CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Cooling Rate on Wear Resistance of Overlay Weld Layer
Fundamental Mechanisms
The cooling rate following overlay welding is a decisive factor governing the microstructure evolution of the weld layer, which in turn dictates wear resistance performance. Rapid cooling promotes the formation of hard phases such as martensite, retained austenite, and fine carbides, while slower cooling rates favor the precipitation of softer phases like ferrite and pearlite. Understanding this relationship is essential for optimizing overlay welding processes in applications ranging from mining equipment to power plant components.
Quantitative Analysis of Cooling Rate Effects
The relationship between cooling rate and hardness can be described through the modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, where the critical cooling rate for martensite formation depends on the carbon equivalent of the weld metal. For typical high-chromium overlay compositions, a cooling rate exceeding 100 °C/s generally produces fully martensitic structures with hardness above 50 HRC, while cooling rates below 10 °C/s result in ferrite-pearlite structures with hardness below 30 HRC.
| Cooling Rate (°C/s) | Dominant Microstructure | Hardness (HRC) | Wear Resistance Rating |
|---|---|---|---|
| > 100 | Martensite + retained austenite | 50–60 | Excellent |
| 30–100 | Mixed martensite + bainite | 40–50 | Good |
| 10–30 | Bainite + fine pearlite | 30–40 | Moderate |
| 3–10 | Ferrite + coarse pearlite | 20–30 | Poor |
| < 3 | Coarse ferrite + pearlite | 15–20 | Very poor |
Process Variables Influencing Cooling Rate
Several process parameters directly affect the cooling rate of the overlay weld layer:
- Preheat temperature: Increasing preheat from 0 °C to 200 °C can reduce the peak cooling rate by approximately 40% to 60%.
- Weld layer thickness: Thicker layers provide better thermal insulation, reducing cooling rate by 20% to 30% compared to thin single-pass deposits.
- Interpass temperature: Maintaining interpass temperature above 150 °C can reduce cooling rate by up to 50%.
- Base material thermal conductivity: Steel substrates with higher thermal conductivity (e.g., stainless steel) extract heat more rapidly than low-alloy steel, increasing cooling rates.
- Welding speed: Higher travel speeds reduce heat input per unit length, increasing cooling rates by 15% to 25%.
Practical Implications for Overlay Process Selection
In engineering practice, the selection of overlay process must be matched to the desired cooling rate. Plasma transferred arc (PTA) welding typically produces cooling rates of 50 to 200 °C/s due to its concentrated heat input and thin layer deposition, making it ideal for hardfacing applications requiring high hardness. Submerged arc welding (SAW) overlay, with its high heat input and thick deposition rates, produces cooling rates of 5 to 30 °C/s, suitable for corrosion-resistant overlays where toughness is more important than hardness.
Laser cladding offers the most extreme cooling rates, often exceeding 1000 °C/s, producing fully martensitic or even amorphous structures. However, this requires careful management of residual stress and cracking susceptibility. Gas tungsten arc welding (GTAW) overlay provides moderate cooling rates of 20 to 80 °C/s, offering a balance between hardness and ductility.
The study underscores that cooling rate is not merely a process parameter but a design variable that must be deliberately controlled. Engineers should establish target cooling rate ranges for each application and select process parameters accordingly, rather than treating cooling rate as an incidental outcome of the welding process.