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

Literature Overview

This 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 in 2012 (Vol. 33, No. 2, pp. 73-76), presents a two-dimensional finite element numerical simulation of the residual stress field in the overlay welding of hot rolling mill roll steel. Funded by the Hebei Provincial Science and Technology Support Program (Project 09215106D) and the Hebei Provincial Top Hundred Talents Support Program (SPRC021), this work addresses a critical engineering problem: the prediction and control of residual stresses in overlay weldments subjected to martensitic phase transformation during cooling.

Core Technical Methodology

The research employs a hybrid experimental-numerical approach. First, the temperature field and residual stress field during overlay welding are measured experimentally using an infrared thermography system and an X-ray stress meter, respectively. These experimental data, combined with the physical and mechanical parameters of the materials, are used to establish a two-dimensional finite element model of the overlay weld stress field. The model is validated by comparing simulated tangential residual stress values with measured values, demonstrating good agreement and confirming the validity of the model.

Simulation Results

The validated model is then used to simulate the tangential stress evolution during the cooling phase of the overlay welding process, specifically examining the effect of martensitic phase transformation. The results reveal a complex stress evolution:

• Early cooling stage (before martensitic transformation): The surface of the overlay weld exhibits tangential compressive stress, which is generated by the thermal contraction of the weld metal as it cools from the solidification temperature to the martensite start temperature (Ms).
• During martensitic transformation: As the cooling temperature drops below the Ms temperature, martensite begins to form. The volume expansion associated with the austenite-to-martensite transformation (typically 1-4% depending on composition) generates compressive stress at the surface.
• Late cooling stage (after substantial martensite formation): As the volume fraction of martensite increases and the transformation proceeds toward completion, the surface stress transitions from compressive to tensile. This tensile stress persists to room temperature and represents the final residual stress state of the overlay weldment.

Physical Mechanism

The transition from compressive to tensile tangential stress is governed by the competition between two effects: thermal stress (which is compressive at the surface due to constrained cooling) and transformation stress (which is initially compressive due to volume expansion but becomes tensile as the transformation progresses and the surrounding material constrains the expanding martensite). The final tensile stress state at the surface is a consequence of the cumulative volume expansion from martensitic transformation being partially offset by the thermal contraction that occurs as the material cools from the finish temperature (Mf) to room temperature.

Engineering Practice Implications

Residual Stress Control in Overlay Welding

For engineers working on overlay welding of hardfacing alloys onto piping components, valve bodies, and pump housings, the final residual stress state is a critical factor in the long-term performance of the overlay. Tensile residual stresses at the surface are detrimental because they:

1. Promote crack initiation: Surface tensile stresses lower the threshold for fatigue crack initiation, reducing the fatigue life of the component.
2. Accelerate stress corrosion cracking: In corrosive environments, tensile residual stresses can initiate and propagate stress corrosion cracks, particularly in susceptible materials such as high-strength steels and austenitic stainless steels.
3. Reduce contact fatigue resistance: In applications involving repeated contact loading (e.g., bearing surfaces, valve seats), tensile residual stresses reduce the resistance to contact fatigue spalling.

Mitigation Strategies

Based on the findings of this study, the following strategies can be employed to manage residual stresses in overlay weldments:

• Post-weld stress relief: A controlled tempering or stress relief heat treatment (typically 550-650°C for 2-4 hours) can reduce tensile residual stresses by 50-80%. However, this must be balanced against the potential for temper embrittlement or softening of the hardfacing alloy.
• Preheating and controlled cooling: Higher preheat temperatures reduce the cooling rate and can shift the transformation to lower temperatures where the volume expansion is more gradual. This results in a lower final tensile stress state.
• Multi-pass welding with lower heat input: Multiple thin passes with lower heat input per pass reduce the volume of material subjected to martensitic transformation in each pass, limiting the cumulative transformation stress.
• Peening or shot peening: Post-weld peening can introduce beneficial surface compressive stresses that counteract the tensile residual stresses from welding and transformation.

Application to Hot Rolling Mill Roll Overlay

The specific application of this study to hot rolling mill roll steel is particularly relevant. Hot rolling mill rolls are subjected to extreme thermal and mechanical loads during operation, and the overlay weld is used to restore worn roll surfaces or to apply a wear-resistant surface layer. The residual stress state of the overlay directly affects the roll's resistance to thermal fatigue cracking and spalling, which are the primary failure modes of overlay welds on hot rolling rolls.

Key Insights and Reflections

The most valuable contribution of this study is the clear demonstration that martensitic transformation during cooling can fundamentally alter the residual stress state of an overlay weldment. Many engineers assume that the residual stress in a weldment is determined solely by the thermal cycle and the mechanical constraint of the base metal. This study proves that phase transformation stresses must be explicitly considered in the prediction and control of residual stresses, particularly for overlay alloys that undergo martensitic transformation during cooling.

The two-dimensional finite element model developed in this study, while validated against experimental data, has inherent limitations. Three-dimensional effects, such as stress concentration at the weld toe and the influence of the base metal's elastic modulus on stress distribution, are not captured. For engineering applications involving complex geometries, three-dimensional finite element models with coupled thermal-metallurgical analysis would provide more accurate predictions. Nevertheless, the qualitative understanding of stress evolution presented here is directly applicable to practical residual stress management strategies.

This literature provides essential insight into the interplay between phase transformation and residual stress in overlay weldments. For engineers involved in the design and repair of overlay-welded piping components, understanding this mechanism is fundamental to achieving reliable long-term performance. The final tensile residual stress state identified in this study must be actively managed through appropriate process parameters and post-weld treatments to ensure the overlay meets its intended service life requirements.