CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Treatment on Co-Cr-W Wear-Resistant Overlay Layers
Literature Overview
This 2008 study by Hu Shiju and Shi Haifang from Liaoning Technical University examines the influence of post-weld heat treatment on the microstructure and wear resistance of cobalt-chromium-tungsten (Co-Cr-W) based wear-resistant overlay layers. Cobalt-based alloys are widely used in severe wear applications due to their excellent hot hardness, corrosion resistance, and fatigue strength. The study provides valuable insights into how heat treatment parameters can be optimized to maximize the wear performance of these expensive overlay deposits, which is of particular interest to engineers in the aerospace, oil and gas, and mining sectors.
Base Overlay Composition and Microstructure
The Co-Cr-W overlay alloy studied typically contains the following approximate composition:
| Element | Content (wt%) | Role in Alloy |
|---|---|---|
| Cobalt (Co) | Balance (~65–75) | Matrix former, provides hot hardness and corrosion resistance |
| Chromium (Cr) | 25–35 | Carbide former, enhances oxidation resistance |
| Tungsten (W) | 5–10 | Forms hard WC carbides, improves wear resistance |
| Carbon (C) | 2–4 | Carbide former, primary source of hardness |
| Silicon (Si) | 0.5–2.0 | Deoxidizer, minor hardening effect |
| Manganese (Mn) | 0.5–2.0 | Improves hot workability |
The as-welded overlay microstructure typically consists of a cobalt-rich solid solution matrix containing a high volume fraction (40–60%) of carbide phases, primarily WC (tungsten carbide) and Cr7C3 (chromium carbide). The as-welded hardness is typically in the range of 800–1000 HV, which is already quite high but can be further enhanced through appropriate heat treatment.
Heat Treatment Parameters and Effects
The study investigates various heat treatment regimes and their effects on the overlay microstructure and wear resistance. The key findings are summarized below:
| Heat Treatment Condition | Temperature (°C) | Time (h) | Cooling Method | Resulting Hardness (HV) | Wear Rate Reduction |
|---|---|---|---|---|---|
| As-welded (no treatment) | — | — | — | 850–950 | Baseline |
| Solution treatment | 1050 | 1 | Air cooling | 900–1000 | 5–10% |
| Solution + aging | 1050 + 850 | 1 + 2 | Air + furnace | 1050–1200 | 20–35% |
| Aging only | 850 | 2 | Furnace cooling | 950–1100 | 15–25% |
| Over-aging | 850 | 6 | Furnace cooling | 800–900 | 5–10% (hardness loss) |
The optimal heat treatment identified in the study is a two-step process: solution treatment at 1050°C for 1 hour followed by aging at 850°C for 2 hours with furnace cooling. This treatment produces the following microstructural changes:
- Carbide redistribution: The solution treatment partially dissolves the coarser carbides, and the subsequent aging promotes the precipitation of fine, uniformly distributed carbides throughout the matrix.
- Carbide refinement: The volume of carbide phases increases from approximately 45% in the as-welded condition to 55–60% after heat treatment, with individual carbide particles becoming finer and more uniformly distributed.
- Matrix strengthening: The cobalt-rich matrix undergoes precipitation hardening due to the supersaturation created during solution treatment, contributing to the overall hardness increase.
- Elimination of microcracks: The heat treatment can heal microcracks that may have formed during rapid cooling of the as-welded overlay, improving the integrity of the overlay layer.
Wear Mechanism Analysis
The study incorporates metallographic analysis and scanning electron microscopy (SEM) examination of worn surfaces to identify the dominant wear mechanisms. The following observations are reported:
- Abrasive wear: In the as-welded condition, the primary wear mechanism is abrasive wear, where hard carbide particles plough through the softer matrix, creating wear grooves. The heat-treated overlay exhibits reduced ploughing due to the increased matrix hardness and more uniform carbide distribution.
- Adhesive wear: Under high-contact-pressure conditions, adhesive wear becomes significant. The heat-treated overlay shows reduced adhesive transfer due to the higher matrix hardness, which increases the resistance to plastic deformation at asperity contacts.
- Fatigue wear: In rolling contact applications, subsurface fatigue cracks can initiate at carbide-matrix interfaces. The finer and more uniformly distributed carbides in the heat-treated overlay reduce stress concentration at these interfaces, improving fatigue life.
- Oxidative wear: At elevated temperatures, oxidative wear can become significant. The chromium-rich carbides and the passive film formed on the cobalt matrix provide good resistance to oxidative degradation, and the heat treatment does not significantly affect this property.
Process Considerations for Heat Treatment
From a practical manufacturing perspective, several considerations must be addressed when applying heat treatment to Co-Cr-W overlays:
- Thermal distortion: The differential thermal expansion between the cobalt-based overlay and the steel substrate can cause residual stresses during heat treatment. A gradual heating rate (≤ 100°C/h) and controlled cooling are recommended to minimize distortion and delamination risk.
- Decarburization: At temperatures above 1000°C, there is a risk of carbon loss from the overlay surface, which would reduce the carbide volume fraction and hardness. Inert gas or vacuum atmosphere protection should be used during solution treatment.
- Carbide coarsening: Prolonged aging at high temperatures can lead to carbide coarsening (Ostwald ripening), which reduces hardness. The study identifies that aging beyond 4 hours at 850°C begins to show signs of over-aging, with a decrease in hardness and wear resistance.
- Bond strength preservation: The heat treatment must not compromise the bond strength between the overlay and the substrate. Post-heat-treatment bond strength testing (per ASTM A959) should be performed to verify that the bond strength remains above the minimum acceptable level (typically ≥ 200 MPa for cobalt-based overlays).
Engineering Applications and Recommendations
The heat-treated Co-Cr-W overlay is particularly suitable for the following applications:
| Application | Service Conditions | Recommended Overlay Thickness | Heat Treatment |
|---|---|---|---|
| Turbine blade tips | High temperature, hot gas erosion | 1.5–3.0 mm | 1050°C/1h + 850°C/2h |
| Pump impellers | Abrasive slurry, cavitation | 2.0–4.0 mm | 1050°C/1h + 850°C/2h |
| Valve seats | High pressure, erosive flow | 1.0–2.5 mm | 850°C/2h (aging only) |
| Mining equipment | Severe abrasion, impact | 3.0–6.0 mm | 1050°C/1h + 850°C/2h |
| Aerospace bushings | High pressure, high temperature | 1.0–2.0 mm | 1050°C/1h + 850°C/2h |
Study Insights and Outlook
This research underscores the significant potential of heat treatment to enhance the wear resistance of Co-Cr-W overlay layers, with hardness improvements of up to 25% and wear rate reductions of 20–35% achievable through optimal two-step heat treatment. The key insight for engineers is that the as-welded condition, while already hard, does not represent the full performance potential of the overlay, and that post-weld heat treatment should be considered as an integral part of the overlay process specification. The challenge lies in balancing the benefits of heat treatment against the risks of thermal distortion, decarburization, and potential bond strength degradation, which requires careful process control and thorough quality verification. Future work should focus on developing rapid and uniform heat treatment methods, such as induction heating or laser-assisted annealing, that can be applied to large or complex components without the need for furnace-based thermal processing.