Numerical Simulation of Martensitic Transformation Effects on Tangential Stress During Overlay Welding Cooling

Literature Overview

The study by Zhou Yefei, Han Chao, Liu Ligang, Yang Yulin, and Yang Qingxiang from Yanshan University, published in the Transactions of the China Welding Institute (2012, Vol. 33, No. 2, pp. 73–76), presents a two-dimensional finite element analysis of residual stress development during overlay welding of hot-rolled strip mill rolls. The research specifically addresses the influence of martensitic phase transformation on tangential residual stress during the cooling stage of the welding process. This work is highly relevant to engineers involved in overlay welding of rolling mill components, thick-section piping, and any application where martensitic transformations occur in the weld or heat-affected zone.

Methodology and Model Development

The authors established a validated finite element model by first measuring the temperature field using an infrared thermal imaging camera and the residual stress field using an X-ray stress meter. The measured data, combined with material physical and mechanical parameters, were used to calibrate and validate the numerical model. The agreement between simulated and measured results confirmed the model's effectiveness for predicting stress development during martensitic transformation.

Stress Evolution During Martensitic Transformation

Cooling Stage	Martensite Volume	Surface Tangential Stress	Stress Character
Initial cooling (above Ms)	0%	Compressive	Thermal contraction dominant
Early transformation	Increasing	Transition to tensile	Transformation strain onset
Mid-transformation	Moderate	Increasing tensile	Transformation strain exceeds thermal
Late transformation	High	Large tensile	Sustained tensile stress
Room temperature	Maximum	Maximum tensile	Final equilibrium state

Technical Interpretation

The key finding is that martensitic transformation introduces a volumetric expansion (typically 3–4% for steel) that, when constrained by surrounding material, generates additional tensile stresses that superimpose on the thermally induced residual stresses. The sequence of events during cooling is:

Above the martensite start temperature (Ms), thermal contraction produces compressive surface stresses as the cooler surface is constrained by the warmer substrate
As martensite forms, the volumetric expansion partially offsets thermal contraction, transitioning the surface stress from compressive to tensile
With continued cooling and increasing martensite volume fraction, the transformation strain becomes dominant, producing progressively larger tensile stresses
At room temperature, with maximum martensite content, the surface exhibits large tensile residual stresses

Engineering Practice Relevance

For overlay welding applications on piping components and fittings, this research has critical implications for weld integrity and service performance:

Components with martensitic weld metals (such as high-carbon steel or low-alloy steels) are prone to cold cracking during cooling
The tensile residual stresses generated by martensitic transformation increase susceptibility to hydrogen-induced cracking (HIC) and stress corrosion cracking (SCC)
Post-weld heat treatment (PWHT) is essential for components where martensitic transformation occurs, to relieve transformation-induced tensile stresses
The orientation of tensile stresses (tangential in this study) has implications for crack propagation direction and fatigue life

For piping systems operating in corrosive environments, such as sour gas service or high-temperature hydrocarbon service, the combination of martensitic microstructure and high tensile residual stress represents a significant risk factor. Engineers should carefully control cooling rates, preheat temperatures, and post-weld treatment to manage the martensitic transformation and associated stress state.

Study Insights

This research demonstrates the power of finite element modeling in understanding complex thermo-mechanical phenomena during welding. The validated model allows prediction of stress states for different process parameters without extensive experimental testing. For engineering practice, the key takeaway is that martensitic transformation is not merely a microstructural event but a significant stress-generating mechanism that must be accounted for in weld design and quality assurance. The tangential tensile stresses identified in this study can drive crack initiation and propagation in overlay welds, particularly in thick-section components where constraint is high.