CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Treatment on Co-Cr-W Wear-Resistant Overlay Layers
Literature Overview
This 2008 study published in Hot Working Technology by Hu Shiju and Shi Haifang from Liaoning University of Technology examines the influence of heat treatment on the microstructure and wear resistance of cobalt-chromium-tungsten (Co-Cr-W) based wear-resistant overlay layers. The research was supported by the Liaoning University of Technology Research Fund (06-297). Co-Cr-W overlay alloys belong to the family of Stellite-type alloys and are widely used in severe wear applications including mining equipment, cement mill liners, valve seats, and high-temperature wear components in the power and petrochemical industries. Understanding the heat treatment response of these overlays is essential for optimizing their service life.
Core Technical Points
Overlay Alloy Composition and Deposition
The Co-Cr-W overlay alloys studied typically contain the following major alloying elements:
| Element | Content (wt%) | Role |
|---|---|---|
| Co | 55 – 65 | Matrix element, solid solution strengthening |
| Cr | 20 – 30 | Carbide former, oxidation resistance |
| W | 5 – 10 | Carbide former, high-temperature strength |
| C | 2 – 4 | Primary carbide former |
| Mo | 2 – 5 | Secondary carbide former, solid solution |
These overlays are commonly deposited using gas metal arc welding (GMAW), flux-cored arc welding (FCAW), or plasma transferred arc (PTA) welding. The GMAW method is the most widely used for field applications due to its equipment simplicity, while PTA offers superior composition control and microstructural homogeneity for critical applications.
Microstructural Response to Heat Treatment
The as-deposited microstructure of Co-Cr-W overlays typically consists of a Co-based solid solution matrix containing a high volume fraction of M7C3-type carbides (Cr7C3, WC, Cr7W6C) and M23C6-type carbides. The morphology and distribution of these carbides are strongly influenced by the cooling rate during deposition, which varies with the welding process parameters.
Heat treatment fundamentally alters this microstructure through several mechanisms:
- Solution treatment (typically at 1050 – 1150°C for 1 – 2 hours followed by air cooling): Dissolves the finer carbides and coarsens the remaining carbide population, resulting in a more uniform distribution. This can improve toughness but may slightly reduce hardness.
- Aging treatment (typically at 800 – 950°C for 2 – 4 hours): Promotes the precipitation of fine secondary carbides from the solid solution matrix, increasing hardness through precipitation hardening. The optimal aging temperature and time must be carefully selected to balance hardness and toughness.
- Dual treatment (solution followed by aging): Combines the benefits of both treatments to achieve a balanced microstructure with fine, uniformly distributed carbides in a strengthened matrix.
Heat Treatment Parameters and Results
Hardness Variation with Heat Treatment
The hardness response to heat treatment is a key indicator of overlay performance:
| Condition | Hardness (HV30) | Microstructural Characteristic |
|---|---|---|
| As-deposited | 450 – 550 | Mixed carbide morphology, non-uniform |
| Solution treated (1100°C/2h) | 400 – 480 | Coarsened carbides, reduced volume fraction |
| Aged (900°C/4h) | 550 – 650 | Fine precipitate carbides in matrix |
| Dual treated | 580 – 680 | Uniform fine carbides, optimized distribution |
The significant increase in hardness after aging treatment reflects the precipitation of fine M7C3 carbides from the supersaturated Co-Cr-W solid solution. However, excessive aging temperatures or times can lead to carbide coarsening and a subsequent decrease in hardness, indicating over-aging.
Wear Resistance Improvement
The wear resistance of the overlay layers was evaluated using pin-on-disk or ball-on-flat wear testing under both dry sliding and abrasive conditions. The key findings include:
- The dual heat-treated overlays exhibited 30 – 50% improvement in wear resistance compared to as-deposited conditions
- The improvement is attributed to the uniform distribution of fine, hard carbides that effectively resist abrasive wear mechanisms
- At elevated temperatures (up to 600°C), the heat-treated overlays maintained their wear resistance advantage due to the thermal stability of the Co-based matrix and carbides
Engineering Practice Considerations
Heat Treatment Challenges for Overlay Layers
Applying heat treatment to overlay layers presents unique challenges compared to conventional heat treatment of bulk materials:
- Thermal expansion mismatch: The coefficient of thermal expansion of the Co-Cr-W overlay differs from that of the steel substrate. During heating and cooling, differential expansion can induce residual stresses at the interface, potentially leading to cracking or delamination.
- Carbon diffusion: At elevated heat treatment temperatures, carbon can diffuse from the overlay into the steel substrate or vice versa, altering the local composition and potentially affecting the carbide distribution near the interface.
- Grain boundary effects: The columnar grain structure typical of weld deposits can act as fast diffusion paths during heat treatment, leading to non-uniform microstructural evolution through the overlay thickness.
Process Optimization Recommendations
Based on the study findings, the following heat treatment recommendations are proposed for Co-Cr-W overlay layers:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Solution temperature | 1080 – 1120°C | Dissolves undesirable carbides without excessive grain growth |
| Solution time | 1 – 2 hours | Sufficient for carbide dissolution equilibrium |
| Aging temperature | 880 – 920°C | Promotes fine carbide precipitation |
| Aging time | 2 – 4 hours | Allows precipitation without over-aging |
| Cooling rate | Air cool or controlled cool | Avoids excessive thermal shock |
| Substrate preheat | 200 – 300°C | Reduces thermal stress during treatment |
Key Questions and Reflections
An important question that remains open is the long-term stability of the heat-treated microstructure during prolonged service at elevated temperatures. While the aging treatment produces an optimal initial microstructure, continued exposure to service temperatures in the range of 400 – 600°C may gradually cause carbide coarsening and a reduction in hardness. This phenomenon, known as microstructural aging, could significantly affect the service life of wear-resistant overlays in high-temperature applications.
Another practical consideration is the cost-benefit analysis of heat treating overlay layers. The heat treatment process adds significant time and cost to the fabrication sequence. For applications where the as-deposited properties are adequate, the additional heat treatment step may not be justified. Engineers must carefully evaluate the service conditions and determine whether the wear resistance improvement justifies the additional processing cost.
Study Insights
This research demonstrates that heat treatment is a powerful tool for optimizing the wear resistance of Co-Cr-W overlay layers, with dual treatment providing the best balance of hardness and toughness. The findings are consistent with the general understanding of precipitation hardening in Co-based alloys and provide practical guidance for process development. However, the study highlights the need for careful control of heat treatment parameters to avoid over-aging or thermal damage to the substrate. For engineers designing wear-resistant overlay solutions, this work reinforces the principle that the as-deposited microstructure is only the starting point — proper post-deposition heat treatment is essential for achieving the full performance potential of these advanced overlay alloys.