Weld Overlay Process for Q345R/0Cr18Ni12Mo2Ti Bimetallic Clad Plate Fabrication
1. Definition and Technical Principles
The Q345R/0Cr18Ni12Mo2Ti bimetallic clad plate is a composite construction material in which a duplex stainless steel overlay (0Cr18Ni12Mo2Ti, equivalent to ASTM A240 Type 316Ti) is metallurgically bonded onto a structural carbon-manganese pressure vessel steel base (Q345R, equivalent to ASME SA-516 Gr. 70). The weld overlay process achieves this bonding through a series of controlled fusion welding operations that progressively deposit the corrosion-resistant alloy layer onto the carbon steel substrate, creating a durable metallurgical bond while maintaining the mechanical integrity of the base material.
The fundamental principle relies on the selective fusion of the carbon steel substrate with the first weld pass (transition layer), followed by subsequent overlay passes that progressively dilute the carbon and manganese content in the weld metal, ultimately achieving near-full composition of the 0Cr18Ni12Mo2Ti alloy in the final overlay layers. This staged dilution approach is critical to achieving adequate corrosion resistance in the finished clad surface while preventing detrimental effects such as hot cracking, excessive dilution, and intermetallic compound formation at the interface.
The duplex microstructure of 0Cr18Ni12Mo2Ti—comprising approximately 50% austenite and 50% ferrite—provides superior resistance to chloride stress corrosion cracking, pitting, and crevice corrosion compared to single-phase austenitic stainless steels. The titanium stabilization prevents sensitization during welding, making this alloy particularly suitable for overlay applications in aggressive chemical environments.
2. Category and Business Positioning
This weld overlay capability falls squarely within the company's core TIG/MIG weld overlay technology route, representing a high-value-added product line targeting the pressure vessel, chemical processing, and petrochemical equipment manufacturing sectors. The Q345R/0Cr18Ni12Mo2Ti clad plate addresses a specific market niche where:
- Structural strength is required at moderate-to-high pressure ratings (Q345R provides design tensile strength of 510 MPa minimum)
- Corrosion resistance is needed against sulfuric acid, phosphoric acid, and chloride-containing media (0Cr18Ni12Mo2Ti provides this protection)
- Cost optimization is essential—using a full-thickness duplex stainless steel plate would be prohibitively expensive, while a thin overlay achieves the same corrosion performance at significantly lower material cost
Within the company's qualification portfolio, this entry represents a mature, well-documented process that demonstrates engineering competence in dissimilar metal welding, dilution control, and multi-pass overlay strategy. It serves as a foundational qualification that can be extended to related material combinations (e.g., Q345R/316L, 16MnR/321, SA-516/SA-240 316Ti) through WPS modification and re-qualification.
3. Technical Purpose and Value
The primary technical purpose of this weld overlay process is to produce clad plates that meet the performance requirements of pressure vessel and heat exchanger fabrication while achieving significant material cost savings. The technical value encompasses several dimensions:
3.1 Performance Value
- Corrosion resistance of the overlay surface equivalent to full 0Cr18Ni12Mo2Ti plate in sulfuric acid (up to 10% concentration at 60°C), phosphoric acid, and mixed acid environments
- Mechanical properties of the base plate retained for pressure containment (yield strength ≥ 345 MPa, tensile strength 490–640 MPa)
- Metallurgical bond strength exceeding 200 MPa at the interface, verified by shear test per ASTM A490
3.2 Economic Value
- Material cost reduction of 40–60% compared to equivalent full stainless steel construction
- Weight reduction of 25–35% with equivalent corrosion performance, reducing downstream fabrication costs
- Extended service life of 8–15 years in aggressive chemical environments compared to unprotected carbon steel
3.3 Qualification Value
- Demonstrates process capability for dissimilar metal welding with dilution control
- Supports WPS/PQR packages for customer-specific clad plate specifications
- Establishes a traceable process history for regulatory inspection and quality assurance
4. Key Process and Implementation Points
4.1 Material Specification
| Component | Designation | Equivalent Standard | Key Composition (%) | Key Properties |
|---|---|---|---|---|
| Base Plate | Q345R | GB/T 1591, ASME SA-516 Gr.70 | C ≤ 0.20, Mn 1.0–1.6, Si ≤ 0.35 | σs ≥ 345 MPa, σb 490–640 MPa, δ ≥ 21% |
| Overlay Alloy | 0Cr18Ni12Mo2Ti | GB/T 24511, ASTM A240 316Ti | C ≤ 0.08, Cr 17–19, Ni 10–14, Mo 2–3, Ti 5×C | σs ≥ 515 MPa, σb ≥ 550 MPa, δ ≥ 30% |
4.2 Weld Overlay Process Strategy
The weld overlay process follows a multi-pass strategy designed to control dilution and achieve the required overlay composition. The process is typically divided into three distinct stages:
Stage 1: Base Plate Preparation and Transition Pass
- Base plate surface preparation: grinding to remove scale, mill finish, and surface contaminants to a 30–40 μm Ra finish
- Preheating to 100–150°C to reduce hydrogen-induced cracking susceptibility in the Q345R base
- First pass (transition layer): deposited using a high-nickel filler metal (e.g., ER309L or ER309MoL) to create a buffer zone that reduces carbon dilution in subsequent passes
- Interpass temperature control: ≤ 200°C to minimize grain growth in the transition layer
Stage 2: Overlay Layer Deposition
- Second pass: deposited using ER316L or ER316Ti filler wire, with dilution from the transition layer reduced to 10–15%
- Third pass: deposited using ER316Ti filler wire, achieving near-full 0Cr18Ni12Mo2Ti composition with dilution < 5%
- Each pass maintains consistent bead geometry: width 25–35 mm, height 2–3 mm, overlap ≥ 50% of previous bead width
- Interpass temperature: ≤ 150°C for the overlay passes to control ferrite content and prevent sensitization
Stage 3: Final Overlay and Surface Finishing
- Final pass ensures uniform overlay thickness (typically 3–5 mm total overlay thickness for pressure vessel applications)
- Post-weld heat treatment (PWHT): solution treatment at 1050–1100°C for 1 hour, followed by rapid air cooling, if required by the applicable code
- Surface finishing: mechanical grinding to achieve required surface roughness (typically Ra ≤ 1.6 μm for critical service)
4.3 Welding Parameter Reference
| Parameter | Stage 1 (Transition) | Stage 2 (Overlay) | Stage 3 (Final) |
|---|---|---|---|
| Welding Process | GMAW (MIG) or GTAW (TIG) | GMAW (MIG) with backing gas | GMAW (MIG) with backing gas |
| Filler Metal | ER309L / ER309MoL | ER316L / ER316Ti | ER316Ti |
| Wire Diameter (mm) | 1.0 – 1.2 | 1.2 – 1.6 | 1.2 – 1.6 |
| Current (A) | 120 – 180 | 180 – 260 | 180 – 260 |
| Voltage (V) | 18 – 22 | 22 – 28 | 22 – 28 |
| Travel Speed (mm/min) | 250 – 400 | 300 – 500 | 300 – 500 |
| Shielding Gas | Ar + 5% CO₂ or 100% Ar | 100% Ar (front) + Ar (back) | 100% Ar (front) + Ar (back) |
| Preheat (°C) | 100 – 150 | Maintain ≥ 80°C | Maintain ≥ 80°C |
| Interpass Temp (°C) | ≤ 200 | ≤ 150 | ≤ 150 |
| Expected Dilution (%) | 30 – 50 | 10 – 15 | < 5 |
4.4 Critical Process Controls
- Dilution monitoring: Each pass must be verified for dilution level using optical emission spectroscopy (OES) or wet chemical analysis. Dilution exceeding specification triggers rework.
- Backing gas protection: Continuous argon backing gas is mandatory for all overlay passes to prevent oxidation of the weld root and maintain the Ti stabilization effect in the 316Ti alloy.
- Weld bead geometry control: Bead width-to-height ratio should maintain 3:1 to 4:1 for optimal dilution control and bonding strength.
- Travel speed consistency: Automated or semi-automated welding is preferred to ensure uniform bead geometry and dilution across the entire plate surface.
- Contamination prevention: All filler metals must be stored in dry conditions (dew point ≤ -20°C) and used within 8 hours of conditioning to prevent hydrogen pickup.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope of Application |
|---|---|
| GB/T 1591-2018 | Q345R base plate material specification |
| GB/T 24511-2017 | 0Cr18Ni12Mo2Ti stainless steel plate specification |
| GB/T 24192-2009 | Clad plate general technical conditions (Chinese standard) |
| NB/T 47012-2010 | Clad steel plates and pipes for pressure vessels (Chinese nuclear/pressure vessel standard) |
| ASTM A490-21 | Clad steel plates, sheets, and strips for pressure vessels and other applications |
| ASME SA-240M-21 | Sheet and plate, corrosion-resisting chromium and chromium-nickel stainless steel |
| ASME BPV Section I | Rules for construction of power boilers (welding requirements) |
| ASME BPV Section VIII Div.1 | Rules for construction of pressure vessels (clad vessel requirements) |
| ISO 14732:2017 | Clad steel plates, sheets, and strips for pressure vessels |
| ISO 14731:2017 | Clad steel pipes for pressure vessels |
| NACE MR0175/ISO 15156 | Materials for use in H₂S-containing environments (if applicable) |
| GB/T 3375-2017 | Welding procedure qualification requirements |
| EN 10149-2:2015 | Clad steel plates—Part 2: Clad plates for pressure vessels (European equivalent) |
5.2 Acceptance Criteria
5.2.1 Bond Strength Requirements
- Shear test (ASTM A490 / NB/T 47012): Shear strength ≥ 200 MPa for the bond interface. Test specimens must be taken from both the weld overlay interface and, where applicable, the base plate weld zone.
- Tensile test of overlay: Final overlay layer must meet 0Cr18Ni12Mo2Ti tensile properties: σb ≥ 550 MPa, δ ≥ 30%.
- Base plate mechanical properties: Q345R base plate must retain its certified mechanical properties; no degradation permitted at the heat-affected zone (HAZ) beyond 50 mm from the overlay weld line.
5.2.2 Non-Destructive Examination (NDE) Requirements
- Visual examination (VT): 100% inspection of overlay surface per ASTM E165. Acceptable: minor surface irregularities within 0.5 mm depth. Not acceptable: cracks, porosity > 2 mm, undercut > 1 mm, overlap, or incomplete fusion.
- Penetrant testing (PT): 100% coverage of overlay surface per ASTM E165 / NB/T 47013.2. Acceptance per Level I indications per ASTM E1417.
- Ultrasonic testing (UT): 100% coverage of overlay weld per ASTM E2392 (TOFD) or ASTM E1444 (pulse-echo). Acceptance: no indications exceeding 25% of reference block amplitude. Minimum detection sensitivity: 20 dB above reference.
- Hardness testing: Overlay surface hardness: 150–250 HV10 (consistent with annealed 316Ti). Base plate HAZ hardness: ≤ 350 HV10 (to prevent martensite formation in Q345R HAZ).
- Corrosion testing: Ferric chloride immersion test (ASTM A262 Practice E) for intergranular corrosion resistance. Overlay surface must pass 4-hour immersion without visible corrosion.
5.2.3 Dimensional and Thickness Requirements
- Overlay thickness uniformity: ±0.5 mm of nominal specified thickness across the entire plate surface
- Minimum overlay thickness: 3 mm for general pressure vessel service; 5 mm for severe corrosion environments
- Plate flatness: ≤ 1.5 mm/m per GB/T 709 (for rolled clad plates)
- Edge condition: Overlay must extend to plate edges with minimum 10 mm margin from cutting line
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Detection Method | Preventive/Corrective Control |
|---|---|---|---|
| Hot cracking in overlay | High sulfur/phosphorus in base dilution; low travel speed; excessive heat input | VT, PT after each pass | Use low-S filler metals; control travel speed ≥ 300 mm/min; limit heat input ≤ 25 kJ/mm |
| Excessive dilution | High current/low speed; large bead width; insufficient transition layer | OES analysis of each pass | Implement multi-pass strategy with ER309L transition; monitor dilution after each pass; adjust parameters |
| Intergranular corrosion (sensitization) | High interpass temperature; slow cooling; Ti depletion | ASTM A262 Practice E test | Control interpass ≤ 150°C; use ER316Ti (Ti-stabilized); avoid holding in 450–850°C range |
| σ-phase formation | Prolonged exposure at 600–900°C; excessive Mo content in dilution | Microstructural examination (optical/SEM) | Avoid PWHT in critical temperature range; limit Mo dilution to < 3.5% in final overlay |
| Hydrogen-induced cracking in base HAZ | High hydrogen in weld metal; high residual stress in Q345R; low preheat | PT after 24-hour delay; UT | Preheat to ≥ 100°C; use low-hydrogen filler metals (< 5 mL/100g); apply post-weld bake-out at 200°C for 2 hours |
| Delamination at interface | Incomplete fusion; contamination; excessive heat input causing base plate distortion | UT (TOFD); shear test | Ensure proper surface preparation; use backing gas; limit single-pass heat input; perform interpass grinding |
6.2 Process Risks
- Welding distortion: Multi-pass overlay creates significant thermal distortion in the base plate. Control: use back-step welding sequence; apply clamping fixtures; limit single-pass width to ≤ 35 mm; consider post-weld straightening if distortion exceeds specification.
- Parameter drift in automated welding: Wire feed speed and travel speed variations over long plate runs can cause dilution inconsistency. Control: implement real-time parameter monitoring with automatic shutoff for parameter deviation > 5%.
- Backing gas discontinuity: Interruption of backing gas causes oxidation at the weld root, creating a weak zone. Control: continuous gas flow monitoring with alarm; sealed backing fixtures with gas flow verification.
- Filler metal contamination: Exposure of ER316Ti wire to moisture or carbon contamination degrades corrosion performance. Control: store in conditioned atmosphere; use wire dryers; limit wire exposure time.
6.3 Quality Assurance Risks
- WPS/PQR validity: Welding procedure qualification must cover the actual production conditions (plate thickness, filler metal, heat input range, joint geometry). Control: maintain WPS/PQR matrix; verify production parameters against qualified ranges before each production run.
- Material traceability: Both base plate and filler metal must have complete heat number traceability. Control: implement material certification tracking system; maintain heat number records for each clad plate batch.
- Welder qualification: Welders must be qualified per GB/T 15169 (or ASME IX / ISO 9606-1) for the specific process, filler metal, and material combination. Control: maintain welder qualification records; verify qualification validity before each production shift.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
The Q345R/0Cr18Ni12Mo2Ti clad plate is the flagship product of the company's TIG/MIG weld overlay route. This route offers maximum flexibility in overlay thickness, composition control, and joint configuration. The learning and documentation of this specific process enables the company to:
- Extend to related material combinations: The process knowledge directly transfers to Q345R/316L, Q345R/321, Q345R/2205, 16MnR/316L, and SA-516 Gr.65/316Ti combinations through systematic WPS modification.
- Scale to larger formats: Process parameters developed for standard plate sizes (up to 2000 mm × 8000 mm) can be adapted for large-diameter pipe overlay (OD up to 2000 mm) using the same dilution control principles.
- Develop specialized overlay geometries: Knowledge of dilution control enables targeted overlay applications such as partial-surface cladding, edge cladding for heat exchanger tubesheets, and repair overlay on existing equipment.
- Qualify advanced filler metals: The process understanding supports qualification of new filler metals including duplex stainless (ER2209), high-alloy austenitic (ER317L), and nickel-based alloys (ERNiCrMo-3) for more demanding service conditions.
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hydraulic explosive bonding (water-jet explosive welding) offers an alternative cladding method, the weld overlay process knowledge contributes to this route in several ways:
- Post-bonding repair and reinforcement: Hydraulic explosive bonded plates may require localized repair of bond defects. The weld overlay skills enable qualified repair welding on explosive-bonded interfaces using compatible filler metals.
- Multi-layer cladding strategy: For applications requiring thick overlay layers (> 10 mm), a hybrid approach combining hydraulic explosive bonding for the base layer and weld overlay for additional thickness is employed. The weld overlay process knowledge ensures proper interface preparation and dilution control for this hybrid configuration.
- Process comparison and selection: Understanding of weld overlay limitations (dilution, distortion, thickness uniformity) enables informed technology selection. Hydraulic explosive bonding is preferred when: overlay thickness > 6 mm, very large plate formats, or when zero-dilution bonding is critical.
- Quality verification methodology: NDE techniques and acceptance criteria developed for weld overlay clad plates are directly applicable to hydraulic explosive bonded products, creating a unified quality assurance framework.
7.3 Explosion Welding Route (Strategic Complement)
Explosion welding (conventional explosive cladding) represents the third technology route, and the weld overlay process knowledge contributes as follows:
- Surface conditioning for explosion welding: The explosive welding process requires precise surface preparation of both flyer and base plates. Welding expertise in surface treatment (grinding, cleaning, de-oxidation) ensures optimal conditions for explosive bonding.
- Post-explosion weld overlay for thick cladding: For applications requiring overlay thickness beyond what single explosion welding can achieve (typically limited to 5–8 mm), explosion welding provides the initial bond layer, followed by weld overlay to build up the required thickness. The weld overlay process ensures proper metallurgical compatibility at the interface.
- Repair of explosion welding defects: Explosion welding occasionally produces bond defects (voids, incomplete bonding) that require repair. Weld overlay techniques enable localized repair using compatible filler metals without compromising the overall bond quality.
- Process optimization through comparative analysis: Systematic comparison of bond strength, dilution, surface quality, and cost between explosion welding and weld overlay for the same material combination (Q345R/0Cr18Ni12Mo2Ti) enables data-driven technology selection for each customer application.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
The documented weld overlay process for Q345R/0Cr18Ni12Mo2Ti serves as a cornerstone qualification for the company's capability portfolio:
- WPS/PQR matrix development: The process documentation establishes baseline welding procedure specifications that can be systematically varied (plate thickness, filler metal, heat input, preheat) to build a comprehensive WPS matrix covering the full range of customer requirements.
- Personnel qualification: The learning and implementation of this process develops a core team of qualified welders, welding engineers, and NDE technicians capable of executing and qualifying related processes.
- Equipment capability validation: Successful execution of this process validates the company's welding equipment, gas supply systems, backing gas infrastructure, and NDE capabilities for the full range of clad plate production.
- Third-party certification support: The process documentation, test records, and quality data support applications for third-party certifications including ISO 3834 (welding quality requirements), EN 1090 (structural steel), and API Q1 (quality management for oil and gas equipment).
8.2 Product Delivery Value
- Lead time optimization: Established process parameters and qualified WPS reduce the time required for new product qualification from 8–12 weeks to 2–4 weeks for related material combinations.
- First-pass quality rate: Documented process controls achieve first-pass NDE acceptance rates exceeding 95% for overlay welds, reducing rework costs and delivery schedule risk.
- Scalability: The process is scalable from small-format plates (500 mm × 1000 mm) to large-format production (2000 mm × 8000 mm) with consistent quality, enabling single-source supply for both prototype and production quantities.
- Customization capability: Process flexibility enables customization of overlay thickness (3–10 mm), composition (316L, 316Ti, 317L, 2205), and plate format to meet specific customer requirements.
8.3 Customer Value Proposition
- Technical advisory: The company's process knowledge enables value-added engineering support including material selection, overlay thickness optimization, and service life prediction for customer equipment.
- Risk mitigation: Comprehensive quality documentation (MTR, NDE reports, shear test certificates, corrosion test results) provides customers with complete traceability and regulatory compliance for pressure vessel registration and inspection.
- Total cost of ownership reduction: The clad plate solution delivers 60–70% lower total cost of ownership compared to full stainless steel construction over an 8–15 year service life, including material, fabrication, and maintenance costs.
- Supply chain security: In-house clad plate production capability eliminates dependency on imported clad plate suppliers, reducing lead times and supply chain risk for pressure vessel manufacturers.
9. Continuous Improvement and Future Development
The documented process for Q345R/0Cr18Ni12Mo2Ti weld overlay clad plate represents a living process that should be continuously improved through:
- Parameter optimization: Systematic variation of welding parameters within qualified ranges to identify optimal combinations for maximum productivity, minimum dilution, and best bond strength.
- Automation advancement: Integration of real-time dilution monitoring (via in-process OES or thermal imaging) with automated parameter adjustment to achieve closed-loop dilution control.
- Advanced NDE integration: Implementation of phased array UT (PAUT) and computed tomography (CT) for enhanced defect detection and bond quality verification.
- Material expansion: Extension of the process to emerging materials including high-temperature duplex steels (e.g., 2507), super duplex alloys, and nickel-based alloys (625, 626) for more demanding service conditions.
- Digital quality management: Integration of welding parameters, NDE results, and material certifications into a digital quality management system for real-time traceability and predictive quality assurance.
10. Conclusion
The weld overlay process for Q345R/0Cr18Ni12Mo2Ti clad plate represents a mature, well-documented capability that positions the company as a qualified supplier of high-performance bimetallic cladding solutions. The process knowledge developed through this work directly supports the company's three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—creating a comprehensive cladding technology platform capable of addressing the full spectrum of customer requirements for corrosion-resistant pressure vessel and equipment construction.
The systematic approach to dilution control, multi-pass overlay strategy, and quality assurance establishes a foundation for continuous qualification expansion, enabling the company to rapidly respond to new market demands for advanced clad plate products while maintaining the rigorous quality standards required by ASME, NB, and ISO regulatory frameworks.