CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Preheat Temperature on Microstructure and Properties of Shield Tunneling Cutter Head Cladding
Literature Overview and Application Context
The research by Wu Qilong from China Railway Tunneling Co., Ltd., published in 2020 in the journal Tunnel Construction, addresses the effect of preheat temperature on the microstructure and mechanical properties of the cladding layer applied to shield tunneling machine (TBM) cutter heads. This is a highly practical study because TBM cutter heads are subjected to extreme wear conditions when excavating hard rock formations, and the cladding layer is the primary means of extending cutter life. The cutter head material is typically low-carbon or medium-carbon steel, and the cladding layer is often a high-chromium cast iron or hardfacing alloy deposited by submerged arc welding or flux-cored arc welding.
Core Technical Points and Preheat Temperature Effects
Preheat temperature is one of the most critical process parameters in hardfacing cladding of steel substrates. Its primary functions are to reduce the cooling rate of the weld metal, minimize thermal gradients between the weld and base material, and reduce the risk of hydrogen-induced cracking (HIC). For TBM cutter head cladding, the preheat temperature directly influences the cooling rate of the cladding layer, which in turn determines the phase composition and microstructure of the deposited material.
At lower preheat temperatures (e.g., 50–100°C), the cooling rate is high, promoting the formation of martensite in the cladding layer. While martensite provides high hardness, it is also brittle and susceptible to cracking under impact loading, which is common in TBM excavation where cutters encounter sudden rock impacts. At moderate preheat temperatures (150–250°C), the cooling rate is reduced, allowing for a more balanced microstructure with a mixture of martensite and tempered carbides, which provides good hardness with improved toughness. At higher preheat temperatures (300°C and above), the cooling rate is further reduced, which may lead to excessive grain growth and reduced hardness, but significantly improves the ductility and crack resistance of the cladding layer.
Microstructural Evolution with Preheat Temperature
| Preheat Temperature (°C) | Cooling Rate (°C/s) | Dominant Phase | Hardness (HV) | Tensile Strength (MPa) | Crack Susceptibility |
|---|---|---|---|---|---|
| 50–100 | 80–150 | Martensite + retained austenite | 800–950 | 1200–1500 | High |
| 150–250 | 40–80 | Martensite + tempered carbides | 650–800 | 1000–1300 | Moderate |
| 300–400 | 20–40 | Bainite + tempered martensite | 500–650 | 800–1100 | Low |
The optimal preheat temperature for TBM cutter head cladding typically falls in the range of 150–250°C. This range provides a good balance between hardness (for wear resistance) and toughness (for impact resistance), while keeping the risk of hydrogen cracking at an acceptable level. The specific selection depends on the geological conditions of the tunnel: harder rock formations (uniaxial compressive strength > 200 MPa) may require lower preheat temperatures to maximize hardness, while softer formations with higher impact loading may benefit from higher preheat temperatures to improve toughness.
Engineering Practice and Quality Assurance
In practice, TBM cutter head cladding is performed in the field or in a workshop after the cutter head is removed from the machine. The welding process is typically flux-cored arc welding (FCAW) or submerged arc welding (SAW) using a high-chromium cast iron wire or flux. The preheat is applied using induction heating or oxy-fuel torches, and the temperature must be monitored at multiple points on the cutter head to ensure uniform heating. The interpass temperature should be maintained at or slightly above the preheat temperature to avoid excessive cooling between passes.
The inspection of the cladding layer includes visual examination for surface defects, ultrasonic testing for internal defects (porosity, lack of fusion, cracks), and hardness testing at regular intervals. The hardness profile should be measured at multiple depths to ensure uniformity across the cladding thickness. Any areas with hardness outside the specified range must be ground off and re-clad.
Summary
This study underscores the critical role of preheat temperature in determining the microstructure and performance of TBM cutter head cladding layers. The findings demonstrate that preheat temperature is not merely a measure to prevent cracking but is a fundamental parameter that controls the phase composition, hardness, and toughness of the deposited material. For engineers involved in TBM maintenance and cutter head refurbishment, the key takeaway is that preheat temperature must be selected based on the specific geological conditions and operating loads, not applied as a blanket value. A systematic approach to preheat temperature selection, combined with rigorous inspection protocols, is essential for maximizing cutter life and minimizing unplanned downtime.