Welding Procedure Specification (WPS) for Bimetallic Cladding and Weld Overlay Fabrication

1. Definition and Fundamental Principles

A Welding Procedure Specification (WPS) is a formal, controlled document that defines the precise set of welding parameters, joint configurations, material specifications, and execution requirements necessary to produce a weld of consistent, qualified quality. In the context of bimetallic cladding and weld overlay fabrication, the WPS serves as the authoritative instruction set that governs every variable influencing weld metal composition, microstructure, dilution, and mechanical integrity. It is not a generic guideline but a binding technical contract between the engineering design intent and the field execution reality.

The fundamental principle underlying a WPS is traceability and repeatability. Every parameter recorded in the WPS—current, voltage, travel speed, wire feed rate, shielding gas flow, interpass temperature, layer sequence, and overlap ratio—must be reproducible by any qualified welder under any shift or production condition. The WPS is derived from and validated against a Procedure Qualification Record (PQR), maintaining a strict one-to-one correspondence between the qualification data and the production procedure. This linkage ensures that the performance demonstrated during qualification testing (mechanical properties, microstructural soundness, corrosion resistance) is carried forward into every production weld.

In bimetallic cladding applications, the WPS additionally governs the critical interface between the base metal and the overlay/transition layers. Dilution control, transition layer composition design, and interpass temperature management are specified with the same rigor as the primary welding parameters, because the metallurgical compatibility at the cladding interface determines the long-term service life of the component.

2. Category and Business Positioning

Within the organizational capability framework of Cladding Technology Shanxi Co., Ltd., the WPS is classified under the category of Process Methods, specifically under the sub-direction of Process Documentation. This positioning reflects its role as the operational bridge between engineering qualification and field execution. While PQRs and material certifications establish technical feasibility, the WPS is the document that enables scalable, auditable, and repeatable production.

From a business perspective, the WPS is a core asset in three dimensions:

Qualification Building: A well-maintained WPS library, each backed by a valid PQR, constitutes the company's qualification portfolio. It demonstrates to customers and regulatory bodies that the company can produce cladding welds meeting specified standards for defined material combinations, joint geometries, and service conditions.
Product Delivery Assurance: The WPS eliminates ambiguity at the point of fabrication. Welders, inspectors, and quality engineers all work from the same authoritative document, reducing rework, scrap, and schedule delays.
Customer Value: For customers in oil and gas, power generation, mining, and chemical processing, a documented WPS with PQR traceability is often a contractual requirement. It provides assurance that the delivered cladding product will perform as specified in its intended service environment.

3. Technical Purpose and Value

The stated technical purpose of the WPS is to guide field operations. This purpose encompasses several critical functions:

3.1 Parameter Control and Consistency

By specifying exact ranges for electrical parameters (current, voltage), mechanical parameters (travel speed, wire feed rate), and environmental parameters (shielding gas flow, interpass temperature), the WPS ensures that every weld pass is deposited under controlled conditions. This is particularly critical in overlay welding where dilution is directly proportional to heat input, which in turn is a function of current, voltage, and travel speed.

3.2 Transition Layer Design Governance

In bimetallic cladding, the transition layer serves as a metallurgical buffer between dissimilar base and overlay metals. The WPS defines the number of transition layers, their composition (typically austenitic stainless grades such as 309L or 312), and the sequence of deposition. This prevents excessive dilution, chromium carbide precipitation at the interface, and cracking due to thermal expansion mismatch.

3.3 Quality Traceability

Each WPS carries a unique identification number and is cross-referenced to its corresponding PQR. When a production weld is inspected, tested, and accepted, the WPS number is recorded on the weld map and test reports. This creates an unbroken audit trail from the final product back to the qualification data, satisfying requirements under ASME Section IX, AWS D10.9/D10.12, and EN ISO 15614.

3.4 Regulatory and Code Compliance

For products fabricated under ASME, API, or NB (Nuclear) codes, the WPS is not optional—it is mandatory. The WPS must be reviewed and approved by a qualified welding engineer and, in many cases, by an authorized inspection agency before production welding commences.

4. Key Process and Implementation Points

4.1 Core Parameter Specification

The following table summarizes the key parameters specified in a typical WPS for weld overlay cladding, along with their engineering rationale and typical values for TIG and MIG processes:

Parameter	Typical Range (TIG)	Typical Range (MIG/GMAW)	Engineering Rationale
Welding Current (A)	120–280	150–400	Controls heat input; directly affects dilution and penetration depth
Welding Voltage (V)	12–22 (AC/DC)	18–30	Influences arc stability and bead width; must be coordinated with current
Travel Speed (mm/min)	200–600	400–1200	Controls deposition rate and heat input; lower speed increases dilution
Wire Feed Rate (m/min)	N/A (manual rod feed)	5–12	Controls deposition rate in MIG; must match arc length and current
Shielding Gas Flow (L/min)	5–10 (Ar or Ar/He mix)	15–30 (Ar/CO₂ or Ar/He)	Prevents atmospheric contamination; higher flow for MIG due to larger arc zone
Interpass Overlap	1/3 to 1/2 of previous bead width	1/3 to 1/2 of previous bead width	Ensures complete fusion between adjacent passes; prevents lack of fusion and porosity
Interpass Temperature (°C)	≤150 (typically ≤100 for overlay)	≤200 (typically ≤150 for overlay)	Controls grain growth and residual stress; critical for transition layer integrity
Number of Layers	2–5 (transition + overlay)	2–5 (transition + overlay)	More layers reduce dilution; transition layers bridge base/overlay composition gap

4.2 Interpass Overlap: The 1/3 to 1/2 Rule

The WPS explicitly mandates an interpass overlap of 1/3 to 1/2 of the previous bead width. This specification is not arbitrary—it is grounded in metallurgical and mechanical principles:

Below 1/3 overlap: Risk of incomplete fusion between adjacent passes, creating linear defects that act as crack initiation sites under cyclic or thermal loading.
Above 1/2 overlap: Excessive heat concentration in the overlap zone, leading to grain coarsening, potential cracking in susceptible alloys, and distortion of the overlay geometry.
1/3 to 1/2 overlap: Provides adequate fusion without excessive thermal cycling, ensuring sound bonding between passes while maintaining a controlled microstructure.

For overlay welds on carbon steel or low-alloy steel bases, the overlap ratio is typically maintained at the upper end (1/2) to ensure complete fusion with the potentially refractory base material. For stainless steel overlay on stainless steel, the overlap is often maintained at 1/3 to minimize heat input and reduce sensitization risk.

4.3 Transition Layer Design

The WPS defines the transition layer architecture based on the specific base metal and overlay metal combination. The following table illustrates typical transition layer designs for common cladding applications:

Base Metal	Overlay Metal	Transition Layer	Number of Layers	Key Consideration
Carbon Steel (Q235/A36)	304/316 Stainless	309L (1–2 layers)	3–4	High dilution; 309L bridges Fe-Cr-Ni composition gap
Low-Alloy Steel (15CrMo/A335 P91)	316L Stainless	309L then 312 (2 layers)	4–5	Cracking susceptibility in P91; low carbon 312 reduces sensitization
Stainless Steel (304/316)	Hastelloy C-276	312 or 625 (1 layer)	3	Moderate dilution; 625 provides Ni-Cr-Mo bridge
Cast Iron	316 Stainless	309L (2 layers) + nickel alloy	4–5	High carbon content; multiple transition layers essential

4.4 Heat Input Management

Heat input is calculated as:

H = (V × I × 60) / (v × η)

where V is voltage (V), I is current (A), v is travel speed (mm/min), and η is the process efficiency factor (η = 0.85 for TIG, η = 0.80 for MIG). The WPS specifies a heat input range (typically in kJ/mm) derived from the PQR. Deviations beyond ±20% of the qualified heat input range require requalification under ASME Section IX.

For overlay welding, the heat input is typically kept at the lower end of the qualified range to minimize dilution. A common target is 0.5–1.5 kJ/mm for TIG overlay and 1.0–3.0 kJ/mm for MIG overlay, depending on the specific material combination and required overlay thickness.

4.5 WPS-to-PQR Correspondence

The WPS is not created in isolation. It is derived from a Procedure Qualification Record (PQR) through a defined engineering process:

PQR Execution: A test weld is deposited under specified parameters and subjected to mechanical testing (tensile, bend, hardness), macrostructural examination, and corrosion testing as required by the applicable code.
WPS Development: A qualified welding engineer reviews the PQR data and develops the WPS, establishing acceptable parameter ranges (essential and non-essential variables) within which production welding may proceed.
One-to-One Mapping: Each WPS is uniquely linked to one or more PQRs. The WPS number, PQR number, and applicable code are recorded in the company's qualification register.
Revision Control: Any change to the WPS that affects essential variables requires a new PQR. Non-essential variable changes require welding engineer approval and documented justification.

5. Applicable Standards and Acceptance Criteria

5.1 Governing Standards for WPS Development and Qualification

Standard	Scope	Relevance to Cladding WPS
ASME Section IX, Part 1	Qualification of welding procedures and welders	Defines essential variables, qualification ranges, and WPS/PQR requirements for pressure vessel cladding
AWS D10.9/D10.12	Welding procedure qualification for overlay welding	Specifically addresses overlay weld qualification, dilution testing, and corrosion testing
EN ISO 15614-1	Qualification testing of welding procedures for metallic materials	European standard for WPS/PQR qualification; widely referenced in international projects
NB/T 20002	Nuclear industry welding procedure qualification	Applies to nuclear-grade cladding; imposes additional requirements for WPS approval and welder qualification
GB/T 985	Welding procedure specification requirements (Chinese national standard)	Defines WPS content requirements for domestic Chinese fabrication projects
API 570/579	Fitting repair and pressure-retaining component examination	References WPS requirements for field overlay repairs on piping and pressure equipment
NACE MR0175/ISO 15156	Materials for H₂S environments	Imposes hardness and microstructural requirements on overlay welds; WPS must control these

5.2 Acceptance Criteria

The WPS defines the acceptance criteria that production welds must meet. These include:

Visual Inspection (VT): No surface cracks, undercut exceeding 0.5 mm (or code-specific limits), porosity, or incomplete fusion visible on the overlay surface. Conformity to AWS D1.1 Table 6.1 or equivalent.
Penetrant Testing (PT): No indications of linear surface defects exceeding 3 mm in length. Per ASME Section V, Article 7.
Magnetic Particle Testing (MT): Applicable to ferromagnetic base metals; no indications of surface or near-surface defects. Per ASME Section V, Article 7.
Hardness Testing: Overlay hardness within specified limits (e.g., ≤250 HV for NACE MR0175 compliance; ≤350 HV for typical stainless overlay). Transition layer hardness gradient must be continuous with no sharp discontinuities.
Dilution Analysis: Macrostructural examination with metallographic etching to verify dilution at the base/overlay interface does not exceed 5% (for high-dilution-sensitive alloys) or 10% (for general stainless overlay). Per AWS D10.9.
Corrosion Testing: Salt spray testing (ASTM B117), intergranular corrosion testing (ASTM A923), or specific media immersion testing as required by the service environment. Minimum pass criteria defined in the WPS.
Ultrasonic Testing (UT): For thick overlay sections (>6 mm), UT per ASME Section V, Article 4, to detect subsurface defects, lack of fusion, or delamination.

6. Common Risks and Controls

6.1 Parameter Drift and Non-Conformance

Risk: Field welders may deviate from WPS-specified parameters due to equipment limitations, ambient conditions, or operator habit. Even small deviations in current or travel speed can significantly affect dilution and weld metal composition in overlay applications.

Control Measures:

Pre-weld parameter verification by a welding inspector before each production shift.
Use of programmable welding power sources with parameter lockout to prevent unauthorized adjustments.
WPS parameters posted at the weld station in a visible, legible format.
Random parameter audits during production with documentation of findings.

6.2 Excessive Dilution

Risk: Dilution beyond the qualified range alters the overlay composition, potentially reducing corrosion resistance, increasing hardness beyond NACE limits, or creating brittle phases at the interface.

Control Measures:

Strict adherence to WPS-specified heat input range, interpass temperature, and overlap ratio.
Macrostructural examination of the first production weld of each shift to verify dilution.
Use of low-dilution consumables (flux-cored wire with low carbon content, or wire with pre-alloyed composition).
Transition layer design with composition buffering (e.g., 309L on carbon steel before 316L overlay).

6.3 Interpass Temperature Exceedance

Risk: Excessive interpass temperature promotes grain coarsening, reduces hardness in HAZ, and can cause cracking in susceptible alloys (e.g., high-strength low-alloy steels, precipitation-hardening alloys).

Control Measures:

Mandatory temperature measurement with infrared pyrometer or thermocouple before each subsequent pass.
WPS specifies maximum interpass temperature (typically 100–150°C for overlay on carbon steel; ≤200°C for stainless overlay).
Passivation or cooling of the weld zone between passes if temperature exceeds limits.
Welding sequence design to minimize heat accumulation in multi-layer builds.

6.4 WPS-PQR Mismatch

Risk: Production welding performed outside the qualified parameter ranges of the corresponding PQR, rendering the weld unqualified and potentially non-compliant with code requirements.

Control Measures:

Mandatory WPS review by a qualified welding engineer before any new production job.
Documented mapping of WPS to PQR in the quality management system.
Procedure for WPS revision with requalification when essential variables change.
Audit trail of all WPS revisions with approval signatures.

6.5 Inadequate Shielding Gas Coverage

Risk: Insufficient or disrupted shielding gas flow leads to atmospheric contamination, porosity, and oxidation of the weld metal, particularly critical for stainless and nickel alloy overlays.

Control Measures:

WPS specifies minimum shielding gas flow rate and gas composition (e.g., 99.99% Ar for TIG; Ar/2% O₂ for MIG stainless overlay).
Gas flow verification with calibrated flowmeter before each weld.
Wind speed monitoring; welding suspended if wind exceeds 1 m/s (or as specified in WPS) without wind shielding.
Back-purge requirements for full-penetration overlay joints specified in WPS.

7. Application Across the Three Technology Routes

7.1 TIG/MIG Weld Overlay

In the TIG/MIG weld overlay route, the WPS is the primary process control document. It governs every aspect of the overlay operation:

TIG Overlay: The WPS specifies electrode diameter (typically 2.4–3.2 mm for overlay), rod diameter and composition (matching the overlay grade), AC/DC polarity, frequency (for AC TIG on aluminum or for cleaning effect on stainless), and fill rod feed technique. TIG overlay is preferred for thin overlay sections (1–3 mm), precision dilution control, and high-purity overlay deposits.
MIG/GMAW Overlay: The WPS specifies wire type (solid, flux-cored, or metal-cored), wire diameter (typically 1.0–1.6 mm), gas composition and flow, voltage, wire feed rate, and gun angle. MIG overlay is preferred for thick overlay builds (3–10+ mm), high deposition rates, and production efficiency.
Multi-Layer Builds: For thick overlays, the WPS defines the layer sequence, composition of each layer (transition, intermediate, surface), and the number of passes per layer. A typical 6 mm stainless overlay on carbon steel might use: Layer 1 (309L, 2 passes), Layer 2 (309L, 2 passes), Layer 3 (316L, 3 passes), Layer 4 (316L, 2 passes).

The WPS for weld overlay is also the document that defines the welding sequence pattern (zig-zag, weave, or spiral) to minimize distortion and ensure uniform coverage across the cladding area.

7.2 Hydraulic Explosive Bonding

In hydraulic explosive bonding, the WPS takes on a modified role. While the primary bonding mechanism is mechanical (hydrodynamic jetting at high velocity), the WPS governs the post-bonding operations and qualification framework:

Post-Bonding Weld Overlay: When hydraulic explosive bonding is followed by a weld overlay layer (e.g., to build up thickness or apply a specific surface alloy), the WPS specifies the overlay welding parameters. The transition from the bonded interface to the weld overlay requires careful dilution control, as the bonded interface may have a different microstructure than the base metal.
Weld Repair and Defect Remediation: If hydraulic explosive bonding produces defects (delamination, contamination) that require repair, the WPS defines the repair welding procedure, including root preparation, fill material selection, and number of repair layers.
Qualification Traceability: The WPS for post-bonding operations must be linked to a PQR that includes testing of the bonded + welded assembly, not just the weld alone. This ensures that the combined interface meets mechanical and corrosion requirements.

In this route, the WPS also specifies the interface preparation requirements (cleaning, surface roughness, temperature) before welding commences on the bonded substrate.

7.3 Explosion Welding

Explosion welding (explosive cladding) produces a solid-state bond between base and cladding metals without melting. The WPS applies to the qualification and post-processing framework:

Explosion Welding Parameter Documentation: While not a "welding" WPS in the traditional sense, the explosion welding procedure specification documents explosive charge weight, stand-off distance, flyer plate velocity, impact angle, and alignment tolerances. This is functionally equivalent to a WPS for the explosion welding process and is linked to PQRs for bond strength, intermetallic layer thickness, and interface quality.
Post-Explosion Welding Overlay: When explosion welding is combined with subsequent weld overlay (e.g., to add a corrosion-resistant surface layer on top of an explosion-welded cladding), the WPS for the weld overlay must account for the unique microstructure at the explosion weld interface. Dilution control is critical to avoid disrupting the clean, metallurgically sound explosion bond.
Weld Overlay on Explosion-Welded Substrates: The WPS specifies interpass temperature limits that are more conservative than for homogeneous substrates, to prevent thermal degradation of the explosion-welded interface. Typical maximum interpass temperature is 100°C, compared to 150–200°C for standard overlay.
Combined Qualification: The PQR for a combined explosion welding + weld overlay process includes testing of the entire layered structure (base metal / explosion weld interface / overlay weld), not just individual layers. The WPS must be consistent with this comprehensive qualification approach.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

The WPS is the cornerstone of the company's qualification infrastructure. Each WPS, backed by a valid PQR, represents a qualified capability for a specific material combination, joint configuration, and service condition. Over time, the accumulation of WPS-PQR pairs creates a comprehensive qualification portfolio that enables the company to:

Accept projects across diverse industries (oil and gas, power, mining, chemical, marine) without requalification delays.
Demonstrate compliance with ASME, API, NB, and ISO requirements during customer audits and regulatory inspections.
Expand capability boundaries by developing new WPS-PQR pairs for emerging material combinations (e.g., high-entropy alloys, advanced refractory metals).

8.2 Product Delivery Assurance

In production, the WPS eliminates variability and ensures consistent quality across all production shifts, welders, and fabrication sites. This translates directly into:

Reduced rework and scrap rates, improving schedule adherence and cost efficiency.
Consistent overlay thickness, composition, and mechanical properties across all delivered products.
Simplified quality inspection, as acceptance criteria are clearly defined in the WPS and can be verified systematically.

8.3 Customer Value

For the end customer, the WPS provides:

Technical Assurance: Proof that the cladding product was fabricated under controlled, qualified conditions.
Regulatory Compliance: Documentation required for code-stamped fabrication, regulatory inspections, and insurance purposes.
Service Life Confidence: Dilution control, transition layer design, and interpass temperature management—all specified in the WPS—directly contribute to the long-term corrosion resistance and mechanical integrity of the cladding in service.
Audit Readiness: Complete traceability from the delivered product back to the qualification data, enabling rapid response to customer quality inquiries or field performance issues.

9. Implementation Recommendations

Establish a WPS Management System: Implement a controlled document management system for all WPS documents, including version control, revision history, approval records, and cross-referencing to PQRs.
Conduct Regular WPS Reviews: Schedule annual reviews of the WPS library to identify obsolete procedures, consolidate overlapping WPS, and develop new WPS for emerging project requirements.
Train Field Personnel: Ensure all welders, welding inspectors, and production supervisors are trained on the specific WPS parameters for their assigned jobs. Conduct periodic competency assessments.
Integrate with Quality Management System: Link WPS compliance verification to the company's quality management system (ISO 9001, ISO 3834, or NB/T 20011), ensuring that WPS adherence is audited as part of routine quality checks.
Develop WPS Templates by Process Route: Create standardized WPS templates for TIG overlay, MIG overlay, post-bonding weld overlay, and explosion welding qualification to streamline WPS development and ensure consistency across the organization.

10. Conclusion

The Welding Procedure Specification (WPS) is not merely a procedural document—it is the technical backbone of the company's cladding fabrication capability. By precisely defining every parameter that influences weld quality, from current and voltage to interpass overlap and transition layer design, the WPS transforms engineering intent into reproducible field execution. Its one-to-one correspondence with the PQR ensures that every production weld is backed by qualification data, providing the traceability and confidence that customers, regulators, and codes demand.

Across the company's three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—the WPS serves as the unifying process control document, adapting its scope and content to the specific metallurgical and mechanical requirements of each route. Maintaining a robust, well-organized WPS library is not a compliance exercise; it is a strategic investment in the company's qualification depth, production efficiency, and customer trust.