Zhuojin Pipe Fitting Co., LtdEffect of Tungsten on Temper Stability of Iron-Based Surfacing Layers
Literature Overview
This study, published in 2002 in Heat Treatment (Vol. 17, No. 3, pp. 20–22), was conducted by Yang Qinghui from Qiqihar No. 1 Machine Tool Factory and Liu Jianhua from the School of Materials Science and Engineering at Yanshan University. The research investigates the influence of tungsten (W) content on the temper stability of iron-based surfacing layers, using metallographic microscopy, X-ray diffraction, and hardness testing to characterize the microstructure and mechanical properties.
Technical Context and Background
Iron-based surfacing layers are widely used in the manufacturing of wear-resistant and high-strength components, particularly in the machine tool industry. The temper stability of these surfacing layers—the resistance to hardness reduction during tempering—is a critical property that determines the service life of the component. In applications where the component is subjected to elevated temperatures during service or during subsequent heat treatment operations, the surfacing layer must retain its hardness and wear resistance.
Tungsten as a Microalloying Element
Tungsten is a strong carbide-forming element that significantly influences the microstructure and properties of iron-based alloys. In surfacing layers, W contributes to:
- Carbide formation: W forms hard, stable carbides (WC, W2C) that resist dissolution during tempering.
- Solid solution strengthening: W in solid solution in the matrix provides additional strengthening that is retained at elevated temperatures.
- Retardation of tempering: W slows the decomposition of martensite and the precipitation of carbides during tempering, maintaining higher hardness at elevated temperatures.
Experimental Methodology
The study examined iron-based surfacing layers with varying W content, using the following characterization techniques:
| Characterization Method | Purpose | Key Observations |
|---|---|---|
| Metallographic microscopy | Microstructure identification | Carbide morphology, martensite decomposition |
| X-ray diffraction (XRD) | Phase identification | Carbide type, martensite/ferrite content |
| Hardness testing | Mechanical property evaluation | Hardness vs. tempering temperature |
Key Results
The study found that:
- W increases temper stability: Surfaces layers with higher W content exhibit greater resistance to hardness reduction during tempering.
- 6% W content: A surfacing layer with 6 wt% W, when tempered at 600°C, retains a hardness of approximately 500 HV (the text indicates "5" which likely refers to 500 HV or a similar high value).
- Carbide stability: W carbides remain stable at elevated temperatures, contributing to retained hardness.
Engineering Practice Implications
Selection of Tungsten Content for Surfacing Applications
The selection of W content in iron-based surfacing layers should be guided by the following considerations:
| Application | Service Temperature | Recommended W Content | Rationale |
|---|---|---|---|
| Room temperature wear | <100°C | 2–4% | Adequate hardness, moderate cost |
| Elevated temperature wear | 100–400°C | 4–6% | Improved temper stability |
| High temperature wear | 400–600°C | 6–8% | High temper stability, retained hardness |
| Cost-sensitive applications | Any | 0–2% | Minimal W, rely on other alloying elements |
Interaction with Other Alloying Elements
The temper stability of iron-based surfacing layers is influenced by the combined effect of multiple alloying elements. W is most effective when combined with:
- Chromium (Cr): Forms Cr-W mixed carbides with enhanced stability.
- Vanadium (V): Forms V-W mixed carbides that resist coarsening.
- Molybdenum (Mo): Synergistic effect on temper stability.
- Carbon (C): Provides carbide-forming capacity; W carbides are more stable than Fe carbides.
Process Considerations
The welding and surfacing process must be carefully controlled to ensure uniform W distribution in the surfacing layer:
- Welding process selection: SMAW, FCAW, and SAW are commonly used for iron-based surfacing. Each process has different dilution characteristics that affect the final W content in the surfacing layer.
- Dilution control: Excessive dilution of the base material reduces the effective W content in the surfacing layer. Process parameters should be optimized to minimize dilution.
- Heat input management: High heat input can cause W carbide dissolution and coarsening, reducing the temper stability benefit.
- Post-weld heat treatment: If the component requires post-weld heat treatment, the W content must be sufficient to maintain hardness at the heat treatment temperature.
Key Questions and Reflections
The study raises an important question regarding the cost-benefit analysis of W addition. Tungsten is a relatively expensive alloying element, and its use should be justified by the performance improvement it provides. For applications where the service temperature is below 200°C, the temper stability benefit of W may not be necessary, and other alloying elements or carbide formers may provide adequate performance at lower cost.
A further consideration is the effect of W on weldability. High W content can increase the hardness of the weld metal and HAZ, potentially increasing susceptibility to cracking. The welding process must be carefully controlled to ensure sound welds with high W content surfacing materials.
Study Insights and Implications
This study provides valuable insight into the role of tungsten in improving the temper stability of iron-based surfacing layers. The finding that 6 wt% W content provides significant hardness retention at 600°C tempering is of practical importance for applications where the surfacing layer is subjected to elevated temperatures. The use of metallographic microscopy, XRD, and hardness testing provides a comprehensive characterization approach that can be applied to other alloying elements and surfacing compositions.
For engineering practice, the key takeaway is that W is an effective microalloying element for improving the temper stability of iron-based surfacing layers. However, the optimal W content must be balanced against cost, weldability, and the specific service requirements of the application. The study reinforces the importance of microstructural characterization in understanding the relationship between composition, processing, and properties in surfacing operations.