CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Preheating Temperature on Microstructure and Performance of Shield Tunneling Cutter Head Overlay
Literature Overview
Published in 2020 by Wu Qilong from China Railway Tunnel Group Co., Ltd., this study investigates the influence of preheating temperature on the microstructure and mechanical performance of hardfacing overlays applied to shield tunneling machine (TBM) cutter heads. The work was presented at the Tunnel Construction (Chinese and English) conference, reflecting the practical engineering context of heavy-duty mining and tunneling equipment. Cutter heads endure extreme abrasion from hard rock, and the overlay layer must provide superior wear resistance while maintaining adequate toughness to resist impact and thermal cycling.
Core Technical Points
Preheating Temperature as a Critical Process Variable
Preheating temperature in hardfacing operations serves multiple metallurgical purposes: it reduces the thermal gradient between the hot weld pool and the cold substrate, minimizes residual stresses, prevents cold cracking in high-carbon or high-alloy steels, and controls the cooling rate of the overlay. For TBM cutter head applications, the base material is typically a high-strength low-alloy steel or cast steel, and the overlay is usually a high-carbon martensitic or austenitic alloy containing carbide-forming elements such as Cr, Mo, and V.
| Preheating Temperature | Cooling Rate | Residual Stress Level | Cracking Risk | Hardness Trend |
|---|---|---|---|---|
| 100–200°C | High (50–100 K/s) | High | Moderate | Very high (900–1100 HV) |
| 300–400°C | Medium (20–50 K/s) | Medium | Low | High (850–1000 HV) |
| 500–600°C | Low (10–20 K/s) | Low | Very low | Moderate (750–900 HV) |
| 700°C+ | Very low (<10 K/s) | Very low | Minimal | Lower (650–800 HV) |
Microstructural Response to Preheating
At low preheating temperatures (100–200°C), the rapid cooling of the overlay promotes the formation of retained austenite and fine martensite. The high cooling rate can also lead to the formation of brittle phases and microcracks, particularly at the weld root and in the heat-affected zone (HAZ). The overlay microstructure consists of lenticular martensite with dispersed M₇C₃ and M₂₃C₆ carbides, providing high hardness but limited toughness.
At moderate preheating temperatures (300–400°C), the cooling rate decreases, allowing for more complete austenitization and a more uniform martensitic transformation. The retained austenite content increases, which can be beneficial for toughness through the TRIP (Transformation-Induced Plasticity) effect. The carbide distribution becomes more uniform, and the overall microstructure is finer due to the reduced thermal gradient.
At high preheating temperatures (500–600°C), the cooling rate is further reduced, promoting the formation of bainitic and tempered martensitic structures. While the hardness decreases, the toughness improves significantly, making the overlay more resistant to impact loading and thermal fatigue. This is particularly important for cutter heads operating in hard rock conditions where the overlay is subjected to both abrasive and impact loading.
Mechanical Performance Correlation
The hardness-toughness trade-off is the central theme of this study. The overlay hardness decreases with increasing preheating temperature, but the impact toughness (measured by Charpy V-notch testing) improves substantially. The optimal preheating temperature for TBM cutter head applications balances these competing requirements based on the specific rock conditions encountered during tunneling.
Process Analysis and Standards Considerations
The study is relevant to standards such as AWS A5.15 (Specification for Welding Consumables for Hardfacing) and GB/T 13814 (Welding Consumables for Hardfacing). These standards specify the chemical composition and mechanical requirements for hardfacing alloys but do not prescribe specific preheating temperatures, which must be determined through process qualification testing in accordance with ASME IX or NB/T 47014.
Defect Analysis and Prevention
| Defect Type | Primary Cause | Preheating Effect |
|---|---|---|
| Cold cracking | High cooling rate, hydrogen embrittlement | Preheating above 300°C significantly reduces risk |
| Hot cracking | Low ductility at high temperature | Moderate preheating (300–500°C) beneficial |
| Excessive hardness gradient | Thermal mismatch | Higher preheating reduces gradient |
| Delamination | Residual stress | Higher preheating reduces residual stress |
| Porosity | Gas inclusion | Minimal effect of preheating |
Integration with Engineering Practice
In the context of TBM cutter head manufacturing, the preheating temperature must be carefully selected based on the expected operating conditions. For soft to medium rock (uniaxial compressive strength below 100 MPa), a preheating temperature of 300–400°C provides an optimal balance of hardness and toughness. For hard rock (UCS above 150 MPa), a lower preheating temperature of 200–300°C is preferred to maximize hardness, accepting the higher risk of cracking.
The study also highlights the importance of post-weld heat treatment (PWHT) as a complementary measure to preheating. A PWHT at 550–650°C for 2–4 hours can further reduce residual stresses and temper the martensitic structure, providing an additional margin for service reliability.
The practical implication for manufacturing is that the preheating temperature should be specified in the welding procedure specification (WPS) and monitored during production. Infrared thermometers or contact thermocouples should be used to verify that the preheating temperature is maintained throughout the welding operation, particularly for large cutter heads where heat loss can be significant.
Study Insights and Implications
This study provides valuable engineering guidance for the optimization of hardfacing processes on heavy-duty equipment. The systematic investigation of preheating temperature effects demonstrates that this single parameter can significantly influence the metallurgical quality and service performance of the overlay. The findings reinforce the principle that hardfacing is not a one-size-fits-all process but requires tailored parameter selection based on the specific application requirements.
The study also underscores the importance of understanding the relationship between process parameters, microstructure, and mechanical properties. Engineers must not rely solely on hardness measurements but should also evaluate toughness, fatigue resistance, and thermal stability to ensure that the overlay performs adequately under the complex loading conditions encountered in tunneling operations. The practical recommendation is to conduct qualification tests at multiple preheating temperatures and select the one that provides the best overall performance for the intended application, rather than simply maximizing hardness.