Numerical Simulation of Preheating Effects on Temperature Field and Stress Field in Medium-High Carbon Steel Overlay Welding

Literature Overview

This paper by Zang Xinliang, Liu Ligang, Wang Yuhui, Qian Yu, and Yang Qingxiang, published in Transactions of Materials and Heat Treatment (2013, Vol. 34, No. 10, pp. 187-192), presents a numerical simulation study of the effects of preheating on the cooling temperature field and residual stress field in overlay welding of 60CrMnMo medium-high carbon steel. The authors are affiliated with Yanshan University, Qinhuangdao Northern Pipe Industry Co., Ltd., and the State Key Laboratory of Metastable Materials Preparation Science and Technology at Yanshan University. The research was supported by the Hebei Provincial Natural Science Foundation (E2012203019) and the Hebei Provincial Top-100 Talents Support Program (SPRC021). The study employs finite element analysis with a two-dimensional model, validated against experimental measurements of temperature and residual stress fields.

Core Technical Findings

Model Development and Validation

The authors first measured the cooling temperature field and residual stress field of a 60CrMnMo steel overlay weld specimen under no-preheat conditions. A two-dimensional finite element model was then developed and validated against these experimental measurements. The model demonstrated good agreement with the measured temperature and residual stress fields, confirming its validity for subsequent simulations under preheated conditions.

The validation of the model is a critical step that lends credibility to the simulation results. The agreement between simulated and measured fields indicates that the model adequately captures the essential physics of the overlay welding process, including heat transfer, phase transformation, and residual stress development.

Temperature Field Evolution

Under no-preheat conditions, the temperature field during cooling shows a characteristic pattern. After preheating to 200 degrees Celsius, the initial temperature distribution shows a higher temperature at the specimen center with the remaining areas at approximately 200 degrees Celsius. At 30 seconds into cooling, the center region experiences a rapid temperature drop. By 180 seconds, the temperature distribution across the entire cross-section becomes relatively uniform.

This temperature evolution pattern is important for understanding the cooling rate and its effects on microstructure and residual stress. The rapid initial cooling at the center followed by gradual uniformization has implications for phase transformation kinetics and the development of transformation-induced stresses.

Residual Stress Distribution

A key finding is that the maximum tensile residual stress occurs in the heat-affected zone (HAZ) rather than in the weld overlay zone. This is counterintuitive, as the weld zone typically experiences the highest thermal gradients and phase transformations. The study attributes this to the specific thermal and metallurgical conditions of the overlay welding process on 60CrMnMo steel.

When martensitic transformation occurs, the tensile stress in the HAZ increases dramatically until it reaches a value comparable to the equilibrium tensile stress. This transformation-induced stress increment is a critical factor in the cracking tendency of the overlay weld.

Preheating Effects on Cracking Tendency

The simulation demonstrates that preheating to 200 degrees Celsius reduces the maximum tensile residual stress in the HAZ, thereby reducing the overlay welding cracking tendency. The mechanism is that preheating reduces the peak temperature gradient during cooling, which in turn reduces the thermal residual stress. Additionally, the slower cooling rate provided by preheating can influence the phase transformation sequence and the associated transformation stresses.

Technical Parameter Summary

Connection to Engineering Practice

This study has direct practical significance for overlay welding operations on medium-high carbon steel components in the steel pipe and piping equipment industry. 60CrMnMo and similar high-carbon alloy steels are commonly used for components requiring high strength and wear resistance, such as valve bodies, pump casings, and pipe fittings. The cracking susceptibility of these materials during overlay welding is a well-known challenge that requires careful process control.

The finding that the maximum tensile stress occurs in the HAZ rather than the weld zone is particularly important for quality control. In practice, inspection efforts are often focused on the weld overlay itself, with the HAZ receiving less attention. This study suggests that the HAZ should be a primary focus of non-destructive testing and quality assessment for overlay welds on medium-high carbon steels.

The demonstration that preheating to 200 degrees Celsius reduces cracking tendency provides a practical process recommendation. However, the optimal preheating temperature may vary depending on the specific base material composition, component geometry, and welding parameters. A systematic study of preheating temperature effects across a range of values would provide more comprehensive guidance for process development.

Key Questions and Reflections

Several important questions arise from this study. First, the simulation is limited to a two-dimensional model, which may not fully capture the three-dimensional stress state in actual production components. The transition from 2D to 3D modeling can significantly affect the predicted stress distribution and magnitude. Second, the study focuses on a single preheating temperature of 200 degrees Celsius, but the optimal preheating temperature may depend on the specific welding parameters and component geometry. Third, the study does not address the post-weld heat treatment effects on residual stress relief, which is often an important part of the overlay welding process for high-carbon steels.

The consideration of martensitic transformation-induced stress is a sophisticated aspect of this study that deserves emphasis. In many practical welding analyses, the focus is on thermal residual stress, with transformation stresses being neglected or approximated. This study demonstrates that transformation stresses can be a dominant factor in the residual stress state, particularly in the HAZ where the transformation sequence is complex.

The validation of the numerical model against experimental measurements is a strength of this work. In my experience with welding simulation studies, the credibility of simulation results depends heavily on the rigor of model validation. The good agreement reported here provides confidence that the simulation results are physically meaningful and can be used for process optimization.

Study Insights and Implications

The most significant contribution of this work is the demonstration that finite element simulation can be effectively used to predict and optimize the preheating requirements for overlay welding of medium-high carbon steels. The identification of the HAZ as the critical region for cracking initiation, combined with the demonstration of preheating effectiveness, provides actionable guidance for welding procedure development.

For engineers involved in overlay welding operations on high-carbon alloy steel components, this study underscores the importance of considering both thermal and transformation-induced stresses in the evaluation of cracking risk. The finite element approach provides a powerful tool for predicting residual stress distributions and optimizing preheating temperatures, potentially reducing the need for extensive trial-and-error qualification testing.

The practical implication is that overlay welding procedures for medium-high carbon steel components should include carefully controlled preheating, with the preheating temperature selected based on a combination of simulation predictions and experimental validation. This approach can significantly improve the quality and reliability of overlay welds while reducing production costs associated with cracking and rework.