Weld Overlay Cladding on Pipe Fittings

Weld overlay deposits a corrosion-resistant alloy (CRA) layer on a carbon or low-alloy steel substrate, combining the structural strength of the steel with the corrosion resistance of the alloy at a fraction of the cost of solid CRA. For pipe fittings in sour gas, high-chloride or acid service, overlay cladding — most often with Alloy 625 — is a standard engineering solution when solid Alloy 825, 625 or Hastelloy C-276 fittings would be uneconomical or hard to obtain in the required size and wall thickness. It is not always equivalent to a solid fitting, though: this article covers when overlay is a sound choice and when solid CRA is required, the overlay alloys, dilution and welding control, heat treatment, NDE, and what to put on the purchase order.

Why Overlay Cladding Rather Than Solid CRA

Solid CRA buttweld fittings in Alloy 625 or Hastelloy C-276 are many times more expensive than carbon steel equivalents, and large-bore, heavy-wall solid nickel-alloy fittings can have long lead times. That premium is fully justified when the entire fitting wall must resist the process fluid. But in many applications only the internal surface is in contact with the corrosive medium, while the external surface is in ambient or benign service. A carbon steel or low-alloy substrate (for example A234 WPB or P235GH) with a full internal CRA overlay then gives essentially the same corrosion-resistant bore at a much lower overall cost. The substrate carries the design pressure and the mechanical and external loads; the CRA layer handles only the corrosive fluid. The overlay is not credited as pressure-bearing wall: the substrate alone must meet the minimum wall calculated to ASME B31.3 or EN 13480. Minimum finished overlay thickness is set by the project specification — 3 mm after final machining is a common requirement, and some sour-service and owner specifications ask for more.

How Weld Overlay Works

Weld overlay (also called weld cladding or internal overlay) deposits filler metal by GTAW, GMAW or, on simple geometries, SAW or electroslag strip, in overlapping beads over the bore of the fitting. The first layer is diluted by the substrate and has a lower alloy content than the filler wire; the second layer is deposited on alloy rather than on steel, so its chemistry is much closer to that of the filler. Most project specifications for corrosive and sour service therefore require at least two layers. The overlay chemistry — particularly the Cr, Mo, Ni and Fe content at the finished surface — must be verified on procedure qualification test pieces before production overlay begins, and on production fittings by PMI.

Common Overlay Alloys and Their Applications

Alloy 625 (ERNiCrMo-3): the most widely used overlay alloy. With a PREN above 50 it resists chloride SCC, pitting and crevice corrosion in acid and sour environments. Used in offshore topsides, subsea, and refinery and gas-plant sour service.
Alloy 825 (matching ERNiFeCr-1 filler): used in phosphoric acid, dilute sulphuric acid and moderate chloride service where performance above 316L is needed at lower cost than 625. Many specifications use 625 filler instead, because its over-alloyed deposit is more tolerant of dilution.
Hastelloy C-276 (ERNiCrMo-4): specified for hydrochloric acid, chloride-contaminated reducing acids and mixed acid environments where 625 is insufficient.
316L (ER309L buffer layer, then ER316L): the lowest-cost option, used in dilute acid, process chemical and similar service where austenitic stainless performance is adequate. The over-alloyed 309L first layer compensates for dilution from the 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 raises the iron content and lowers the chromium, molybdenum and nickel content of the overlay, degrading corrosion resistance. Dilution depends strongly on process and parameters: GTAW typically gives the lowest dilution (around 5–10%), GMAW somewhat more (around 10–20%), and high-deposition processes such as submerged arc can reach 25–40% in a single layer unless parameters are tightly controlled. For Alloy 625 overlay, project specifications set a maximum iron content at the finished surface — commonly 5% or 10% Fe depending on the service — which in practice determines the number of layers and the process. This is why two-layer overlay is the standard for sour service and highly corrosive applications: the first layer is accepted as a transition layer, and the second layer provides the specified corrosion-resistant composition. The bond line also matters at high temperature: in long-term service at elevated temperature, carbon migration from the steel into the overlay can degrade the region next to the bond line.

Welding Procedures and Heat Input Control

Corrosion-resistant overlay procedures are qualified to ASME Section IX (QW-214), with essential variables including base metal P-Number, filler metal classification, process, heat input, number of layers, preheat and interpass temperature, and the chemistry required at the specified depth. For Alloy 625 on carbon steel, preheat is applied where the substrate thickness and carbon equivalent call for it, to avoid hydrogen cracking in the steel HAZ. Interpass temperature is kept low for nickel-alloy overlays to limit dilution, hot cracking and distortion. Excessive heat input increases dilution, promotes grain growth in the overlay and widens the substrate HAZ. Automated GTAW with oscillation is typical for fittings — elbows, tees and reducers need robotic or manual welding on contoured surfaces where strip or SAW cladding is impractical.

The Overlay Geometry Challenge for Elbows

Internal overlay of a straight pipe section is relatively straightforward — the bore is uniform and the automated overlay head progresses along the axis. An elbow bore changes direction through 45° or 90°, and the intrados (inner radius) has a shorter arc length than the extrados. This creates three specific challenges: (1) the overlay head must be repositioned or angled for different sections of the elbow; (2) the intrados wall of a hot-formed long-radius elbow is thicker than the extrados, and the shorter arc means the overlay pass width and overlap must change around the circumference; and (3) the extrados — the critical zone for erosion-corrosion — is the most difficult zone for verifying overlay bond quality and thickness. For these reasons, elbow overlay needs more procedure qualification work and more NDE than straight-pipe overlay, and costs correspondingly more per fitting.

When Overlay Is Equivalent to a Solid Fitting

Internal corrosion only — the external surface is in ambient or insulated service with no corrosive exposure and no significant CUI risk
Clean corrosive liquid or gas service without significant solids, in sizes large enough for reliable overlay (NPS 6 and above is the comfortable range for elbows)
The fitting wall allows the full overlay without reducing the pressure-carrying steel wall below its calculated minimum
Service temperature is within the design range of the carbon or low-alloy steel substrate
No severe cyclic thermal service that would fatigue the overlay/substrate interface through differential expansion
Where the schedule cannot accommodate the lead time of large solid CRA fittings — overlaid carbon steel fittings can often be produced faster

When a Solid CRA Fitting Is the Better Choice

External surface also in corrosive service (subsea, fully immersed, or insulated with CUI risk)
Small bore — internal overlay becomes geometrically difficult around NPS 4 and below and is generally impractical at NPS 2 and below, where consistent coverage is also hard to verify; the cost advantage of overlay also shrinks at small sizes
High-erosion service (sand-laden multiphase, slurry) — the overlay thickness is consumed at the extrados, while a solid CRA fitting keeps its full corrosion-resistant wall
Cyclic thermal service — differential thermal expansion between overlay and substrate causes bond-line fatigue; solid CRA eliminates the dissimilar-metal interface
Cryogenic service — differential contraction at the interface can cause disbonding, and qualification data for overlay bonds under cryogenic cycling are limited
The project or owner specification requires solid CRA — some operators prohibit overlay on elbows in sour or HPHT service

Post-Weld Heat Treatment of Clad Fittings

PWHT of clad fittings needs careful planning. Carbon steel substrates (P-No. 1) require PWHT above the thickness threshold in the applicable code, or when the corrosion engineer specifies it (for example for wet H₂S service). However, the PWHT thermal cycle can sensitise austenitic stainless overlays (chromium carbide precipitation at grain boundaries) and can affect the properties and corrosion resistance of nickel-alloy overlays. Where possible, choose substrate material and thickness so that code PWHT is not triggered. Where PWHT is unavoidable, include it in the overlay procedure qualification and verify the corrosion resistance of the overlay after the PWHT cycle — for example by ASTM A262 testing for stainless overlays or ASTM G28 for nickel-alloy overlays, as the specification requires.

NDE of Overlay Cladding Integrity

Ultrasonic testing (UT) of the bond line detects disbonding (lack of fusion between overlay and substrate) and laminations in the substrate. Straight-beam UT scans the bond line; any area where the bond signal changes or the back-wall echo is lost indicates possible disbonding and is evaluated against the specification acceptance criteria.
Liquid penetrant testing (PT) of the finished overlay surface detects surface cracks and porosity.
Chemistry and PMI (portable XRF or OES, or laboratory analysis on test pieces) verify the overlay composition at the finished surface, including the iron content limit for Alloy 625 overlay.
Ferrite measurement (Feritscope) applies to stainless overlays such as 309L/316L, to confirm the ferrite number is within the specified range; it is not a meaningful check on fully austenitic nickel-alloy overlays.
Thickness measurement of the overlay after final machining confirms the minimum finished thickness.

Dimensional and Bore Finish Considerations

Overlay adds several millimetres to the bore wall before machining. The fitting substrate must therefore be ordered with a compensating bore oversize, so that after overlay and final machining to the specified bore (matching the ASME B16.9 dimensions or the connecting pipe), the minimum overlay thickness is still achieved. Elbow and reducer bores are finished by CNC machining or grinding after overlay; an as-machined bore finish around Ra 3.2–6.3 µm is adequate for most process service, and additional polishing can bring the bore to Ra 0.8 µm or better for high-purity service. All dimensional checks — wall and overlay thickness, bore diameter, end preparation — are performed after final machining and before hydrostatic testing. The weld ends need particular attention: the overlay must be carried onto the weld bevel so that the girth weld to the connecting pipe, made with a CRA filler, gives a continuous corrosion-resistant bore.

Specification of Overlay Fittings

On the purchase order, state for example: "Internal weld overlay, ERNiCrMo-3, minimum two layers, minimum finished overlay thickness 3.0 mm, overlay procedure qualified to ASME Section IX QW-214, maximum Fe content at the finished surface as per project specification. Chemistry verification of the finished overlay by XRF or spectrometric analysis — results to be included in the EN 10204 3.1 certificate." The 3.1 certificate for an overlay fitting must cover both the base fitting (e.g. ASTM A234 WPB chemistry and mechanical properties) and the overlay (filler batch, chemistry at the specified depth, overlay thickness and NDE results). Both must be present — a 3.1 that only addresses the base metal is incomplete for an overlay-specified fitting.