Materials for Thermal Spray, Spray Welding, and Weld Overlay Coatings
1. Definition and Fundamental Principles
Thermal spray, spray welding, and weld overlay materials constitute the foundational material science knowledge base that governs all cladding and overlay operations. These three material application methodologies differ in process mechanism, microstructure formation, and bonding integrity, yet they share a common objective: to deposit a metallurgically or mechanically bonded corrosion-resistant, wear-resistant, or functionally graded layer onto a substrate to extend service life or enhance performance.
Thermal Spray Coatings involve the projection of molten or semi-molten particles onto a prepared substrate surface at high velocity, forming a coating through mechanical interlocking and partial bonding. Common variants include Arc Spray (AS), Flame Spray (FS), High-Velocity Oxy-Fuel (HVOF), and Cold Spray (CS).
Spray Welding refers specifically to thermal spray processes (primarily Arc Spray) where the deposited material achieves a true metallurgical bond with the substrate, producing a coating that is part of the base metal rather than merely adhered. This distinction is critical for pressure-containing components and structural applications.
Weld Overlay Materials (also termed surfacing or cladding weld deposits) are filler metals applied through arc welding processes (TIG/GTAW, MIG/GMAW, or SAW) to build up a corrosion-resistant or wear-resistant surface layer with full metallurgical fusion to the base material.
2. Category and Business Positioning
Within the cladding industry ecosystem, material selection knowledge sits at the intersection of metallurgy, process engineering, and quality assurance. For Cladding Technology Shanxi Co., Ltd., this knowledge domain serves as the intellectual backbone that enables:
- Material specification and selection for customer inquiries across oil and gas, power generation, chemical processing, and mining sectors
- WPS/PQR development for qualified weld overlay procedures under ASME, API, and GB standards
- Substitution and optimization of overlay materials to reduce cost while maintaining performance
- Technical support for NDT acceptance and post-weld heat treatment decisions
3. Technical Purpose and Value
Mastery of spray, spray welding, and weld overlay materials delivers measurable value in three dimensions:
3.1 Engineering Performance
Correct material selection ensures the deposited layer achieves the required hardness (HV), corrosion resistance (per NACE MR0175/ISO 15156), thermal stability, and fatigue life. For example, selecting a duplex stainless steel overlay (UNS S31803) over a 316L overlay in chloride-containing environments can extend service life by 5–10 times.
3.2 Economic Efficiency
Understanding material properties enables optimization of deposit thickness, reducing expensive alloy consumption while meeting design life requirements. A systematic materials knowledge base reduces trial-and-error, minimizes rework, and accelerates project timelines.
3.3 Compliance and Risk Mitigation
Thorough material knowledge ensures compliance with governing codes including ASME B31.3, API 570, NB/T 20011, and GB/T 8169, thereby reducing regulatory risk and insurance exposure.
4. Key Material Categories and Selection Criteria
4.1 Weld Overlay Filler Metals (for TIG/MIG Overlay)
| Material Class | UNS/Grade | Typical Application | Key Properties | Governing Standard |
|---|---|---|---|---|
| Transition Layer | UNS E309L / ER309L | Carbon steel to SS cladding transition | High Cr-Ni, strain compatibility | ASME SFA-5.4 / GB/T 8110 |
| Austenitic SS | UNS E316L / ER316L | General corrosion resistance | 12–13% Mo, low C, pitting resistance | ASME SFA-5.4 / AWS A5.9 |
| Duplex SS | UNS E2209 / ER2209 | High-strength corrosion service | 22% Cr, 5% Mo, 3% N | ASME SFA-5.4 / ISO 12535 |
| High-Nickel Alloy | UNS E617 / ERNiCrMo-16 | Severe reducing acid environments | 60% Ni, 16% Cr, 6% Mo | ASME SFA-5.14 / ASTM B335 |
| Cast Iron Overlay | UNS E509 / ER509 | Repair of cracked castings | High ductility, low hardness | ASME SFA-5.18 |
| Hardfacing | UNS E517 / ER517 | Wear-resistant surfaces | High Cr carbide, HV 800–1000 | ASME SFA-5.18 / GB/T 12469 |
4.2 Thermal Spray Materials (for Arc Spray / HVOF / Cold Spray)
| Material Class | Typical Form | Process | Bond Strength (MPa) | Application |
|---|---|---|---|---|
| Stainless Steel (309/316) | Wire (1.2–1.6 mm) | Arc Spray | 40–60 (metallurgical) | Corrosion protection, repair |
| High-Nickel Alloy (Alloy 6/7/8) | Wire (1.0–1.6 mm) | Arc Spray | 35–55 (metallurgical) | Chemical plant repair |
| Hardfacing (CrC/B4C) | Wire (1.2–1.6 mm) | HVOF | 70–100 | Valve seats, pump impellers |
| Titanium / Ti-6Al-4V | Powder (30–45 μm) | Plasma Spray / HVOF | 15–30 | Bio-implants, aerospace |
| Copper / Cu-Ni | Wire (1.0–1.2 mm) | Arc Spray | 30–50 (metallurgical) | Electrical contact, heat exchangers |
4.3 Selection Decision Matrix
- Corrosion Environment Severity: Mild → 309L/316L overlay; Moderate → 316L/duplex; Severe → Alloy 6/8/C-276
- Mechanical Load Requirement: Static → austenitic SS; Dynamic/cyclic → duplex or high-Ni; Impact → duplex with controlled thickness
- Temperature Range: Ambient → 316L; Up to 400°C → Alloy 6; Above 400°C → Alloy 800/617
- Base Material Compatibility: CS → 309L transition; SS → matching grade; Ni-alloy → Ni-base overlay
- Regulatory Requirement: Pressure vessel → ASME qualified filler; Pipeline → API 5L/NACE compliant
5. Applicable Standards and Acceptance Criteria
5.1 Material Specification Standards
- ASME SFA-5.4 — Filler metal specifications for stainless steel welding (including overlay)
- ASME SFA-5.14 — Nickel and nickel alloy filler metals
- ASME SFA-5.18 — Filler metal specifications for cast iron and hardfacing
- AWS A5.9 / AWS A5.22 — Carbon/mild steel and stainless steel wire electrodes
- GB/T 8110 — Carbon steel and low-alloy steel solid wire electrodes for arc welding
- GB/T 12469 — Filler metals for surfacing and repair welding
- ASTM B335 / B564 / B565 — Nickel alloy wire for thermal spray
- ASTM B1027 — Standard specification for cast iron filler metal for hardfacing
5.2 Process and Performance Standards
- ASME Section IX — Qualification of welding procedures (WPS/PQR) for overlay
- API 570 — Piping inspection (overlay repair acceptance)
- ASME B31.3 — Process piping overlay requirements
- NB/T 20011 — Nuclear piping weld overlay procedure qualification
- ISO 14555 — Thermal spray — General recommendations for thermal spray coatings
- ISO 14801 — Thermal spray — Requirements for arc spray coatings on steel
- ASTM C235 / C264 — Thermal spray coating thickness and bond strength testing
- GB/T 11366 — Thermal spray — Coating requirements
5.3 Acceptance Criteria Summary
| Test Parameter | Weld Overlay (TIG/MIG) | Arc Spray (Metallurgical) | HVOF Spray | Reference Standard |
|---|---|---|---|---|
| Bond Strength | Full fusion (metallurgical) | ≥ 40 MPa (ASTM C633) | ≥ 70 MPa (ASTM C633) | ASTM C633 / ISO 4624 |
| Porosity | ≤ 1% per ASME IX | ≤ 5% (open), ≤ 10% (total) | ≤ 3% (open) | ASME IX / ISO 14555 |
| Crack Acceptance | Zero longitudinal cracks | Per AWS C10.14 | Per ASTM C264 | AWS C10.14 / ASTM C264 |
| Hardness | Per material spec ±10% | Per material spec ±15% | Per material spec ±10% | ASTM A262 / ISO 6508 |
| NDT Method | PT + MT + UT (thickness) | PT + MT + UT | PT + UT (bond) | ASME V / ISO 17638 |
6. Common Risks and Controls
6.1 Material-Related Risks
- Sensitization (Sigma phase formation): Duplex and austenitic overlays exposed to 450–850°C risk intermetallic precipitation. Control: limit interpass temperature to ≤ 150°C; select low-C or stabilized grades (321, 347).
- Cracking in overlay welds: High dilution from carbon steel base can cause cracking in austenitic overlay. Control: use 309L transition layer with ≥ 2 passes; limit single-pass dilution to ≤ 30%.
- Galvanic corrosion: Dissimilar material overlay in electrolyte environments. Control: ensure proper design of joints; apply compatible sealant at transition boundaries.
- Hydrogen-induced cracking (HIC): Nickel-based and high-Cr overlays on HIC-sensitive steels. Control: preheat to 100–200°C; post-weld bake at 200°C for 2 hours; use low-hydrogen filler.
6.2 Process-Related Risks
- Insufficient preheat: Cold base material causes thermal shock and cracking. Control: minimum preheat per ASME IX table (typically 100–250°C for overlay on CS).
- Excessive interpass temperature: Leads to grain coarsening and loss of mechanical properties. Control: monitor with IR thermometer; maintain ≤ 150°C for most SS overlays.
- Contamination (oxygen, moisture): Causes porosity and oxidation in overlay deposits. Control: use dry shielding gas (Ar or Ar/CO₂ mix); clean base material to Sa 2.5 per ISO 8501-1.
- Thermal spray oxide inclusions: Particles oxidize during flight, creating weak spots. Control: use HVOF with high velocity (>700 m/s) to minimize residence time; optimize powder feed rate.
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route
In the TIG/MIG overlay route, material selection directly determines WPS qualification parameters. Key considerations include:
- Filler wire diameter and form: 1.0–2.4 mm solid wire for TIG; 0.9–1.2 mm solid or flux-cored for MIG. Selection depends on deposit thickness requirement and travel speed.
- Shielding gas composition: Pure Ar for most SS/Ni overlays; Ar + 5% O₂ for hardfacing; Ar + 2% H₂ for high-deposition-rate MIG.
- Heat input management: Typical range 0.5–2.5 kJ/mm for overlay; lower values for thin sections, higher for thick multi-pass builds.
- Layer build strategy: Transition layer (309L) → Intermediate layer (316L) → Final layer (specified grade). Each layer's material and thickness must be documented in the WPS per ASME IX QW-451.
7.2 Hydraulic Explosive Bonding Route
In hydraulic explosive bonding (hydraulic explosion cladding), material selection focuses on the laminate pair compatibility:
- Explosive layer (fly plate): Typically 304/316 SS, Alloy 6, or Alloy 800 — must match the corrosion resistance requirement.
- Base plate: Carbon steel (Q235, A36) or low-alloy steel (16Mn, A516 Gr.70).
- Velocity matching: Impact velocity must exceed the critical bonding velocity (typically 200–300 m/s for SS-CS pairs). Material density and thickness determine the achievable velocity.
- Material thickness ratios: Fly plate thickness typically 15–25% of total laminate thickness. Material ductility must accommodate the plastic deformation during bonding.
7.3 Explosion Welding Route
For air-gap explosion welding, material selection follows similar principles to hydraulic explosive bonding but with additional considerations:
- Material pair database: Over 200 qualified pairs exist (e.g., 304/CS, 316/CS, Alloy 6/CS, Ti/CS, Cu/CS, Ni/CS). Each pair requires qualification testing per ASTM A377 or ISO 16761.
- Explosive charge design: Material density, thickness, and stand-off distance determine charge geometry and mass. Material properties directly feed into the detonation simulation.
- Post-bond characterization: Shear strength (≥ 150 MPa for CS-SS per ASTM A377), hardness profile across interface, and microstructural examination confirm bond quality.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
A comprehensive materials knowledge base enables the company to:
- Develop and qualify WPS/PQR packages for diverse material combinations without external consultation
- Accelerate ASME IX qualification cycles by pre-selecting appropriate filler metals and parameters
- Maintain current qualification records across multiple standards (ASME, API, NB, GB) simultaneously
- Respond to customer inquiries with technically validated material recommendations within 24–48 hours
8.2 Product Delivery Excellence
Material expertise translates directly into:
- First-time-right manufacturing: Correct material selection reduces rework rate by 40–60%, directly improving schedule adherence
- Cost optimization: Substitution of premium alloys with equivalent-performance alternatives where justified by service conditions
- Design life assurance: Material selection aligned with actual operating conditions ensures cladding delivers its designed service life
8.3 Customer Value Creation
- Technical consultation: Providing material selection guidance during the design phase positions the company as a value-added partner rather than a pure contractor
- Risk mitigation: Identifying potential failure modes (HIC, SCC, sensitization) and specifying preventive material strategies reduces customer downtime risk
- Regulatory support: Ensuring all material certifications (mill test reports, composition analysis, mechanical testing) meet code requirements for customer inspection authority approval
- Lifecycle cost reduction: Optimized material selection extends equipment service intervals, reducing total cost of ownership for the customer
9. Continuous Improvement and Knowledge Management
The materials knowledge base must be treated as a living document subject to continuous improvement:
- Post-project feedback loops: Each completed project's material performance data (field failures, premature wear, corrosion rates) must feed back into the materials database.
- Supplier qualification updates: Regular review of filler metal and spray material suppliers for composition consistency, traceability, and certification currency.
- Standards monitoring: Tracking revisions to ASME SFA, AWS A5, GB/T, and ISO standards to ensure material specifications remain current.
- Training integration: Materials knowledge must be embedded in operator training programs to ensure correct material handling, storage (especially low-hydrogen electrodes requiring 150°C ovens), and identification on the shop floor.
- Failure analysis integration: Every field failure investigation should include metallurgical examination of the overlay material to identify material-related root causes.
10. Conclusion
Mastery of spray coating, spray welding, and weld overlay materials is not merely an academic exercise — it is the operational foundation upon which all cladding technology capabilities rest. For Cladding Technology Shanxi Co., Ltd., this knowledge domain directly enables WPS qualification, manufacturing excellence, regulatory compliance, and customer trust. By maintaining a rigorous, continuously updated materials knowledge base aligned with ASME, API, AWS, ISO, GB, and NB standards, the company ensures that every overlay deposit, thermal spray coating, and explosive bond delivers its intended performance in the most demanding industrial environments.