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

Literature Overview

The study 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), presents a finite element numerical simulation of the cooling temperature field and residual stress field during surfacing of 60CrMnMo medium-high carbon steel. The work was supported by the Hebei Natural Science Foundation (E2012203019) and the Hebei Province Hundred Talents Support Program (SPRC021). The research bridges experimental measurement with computational modeling to establish a predictive framework for surfacing process optimization.

Research Background and Motivation

60CrMnMo is a medium-high carbon alloy steel widely used in heavy-duty mechanical components such as gear shafts, crankshafts, and high-strength fasteners. Surfacing repair of such components is challenging due to:

• High hardenability of the base metal, leading to hard and brittle heat-affected zones.
• Significant thermal gradients during surfacing causing high residual stresses.
• Propensity for cold cracking in the HAZ due to martensitic transformation.
• Preheating is a standard practice to mitigate these issues, but its quantitative effects on temperature and stress fields have not been extensively modeled.

Methodology and Model Development

The study employed a two-dimensional finite element model to simulate the surfacing cooling process. The methodology involved:

1. Experimental baseline: Temperature fields and residual stress fields were measured under no-preheat conditions on 60CrMnMo surfacing specimens.
2. Model validation: The finite element model was calibrated against experimental data to ensure accuracy.
3. Preheat simulation: The validated model was then used to simulate the 200°C preheating condition.
4. Phase transformation coupling: The model incorporated martensitic transformation kinetics and associated volumetric expansion effects on stress development.

Temperature Field Analysis

The temperature field evolution during surfacing cooling was analyzed at multiple time points:

Without preheating, the cooling rate is significantly higher due to the greater temperature difference between the molten pool and the room-temperature substrate. With 200°C preheating, the effective cooling rate is reduced, particularly in the early cooling stage when the HAZ is most vulnerable to cracking.

Residual Stress Field Analysis

The residual stress analysis revealed critical findings:

The maximum tensile stress occurs in the HAZ rather than in the surfacing zone itself. This is a crucial finding because it indicates that:

1. The HAZ is the most vulnerable region for cold cracking initiation.
2. Martensitic transformation in the HAZ causes significant volumetric expansion, generating additional tensile stress.
3. The stress in the HAZ increases dramatically upon martensitic transformation, reaching values comparable to equilibrium stress levels.

Effect of Preheating on Stress Reduction

The 200°C preheating reduces the maximum tensile stress in the HAZ through several mechanisms:

• Reduced cooling rate decreases the driving force for rapid martensitic transformation.
• Higher initial temperature allows for more complete austenite homogenization before transformation.
• Thermal stresses during cooling are reduced due to the smaller temperature differential.
• The overall stress state is shifted toward a more compressive regime in the HAZ.

Model Validation and Accuracy

The comparison between simulated and experimental results demonstrated good agreement:

The validated model provides confidence for predictive simulations of different preheating temperatures, surfacing geometries, and alloy compositions.

Engineering Applications and Process Optimization

Based on the simulation results, the following process recommendations emerge for surfacing medium-high carbon steel components:

1. Preheating temperature: 200°C is effective for 60CrMnMo; higher temperatures (250–300°C) may be required for higher carbon equivalents.
2. Interpass temperature control: Maintaining interpass temperature above 150°C prevents excessive cooling between passes.
3. Post-weld heat treatment: Stress relief at 550–650°C can further reduce residual stresses in the HAZ.
4. Weld sequence optimization: Multi-pass surfacing with appropriate pass geometry can distribute thermal input more uniformly.
5. Monitoring: In-situ thermocouple monitoring during surfacing can validate the predicted thermal cycles.

Key Reflections and Methodological Insights

The coupling of thermal, mechanical, and metallurgical phenomena in surfacing residual stress development is complex. This study demonstrates that a properly formulated finite element model can capture the essential physics, including the critical role of martensitic transformation in HAZ stress development. The finding that maximum tensile stress occurs in the HAZ rather than the weld metal itself is particularly important for quality control—conventional inspection focuses on the weld metal, but the HAZ may be the critical failure region.

The approach of validating against experimental data before extending simulations to different process conditions is methodologically sound and provides confidence in the predictive capability of the model.

Study Insights and Outlook

This work establishes a computational framework that can be extended to three-dimensional models for complex component geometries. The methodology is directly applicable to surfacing repair of large piping components, pressure vessels, and structural steel members where preheating decisions significantly impact service integrity. For piping engineering applications, where surfacing is used for corrosion repair or dimensional restoration, understanding the stress state in the HAZ is essential for predicting long-term fatigue and fracture behavior.

The integration of numerical simulation with experimental validation represents the modern approach to welding process development, reducing reliance on costly trial-and-error while providing detailed insight into the underlying physical mechanisms that govern weld quality and integrity.