Composite Weld Overlay and Thermal Spray Melting for Wear-Resistant Component Reinforcement
1. Definition and Fundamental Principles
The composite process of weld overlay (surfacing) and thermal spray melting represents a hybrid surface engineering strategy in which a molten metal or alloy is deposited onto a base substrate through two complementary mechanisms: arc-driven weld overlay and high-velocity thermal spray melting. In this approach, the weld overlay stage establishes a metallurgically bonded transition and substrate reinforcement layer, while the subsequent thermal spray melting stage deposits a refined, high-performance wear-resistant surface layer with superior microstructural homogeneity and reduced dilution.
The fundamental principle relies on exploiting the synergistic advantages of both processes. Weld overlay (TIG or MIG) provides deep thermal input, excellent metallurgical bonding, and the ability to build substantial overlay thicknesses with controlled dilution. Thermal spray melting—particularly flame-sprayed or plasma-sprayed molten wire—delivers a rapidly solidified microstructure with fine grain morphology, high hardness, and reduced carbide coarsening compared to conventional arc weld overlay alone. The combination addresses a persistent engineering challenge: achieving both sufficient overlay thickness for structural durability and a refined surface microstructure for maximum wear resistance.
2. Category and Business Positioning
This composite process falls under the broader category of surface hardening and wear-resistant cladding technologies, positioned within the company's TIG/MIG weld overlay technology route. It serves as an advanced variant of conventional weld overlay, targeting applications where single-process overlay solutions are insufficient—specifically, components subject to severe abrasive, erosive, or adhesive wear where both bond strength and surface integrity are critical.
Within Cladding Technology Shanxi Co., Ltd's capability portfolio, this process occupies a specialized niche between standard weld overlay and advanced explosion welding. It is particularly suited for restoration and upgrade of wear parts in mining, cement, power generation, and heavy machinery sectors, where cost-effective refurbishment of existing components is preferred over full replacement.
3. Technical Purpose and Value
The composite process addresses several critical engineering objectives:
- Enhanced wear resistance: The rapidly solidified microstructure from the thermal spray melting stage produces fine, uniformly distributed carbides (Cr7C3, Cr3C, or WC) that resist abrasive and erosive degradation far more effectively than coarser carbide distributions typical of arc-only overlay.
- Reduced dilution of the final wear layer: By separating the transition layer (weld overlay) from the wear layer (thermal spray), dilution is confined to the transition zone, preserving the alloy chemistry and properties of the surface wear layer.
- Improved spall resistance: The weld overlay transition layer provides a ductile buffer between the hard thermal spray surface and the base material, mitigating thermal stress cracking and spallation under cyclic loading.
- Cost-effective component restoration: The process enables recovery of worn components—such as crusher hammers, ball mill liners, slurry pump impellers, and boiler furnace components—extending service life by 2–5× compared to uncoated replacements.
4. Key Process and Implementation Points
4.1 Process Sequence
- Base material preparation: Machining or grinding of the wear surface to remove oxide, rust, and non-metallic inclusions. Surface roughness Ra ≤ 12.5 μm is recommended for optimal bonding. Preheating per WPS requirements (typically 100–250°C depending on base material).
- Transition layer weld overlay: Application of 1–3 passes of transition alloy (e.g., 309L, 310, or low-alloy nickel-based) using TIG or MIG arc welding. This layer establishes metallurgical bonding and acts as a dilution buffer. Target thickness: 2–4 mm per pass, total 4–12 mm.
- Wear layer thermal spray melting: Deposition of the final wear-resistant alloy (e.g., Cr-C, Cr-Ni-C, Ni-Cr-Mo, or tungsten carbide composite) using flame-sprayed molten wire or plasma-sprayed wire. Target thickness: 0.5–3.0 mm per layer, total 1.0–6.0 mm.
- Post-deposition heat treatment: Stress-relief annealing at 550–650°C for 1–4 hours (for martensitic Cr-C systems) to relieve residual stresses and optimize carbide distribution. For austenitic or nickel-based systems, solution treatment may be specified.
- Machining and finishing: Grinding or machining of the final surface to dimensional tolerance (typically ±0.1 mm) and surface finish requirements (Ra ≤ 3.2 μm for precision components).
4.2 Typical Process Parameters
| Parameter | Weld Overlay Stage (Transition) | Thermal Spray Melting Stage (Wear Layer) |
|---|---|---|
| Process Type | TIG (GTAW) or MIG (GMAW) | Flame-sprayed molten wire or plasma-sprayed wire |
| Typical Alloy | 309L, 310, Ni-based (Stellite 6) | Cr-C (e.g., 57Cr), Cr-Ni-C, Ni-Cr-Mo, WC-Co |
| Heat Input | 1.5–5.0 kJ/mm | High velocity (150–300 m/s particle velocity) |
| Layer Thickness | 2–4 mm per pass | 0.3–1.0 mm per layer |
| Total Build-up | 4–12 mm | 1.0–6.0 mm |
| Dilution Control | Managed via interpass temperature (≤350°C) | Minimized by single-layer or few-layer deposition |
| Post-Heat Treatment | 550–650°C / 1–4 h (martensitic systems) | Same as above, or air cool for retained austenite systems |
4.3 Critical Implementation Controls
- Interpass temperature management: Must not exceed 350°C between weld overlay passes to prevent excessive grain growth and dilution. Thermocouple monitoring is mandatory.
- Substrate preheat: Carbon steel substrates require 150–250°C preheat to reduce hydrogen-induced cracking risk. Austenitic stainless substrates require 50–100°C preheat to minimize thermal cracking.
- Wire feed and travel speed matching: For thermal spray melting, wire feed rate and torch travel speed must be precisely synchronized to achieve uniform layer thickness and minimize unmelted or partially melted particles.
- Atmosphere control: Inert gas shielding (Ar or Ar/He mix) is required during both stages to prevent oxidation and porosity. For plasma spray, process gas composition (Ar, H2, N2) directly affects particle temperature and velocity.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope of Applicability |
|---|---|
| ASTM A562 | Standard specification for high-chromium castings for corrosion and wear resistance (reference for Cr-C alloy composition) |
| ASTM A240 | Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip (for substrate qualification) |
| ASTM B476 | Standard specification for nickel-cobalt-chromium-columbium (molybdenum) castings for corrosion and wear resistance (Stellite-type alloys) |
| ASTM A388 | Standard specification for high-nickel alloy castings for corrosion resistance (Ni-based overlay qualification) |
| ASME Section IX | Qualification of welding procedures and welders (WPS/PQR for weld overlay stages) |
| ASME Section II, Part D | Welding consumables specifications (filler metal qualification) |
| ISO 14274 | Thermal spraying — General recommendations for flame-sprayed and plasma-sprayed coatings |
| ISO 2361 | Thermal spray — Terminology |
| ISO 2807 | Thermal spray — Determination of adhesion strength (pull-off test) |
| ISO 6425 | Thermal spray — Determination of coating thickness and density |
| NACE No. 211 | Recommended practice for application and performance of thermal-sprayed coatings for chemical process industries |
| GB/T 11366 | Surfacing — General technical conditions (Chinese national standard for weld overlay) |
| NB/T 47014 | Rules for qualification of welding procedures for pressure vessels (for pressure-containing components) |
5.2 Acceptance Criteria
- Adhesion strength: Pull-off test per ISO 2807 — minimum 25 MPa for thermal spray layer on weld overlay substrate; substrate failure preferred over coating failure.
- Porosity: Maximum 5% volume fraction per ASTM E140 (metallographic examination). No planar or linear porosity defects at the interface.
- Hardness: Vickers hardness (HV30) per ISO 6507 — target 600–900 HV for Cr-C martensitic systems; 450–650 HV for Ni-Cr-Mo austenitic systems. Measured at 0.25 mm depth below surface.
- Crack-free interface: 100% visual and dye penetrant examination (PT) per ASTM E709 of all surfaces and interfaces. No transverse or longitudinal cracks permitted.
- Dimensional tolerance: Final machined surface within ±0.1 mm of drawing dimensions; surface finish Ra ≤ 3.2 μm.
- Chemical composition: Spectrographic analysis per ASTM E415 — overlay composition within ±0.5% of specified alloy chemistry.
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Delamination at weld overlay / spray interface | Contamination between stages; insufficient bond strength of transition layer | Mandatory cleaning and light grinding between stages; verify transition layer hardness and dilution before spray stage |
| Excessive dilution of wear layer | Multiple spray passes; excessive torch dwell time | Single-pass or limited multi-pass deposition; real-time thickness monitoring; adjust wire feed and travel speed |
| Residual stress cracking | Thermal mismatch between hard overlay and ductile base; lack of post-heat treatment | Mandatory stress-relief anneal; gradual cool from 550–650°C; design ductile transition layer |
| High porosity in spray layer | Inadequate shielding; poor wire feed consistency; substrate porosity | Verify gas flow rates; calibrate wire feed mechanism; inspect substrate for casting defects |
| Hydrogen-induced cold cracking (carbon steel substrates) | Hydrogen entrapment from moisture or flux; high heat input | Dry electrodes/wire; preheat to 200–250°C; post-weld bake at 200°C for 2 h |
| Hot cracking in austenitic overlay | Sulfur/phosphorus segregation; excessive heat input | Use low-sulfur filler metals; reduce heat input; increase travel speed |
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
The composite weld overlay + thermal spray melting process is the flagship application of this technology entry. It extends the company's core TIG/MIG weld overlay capabilities by adding a high-performance surface finishing stage. Key applications include:
- Crusher hammers and jaws: Base component of manganese steel or high-carbon steel; transition layer of 309L (2–4 mm); wear layer of 57Cr or 55Cr (3–5 mm). Service life improvement of 3–5×.
- Ball mill liners and grinding rings: Low-alloy steel substrate; Ni-Cr-Mo transition (3–5 mm); Cr-C or WC-Co wear layer (2–4 mm). Applicable to cement, mining, and mineral processing.
- Slurry pump impellers and wear rings: Ductile iron or stainless substrate; austenitic transition; hardfacing wear layer. Resistant to abrasive slurry erosion.
- Boiler furnace components and burners: Alloy steel substrate; high-temperature alloy transition; erosion-resistant wear layer for burner tips and furnace walls.
- Excavator bucket teeth and dozer blades: High-strength steel substrate; transition layer; Cr-C or Ni-Cr-Mo wear layer for soil and rock abrasion resistance.
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hydraulic explosive bonding (hydroforming-based explosive cladding) is primarily used for large-area, high-integrity clad plate and pipe fabrication, the composite weld overlay + spray melting technology serves as a post-processing enhancement step for components produced by this route. For example, a hydraulic explosive bonded duplex stainless steel clad pipe can receive a localized thermal spray melting overlay at high-wear zones (flanges, elbows, or instrument connections) to provide additional erosion resistance without compromising the bulk cladding integrity. This hybrid approach leverages the company's capability in both bulk cladding and surface engineering.
7.3 Explosion Welding Route (Complementary Application)
Explosion welding produces monolithic clad plates with excellent metallurgical bonding and minimal dilution. The composite process complements explosion welding in scenarios where the explosion-welded component subsequently requires localized wear-resistant surface enhancement. For instance, an explosion-welded Ni-Cr-Mo/steel clad plate used in a mining slurry tank may receive additional thermal spray melting overlay at high-velocity flow zones where the base cladding thickness is insufficient for long-term erosion resistance. This ensures the explosion welding route benefits from surface engineering enhancements where needed.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This composite process technology strengthens the company's qualification portfolio in several ways:
- WPS/PQR development: Each composite process configuration requires a qualified welding procedure specification (WPS) per ASME Section IX for the weld overlay stage, supplemented by thermal spray procedure qualification per ISO 14274. Accumulating qualified procedures for multiple substrate/alloy combinations (e.g., carbon steel/309L/57Cr, duplex/310/Ni-Cr-Mo, austenitic/310/Stellite 6) builds a comprehensive procedure library.
- Welder certification: Operators must be qualified in both TIG/MIG weld overlay and thermal spray melting, creating a specialized workforce with dual competency. This differentiates the company from competitors offering only single-process services.
- NDT capability integration: The process requires proficiency in multiple NDT methods—PT, MT, UT, and radiographic testing—for both weld overlay and spray coating inspection, enhancing the company's overall quality assurance infrastructure.
8.2 Product Delivery
The composite process enables delivery of high-value wear-resistant components that command premium pricing in the market. Key delivery advantages include:
- Customization capability: The process allows tailoring of transition layer alloy, wear layer alloy, and thickness combinations to specific wear mechanisms (abrasive, erosive, adhesive, corrosive-abrasive), enabling solution-based product development.
- Restoration services: The technology supports on-site or shop-based restoration of worn components, providing a revenue stream beyond new component manufacturing. Customers benefit from reduced downtime and lower replacement costs.
- Scalability: The process is applicable to components ranging from small parts (impeller wear rings, < 50 kg) to large structural components (crusher hammers, > 500 kg), providing broad market coverage.
8.3 Customer Value
The composite weld overlay and thermal spray melting process delivers quantifiable customer value through: (1) 3–5× extension of component service life, (2) 40–60% reduction in total cost of ownership compared to frequent replacement with standard materials, (3) reduced unplanned downtime through predictable wear-life management, and (4) environmental benefit through resource conservation and reduced scrap generation. These metrics provide a compelling return-on-investment case for industrial customers in mining, cement, power, and heavy machinery sectors.
9. Conclusion
The composite weld overlay and thermal spray melting process represents a high-value, technically differentiated capability within Cladding Technology Shanxi Co., Ltd's portfolio. By integrating arc weld overlay with thermal spray melting, the company delivers wear-resistant components that combine the structural integrity of deep weld overlay with the microstructural refinement of rapid solidification. This technology strengthens the company's position in the surface engineering market, supports qualification building through ASME Section IX and ISO 14274 procedure development, and creates significant customer value through extended service life and reduced lifecycle costs. Continued investment in process optimization, WPS development, and operator training in this composite technology will be essential to maintaining competitive advantage in the wear-resistant cladding market.