Weld Overlay / Cladding — Types, Process & ASME Requirements: The Complete Technical Guide
Weld overlay and cladding represent one of the most technically demanding and commercially significant niche applications in pressure equipment fabrication and maintenance. In refinery pressure vessels, hydroprocessing reactors, heat exchanger shells, sour gas pipelines, and nuclear pressure boundaries, the ability to apply a thin but metallurgically sound layer of corrosion-resistant or wear-resistant alloy to a carbon steel or low-alloy steel structural substrate — rather than constructing the entire component from expensive alloy — is the foundation of cost-effective pressure equipment design in corrosive service.
A hydroprocessing reactor operating at 480 deg C in high-pressure hydrogen and hydrogen sulphide service might have a vessel shell wall of 200 mm of 2.25Cr-1Mo low-alloy steel — chosen for its strength and hydrogen resistance — lined internally with 7 mm of Type 347 stainless steel weld overlay. The 2.25Cr-1Mo provides all the structural integrity at a fraction of the cost of a solid alloy vessel; the 347 overlay protects against naphthenic acid and sulphidic corrosion at the operating temperature. The interface between these two very different metallurgical systems — the bond between the carbon-rich ferritic steel and the chromium-nickel austenitic overlay — is what weld overlay technology creates, and the quality of that bond is what determines whether the vessel remains in service for its designed 25-year life or fails prematurely.
This guide covers the complete technical picture: the metallurgical principles of overlay bonding and dilution, every major overlay type and the processes used to deposit them, filler metal selection tables, preheat and PWHT strategies for the most common overlay combinations, ASME Section IX qualification requirements including the essential variables unique to overlay procedures, ASME Section VIII cladding requirements, NDE requirements and acceptance criteria, and a complete description of the common defects found in overlay work and how to detect them.
What is Weld Overlay and Cladding?
Weld overlay is the application of one or more layers of weld metal onto the surface of a base material to produce a composite structure whose surface has different properties from the substrate. The substrate (base material) provides structural integrity and dimensional form; the overlay provides the surface property — corrosion resistance, wear resistance, erosion resistance, high-temperature oxidation resistance, or dimensional restoration.
Cladding is a specific term within weld overlay referring to the application of a corrosion-resistant alloy layer. The distinction in industry usage is:
- Weld overlay — the general term for any weld-deposited surface layer
- Cladding / CRO (Corrosion Resistant Overlay) — overlay applied for corrosion protection, typically austenitic stainless steel or nickel alloy on carbon or low-alloy steel
- Hardfacing — overlay applied for wear/erosion/abrasion resistance
- Buttering — overlay on one face of a joint to provide a compatible metallurgical interface for a subsequent dissimilar metal weld
- Dimensional restoration — overlay to rebuild worn or corroded areas to original dimensions
All of these are weld overlay in the broad sense, but each has distinct technical requirements, filler metal selections, process choices, and qualification requirements under ASME Section IX.
Types of Weld Overlay
Corrosion Resistant Overlay (CRO)
The most common overlay type in refinery, petrochemical, and pressure vessel fabrication. Austenitic stainless steel (Type 304/308, 316/316L, 321, 347) or nickel alloys (Inconel 625, Alloy 625, C-276, Alloy 59) deposited on carbon steel, low-alloy steel, or Cr-Mo steel to provide a corrosion barrier. The overlay protects against aqueous corrosion, acid attack, naphthenic acid, high-temperature sulphidation, and chloride corrosion depending on alloy selection.
Hardfacing Overlay
High-hardness overlay for wear, abrasion, erosion, and cavitation resistance. Materials include cobalt-base alloys (Stellite 6/12/21), tungsten carbide composites, chromium carbide alloys, and high-chromium iron alloys. Applied to valve seats, pump impellers, fan blades, choke valve trims, catalytic cracker slide valves, and drilling equipment. Maximum hardness of 60+ HRC for cobalt and tungsten carbide hardfacing.
Buttering Layer
Transitional overlay deposited on one or both faces of a dissimilar metal joint interface before the main butt weld. Alloy 82/182 (ERNiCr-3/ENiCrFe-3), Alloy 52M, or Alloy 625 buttered onto P91 or P22 ferritic steel nozzle ends before joining to austenitic stainless piping. Allows PWHT of the ferritic component independently before the final joint weld. Critical in nuclear DMW applications.
Dimensional Restoration Overlay
Overlay to rebuild components worn or corroded beyond minimum thickness or dimensional tolerance. Pump shafts, valve bodies, flange faces, and pressure vessel nozzles restored to original dimensions by depositing compatible weld metal and machining back to drawing dimensions. Material selection matches or is compatible with the original base material. Documented as a repair procedure under applicable design code.
Dilution — The Critical Variable in Weld Overlay ASME IX QW-216
Dilution is the single most important metallurgical variable in corrosion-resistant overlay work. Understanding it — and controlling it — is what separates overlay work that will protect the vessel for its design life from overlay that looks visually acceptable but will corrode through within a few years of service.
Dilution (%) = [A_base / (A_base + A_filler)] × 100
Where:
A_base = Cross-sectional area of base metal melted into weld bead
A_filler = Cross-sectional area of filler metal deposited
Effect on overlay chemistry (example: ER308L on carbon steel):
ER308L nominal Cr content: 20% | Carbon steel Cr content: ~0%
At 30% dilution: actual overlay Cr = 0.70 × 20 + 0.30 × 0 = 14% Cr
At 15% dilution: actual overlay Cr = 0.85 × 20 + 0.15 × 0 = 17% Cr
At 5% dilution: actual overlay Cr = 0.95 × 20 + 0.05 × 0 = 19% Cr
High dilution directly reduces the Cr, Ni, Mo content of the overlay — degrading corrosion resistance.
Minimum Cr content in finished overlay (ASME/Owner spec): typically ≥13% for 410, ≥18% for 308/309.
Typical Dilution by Welding Process
| Welding Process | Typical Dilution Range | Dilution Control Strategy | Relative Deposition Rate |
|---|---|---|---|
| GTAW (TIG) | 3–10% | Low current, short arc — naturally low dilution; ideal for thin precision overlay | Very low — manual or orbital automated |
| SMAW (stick) | 10–20% | Low current settings, weave bead technique; first layer typically higher dilution | Low — site repair and small area applications |
| GMAW (MIG/GMAW-P) | 15–30% | Pulsed GMAW reduces dilution vs spray transfer; wider bead reduces depth of fusion | Medium — production overlay on medium components |
| SAW (wire electrode) | 20–40% | Tandem wire, oscillating head, reduced current — partial dilution control | High — large area production |
| SAW (strip electrode) | 10–20% | Wide strip (30–90 mm) distributes heat over large area — lower dilution than wire SAW | Very high — pressure vessel shell/heads |
| ESSC (Electroslag Strip Cladding) | 5–15% | Resistive heating through slag pool — very low arc energy, lowest dilution of all high-deposition processes | Highest — standard for large pressure vessel CRO |
| PTAW (Plasma Transferred Arc) | 3–8% | Concentrated plasma — very low dilution; used for precision hardfacing | Low-medium — precision hardfacing applications |
Welding Processes for Overlay — Selection and Comparison
Process selection for weld overlay is driven by the component geometry (flat, curved, nozzle bore), the area to be covered (small repair area vs full vessel shell), the required dilution level (determined by overlay type and filler/base combination), and whether the work is in a fabrication shop or on-site during a maintenance turnaround.
Electroslag Strip Cladding (ESSC) — The Production Standard
For large-area CRO on flat or moderately curved surfaces — pressure vessel shells, heads, and transition rings in a fabrication shop — Electroslag Strip Cladding is the dominant process. ESSC uses a wide stainless steel or nickel alloy strip electrode (typically 30 mm, 60 mm, or 90 mm wide by 0.5 mm thick) fed through a flux-submerged slag pool. The current passes through the resistive slag rather than through an electric arc, generating heat that melts both the strip electrode and a shallow layer of the base material. The result is exceptionally uniform coverage, very low dilution (5–15%), and very high deposition rates — a 90 mm strip can deposit 30 to 40 kg/hr, covering approximately 1 m²/hr of vessel shell.
SAW Strip Cladding — The Alternative Production Process
Submerged Arc Welding with strip electrode (strip SAW) is similar in appearance to ESSC but uses an electric arc rather than resistive slag heating. Strip SAW achieves higher deposition rates than ESSC but at slightly higher dilution (10–20%). It is better suited to materials and applications where dilution control is less critical or where the higher heat input of arc heating is acceptable. Strip SAW has better penetration and can be used on moderately contaminated surfaces where ESSC’s more sensitive slag process would be disrupted.
SMAW for Site and Repair Overlay
For on-site maintenance and repair overlay — heat exchanger tubesheet cladding repair, nozzle bore overlay, localised vessel shell repair — SMAW is the most practical process. Electrode types include E308/308L, E309/309L, E316/316L for austenitic SS overlay, and ENiCrFe-3 (Alloy 182) for nickel alloy overlay. SMAW is limited in deposition rate and requires more skill to control dilution than strip processes, but its flexibility — any position, any access — makes it indispensable for maintenance applications. Stringer beads minimise dilution; weave beads increase dilution but improve coverage rate.
GTAW for Precision and Thin Overlay
GTAW (TIG) with filler wire is used for precision thin-section overlay applications: heat exchanger tube-to-tubesheet joint overlay, nozzle bore CRO in small-bore components, and the first transitional layer on high-alloy or reactive materials where dilution control is paramount. GTAW achieves the lowest dilution of any process (3–10%) but at very low deposition rates. For tube-to-tubesheet overlay specifically, orbital GTAW with programmed current and wire feed ensures consistent coverage around the full tube bore circumference without the arc wander that manual GTAW would produce at the low currents required.
GMAW and FCAW for Medium-Area Production Overlay
GMAW (MIG) with solid wire or FCAW with flux-cored wire is used for medium-area production overlay — nozzle internal surfaces, vessel head area above and below the tangent line, and complex geometry surfaces where strip processes cannot reach. Pulsed GMAW achieves lower dilution than spray transfer GMAW and is preferred for CRO applications. FCAW with stainless steel or nickel alloy tubular electrodes offers higher deposition rates than solid wire GMAW for the same position capability but requires more attention to slag removal between passes.
| Process | Dilution (%) | Deposition Rate | Best For | Limitations |
|---|---|---|---|---|
| ESSC | 5–15 | Very high (30–40 kg/hr) | Large flat/curved vessel shell and head CRO in shop | Flat position only; not portable; sensitive to base surface condition |
| SAW Strip | 10–20 | High (20–35 kg/hr) | Production CRO in shop; slightly better penetration than ESSC | Flat position; not portable; higher dilution than ESSC |
| SAW Wire | 20–40 | High | Restoration overlay; applications where dilution less critical | Higher dilution requires more layers; flat/horizontal only |
| SMAW | 10–20 | Low | On-site repair, any position, complex geometry | Low deposition; slag removal between passes; welder skill dependent |
| GTAW | 3–10 | Very low | Precision thin overlay; tube-to-tubesheet; low dilution critical | Very low speed; expensive for large areas |
| GMAW/FCAW | 15–30 | Medium-high | Nozzle bores, complex shapes, medium area production | Higher dilution; FCAW requires slag removal |
| PTAW | 3–8 | Low-medium | Precision hardfacing (valve seats, pump wear rings) | Specialised equipment; skilled operator required |
CRO Layer Strategy — First Layer, Second Layer, Minimum Thickness
Why Two Layers Are Almost Always Required
For stainless steel overlay on carbon steel or low-alloy steel, a single-layer approach deposited with any of the commonly available processes at practical parameters will almost always produce unacceptable dilution in the first layer — the iron content from the base will reduce the chromium content of the overlay below the specified minimum. The standard industry solution is a two-layer strategy:
The first layer uses a transitional filler metal — typically ER309L (22Cr-12Ni) or ENiCrFe-3 (Alloy 182) — which has a higher initial alloy content than the target composition, specifically selected to accommodate the iron dilution from the base while still producing a layer with adequate chromium content to resist further dilution of the second layer. The first layer is sometimes called the butter layer, the hot layer, or the transitional layer. Chemical analysis of the first layer alone is typically not required by the code because this layer is understood to be transitional — but some Owner specifications require it to demonstrate a minimum composition even in the first layer.
The second layer is deposited using the target composition filler metal — ER308L for Type 304/308 service, ER316L for Type 316 service, ERNiCrMo-3 for Alloy 625 service — on top of the already-alloyed first layer substrate. The dilution of the second layer is now from the alloy-rich first layer rather than from the carbon steel base, so the composition of the second layer closely matches the target filler metal composition. Chemical analysis of the finished overlay is taken from the surface of this second (or final) layer.
Minimum Overlay Thickness
| Service / Application | Minimum Finished Overlay Thickness | Basis |
|---|---|---|
| General CRO on pressure vessels (ASME VIII) | 3 mm minimum, typically 5–8 mm specified | ASME Section VIII App. F; Owner company specification |
| Hydroprocessing reactor lining (Type 347) | Minimum 7 mm finished overlay (2 layers) | API 582; NACE TM-01-69; Owner specifications for hydroprocessing service |
| Sour service CRO (H2S-containing) | Minimum 3 mm; hardness HRC ≤22 in HAZ | NACE MR0175/ISO 15156-2; Owner specification |
| Nickel alloy overlay (Alloy 625, C-276) | Minimum 3–5 mm finished overlay | Owner specification; determined by corrosion allowance calculation |
| Hardfacing overlay (valve seat, wear surface) | Application-specific; typically 2–6 mm minimum | OEM specification; API 6A/6D for valves; Owner specification |
Filler Metal Selection for Common Overlay Combinations
| Base Material | Target Overlay (Service) | Layer 1 Filler (Butter/Transitional) | Layer 2+ Filler (Target) | Min Finished Cr (%) |
|---|---|---|---|---|
| Carbon steel (P-No. 1) | Type 304/308 SS (general corrosion) | ER309L / E309L-16 | ER308L / E308L-16 | ≥18% Cr |
| Carbon steel (P-No. 1) | Type 316/316L (Cl⁻ and Mo service) | ER309LMo / E309LMo-16 | ER316L / E316L-16 | ≥18% Cr, ≥2% Mo |
| Carbon steel (P-No. 1) | Type 347 (sensitisation-resistant, high temp) | ER309L / E309Cb-16 | ER347 / E347-16 | ≥18% Cr, Nb ≥8×C |
| 2.25Cr-1Mo (P-No. 5A) | Type 347 or 321 (hydroprocessing) | ER309L (first pass) or direct ER347 with preheat | ER347 / E347-16 | ≥18% Cr, Nb content verified |
| Carbon/Low-alloy steel | Alloy 625 (severe corrosion, sour service) | ERNiCrMo-3 (direct, 1 layer often sufficient) | ERNiCrMo-3 / ENiCrMo-3 | ≥20% Cr, ≥8% Mo, ≥55% Ni |
| Carbon/Low-alloy steel | Alloy C-276 (HCl, wet Cl₂ service) | ERNiCrMo-4 | ERNiCrMo-4 / ENiCrMo-4 | ≥14.5% Cr, ≥15% Mo |
| P91 / P22 (dissimilar joint buttering) | Alloy 82/182 butter for DMW to SS | ERNiCr-3 / ENiCrFe-3 (Alloy 82/182) | ERNiCr-3 (Alloy 82) | ≥14% Cr, ≥67% Ni |
| Any substrate | Stellite 6 hardfacing (wear/erosion) | Not required (direct deposit) | ERCoCr-A (Stellite 6) / ECoCr-A | N/A — hardness ≥38 HRC verified |
Preheat, Interpass Temperature, and PWHT
Preheat Requirements for Overlay
Preheat requirements for weld overlay are governed by the base material carbon equivalent and the applicable code, not by the overlay filler metal. The preheat ensures that the heat-affected zone in the base material does not undergo rapid cooling and form hard, crack-susceptible martensite. For carbon steel (P-No. 1) base, preheat is typically 10–80 deg C depending on carbon equivalent and thickness per ASME Section VIII / AWS D1.1. For Cr-Mo low-alloy steel (P-No. 4, 5A, 5B) — 2.25Cr-1Mo, P91 — preheat of 200–300 deg C is mandatory and is one of the most critical quality controls in CRO work on hydroprocessing reactors.
Maximum Interpass Temperature
Interpass temperature control in CRO work is critical for two reasons: excessive interpass temperature in austenitic stainless overlay can promote sensitisation (chromium carbide precipitation at grain boundaries), and in P91 base materials, high interpass temperature can affect the parent HAZ transformation behaviour. Maximum interpass temperature is typically specified as 175 deg C for stainless overlay and as low as 150 deg C for some Owner specifications on sensitisation-sensitive Type 321 or 347 service. Interpass temperature must be measured with calibrated contact thermometers or temperature-indicating sticks across the full weld area.
PWHT Strategy — The Critical Decision
Post-weld heat treatment of overlay-clad components requires careful engineering because the base material (carbon or low-alloy steel) typically requires PWHT, while the overlay material (austenitic stainless or nickel alloy) may be damaged or sensitised by PWHT.
| Base Material | Overlay Material | PWHT Requirement | PWHT Approach |
|---|---|---|---|
| Carbon steel (P-No. 1, t ≤38 mm) | Austenitic SS (308/316/347) | Not mandatory per ASME VIII for t ≤38 mm | PWHT typically not required unless specified by Owner or needed for NACE compliance. If done: 600–650 deg C for carbon steel. |
| Carbon steel (P-No. 1, t >38 mm) | Austenitic SS (308/316/347) | PWHT required per ASME VIII UCS-56 | 600–650 deg C PWHT acceptable for austenitic SS overlay — does not sensitise at these temperatures if low-carbon (L-grade) filler used. Must use ER308L/316L, not ER308/316. |
| 2.25Cr-1Mo (P-No. 5A) | Type 347 SS | PWHT mandatory per ASME VIII for all thicknesses | 690–760 deg C for P-No. 5A base. At these temperatures, Type 347 overlay is resistant to sensitisation due to Nb stabilisation. Must not use non-stabilised grades. |
| P91 (P-No. 5B Gr. 2) | Alloy 82 butter layer | PWHT mandatory: 730–775 deg C for P91 | Alloy 82/182 butter tolerated at P91 PWHT temperatures without significant degradation. This is specifically why Alloy 82 is used for P91 buttering — it survives the mandatory PWHT cycle. |
| Carbon/Low-alloy steel | Alloy 625 (ERNiCrMo-3) | Per base material requirement | Alloy 625 overlay is stable through all standard PWHT temperatures for carbon and Cr-Mo steels. No sensitisation or phase transformation concerns at PWHT temperatures up to 760 deg C. |
ASME Section IX — Overlay WPS Qualification QW-216
Weld overlay procedures must be qualified under ASME Section IX using a Procedure Qualification Record (PQR) that demonstrates the overlay meets the required properties. Section IX QW-216 addresses cladding (corrosion-resistant overlay) and hardsurfacing essential variables specifically. A separate WPS and PQR are required for each distinct overlay application — they cannot be derived from the standard fusion weld qualification.
Essential Variables Unique to Overlay WPS
In addition to the standard welding essential variables (base material P-Number and Group Number, welding process, current type and polarity, preheat and PWHT), QW-216 specifies overlay-specific essential variables:
| Essential Variable | Applies To | Code Clause | Why It Is Essential |
|---|---|---|---|
| Overlay material (filler classification or composition) | All overlay types | QW-216.1 | Different filler metal compositions produce fundamentally different overlay properties — corrosion resistance, hardness, or interface chemistry |
| Base material P-Number and Group Number | All overlay types | QW-216.1 | Dilution from different base materials produces different overlay compositions even with the same filler metal — P-No. 1 vs P-No. 5A base produces different dilution chemistry |
| Number of layers (increase above qualified) | CRO and hardsurfacing | QW-216.1(a) | Additional layers affect the cumulative dilution and the final surface composition |
| Chemical composition of the deposited overlay (for CRO) | CRO (corrosion resistant) | QW-216.1(b) | The PQR must demonstrate by chemical analysis that the qualified overlay achieves the minimum specified alloy content |
| Hardness of the finished overlay (for hardsurfacing) | Hardfacing/hardsurfacing | QW-216.2(a) | PQR must demonstrate the hardness range; increasing hardness below qualified minimum is an essential variable |
| Heat input (decrease below 0.75 × qualified) | All overlay types | QW-216.1(c) | Lower heat input changes dilution level and HAZ characteristics — a significant decrease could change the overlay composition by reducing dilution from the base |
| PWHT change (temperature, time) | All overlay types | QW-216.1 | PWHT affects sensitisation of stainless overlay and hardness of hardfacing; changes outside qualified range require re-qualification |
| Addition or deletion of buttering | All overlay on dissimilar joints | QW-216.1 | Buttering changes the interface composition and metallurgy; its presence or absence is a fundamental change to the overlay procedure |
PQR Test Requirements for CRO
The PQR for corrosion-resistant overlay must include chemical analysis of the finished overlay demonstrating that the minimum specified alloy content is achieved. The chemical analysis sample is taken from a location on the test coupon corresponding to the surface of the final overlay layer, using drillings or other sampling methods that collect metal from the full overlay depth above the fusion line without including base material. The analysis must be performed by a certified laboratory and included in the PQR documentation.
ASME Section IX does not specify the minimum alloy content — that requirement comes from the design specification, Owner requirements, or referenced material standard (such as ASME Section II Part C for the overlay filler metal). The PQR must demonstrate compliance with whatever minimum composition is specified. If no minimum is specified in the design document, the applicable corrosion standard (NACE MR0175/ISO 15156 for sour service) or the API standard (API 582 for process plant overlay) typically provides the guidance.
ASME Section VIII — Cladding Requirements for Pressure Vessels
Appendix F — Mandatory Requirements for Clad Vessels
ASME Section VIII Division 1 Appendix F provides mandatory requirements for the design and fabrication of vessels that use clad material — both mill-applied integrally bonded clad (roll-bonded or explosion-bonded) and weld overlay cladding. The key provisions relevant to weld overlay are:
- Design credit for cladding: For corrosion-resistant overlay where the overlay is not given structural credit in the pressure design calculations, the vessel is designed based on the full base material wall thickness. The cladding thickness is additional to the design wall thickness and provides only the corrosion allowance.
- Design credit for integral cladding: When mill-applied clad plate (integrally bonded) is used and given structural credit, the combined thickness may be used in pressure design calculations subject to the conditions in Appendix F — including shear stress limitations at the bond interface and demonstration of adequate bond strength by shear testing.
- Weld joint design for clad vessels: Butt welds through clad plates must be designed to maintain the cladding continuity across the joint. The cladding side of the weld must be finished with a cladding-compatible filler metal.
UCS-6 and Related Provisions
ASME Section VIII UCS-56 specifies the PWHT requirements for carbon and low-alloy steel vessels. When a carbon steel vessel has a weld overlay applied, the PWHT requirements apply to the base material weld joints — including any weld joints in the base material made through the overlay, such as nozzle-to-shell welds. The overlay procedure must be compatible with the PWHT temperature required for the base material weld joints.
UHA — Requirements for Austenitic Stainless Steel
UHA-51 specifies that for austenitic stainless steel overlay (and for stainless-clad vessels), the corrosion test requirements of UHA-51 apply. This means that for certain corrosive services, a corrosion test (such as ASTM A262 Practice E for intergranular corrosion testing) may be required to demonstrate that the overlay is not sensitised. This is particularly relevant for overlays that have undergone PWHT at temperatures in the sensitisation range.
Hardness Requirements
Hardness Limits for Sour Service Overlay — NACE MR0175
For weld overlay on equipment in sour service (wet H2S environments covered by NACE MR0175/ISO 15156), hardness requirements apply not only to the base material HAZ but also to the overlay itself and the transition zone between overlay and base. The key hardness limits are:
| Material Zone | Maximum Hardness | Standard | Notes |
|---|---|---|---|
| Base metal HAZ (carbon and low-alloy steel) | HRC 22 (248 HV10, 237 HB) | NACE MR0175/ISO 15156-2 | Must be demonstrated in the HAZ immediately beneath the overlay by hardness traverse |
| Weld metal (carbon steel base joints) | HRC 22 | NACE MR0175/ISO 15156-2 | Base metal weld joints — not the overlay weld metal itself |
| Stainless steel overlay (Type 308/316/347) | No maximum hardness limit per NACE | NACE MR0175/ISO 15156-3 | Austenitic SS overlay is generally acceptable in sour service without hardness limits, subject to composition requirements |
| Nickel alloy overlay (Alloy 625, C-276) | No maximum per NACE for most alloys | NACE MR0175/ISO 15156-3 | Alloy 625 overlay acceptable; verify specific alloy qualification status in ISO 15156-3 Tables |
| Hardfacing (Stellite 6) | Up to 50+ HRC acceptable in non-wetted surfaces | Not NACE-restricted for valve internals not in contact with sour fluid | For surfaces in direct contact with sour fluid, hardness limits apply per equipment design code |
Hardness Testing in Overlay PQR
The PQR for overlay on sour service components must include a hardness traverse demonstrating that the base material HAZ hardness does not exceed HRC 22. The traverse is performed on a section cut transverse to the overlay weld beads, etched to reveal the HAZ, and tested at 0.5 mm intervals through the HAZ zone. Preheat and PWHT parameters in the WPS must be set to ensure the HAZ hardness is consistently controlled within this limit — the WPS qualified preheat minimum and PWHT temperature range are therefore directly linked to the hardness performance demonstrated in the PQR.
NDE Requirements for Weld Overlay ASME V Art. 6/8/4
NDE of completed weld overlay is required by ASME Section VIII and Owner company specifications to verify bond integrity, surface condition, and absence of detrimental discontinuities. The applicable NDE methods and their specific roles in overlay inspection are:
Visual Examination (VT)
100% visual examination of the completed overlay surface is always required as the first examination. The examiner checks for: surface cracks, pinholes, porosity, incomplete coverage (missed areas, gaps between weld beads), surface irregularities (excessive convexity, undulations), and weld starts and stops. The overlay surface finish must be smooth enough for PT or UT examination — rough as-welded surfaces on ESSC or SAW strip overlay are typically lightly dressed before examination.
Liquid Penetrant Testing (PT)
100% PT of the completed overlay surface is standard for austenitic stainless and nickel alloy overlays where MT is not applicable. PT detects surface-breaking cracks, porosity clusters, and lack-of-fusion defects that break the surface. PT is performed after any PWHT (because PWHT can cause cracking in sensitised or stress-corrosion-susceptible overlays that would not be present before PWHT). Method: colour contrast or fluorescent penetrant per ASME Section V Article 6, with acceptance criteria per ASME Section VIII UHA-34 or Owner specification — typically no linear indications and no rounded indication pattern exceeding applicable limits.
Magnetic Particle Testing (MT)
MT is used for ferritic overlay materials — hardfacing alloys and some dimensional restoration overlays where the deposit is ferromagnetic. MT is not applicable to austenitic stainless steel or nickel alloy overlays because these materials are non-ferromagnetic. For hardfacing overlay inspection, MT provides superior sensitivity to subsurface cracking compared to PT alone.
Ultrasonic Testing (UT) for Bond Integrity
UT examination of the overlay-to-base-material bond is specified for critical applications — hydroprocessing reactors, nuclear pressure boundary components, and high-integrity sour service vessels. The UT technique must detect lack-of-fusion at the overlay/base interface, which appears as a planar reflector at the bond line depth. Straight-beam (normal incidence) UT scanning from the overlay surface is the standard approach, using calibrated sensitivity referenced to a flat-bottom hole at the bond line depth in a test block. Immersion UT or phased array UT (PAUT) with encoded scanning is used for high-sensitivity applications requiring documented coverage maps.
| NDE Method | What It Detects | When Applied | Applicability |
|---|---|---|---|
| Visual (VT) | Surface defects, coverage gaps, gross irregularities | After completion of each layer (optional) and final surface | All overlay types |
| Liquid Penetrant (PT) | Surface-breaking cracks, porosity, LOF at surface | Final overlay surface; after PWHT | Non-ferromagnetic overlay (SS, Ni alloys) |
| Magnetic Particle (MT) | Surface and near-surface cracks | Final overlay surface; after PWHT | Ferromagnetic overlay (hardfacing, some restoration) |
| UT bond inspection | Lack-of-fusion at overlay/base interface; delamination | After PWHT on critical vessels; pre-service on reactors | All overlay types on flat/curved surfaces |
| Radiography (RT) | Generally NOT specified for overlay | N/A for routine overlay | RT cannot reliably detect overlay interface defects; PT+UT combination is standard |
| PAUT encoded | Full coverage bond map; sizing of interface defects | Nuclear, hydroprocessing reactors — highest integrity requirement | Critical flat/curved surfaces where full coverage documentation required |
| Chemical analysis | Overlay composition (Cr, Ni, Mo, Nb) to verify dilution compliance | PQR qualification and production verification per specification | CRO (corrosion resistant overlay) — mandatory for qualification |
Common Weld Overlay Defects — Causes and Prevention
Buttering — Overlay for Dissimilar Metal Joints
Buttering is a specialised overlay application that creates a transitional metallurgical interface on one or both faces of a dissimilar metal weld joint. While it is technically a form of weld overlay, its purpose and qualification requirements differ significantly from CRO or hardfacing, and it is addressed separately here because it is one of the most technically complex and failure-prone operations in pressure piping and vessel fabrication.
The Standard Buttering Application — P91 to Austenitic SS
The most commonly encountered buttering application in process plant fabrication involves joining P91 or P22 Cr-Mo steel pipe or nozzles to austenitic stainless steel piping or vessel shells. These two material families are metallurgically incompatible in several ways: they have very different thermal expansion coefficients (P91 at 12 ×10⁻⁶ /°C vs 304SS at 17.5 ×10⁻⁶ /°C), different PWHT requirements (P91 requires 730–775 deg C PWHT; austenitic SS must not be PWHT above 300 deg C to avoid sensitisation), and different microstructural behaviour under creep conditions.
The buttering procedure addresses the PWHT incompatibility specifically: Alloy 82 filler metal (ERNiCr-3) is deposited on the P91 pipe end as a butter layer 3–5 mm thick, building up a nickel alloy interface over several passes. The buttered P91 component is then PWHT at the required 730–775 deg C — the Alloy 82 butter layer survives this PWHT without cracking or property loss. After PWHT, the final butt weld joint is made between the Alloy 82 butter on the P91 side and the austenitic stainless steel pipe, using Alloy 82 filler metal for the root and fill passes. No further PWHT is required after the final joint weld — which is essential because a second PWHT would sensitise the austenitic stainless steel.
ASME Section IX Buttering Qualification
Buttering procedures require qualification under ASME Section IX QW-216 as overlay procedures, with the additional requirement that the final joint weld WPS (which makes the butt weld between the buttered surface and the opposite material) must also be separately qualified referencing the buttered base material surface as the qualification basis. The buttering PQR and the final joint WPS/PQR must both be reviewed together — a common oversight is qualifying each in isolation without confirming that the buttered coupon surface chemistry and hardness from the PQR are representative of the production buttered surface.
For detailed technical coverage of P91 and P22 to austenitic stainless dissimilar metal welding, including the role of buttering in managing carbon migration, oxide notching, and Type IV creep, see our comprehensive dissimilar metal welding guide.
Industrial Applications of Weld Overlay
| Component | Overlay Type | Overlay Material | Why Overlay vs Solid Alloy |
|---|---|---|---|
| Hydroprocessing reactor vessel shell (refinery) | CRO — corrosion and high-temp sulphidation protection | Type 347 SS or Alloy 825 | 2.25Cr-1Mo base provides structural strength vs hydrogen attack; 347 overlay provides corrosion barrier. Solid alloy reactor would cost 5–8× more. |
| Pressure vessel for sour gas service | CRO — H2S corrosion and SSC protection | Alloy 625 or Type 316L | Carbon steel provides strength at low cost; Alloy 625 overlay provides SSC and H2S corrosion resistance that carbon steel cannot. NACE MR0175 compliance. |
| Heat exchanger tubesheet cladding | CRO — process-side corrosion protection | Type 316L or Alloy 825 | Carbon steel tubesheet with 316L overlay provides the required corrosion protection in a wide range of aqueous corrosive service at a fraction of the solid 316L cost. |
| Valve seats (refinery, power plant, mining) | Hardfacing — wear and erosion resistance | Stellite 6 / Stellite 12 (CoCr alloy) | Carbon steel or low-alloy steel valve body with Stellite seat overlay provides the hard, wear-resistant contact surface without the cost of an all-Stellite valve. |
| P91 nozzle-to-SS pipe joint (power plant) | Buttering — dissimilar metal weld interface | Alloy 82 (ERNiCr-3) | Allows independent PWHT of P91 side at 730–775 deg C before making the final joint weld to stainless steel — without sensitising the SS. See DMW guide. |
| Offshore subsea pipeline flanges | CRO — seawater and produced-fluid corrosion | Alloy 625 or Alloy C-276 | Carbon steel flange body with Alloy 625 bore and face overlay provides corrosion protection in produced fluid (H2S/CO2/Cl⁻) at substantially lower cost than solid Alloy 625 flanges. |
| Pump impellers (slurry service) | Hardfacing — abrasion and cavitation resistance | Chromium carbide or tungsten carbide composite | Overlaying wear surfaces of fabricated impellers extends service life by 3–5× vs bare carbon steel with minimal added cost compared to replacement. |
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