Numerical Simulation of Submerged Arc Overlay Welding Considering Phase Transformation Induced Plasticity

Literature Overview

This paper by Huang Qingchun and colleagues from Liaoning University of Science and Technology (2021, Surface Technology, Vol. 50, No. 3, pp. 261-269) presents a comprehensive numerical simulation method for submerged arc overlay welding (SAW) of failed rolling mill rolls, incorporating phase transformation induced plasticity (TRIP) stress. The work was supported by the National Natural Science Foundation of China (Grants 51105187, 51205187) and other provincial funding sources. The study addresses a critical challenge in numerical simulation of welding processes: the coupling between temperature field, phase transformation, and mechanical stress, particularly the TRIP effect that occurs during martensitic transformation.

Methodology and Technical Framework

The study employed a multi-physics coupled approach using COMSOL Multiphysics platform, with material properties calculated using the CALPHAD (Calculation of Phase Diagram) method. The simulation framework includes:

Key Simulation Results

Phase Transformation Induced Plasticity (TRIP) Effect

The TRIP effect is a critical phenomenon in welding simulation that is often neglected in conventional thermal-mechanical analyses. During the cooling process, the austenite-to-martensite transformation causes a volume expansion (approximately 1-4% depending on composition). If this transformation occurs under a stress state that exceeds the yield strength of the material, plastic deformation occurs during the transformation, which is the TRIP effect.

TRIP Stress Characteristics

The paper identifies several important characteristics of TRIP stress in the overlay welding process:

1. Spatial variation: The TRIP stress distribution is wider near the substrate side because the heat transfer rate is faster in this region, leading to more rapid martensitic transformation.
2. Temporal evolution: The TRIP stress develops during cooling as the austenite transforms to martensite, and it partially relaxes the thermal stress.
3. Phase fraction dependence: With 94% martensite phase fraction, the TRIP effect is significant and must be included in the stress analysis.

Comparison of Simulation Approaches

Engineering Applications to Rolling Mill Roll Repair

Rolling mill rolls are critical components in steel production, and their failure requires rapid repair to minimize production downtime. The overlay welding repair process must address the following challenges:

• Residual stress control: High residual stresses can lead to overlay layer cracking and spalling during service.
• Phase transformation management: The martensitic transformation causes volume expansion and stress generation that must be accounted for in process design.
• Thermal cycle optimization: The cooling rate must be controlled to achieve the desired microstructure and minimize cracking risk.

Process Optimization Recommendations

Based on the simulation results, the following process parameters can be optimized to reduce residual stress and prevent cracking:

1. Preheat temperature: Increasing preheat to 200-300°C reduces the peak thermal gradient and slows the cooling rate, which reduces the TRIP stress magnitude.
2. Interpass temperature: Maintaining interpass temperature above 150°C allows partial stress relaxation between passes.
3. Post-weld heat treatment: Tempering at 550-650°C for 2-4 hours can relieve residual stresses and temper the martensite to a more ductile structure.
4. Weld sequencing: Using a multi-pass strategy with proper sequencing can distribute residual stresses more uniformly.

Model Validation and Limitations

The simulation results were validated against metallographic observations using Zeiss Sigma HD field emission SEM. The agreement between predicted and observed microstructures confirms the accuracy of the CALPHAD-based material property calculations and the multiphysics coupling approach.

However, several limitations should be acknowledged:

• The double-ellipsoidal heat source model is a simplification of the actual arc heat input, which varies with welding conditions.
• The CALPHAD method provides equilibrium phase fractions, but the actual transformation kinetics during welding cooling may deviate from equilibrium.
• The model does not account for texture development, which can affect the anisotropy of mechanical properties.
• The boundary conditions are simplified and may not fully capture the actual thermal and mechanical constraints of the roll.

Key Reflections

The incorporation of TRIP stress into the welding simulation is a significant advancement over conventional thermal-mechanical analyses. The finding that the TRIP stress has a wider bandwidth near the substrate side is particularly important because this region is the most critical for overlay layer adhesion and cracking resistance. The faster heat transfer rate near the substrate, caused by the higher thermal conductivity of the base material compared to the overlay layer, leads to more rapid martensitic transformation and a more intense TRIP effect.

The maximum residual stress of 376 MPa is relatively moderate compared to the yield strength of typical overlay materials (which can exceed 800 MPa for martensitic steels). However, this value represents the stress after complete cooling, and the peak stress during the welding process may be significantly higher. The inclusion of TRIP stress in the analysis provides a more realistic prediction of the stress state, which is essential for predicting cracking and spalling behavior.

The CALPHAD-based approach to material property calculation is a powerful methodology that ensures thermodynamic consistency between phase fractions and material properties. However, the computational cost is significantly higher than using empirical property databases, and the accuracy depends on the quality of the thermodynamic database used. For industrial applications, a balance between computational efficiency and accuracy must be struck.