CLADDING TECHNOLOGY SHANXI CO., LTDWear Resistance of Hard Alloy Overlay Layer After Heat Treatment
Literature Overview
This 2002 study published in China Surface Engineering by researchers from the Department of Materials Science and Engineering at Luoyang Institute of Technology (now Luoyang University of Technology) investigates the effect of post-weld heat treatment on the wear resistance of hard alloy overlay layers. The research addresses a fundamental question in cladding technology: how does heat treatment modify the as-welded microstructure of hard alloy overlays to optimize wear performance? Hard alloy overlays, typically based on cobalt-based (Stellite-type) or nickel-based systems, are widely used in high-temperature, high-wear applications such as gas turbine components, extrusion dies, and hot-working tools.
Core Technical Content
Hard Alloy Overlay Systems
Hard alloy overlay layers typically fall into several categories:
| Overlay System | Base Alloy | Hard Phase | Typical Hardness (HV) | Application |
|---|---|---|---|---|
| Co-Cr-C | Cobalt | Cr7C3 | 800–1000 | High-temperature wear |
| Co-W-C | Cobalt | WC, Co3W | 900–1100 | Abrasive wear |
| Ni-Cr-B-Si | Nickel | Ni3B, NiSi | 600–800 | Sliding wear |
| Fe-Cr-C | Iron | Cr7C3, Cr23C6 | 700–900 | General wear |
| Co-Cr-W-C | Cobalt | Cr7C3, Co3W | 950–1150 | Severe abrasive wear |
The study focused on cobalt-based and iron-based hard alloy overlays, examining how different heat treatment cycles affect the as-welded microstructure and resulting wear performance.
Effect of Heat Treatment on Microstructure
The as-welded microstructure of hard alloy overlays typically contains:
- Primary carbides: Coarse carbide particles formed during solidification
- Matrix: Cobalt or nickel-based solid solution
- Secondary phases: Eutectic carbides and intermetallics
- Residual stresses: Thermal stresses from welding
Post-weld heat treatment modifies this microstructure through:
- Solution treatment: Dissolves coarse primary carbides and homogenizes the matrix
- Aging/precipitation: Forms fine, dispersed secondary carbides that provide optimal strengthening
- Stress relief: Reduces residual welding stresses that can cause overlay spalling or cracking
| Heat Treatment Cycle | Temperature (°C) | Time (h) | Hardness (HV) | Wear Resistance Improvement |
|---|---|---|---|---|
| As-welded | — | — | 850–950 | Baseline |
| Solution + Quench | 1100–1150 | 2–4 | 700–800 | -10% (softened) |
| Solution + Aging | 1100 °C + 850 °C | 4h + 4h | 950–1050 | +25–35% |
| Stress Relief Only | 700–800 | 2–4 | 820–900 | +5–10% |
| Optimized Aging | 1100 °C + 800 °C | 4h + 8h | 1000–1100 | +30–40% |
The optimized heat treatment (solution treatment followed by double aging) produces the finest and most uniformly distributed carbide particles, maximizing wear resistance through the Hall-Petch strengthening mechanism and dispersion strengthening.
Wear Mechanism Analysis
The wear resistance improvement after heat treatment is attributed to several mechanisms:
- Carbide refinement: Fine carbide particles (50–200 nm) provide more effective resistance to abrasive material removal compared to coarse primary carbides (1–10 μm)
- Matrix strengthening: Precipitation hardening of the cobalt or nickel matrix increases the resistance of the matrix phase to deformation
- Stress relief: Reduced residual stresses prevent microcrack initiation and propagation
- Improved bonding: Better metallurgical bond between overlay and substrate reduces spalling risk
Process and Standards Analysis
Heat Treatment Process Control
The heat treatment of hard alloy overlay layers requires careful process control:
| Process Variable | Recommended Range | Critical Consideration |
|---|---|---|
| Heating rate | 100–200 °C/h | Prevent thermal shock and cracking |
| Solution temperature | 1050–1150 °C | Dissolve carbides without melting |
| Quenching medium | Air or oil | Avoid excessive thermal stress |
| Aging temperature | 750–900 °C | Precipitate fine carbides |
| Aging time | 4–12 h | Optimize particle size |
| Cooling rate | Furnace cool | Prevent residual stress |
Quality Inspection After Heat Treatment
Post-heat treatment inspection should include:
- Hardness verification: Confirm hardness improvement and uniformity across the overlay
- Microstructural examination: Verify carbide morphology and distribution
- Bond strength testing: Ensure heat treatment did not degrade the overlay-substrate bond
- Crack detection: MT or PT to detect any heat treatment-induced cracking
- Dimensional stability: Verify that heat treatment did not cause excessive distortion
Engineering Practice Integration
Heat treatment of hard alloy overlays is critical for maximizing service life in demanding applications:
- Gas turbine blades and vanes: Co-based overlays with optimized heat treatment achieve 2–3× the wear life of as-welded condition
- Extrusion dies: Ni-based overlays with solution-aging treatment extend die life by 30–40%
- Hot rolling mill rolls: Hard alloy overlays with stress relief and aging improve surface durability
- Valve trim components: Co-based overlays with optimized heat treatment resist cavitation erosion and sliding wear
The heat treatment cycle must be tailored to the specific overlay system and application requirements. For cobalt-based overlays, solution treatment at 1100–1150 °C followed by aging at 800–850 °C typically provides optimal results. For nickel-based overlays, slightly lower temperatures are used to avoid excessive grain growth.
Key Technical Reflections
The heat treatment of hard alloy overlays represents a critical post-processing step that can dramatically improve wear performance. The as-welded microstructure, while hard, often contains coarse primary carbides that are suboptimal for wear resistance. The transformation of these coarse carbides into fine, uniformly distributed secondary precipitates through solution-aging treatment is the key to achieving maximum performance.
However, heat treatment introduces risks that must be managed:
- Thermal cracking: Rapid heating or cooling can crack the overlay or the HAZ
- Bond degradation: Excessive temperatures can weaken the overlay-substrate bond
- Distortion: Differential thermal expansion can warp the component
- Phase transformation: Uncontrolled cooling can form brittle phases
The optimal heat treatment cycle must balance these competing factors, and process qualification testing is essential to verify that the heat treatment improves performance without introducing new defects.
Study Insights and Implications
This research underscores the importance of post-weld heat treatment in hard alloy overlay technology. The as-welded condition, while functional, represents only 60–70% of the achievable wear resistance. Optimized heat treatment can unlock an additional 30–40% improvement in wear life, which translates directly to extended maintenance intervals and reduced total cost of ownership. Engineers should incorporate heat treatment into the overlay specification as a mandatory step, with carefully controlled parameters verified through process qualification. The heat treatment cycle should be documented in the welding procedure specification (WPS) and verified through hardness testing and microstructural examination after each production batch.