Effect of Preheating Temperature on Microstructure and Properties of Surfacing on Shield Tunneling Cutter Heads

Literature Overview

This paper by Wu Qilong, published in Tunnel Construction (Volume 40, Issue 8, 2020, pages 1154-1159), addresses a critical practical problem in tunnel boring machine (TBM) maintenance: the occurrence of delayed cracks in large-diameter cutter heads after assembly welding. The study investigates the effect of preheating temperature on the microstructure and mechanical properties of surfacing welds deposited on Q345R steel cutter heads using self-shielded flux-cored wire.

Problem Background and Engineering Significance

Large-diameter shield tunneling cutter heads are assembled from multiple plate segments welded together, creating extensive weld zones with high residual stresses. The combination of high residual stresses, the Q345R material's susceptibility to hydrogen-induced delayed cracking, and the complex loading conditions during tunneling operations creates a significant risk of delayed cracking. This phenomenon is particularly dangerous because cracks may not manifest immediately after welding but can develop over days or weeks under operational stresses, leading to catastrophic failure during tunneling operations.

Experimental Design and Results

The study employed a systematic experimental approach, varying the preheating temperature across four levels: 50°C, 100°C, 150°C, and 200°C. The surfacing process used self-shielded flux-cored wire, which is a practical choice for field repair conditions where the availability of shielding gas is limited.

Preheating Temperature	Residual Stress	Impact Toughness	Hardness Gradient	UT Pass Rate
50°C	Higher	Lower	Steeper gradient	Not specified
100°C	Minimum	Best	Smallest variation	100% first-pass
150°C	Moderate	Good	Moderate gradient	Not specified
200°C	Higher	Lower	Steeper gradient	Not specified

The results demonstrate that 100°C preheating provides the optimal balance of residual stress reduction, impact toughness improvement, and hardness gradient minimization. This temperature represents a practical and achievable preheat level that can be implemented using portable heating equipment available at tunnel construction sites.

Microstructural Analysis and Mechanism Interpretation

The selection of 100°C as the optimal preheat temperature can be understood through the metallurgical mechanisms involved:

Hydrogen diffusion and escape: The elevated temperature promotes hydrogen diffusion out of the weld metal and heat-affected zone, reducing the risk of hydrogen-induced delayed cracking. At 100°C, hydrogen mobility is significantly enhanced compared to room temperature conditions.
Cooling rate reduction: Preheating reduces the cooling rate of the weld, which helps prevent the formation of brittle martensitic microstructures in the heat-affected zone. For Q345R steel, a cooling rate below the critical rate for martensite formation is essential for maintaining adequate toughness.
Thermal gradient reduction: The preheat temperature reduces the thermal gradient between the hot weld zone and the cooler base material, which directly reduces the magnitude of thermal stresses and the associated residual stresses.
Dilution and microstructure control: The controlled cooling rate at 100°C preheat promotes the formation of a more favorable microstructure with appropriate grain size and phase distribution in both the weld metal and HAZ.

The observation that 200°C preheat does not provide additional benefits and may actually be detrimental is noteworthy. Excessive preheating can lead to grain coarsening in the HAZ, increased dilution, and potentially adverse effects on the mechanical properties of the deposited layer. The hardness gradient data supports this interpretation, showing that higher preheat temperatures lead to steeper hardness transitions between the weld and base material.

Practical Application and Quality Control

The most significant practical outcome of this study is the demonstration that 100°C preheating achieves a 100% first-pass UT inspection pass rate for cutter head surfacing repairs. This is a remarkable quality metric that demonstrates the effectiveness of the optimized preheat temperature in preventing weld defects. For tunneling operations where project schedules are critical and equipment availability is limited, achieving first-pass quality is economically and operationally significant.

Study Insights and Recommendations

This study provides a clear, actionable process parameter for field repair of TBM cutter heads. The 100°C preheat temperature is easily achievable with portable induction or resistance heating equipment and provides substantial improvements in weld quality without requiring specialized equipment or extended heating cycles. Engineers working on TBM maintenance should adopt this preheat temperature as a standard practice for Q345R cutter head repairs. The study also underscores the importance of preheat temperature optimization as a fundamental quality control measure in welding operations, particularly for materials susceptible to delayed cracking. The systematic approach of varying preheat temperature and measuring multiple quality indicators provides a methodology that can be applied to other welding applications requiring preheat optimization.