What Is Weld Overlay? Cladding Process

Weld overlay (also called weld cladding or hardfacing) is a process that deposits one or more layers of corrosion-resistant or wear-resistant alloy onto a carbon or low-alloy steel substrate using welding. The overlay provides the chemical resistance of an expensive alloy at a fraction of the cost of solid alloy construction. It is widely used in pipe coating and cladding for oil and gas, petrochemical, and power generation equipment.

Weld Overlay Processes

Process	Application	Deposition Rate	Dilution
GTAW (hot wire)	Pipe bore, nozzle bores, precision overlay	1-3 kg/h	5-10% (low)
GMAW (pulsed)	Pipe bore, fittings, valve internals	2-5 kg/h	10-20%
SAW (strip)	Vessel shells, heads, large flat surfaces	10-25 kg/h	10-15%
SAW (wire)	Circumferential seams, nozzles	5-15 kg/h	15-25%
FCAW	Field repairs, hardfacing	3-8 kg/h	15-25%
Electroslag (strip)	Large vessel shells	15-30 kg/h	8-12%
Plasma transferred arc (PTA)	Valve seats, wear surfaces	2-6 kg/h	5-10%

Common Overlay Alloys

Overlay Alloy	UNS	Typical Application
Alloy 625 (Inconel 625)	N06625	Sour service, subsea, high-chloride environments
Alloy 825	N08825	Moderate sour service, sulfuric acid
316L stainless	S31603	General corrosion protection
Alloy 309L + 347	S30903/S34700	Transition layer + final layer on CS vessels
Stellite 6	R30006	Valve seats, trim, wear surfaces
Alloy C-276 (Hastelloy)	N10276	Severe chemical service, FGD systems

Minimum Overlay Thickness

ASME Section VIII, Division 1 and ASME B31.3 require the weld overlay to meet the specified chemistry at a minimum distance from the fusion line. After dilution with the base metal, the first layer may not meet the target composition; a second layer is typically required.

Specification	Minimum Overlay Thickness After Machining
ASME Section VIII (corrosion overlay)	3 mm (1/8”) minimum of undiluted alloy composition
NORSOK M-001	3 mm CRA after final machining
Client specifications (typical)	3-6 mm depending on service severity

Pro Tip

The first overlay layer on carbon steel is diluted by iron from the substrate (typically 10-25% dilution depending on process). Chemical analysis must be performed at the required depth from the surface to confirm the overlay meets the target alloy composition. GTAW hot wire and electroslag strip processes minimize dilution, reducing the number of layers needed.

Quality Requirements

All weld overlays require procedure qualification (PQR) per ASME Section IX with chemical analysis, bend tests, and (for pressure equipment) side bend or shear tests of the bond. Non-destructive testing includes PT (liquid penetrant) on 100% of the overlay surface and UT to verify bond integrity.

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