Weld Overlay Process Qualification (PQR/WPQR) for Bimetallic Cladding Systems
1. Definition and Fundamental Principles
Weld overlay process qualification is a formal, standards-based procedure that demonstrates the reproducibility and legality of a specific weld overlay method under defined parameters. The resulting documents—Welder Performance Qualification Record (PQR) and Welding Procedure Qualification Record (WPQR)—serve as the evidentiary foundation proving that a given overlay process, when executed within qualified parameter ranges, will consistently produce joints meeting all applicable acceptance criteria. Unlike base-metal welding qualification, overlay qualification introduces unique metallurgical and performance challenges: the interface between the parent material (PM) and the overlay layer is inherently a dissimilar-material weld, where dilution, intermetallic formation, and residual stress distributions differ fundamentally from homogeneous welds.
The fundamental principle governing overlay PQR/WPQR is that process variables—heat input, preheat temperature, interpass temperature, travel speed, wire feed rate, shielding gas composition, and number of passes—must be bracketed within qualified limits such that any future production weld executed within those limits is deemed equivalent to the qualified procedure. The qualification coupon is a controlled, instrumented demonstration that the process achieves the required mechanical, metallurgical, and chemical performance at the overlay-to-parent interface and throughout the overlay cross-section.
2. Category and Business Positioning
Within the company's technical capability taxonomy, weld overlay process qualification belongs to the Process Method (工艺方法) category under the Process Qualification (工艺评定) technology direction. Its primary technical purpose is process legality and repeatability (工艺合法性可重复). This positioning is critical because:
- Regulatory compliance: Pressure vessels, pipelines, and critical equipment cannot be legally manufactured or commissioned without a valid, current WPS backed by a qualified PQR. The qualification record is the legal permit to weld.
- Customer assurance: End-users in oil & gas, power generation, mining, and chemical processing require documented proof that overlay processes are qualified per recognized codes before accepting cladded components.
- Risk mitigation: A properly executed PQR eliminates ambiguity about process capability, reducing the probability of field failures, rework, and warranty claims.
The entry explicitly notes that any change in parent material or filler material triggers requalification, underscoring the sensitivity of overlay systems to material combinations and reinforcing the need for a rigorous qualification management system.
3. Technical Purpose and Value
The weld overlay PQR/WPQR process serves several interrelated value propositions:
3.1 Legal and Code Compliance
ASME Section IX, ISO 15614-7, and NB/T 47014 all mandate that welders and welding procedures be qualified before production work commences. For overlay applications specifically, QW-451 (ASME IX) establishes that overlay welds require qualification demonstrating the ability to produce the required thickness, chemical composition, and performance properties. Without a valid PQR, no WPS can be issued, and no production component can be certified.
3.2 Reproducibility Assurance
Overlay welding is inherently variable—dilution rates fluctuate with heat input, dilution affects hardness profiles, and interpass temperature influences microstructure evolution. The PQR process systematically tests these variables under controlled conditions, establishing parameter envelopes within which the process is proven repeatable. This transforms overlay welding from an artisanal craft into a controlled, engineering-managed process.
3.3 Customer and Market Value
For Cladding Technology Shanxi Co., Ltd., maintaining a comprehensive portfolio of qualified PQRs across multiple material combinations, overlay thicknesses, and process routes (TIG, MIG, explosive bonding) is a core competitive differentiator. It enables:
- Rapid quotation and project acceptance because the process is already qualified
- Reduced project lead times by eliminating the need for ad-hoc qualification during production
- Credibility in competitive bids where customers require proof of qualified capability
- Facilitated audits by regulatory bodies, TUV organizations, and customer quality teams
4. Key Process and Implementation Points
4.1 Applicable Standards Framework
| Standard | Jurisdiction | Scope for Overlay Qualification |
|---|---|---|
| ASME IX, QW-451 | USA / International | Overlay weld qualification requirements, essential variables, performance tests |
| ISO 15614-7 | International | Qualification of welding procedures for hardfacing/overlay welding |
| NB/T 47014 | China (National Boiler) | Qualification of welding procedures for pressure vessels (including overlay) |
| ASME IX, QW-250/QW-251 | USA / International | Qualification limits and essential variables for welding processes |
| ASME VIII Div. 1, UW-4 | USA / International | Welding procedure qualification for pressure vessels |
| NB/T 47014.2 | China (National Boiler) | Method for welding procedure qualification (supplemental) |
4.2 Essential Variables for Overlay PQR
The following essential variables must be recorded and controlled during PQR execution. Any deviation beyond qualified limits invalidates the procedure:
| Essential Variable | TIG Overlay (GTAW) | MIG Overlay (GMAW) | Explosive Bonding (Reference) |
|---|---|---|---|
| Process | GTAW (Process No. 11) | GMAW (Process No. 12) | N/A (non-weld process) |
| Filler metal classification | As specified (e.g., ER309L, ERNiCr-3) | As specified | N/A |
| Filler metal form | Wire, rod, strip | Wire | N/A |
| Filler metal diameter | 0 to +2.5 mm | 0 to +2.5 mm | N/A |
| Preheat temperature | 0 to +150°C above qualified | 0 to +150°C above qualified | N/A |
| Interpass temperature | 0 to +150°C above qualified | 0 to +150°C above qualified | N/A |
| Heat input (GTAW) | 0 to +25% above qualified | N/A | N/A |
| Heat input (GMAW) | N/A | 0 to +25% above qualified | N/A |
| Shielding gas type/composition | As qualified | As qualified | N/A |
| Shielding gas flow rate | 0 to +50% above qualified | 0 to +50% above qualified | N/A |
| Travel speed | 0 to +25% above qualified (GTAW) | 0 to +25% above qualified (GMAW) | N/A |
| Current/voltage (GTAW) | 0 to +15% / 0 to +15% | N/A | N/A |
| Wire feed speed (GMAW) | N/A | 0 to +25% above qualified | N/A |
| Overlay thickness | As qualified (minimum and maximum) | As qualified | As qualified (bond line + clad) |
| Number of overlay passes/layers | As qualified | As qualified | N/A |
| Backing material | None / as qualified | None / as qualified | N/A |
4.3 Coupon Preparation and Test Configuration
The PQR coupon must be prepared from the same base material grade, heat number (or equivalent), and thickness as the intended production application. For overlay qualification per ASME QW-451, the coupon typically includes:
- Flat coupon: A plate specimen of sufficient thickness (minimum 19 mm or as specified) with overlay deposited on one surface
- Overlay thickness: Representative of the intended production thickness (e.g., 3 mm, 6 mm, 12 mm)
- Test preparation: Cross-sections prepared for metallographic examination, hardness testing, and chemical analysis
- Multiple specimens: Typically 2-3 coupons prepared per PQR to allow for destructive testing without compromising the record
4.4 Performance Tests Required
| Test | Standard Reference | Purpose | Typical Acceptance Criteria |
|---|---|---|---|
| Visual Examination (VT) | ASME IX QW-191 / ISO 17637 | Surface defects, undercut, porosity, overlap | No cracks, no surface defects exceeding code limits |
| Hardness Testing (HV/HRB) | ASTM E92 / ASTM E18 | Overlay hardness uniformity, dilution gradient | Overlay hardness within specified range (e.g., 250-450 HV for 316L overlay); dilution zone hardness < 500 HV |
| Chemical Analysis | ASTM E415 / ASTM E1251 | Dilution rate, overlay composition | Dilution < 15-25% (per application requirement); overlay composition within specified grade range |
| Metallographic Examination | ASTM E3 / ASTM E407 | Microstructure, intermetallics, grain structure | No continuous intermetallic phases at interface; acceptable grain morphology |
| Tensile Test (transverse) | ASTM E8 / ASTM E8M | Overlay joint strength | UTS ≥ 90% of filler metal specified minimum tensile strength |
| Bend Test (face/transverse) | ASTM E16 | Ductility of overlay and interface | No cracks > 1 mm at bend surface |
| Impact Test (if required) | ASTM E23 / ASTM E23M | Toughness at low temperature | Min. 27 J at test temperature (per code requirement) |
| Corrosion Testing | ASTM G48 / ASTM A262 | Overlay corrosion resistance, sensitization | No intergranular corrosion; pitting resistance within specification |
| NT (Non-Destructive Testing) | ISO 17640 (MT) / ISO 17638 (UT) | Internal defects, bond quality | No indications exceeding acceptance criteria per relevant code |
4.5 Critical Process Parameters for Overlay Control
4.5.1 Dilution Rate Control
Dilution—the percentage of parent material alloyed into the overlay—is the single most critical variable in overlay PQR. Dilution directly affects:
- Corrosion resistance: Higher dilution introduces base metal elements (Fe, Cr, Ni ratios shift) that may reduce pitting resistance or promote intermetallic formation
- Hardness: Dilution softens the overlay, potentially below required wear resistance
- Intermetallic formation: At the dilution interface, chromium carbides (M7C3, M23C6) or sigma phase may precipitate, embrittling the joint
Dilution rate is controlled through:
- Number of overlay passes (more passes = lower dilution)
- Heat input per pass (lower heat input = lower dilution)
- Weld bead geometry and overlap pattern
- Filler metal selection (matching or deliberately mismatched composition)
- Travel speed and arc voltage
4.5.2 Interpass Temperature Management
Interpass temperature is a critical essential variable because it governs:
- Cooling rate of the previous pass (affects microstructure and residual stress)
- Dilution in subsequent passes (higher interpass = more PM melting = higher dilution)
- Hydrogen diffusion and cracking susceptibility
- Phase transformation behavior in the HAZ
For austenitic overlay on carbon steel, interpass temperature is typically limited to <150°C to minimize sensitization. For nickel-based overlays, interpass may be limited to <100°C to prevent excessive grain growth and intermetallic formation. The qualified interpass temperature and its allowable range must be documented on the PQR.
4.5.3 Preheat Requirements
Preheat serves to reduce thermal gradients, slow cooling rates, and minimize residual stress. Preheat is essential for:
- High-carbon or high-hardness parent materials (HRC > 30)
- Thick sections (risk of hydrogen cracking)
- Low-temperature service environments
- Materials with high thermal expansion mismatch
Preheat temperature is an essential variable in all major qualification standards. The PQR must record the actual preheat temperature applied and the qualified range. For carbon steel parent materials, preheat of 100-250°C is typical; for stainless steel, preheat is generally 0-100°C or omitted.
4.5.4 Multi-Pass Overlay Strategy
Overlay qualification typically involves multi-pass welding to achieve required thickness with controlled dilution. The qualified sequence includes:
- First pass (highest dilution): Directly on parent material, typically with a transition filler (e.g., ER309L on carbon steel) to accommodate thermal mismatch and reduce cracking risk
- Intermediate passes: Increasingly matching the target overlay composition, with controlled overlap to manage dilution
- Final passes: Full target composition (e.g., ER316L, ERNiCr-3, ER2594), with minimum dilution
The PQR must document the number of passes, the sequence of filler metals used, and the resulting dilution profile at each layer. This information is essential for WPS development and production replication.
5. Applicable Standards and Acceptance Criteria
5.1 ASME Section IX, QW-451
QW-451 is the primary reference for overlay weld qualification in ASME code construction. Key requirements include:
- Overlay welds must be qualified for the specific filler metal and base metal combination
- Qualification must demonstrate the ability to produce the required overlay thickness
- Performance tests include visual examination, hardness testing, and chemical analysis of the dilution zone
- Qualification limits for essential variables follow QW-250/QW-251
- Welding position qualification follows QW-461 (overlay may be qualified in the position of use or all positions)
5.2 ISO 15614-7
ISO 15614-7 specifically addresses qualification of welding procedures for hardfacing and overlay welding. It provides:
- Essential variables specific to overlay processes (GTAW, GMAW, SAW, FCAW)
- Qualification rules for different overlay thicknesses
- Performance test requirements including hardness, dilution, and microstructure
- Guidance on qualification for different joint configurations (flat, pipe, etc.)
5.3 NB/T 47014 (China National Boiler Standard)
NB/T 47014 governs welding procedure qualification for pressure vessels and components in China. For overlay applications:
- The standard references ASME IX and ISO 15614-7 for overlay-specific requirements
- Chinese regulatory bodies (TSG) require PQR documentation for all pressure vessel overlay welding
- Additional requirements may apply for nuclear, boiler, or high-pressure applications
5.4 Acceptance Criteria Summary
| Acceptance Parameter | Typical Criteria | Standard Reference |
|---|---|---|
| Visual surface quality | No cracks, undercut < 0.5 mm, porosity < 10% | ASME IX QW-191 / ISO 17637 |
| Overlay hardness | Within specified range (e.g., 200-400 HV for 316L) | ASTM E92 |
| Dilution rate | < 15-25% (per application) | ASTM E415 |
| Overlay thickness | ≥ specified minimum, uniform within ±10% | ASME QW-451 |
| Chemical composition | Within specified grade limits (e.g., ASTM A240 316L) | ASTM E1251 |
| Microstructure | No continuous intermetallics, acceptable grain size | ASTM E3 / E407 |
| Tensile strength (if tested) | ≥ 90% of filler metal SMTS | ASTM E8 |
| Bend test (if tested) | No cracks > 1 mm | ASTM E16 |
| Corrosion resistance | No IGSCC, acceptable pitting resistance | ASTM G48 / A262 |
| UT/MT internal defects | No indications exceeding code limits | ISO 17638 / 17640 |
6. Common Risks and Controls
6.1 Dilution Exceedance
Risk: If dilution exceeds the qualified limit, the overlay composition deviates from specification, potentially compromising corrosion resistance, hardness, or mechanical properties.
Controls: Multi-pass welding with increasing composition matching; controlled heat input per pass; documented dilution testing at each qualification; WPS specifying minimum number of passes.
6.2 Intermetallic Formation at Interface
Risk: Chromium carbides (M7C3, M23C6), sigma phase, or Laves phase may form at the dilution interface, causing embrittlement and potential intergranular cracking.
Controls: Transition layer welding (e.g., ER309L on carbon steel before 316L overlay); controlled interpass temperature; avoidance of excessive heat input; metallurgical examination during PQR to verify absence of continuous intermetallics.
6.3 Cracking in Overlay or HAZ
Risk: Hot cracking (austenite grain boundary liquation), cold cracking (hydrogen-induced), or solidification cracking in the overlay or HAZ.
Controls: Preheat per qualified procedure; controlled interpass temperature; low-hydrogen filler metals; appropriate travel speed; welding sequence to minimize restraint; post-weld heat treatment if required by WPS.
6.4 Material or Filler Change Without Requalification
Risk: Substitution of parent material grade or filler metal classification without requalification invalidates the WPS and renders production welds non-compliant.
Controls: Strict material traceability; documented material approval process; PQR database with clear coverage limits; quality system requiring requalification trigger assessment for any material change.
6.5 Parameter Drift During Production
Risk: Production welders deviating from qualified parameters (heat input, travel speed, interpass temperature) outside qualified limits.
Controls: Parameter monitoring and recording; welder qualification maintenance; WPS with clear parameter ranges; periodic production audit sampling; instrumentation for automated processes.
6.6 Inadequate Qualification Coverage
Risk: PQR not covering the actual production conditions (material thickness, welding position, joint configuration).
Controls: Comprehensive PQR database mapped to product portfolio; qualification gap analysis before project acceptance; proactive qualification of anticipated material/process combinations.
7. Application Across Company Technology Routes
7.1 TIG (GTAW) Weld Overlay
TIG overlay is the primary process for high-quality, low-dilution overlay applications where metallurgical integrity and surface finish are critical. PQR/WPQR for TIG overlay is governed by ASME QW-451 and ISO 15614-7, with the following specific considerations:
- Process advantages: Precise heat input control, low dilution, excellent surface quality, suitable for thin overlays and high-alloy overlays (Ni-based, Co-based)
- Essential variables: Current, voltage, travel speed, shielding gas composition and flow rate, filler rod diameter and classification, preheat, interpass temperature, number of passes
- Typical applications: 316L/304L overlay on carbon steel for corrosion resistance; Ni-based overlay (Stellite, Inconel) for wear/erosion resistance; transition layers for dissimilar material joints
- PQR-specific considerations: TIG overlay PQR must demonstrate consistent dilution control across multiple passes; metallographic examination is critical to verify absence of intermetallics; hardness profiling across the overlay cross-section is mandatory
For TIG overlay, the PQR typically involves 3-5 passes to achieve 3-6 mm overlay thickness, with the first pass using a transition filler (e.g., ER309L) and subsequent passes using the target composition (e.g., ER316L). The qualified heat input range is narrow (typically 0.5-1.5 kJ/mm), and the WPS must specify precise current, voltage, and travel speed to maintain this range.
7.2 MIG (GMAW) Weld Overlay
MIG overlay is used for thicker overlay deposits, higher productivity applications, and automated/robotic overlay systems. PQR/WPQR for MIG overlay follows the same standards framework but with different essential variable ranges:
- Process advantages: Higher deposition rate, thicker overlay in fewer passes, suitable for automated systems, lower cost per kg of overlay
- Essential variables: Wire feed speed, voltage, travel speed, shielding gas composition and flow rate, wire diameter and classification, preheat, interpass temperature, number of passes, contact tip to work distance
- Typical applications: Thick overlay (6-25 mm) for severe erosion/corrosion service; automated overlay on large components (vessels, pipes, structural steel); hardfacing with high-carbon or Ni-based consumables
- PQR-specific considerations: MIG overlay PQR must account for higher heat input and potentially higher dilution; spatter control and gas shielding effectiveness are critical; automated systems require parameter stability verification
For MIG overlay, the PQR typically involves 2-4 passes to achieve 6-12 mm overlay thickness, with controlled overlap and interpass temperature. The qualified heat input range is wider (typically 2-8 kJ/mm), and the WPS must specify wire feed speed, voltage, and travel speed within tight tolerances to ensure consistent dilution and bead geometry.
7.3 Hydraulic Explosive Bonding and Explosion Welding
Explosive bonding (including hydraulic explosive bonding) is a solid-state joining process that does not involve melting of the parent materials. However, PQR/WPQR principles still apply to explosive bonding qualification, with significant differences in scope and testing:
- Qualification basis: Explosive bonding qualification follows standards such as ASTM A402 (for explosion-welded clad plate), ASTM A418 (for explosion-welded pipe), and ASTM F2321 (for explosion welding of tubular components). These standards define qualification requirements for explosive bonding processes.
- Essential variables: Charge mass per unit area, flyer-to-target spacing, flyer velocity, target material and thickness, surface preparation, environmental conditions (temperature, humidity)
- Performance tests: Peel/shear strength testing, macroscopic and microscopic bond examination, chemical composition verification, corrosion testing, mechanical property testing of base materials
- Dilution: Explosive bonding produces a diffusion bond with minimal intermixing (typically < 5% interdiffusion at the bond interface), eliminating dilution as a critical variable
For explosive bonding, the PQR focuses on demonstrating consistent bond quality across the clad surface, verifying the absence of voids, cracks, or unbonded areas, and confirming that the bond interface meets mechanical and metallurgical acceptance criteria. The qualification coupon typically includes peel/shear specimens prepared at multiple locations across the clad surface, as well as cross-sections for metallographic examination.
| Qualification Aspect | TIG Overlay PQR | MIG Overlay PQR | Explosive Bonding PQR |
|---|---|---|---|
| Governing Standard | ASME IX QW-451, ISO 15614-7, NB/T 47014 | ASME IX QW-451, ISO 15614-7, NB/T 47014 | ASTM A402, A418, F2321 |
| Primary Variable | Heat input, dilution rate | Heat input, dilution rate | Charge mass, flyer velocity, spacing |
| Dilution Concern | High (5-25% typical) | High (10-30% typical) | Minimal (< 5% interdiffusion) |
| Key Performance Test | Hardness, chemical analysis, microstructure | Hardness, chemical analysis, microstructure | Peel/shear strength, bond examination |
| Typical Overlay Thickness | 1-6 mm | 3-25 mm | 3-25 mm (clad thickness) |
| Requalification Trigger | Material change, process change, parameter change | Material change, process change, parameter change | Material change, charge design change, process change |
8. Qualification Management System
8.1 PQR Database and Coverage Mapping
A robust qualification management system requires maintaining a comprehensive PQR database that maps each qualified procedure to its coverage limits. This includes:
- Base material grade, heat number, and thickness range
- Filler metal classification, form, and diameter
- Process type and essential variable ranges
- Overlay thickness range
- Welding position qualification
- Performance test results and acceptance verification
- Expiry date (if applicable per customer or regulatory requirement)
8.2 Requalification Triggers
Per the entry's explicit note ("母材或焊材变更重评"), any change in parent material or filler material triggers requalification. Additional triggers include:
- Change in welding process (e.g., GTAW to GMAW)
- Change in essential variable outside qualified limits
- Change in overlay thickness outside qualified range
- Change in welding position outside qualified coverage
- Change in base material thickness outside qualified range
- Customer or regulatory requirement for requalification
8.3 WPS Development from PQR
The PQR is the input to WPS (Welding Procedure Specification) development. The WPS translates the qualified PQR into a production-ready document that specifies:
- Essential variables within qualified ranges (with production setpoints)
- Non-essential variables (equipment, technique, sequence)
- Preheat and interpass temperature requirements
- Post-weld heat treatment (if applicable)
- NT requirements and acceptance criteria
- Welder qualification requirements
The WPS must be traceable to the PQR, with clear documentation of which PQR supports which WPS, and which production components are covered by which WPS.
9. Conclusion
Weld overlay process qualification (PQR/WPQR) is not merely a regulatory formality—it is the technical foundation that transforms overlay welding from a variable artisanal process into a controlled, repeatable, and legally compliant manufacturing operation. For Cladding Technology Shanxi Co., Ltd., maintaining a comprehensive, well-organized PQR portfolio across TIG overlay, MIG overlay, and explosive bonding routes is essential to:
- Legal compliance: Meeting ASME IX, ISO 15614-7, and NB/T 47014 requirements for pressure vessel and component construction
- Process control: Demonstrating that overlay processes achieve consistent dilution, hardness, composition, and mechanical properties
- Customer confidence: Providing documented proof of qualified capability to support competitive bids and project acceptance
- Risk management: Preventing non-conformances, rework, and field failures through systematic qualification
- Business agility: Enabling rapid project execution because processes are pre-qualified and WPS documents are readily available
The company's commitment to rigorous PQR/WPQR management—encompassing strict adherence to essential variable control, comprehensive performance testing, clear requalification triggers, and systematic qualification database management—positions it as a technically credible and compliant supplier in the bimetallic cladding and weld overlay market.