Influence of Preheating Temperature on Overlay Welding Microstructure and Properties of Shield Tunnel Machine Cutterheads

Literature Overview

This paper by Wu Qilong, published in Tunnel Construction (2020, Vol. 40, No. 8), addresses a critical engineering challenge encountered in the assembly and repair of large-diameter shield tunneling machine (TBM) cutterheads. The Q345R low-alloy steel cutterhead, when assembled through welding, accumulates substantial residual stresses that frequently lead to delayed cracking. The study systematically investigates how preheating temperatures of 50, 100, 150, and 200°C affect the overlay weld microstructure, residual stress, impact toughness, and microhardness gradient when using self-shielded flux-cored wire (SSFCW). The findings demonstrate that 100°C preheating yields optimal comprehensive mechanical performance, and field application confirmed a 100% first-pass UT qualification rate for cutterhead overlay welds.

Core Technical Analysis

Material and Process Parameters

Microstructural Response to Preheating

The Q345R steel contains approximately 1.0–1.6% Mn, 0.5–0.8% Si, and trace amounts of Mo, Nb, and V. During overlay welding, the heat-affected zone (HAZ) and weld metal undergo complex phase transformations. Without adequate preheating, the rapid cooling rate promotes the formation of hard, brittle phases such as martensite and bainite, which elevate residual stress and increase susceptibility to delayed cracking.

At 50°C preheating, the cooling rate remains high enough to produce a significant fraction of upper bainite and some martensite in the HAZ. The residual stress distribution remains steep, with peak values exceeding 200 MPa near the weld root. At 100°C preheating, the cooling rate decreases sufficiently to suppress martensite formation while retaining adequate hardness for wear resistance. The microhardness gradient across the weld interface becomes the most gradual, indicating a smoother stress transition. At 150°C and 200°C, the cooling rate is further reduced, leading to increased ferrite content in the HAZ, which lowers hardness and reduces the wear resistance of the overlay layer. The residual stress at these higher preheating temperatures is comparable to the 100°C case but does not show further significant improvement.

Residual Stress and Delayed Cracking Mechanism

Delayed cracking in Q345R overlay welds is primarily driven by three factors: hydrogen embrittlement, residual stress concentration, and microstructural brittleness. The SSFCW introduces hydrogen into the weld metal from the flux coating. Without adequate preheating, hydrogen cannot diffuse out effectively during cooling, concentrating at the weld root and HAZ boundaries. The combination of high residual tensile stress and trapped hydrogen leads to delayed cracking hours or even days after welding.

The 100°C preheating temperature provides a critical balance: it raises the base metal temperature sufficiently to allow hydrogen diffusion (the diffusion coefficient of hydrogen in steel increases exponentially with temperature), reduces the cooling rate to a level that suppresses martensite formation, and simultaneously does not excessively soften the HAZ. The microhardness gradient data confirm that the transition zone between base metal and overlay weld at 100°C shows the smallest amplitude of hardness change, indicating a more uniform stress state.

Engineering Practice Integration

Field Application Results

The study reports that applying the 100°C preheating recommendation to actual cutterhead overlay welding operations resulted in a 100% first-pass UT qualification rate. This is a significant improvement over the baseline condition where preheating was either omitted or performed at ad-hoc temperatures. The practical implementation involved using gas torches or induction heating to uniformly preheat the welding area to 100°C, verified by infrared pyrometers or thermocouples.

Process Control Recommendations

1. Preheating uniformity: The entire welding zone, including a minimum 150 mm radius around the weld path, should be preheated to avoid thermal gradients that could induce additional stresses.
2. Interpass temperature: Maintain the interpass temperature above 80°C throughout multi-pass overlay welding to prevent localized cooling that could re-introduce high cooling rates.
3. Post-weld treatment: For critical applications, a low-temperature stress relief treatment at 550–600°C for 2 hours per 25 mm of thickness is recommended to further reduce residual stress below 100 MPa.
4. Hydrogen control: Use low-hydrogen flux-cored wires and ensure thorough drying of the wire spools before use. A post-weld bake at 250–300°C for 1–2 hours can extract residual hydrogen.

Defect Analysis and Countermeasures

Study Insights and Reflections

This research provides a clear, practical methodology for optimizing overlay welding on Q345R cutterheads. The finding that 100°C is the optimal preheating temperature rather than the conventional approach of using higher temperatures (150–250°C commonly specified in welding codes for this steel grade) is particularly noteworthy. It demonstrates that for overlay welding, where the primary objective is surface performance rather than joint strength, a moderate preheat that balances toughness and hardness is superior to aggressive preheating that softens the weld too much.

The engineering significance extends beyond cutterhead repair to any large-scale steel structure where overlay welding is used for wear protection or surface hardening. The methodology of systematically varying preheating temperature and correlating it with microstructure, residual stress, and NDT results is a rigorous approach that should be adopted as a standard procedure in welding qualification programs.

One limitation of the study is the absence of long-term service performance data. While the UT qualification rate is an excellent indicator of weld soundness, the actual resistance to delayed cracking in field service over months or years would provide additional confidence. Future work should incorporate accelerated aging tests and field monitoring to validate the long-term durability of the optimized process.

The study also highlights the importance of material-specific welding procedure development. Generic preheating recommendations from welding codes may not be optimal for overlay welding applications, and tailored experimental determination, as demonstrated here, yields superior results. This approach aligns with the philosophy of welding procedure qualification under standards such as ISO 15614-1 and ASME Section IX, where the specific combination of base material, filler metal, and process parameters must be experimentally validated rather than assumed from general guidelines.