Weld Overlay Cladding on Pipe Fittings: CRA Lining for Hot Corrosive Service, Overlay Alloys, and NDE of Cladding Integrity

Why Overlay Cladding Rather Than Solid CRA

Solid CRA buttweld fittings in Alloy 625 or Hastelloy C-276 are available only to NPS 10–12 from most manufacturers, and the material cost per fitting can be five to ten times that of a solid 316L equivalent. Weld overlay on a P235GH or A234 WPB carbon steel substrate achieves the same corrosion-resistant bore at 15–30% of the solid CRA cost. The carbon steel substrate carries mechanical loads, design pressure, and external loads — the CRA layer handles only the corrosive fluid. Minimum overlay thickness after final machining is typically 3 mm per ASME B31.3, with some specifications (particularly sour service per NACE MR0175) requiring 5 mm minimum to ensure that dilution from the substrate does not degrade corrosion resistance at the bond line.

Common Overlay Alloys and Their Applications

Alloy 625 (ERNiCrMo-3, UNS N06625) is the most widely used overlay alloy. Its PRE exceeds 50 and it resists chloride SCC, pitting, and crevice corrosion in acids and sour environments. Used in offshore topsides, subsea, and refinery sour gas service. Alloy 825 (ERNiCrMo-3 analog, but 825 is actually ERNiCrMo-7 or similar — typical filler is AWS A5.14 ERNiCrMo-3 for 625 or ERNiCrFe-7 for 825) is used in phosphoric acid, sulfuric acid dilute, and seawater service where cost needs to be lower than 625 but performance exceeds 316L. Hastelloy C-276 (ERNiCrMo-4) overlay is specified for concentrated sulfuric acid, HCl, and mixed acid environments where 625 is insufficient. 316L (ER316L) overlay is the lowest-cost option — used in dilute acid, process chemical, and pharmaceutical service where austenitic performance is adequate and the substrate is carbon steel.

Dilution Control and Bond Line Chemistry

The critical metallurgical issue in weld overlay is dilution — the degree to which the carbon steel substrate mixes into the overlay deposit. High dilution reduces the chromium, molybdenum, and nickel content of the overlay, degrading corrosion resistance. For Alloy 625 overlay, dilution in the first layer should not exceed 10–15%; second-layer dilution from the first layer is lower because the first layer acts as a buffer. GTAW (TIG) overlay achieves 5–10% dilution. GMAW (MIG) achieves 10–20%. Submerged arc overlay (SAW) can reach 25–40% dilution in a single layer and usually requires a butter layer first. For this reason, two-layer overlay is the standard for sour service and highly corrosive applications: the first layer (butter layer) dilutes into the substrate but is accepted as transitional; the second layer provides the specified corrosion-resistant composition.

Welding Procedures and Heat Input Control

Overlay welding procedures are qualified per ASME Section IX, with essential variables including base metal P-number, filler metal classification, heat input range, preheat, and interpass temperature. For Alloy 625 on carbon steel, preheat is typically 100–150°C to avoid hydrogen cracking in the HAZ of the carbon steel substrate. Interpass temperature should not exceed 150°C for austenitic nickel overlays to avoid sensitisation of the deposited layer and to control distortion. Heat input control is critical: excessive heat input promotes grain growth in the overlay, increases dilution, and creates wider HAZ in the substrate. GTAW with oscillation (weave bead) is typical for complex fitting geometries — elbows, tees, and reducers require robotic or manual welding on contoured surfaces where SAW is impractical.

Post-Weld Heat Treatment of Clad Fittings

PWHT of clad fittings is a complex issue. Carbon steel substrates (P1 material) may require PWHT for wall thickness above 32 mm (per ASME B31.3) or when specified by the corrosion engineer. However, PWHT can sensitise austenitic overlays (precipitation of Cr23C6 at grain boundaries in the 425–850°C range) and can embrittle high-nickel alloys. The standard approach is to design the substrate so that PWHT is not required: use normalised carbon steel, control carbon equivalent, and keep welding heat input within limits that avoid a HAZ requiring stress relief. If PWHT is unavoidable, the overlay alloy must be verified by ASTM A262 Practice E or Huey test after the PWHT thermal cycle to confirm that sensitisation has not occurred.

NDE of Overlay Cladding Integrity

Three NDE methods are used on clad fittings. Ultrasonic testing (UT) of the bond line detects disbanding (lack of fusion between overlay and substrate) and laminations in the substrate. Straight-beam UT from the OD surface scans the bond line; any area where the bond echo disappears or shows amplitude loss indicates disbanding and is cause for rejection. Ferrite measurement (Feritscope or ASTM E1044) verifies that the overlay has not been diluted to a composition where sensitisation risk increases — Alloy 625 overlay should show <0.1 FN; 316L overlay is typically 3–8 FN. PMI (portable XRF or OES) verifies the chemical composition of the overlay surface. For sour service, iron content of the overlay surface must not exceed the specified limit (e.g., <1% Fe at the bore surface for Alloy 625 to ensure NACE MR0175 compliance of the wetted surface).

Dimensional and Bore Finish Considerations

Overlay adds 5–8 mm to the bore wall before machining. The fitting substrate must be ordered with a compensating bore oversize so that after overlay deposition and final machining to the specified bore (matching ASME B16.9 bore dimensions), the minimum overlay thickness remains 3 mm (or 5 mm per specification). Elbow and reducer bore shapes require machining by CNC turning or grinding after overlay; bore finish is typically Ra 3.2–6.3 µm as-machined, which is adequate for most process service. For pharmaceutical or high-purity overlay service, additional polishing steps bring the bore to Ra ≤0.8 µm. All dimensional checks — wall thickness, bore diameter, end prep dimensions — are performed after final machining and before hydrostatic test.