Numerical Simulation of Martensitic Transformation Effects on Cladding Residual Stress

Literature Overview

This study by Zhou Yefei, Han Chao, Liu Ligang, Yang Yulin, and Yang Qingxiang from Yanshan University was published in the Transactions of the China Welding Institution in 2012. Funded by the Hebei Provincial Science and Technology Support Program (09215106D) and the Hebei Provincial Hundred Talents Program (SPRC021), this research employs finite element analysis to investigate the influence of martensitic phase transformation on the residual stress distribution in weld overlay cladding deposits. The work is particularly significant because martensitic transformation is a major contributor to residual stresses in hardfacing alloys and can lead to cracking, distortion, and reduced service life.

Core Technical Content

The researchers developed a coupled thermo-mechanical finite element model that incorporates the effects of martensitic phase transformation on residual stress development during the cladding welding process. The model was validated against experimental measurements of residual stresses obtained using X-ray diffraction (XRD) and the hole-drilling method. The study considered a multi-layer cladding deposit of a Ni-based hardfacing alloy (similar to Stellite) on a carbon steel substrate, with each layer deposited by GMAW welding.

The key innovation of this research was the incorporation of a transformation plasticity model into the finite element analysis. The transformation plasticity model accounts for the fact that the martensitic phase transformation (austenite to martensite) produces a volume expansion of approximately 4% (from the specific volume difference between the two phases). This volume expansion, combined with the constraints imposed by the surrounding material and the thermal gradients, generates significant transformation-induced stresses that can either add to or subtract from the thermal residual stresses.

Residual Stress Analysis and Results

The finite element simulation results showed that the residual stress distribution in the cladding deposit is highly complex and varies significantly with position. The maximum tensile residual stress was found at the cladding surface near the weld toe, reaching values of 600-800 MPa, which is well above the yield strength of the cladding material at room temperature. The residual stress at the cladding-substrate interface was predominantly compressive, with values of -200 to -400 MPa, which is beneficial for fatigue resistance but can cause interfacial cracking if the compressive stress exceeds the bond strength.

The most important finding of this research was that the martensitic transformation contributes 30-50% of the total residual stress in the cladding deposit. Without considering the transformation plasticity effect, the simulated residual stresses were significantly underestimated, particularly in the cladding surface region where the cooling rate is highest and the martensitic transformation is most complete. This finding underscores the importance of including phase transformation effects in residual stress predictions for hardfacing alloys.

The researchers also investigated the effect of interpass temperature on the residual stress distribution. Increasing the interpass temperature from 150°C to 250°C reduced the peak residual stress by approximately 15-20%, as the higher interpass temperature allows for partial stress relief during the welding process. However, excessive interpass temperature (above 300°C) can cause softening of the previously deposited layers and reduce the hardness of the cladding deposit, creating a trade-off between residual stress control and hardness maintenance.

Process Optimization and Engineering Recommendations

Based on the simulation results, the researchers proposed several strategies for reducing residual stresses in cladding deposits: (1) increasing the interpass temperature to 200-250°C to promote partial stress relief; (2) using a multi-pass welding strategy with decreasing current in the final pass to reduce the thermal input and transformation-induced stresses; (3) performing post-weld stress relief annealing at 600-650°C for 2 hours per 25 mm of thickness; and (4) designing the cladding geometry to minimize stress concentrations at the weld toe.

The simulation was also used to predict the residual stress distribution in a real industrial cladding application: the overlay of a Ni-based hardfacing alloy on a pump impeller. The predicted residual stresses were in good agreement with experimental measurements, validating the model for practical engineering applications. The researchers emphasized that residual stress management is critical for preventing cracking in hardfacing deposits, particularly in applications where the cladding deposit is subjected to cyclic loading or thermal cycling.

Study Insights and Reflections

This research makes a significant contribution to the understanding of residual stress development in hardfacing alloys by demonstrating the quantitative importance of martensitic transformation in residual stress formation. The practical implication is that residual stress predictions for hardfacing alloys must include phase transformation effects to be accurate, and that process optimization strategies should account for the transformation-induced stresses. For engineering practice, the key recommendation is to perform post-weld stress relief annealing for all hardfacing deposits that are expected to be subjected to significant residual stresses, and to use finite element analysis to predict residual stress distributions and identify potential cracking locations before production welding begins.