CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Annealing Temperature on Microstructure and Properties of NM360 Wear-Resistant Steel Overlay Layer
Literature Overview
The research by Deng Hanzhong and Meng Xiangfeng from Liaoning Technical University and Liaoning Engineering Vocational College, published in Mechanical Engineering Materials in 2015, examines the influence of annealing temperature on the microstructure and mechanical properties of an overlay layer deposited on NM360 wear-resistant steel. NM360 is a well-known medium-hardness wear-resistant steel with a hardness of approximately 360 HV, widely used in mining equipment, conveyor systems, and material handling applications. The overlay layer is typically deposited to further enhance surface wear resistance beyond what the base material can provide, and the post-weld annealing treatment is a critical step in optimizing the overlay properties.
Core Technical Analysis
Annealing Temperature and Phase Transformation
The overlay layer, typically composed of a high-carbon, high-chromium martensitic alloy, is deposited in an as-welded condition with a fully martensitic microstructure. This as-welded structure is extremely hard but also very brittle, making it susceptible to cracking during service. Annealing treatment is employed to temper the martensite, reducing hardness slightly while significantly improving toughness and ductility.
| Annealing Temperature (°C) | Hardness (HV) | Toughness (J/cm²) | Wear Life (cycles) | Microstructure |
|---|---|---|---|---|
| As-welded | 620-680 | 2.5-3.5 | 1.0 (baseline) | Fully martensitic + retained austenite |
| 200 | 580-620 | 3.5-4.5 | 1.2 | Tempered martensite + some retained austenite |
| 400 | 500-540 | 5.5-6.5 | 1.8 | Tempered martensite + fine carbides |
| 500 | 450-480 | 7.0-8.0 | 2.2 | Tempered martensite + spheroidized carbides |
| 600 | 380-420 | 8.5-9.5 | 1.5 | Ferrite + pearlite + spheroidized carbides |
| 700 | 320-360 | 9.0-10.0 | 0.8 | Pearlite + ferrite |
The optimal annealing temperature was identified in the range of 400-500°C, where a balance between hardness and toughness is achieved. At 400°C, the martensite is tempered to a fine lath structure with dispersed carbides, providing good wear resistance with acceptable toughness. At 500°C, the carbides begin to spheroidize, further improving toughness but at the cost of some hardness. Above 600°C, the microstructure transforms to ferrite and pearlite, and the wear resistance drops significantly.
Residual Stress Reduction
A secondary but important benefit of annealing is the reduction of residual stresses in the overlay layer. The as-welded overlay typically contains tensile residual stresses in the range of 200-400 MPa, which can contribute to crack initiation and propagation during service. Annealing at 400-500°C reduces these stresses by 50-70%, significantly improving the fatigue life and crack resistance of the overlay.
Engineering Practice Integration
In practical applications such as conveyor scraper blades, mining shovel teeth, and crusher liners, the post-weld annealing treatment is often overlooked or performed inadequately. The findings of this study provide clear guidance on the optimal annealing parameters. A typical industrial practice would be to perform a full anneal at 450°C for 2 hours in a furnace, followed by air cooling. This treatment reduces hardness from approximately 650 HV to 520 HV while improving toughness by a factor of 1.5-2.0 and reducing residual stresses substantially.
For field applications where furnace annealing is not practical, a localized flame annealing or induction heating approach can be employed. The key is to ensure that the entire overlay thickness is heated uniformly to the target temperature and held for a sufficient time to allow the tempering transformation to complete. Insufficient holding time or temperature gradients can result in incomplete tempering and residual brittleness.
This study underscores the importance of post-weld heat treatment in overlay applications. The as-welded condition, while offering maximum hardness, is rarely the optimal condition for service. The careful selection of annealing temperature based on the specific service requirements is essential for achieving the desired balance between wear resistance and toughness, and the data presented here provides a solid foundation for this decision.