Numerical Simulation of Temperature and Stress Fields in Medium-High Carbon Steel Overlay Welding

Literature Overview

The paper by Yang Qingxiang, Gao Jun, Li Da, and Chen Xiaojun from the State Key Laboratory of Metastable Materials Preparation Science at Yanshan University presents a numerical simulation study of the temperature field and stress field during overlay welding on medium-high carbon steel substrates. Published in the journal "Welding Journal" in 2006, this work was supported by the Hebei Provincial Science and Technology Program (04212201D) and the Ministry of Education Overseas Research Fellowship. The research addresses the critical challenge of predicting and controlling residual stresses and thermal distortions in overlay welding operations on high-carbon steel components, where cracking susceptibility is a major concern.

Core Technical Content

Medium-high carbon steel substrates, typically in the range of 0.4–0.7% carbon, present unique challenges for overlay welding due to their high hardenability and susceptibility to hydrogen-induced cracking. The high carbon content promotes the formation of hard, brittle martensite in the heat-affected zone (HAZ) during rapid cooling, which can lead to cracking if not properly managed. The numerical simulation approach allows engineers to predict the thermal cycle and stress distribution before actual welding, enabling optimization of process parameters to minimize cracking risk.

The finite element method (FEM) is employed to model the transient temperature field during the welding process, considering the moving heat source, material properties that vary with temperature, and the boundary conditions imposed by the workpiece geometry. The thermal model is then coupled with a mechanical model to compute the stress and strain fields, accounting for elastic-plastic deformation, thermal expansion, and phase transformation effects.

Thermal Field Analysis

The temperature field simulation reveals that the peak temperature at the weld center can reach 2000–2500°C, while the temperature drops rapidly with distance from the weld centerline. The cooling rate in the HAZ is the most critical parameter influencing the microstructure and cracking susceptibility. For medium-high carbon steel, the critical cooling rate for avoiding martensite formation is approximately 10–15°C/s, below which the HAZ retains a ferrite-pearlite microstructure with adequate toughness. Above this rate, martensite begins to form, increasing hardness to 400–500 HV and significantly reducing toughness.

The simulation results show that the thermal cycle is highly asymmetric, with rapid heating (rates of 100–500°C/s) followed by slower cooling (5–50°C/s depending on location and preheat). This asymmetry is characteristic of arc welding processes and has significant implications for residual stress development. The peak temperature distribution follows a Gaussian-like profile perpendicular to the weld direction, with the highest temperatures concentrated in a narrow zone around the weld centerline.

The effect of preheat temperature on the thermal cycle is substantial. Increasing preheat from 100°C to 300°C reduces the peak cooling rate in the HAZ by approximately 40–50%, effectively suppressing martensite formation. However, excessive preheat can lead to grain coarsening in the HAZ and increased distortion of the workpiece, creating a trade-off that must be carefully managed.

Stress Field Analysis and Cracking Prediction

The residual stress field developed during overlay welding is complex, with longitudinal tensile stresses along the weld centerline reaching 300–500 MPa, while transverse stresses can be either tensile or compressive depending on the welding sequence and constraint conditions. The high residual stresses combined with the hard, brittle martensitic microstructure in the HAZ create a favorable condition for hydrogen-induced cracking, particularly in high-strength medium-high carbon steels.

The simulation incorporates the effect of phase transformation on residual stress through the dilatometry model, which accounts for the volume change associated with austenite-to-martensite transformation. This transformation-induced plasticity (TRIP) effect can either relax or intensify residual stresses depending on the sequence of transformation and mechanical loading. In the HAZ of medium-high carbon steel, the transformation from austenite to martensite typically occurs during cooling at temperatures below the martensite start temperature (Ms), which for 0.5% carbon steel is approximately 250–300°C.

The cracking risk assessment based on the simulation results employs the hydrogen cracking criterion, which considers the combined effect of residual stress, hardness, and hydrogen concentration. The critical condition for cracking is when the maximum tensile stress exceeds the material's yield strength at the temperature of maximum hydrogen diffusion, which typically occurs in the 200–400°C range. The simulation provides a predictive tool for identifying high-risk zones and optimizing process parameters to reduce cracking probability.

Process Optimization Based on Simulation Results

The numerical simulation enables systematic optimization of welding parameters to minimize cracking risk while maintaining adequate weld quality. The key parameters identified for optimization include preheat temperature, welding current, welding speed, and interpass temperature. The simulation results indicate that a preheat temperature of 250–300°C combined with a welding current of 200–250 A and a welding speed of 200–250 mm/min provides an optimal balance between cracking resistance and distortion control.

The interpass temperature control is another critical factor identified by the simulation. Maintaining the interpass temperature below 250°C prevents excessive softening of the previously deposited weld metal while still providing adequate thermal mass to reduce the cooling rate of subsequent passes. The simulation also reveals that multi-pass welding with controlled thermal input per pass is more effective than single-pass welding in managing residual stresses, as each subsequent pass partially relieves the stresses from the previous pass.

Engineering Practice Integration

In practical engineering applications, the simulation results are used to develop welding procedure specifications (WPS) that incorporate the optimized parameters identified through numerical analysis. The simulation also aids in the selection of post-weld heat treatment (PWHT) parameters, such as temperature and duration, to further reduce residual stresses and improve the toughness of the HAZ. A typical PWHT for medium-high carbon steel overlay welds involves heating to 550–650°C for 1–2 hours per 25 mm of thickness, followed by controlled cooling in the furnace.

The integration of numerical simulation with experimental validation is essential for developing reliable predictive models. The simulation results are typically validated by comparing predicted temperature and stress distributions with experimental measurements obtained from thermocouples, strain gauges, and X-ray diffraction residual stress measurements. Discrepancies between predicted and measured values are used to refine the material property models and boundary conditions, improving the accuracy of subsequent simulations.

Study Insights and Conclusions

This literature demonstrates the power of numerical simulation as a tool for understanding and controlling the complex thermal-mechanical phenomena in overlay welding of medium-high carbon steel. The simulation approach provides insights that are difficult or impossible to obtain through experimental methods alone, particularly regarding the transient stress states and phase transformation sequences during welding. For engineers, the key insight is that numerical simulation should be used as a complement to, not a replacement for, experimental validation, with both approaches working together to develop robust welding procedures. The study also highlights the importance of considering the entire thermal-mechanical history of the weld, including the effects of phase transformation, in predicting cracking susceptibility.