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

Literature Overview

The study by Wu Qilong, published in Tunnel Construction (2020, Vol. 40, No. 8, pp. 1154-1159), addresses a critical engineering challenge in large-diameter shield tunneling machine (TBM) cutterhead fabrication and repair. The Q345R structural steel used for cutterheads is subjected to extensive assembly welding, which generates high residual stresses and frequently results in delayed cracking. The author proposes controlling preheating temperature as a key measure to improve overlay welding quality, using self-shielded flux-cored wire (SSFCW) for overlay welding on Q345R substrates. Four preheating temperatures—50°C, 100°C, 150°C, and 200°C—were systematically investigated to evaluate their effects on weld residual stress, impact toughness, microhardness gradient, and overall mechanical performance.

Core Technical Findings

The experimental results demonstrate that a preheating temperature of 100°C yields the optimal combination of weld quality indicators. At this temperature, residual stress reaches its minimum value, impact toughness achieves its peak, and the microhardness gradient transition zone exhibits the smallest variation amplitude. The following table summarizes the comparative performance across preheating conditions:

Metallurgical Interpretation

The improvement in weld quality at 100°C can be attributed to several metallurgical mechanisms. First, the preheating temperature controls the cooling rate of the weld metal and heat-affected zone (HAZ). At 50°C, the cooling rate remains too rapid, promoting the formation of hard, brittle martensitic structures in the HAZ that are susceptible to hydrogen-assisted delayed cracking. At 100°C, the cooling rate is sufficiently reduced to allow partial transformation of austenite to bainite and tempered martensite, which provides a favorable balance between strength and toughness.

Second, preheating reduces the thermal gradient between the base metal and the weld pool, thereby lowering thermal stresses. The microhardness gradient data indicate that at 100°C, the transition from base metal hardness through the HAZ to the weld metal hardness is more gradual, reducing the risk of cracking at the interface where hardness differentials are most severe.

Third, the hydrogen diffusion behavior is significantly affected by preheating. Q345R steel contains carbon and alloying elements that increase its susceptibility to hydrogen embrittlement. Preheating to 100°C facilitates hydrogen diffusion out of the weld zone during the welding and post-weld cooling stages, reducing the trapped hydrogen concentration that drives delayed cracking.

Engineering Practice Integration

The field application results reported by the author are particularly noteworthy. When the 100°C preheating recommendation was implemented in actual cutterhead overlay welding operations, the ultrasonic testing (UT) first-pass acceptance rate reached 100%. This is a remarkable achievement given that large-diameter cutterheads involve thousands of weld joints, and even a 1-2% defect rate would result in significant rework costs. The practical success validates the laboratory findings and demonstrates that systematic preheating temperature optimization can eliminate delayed cracking defects entirely in this application.

For engineering practice, the following procedural guidelines should be considered:

1. Preheat the entire workpiece to a uniform temperature of 100°C, verified by infrared thermometers or contact thermocouples at multiple locations.
2. Maintain interpass temperature between 100°C and 150°C to prevent excessive cooling between passes.
3. Use low-hydrogen flux-cored wire with hydrogen content below 5 mL/100 g to minimize hydrogen-induced cracking risk.
4. Apply post-weld heat treatment (PWHT) at 600-650°C for stress relief if residual stress levels still exceed design thresholds after welding.

Key Questions and Reflections

An important question arises regarding the applicability of the 100°C preheating temperature to different environmental conditions. In cold-weather construction environments where ambient temperatures may fall below 0°C, the effective preheating temperature at the weld zone may be significantly lower than the measured surface temperature. Engineers should consider using higher preheating temperatures (150-200°C) in cold environments to compensate for heat dissipation to the surrounding air and supporting structures.

Additionally, the study focuses on a single substrate material (Q345R) and a single welding process (SSFCW). The findings may not directly transfer to other cutting-edge materials such as high-manganese steel or to other welding processes such as submerged arc welding (SAW) or flux-cored arc welding (FCAW). Further parametric studies across material-process combinations would be valuable for developing a comprehensive preheating temperature selection chart.

Study Insights and Implications

This study provides a clear, practical, and cost-effective solution to a persistent problem in shield tunneling cutterhead manufacturing. The simplicity of the recommendation—preheat to 100°C—belies the depth of metallurgical understanding behind it. The study exemplifies how fundamental welding metallurgy principles, when applied systematically, can resolve complex field problems. For engineers involved in large-scale structural welding, this work reinforces the importance of preheating temperature as a primary control variable for preventing hydrogen-induced delayed cracking, and it provides quantitative data to support process specification development. The 100% UT first-pass acceptance rate achieved in field application serves as compelling evidence that disciplined adherence to optimized welding parameters can achieve defect-free results even in demanding industrial environments.