface and the cladding separates. Mitigation requires qualified overlay procedures with disbonding testing and a controlled cooldown rate, commonly limited to 20 to 30°C per hour below about 200°C.

Q: Can mechanically lined pipe replace metallurgically clad pipe?
A: Only within a defined envelope. Mechanically lined pipe holds a corrosion-resistant liner in place by residual contact stress after hydraulic or thermal expansion, which is much cheaper but can collapse under external pressure or rapid depressurisation, and the fit can relax under temperature cycling. Metallurgically clad pipe has a bonded interface that cannot separate, and is required where vacuum, rapid depressurisation or significant thermal cycling occur.