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

Literature Overview

The paper by Zang Xinliang and colleagues from Yanshan University, Qinhuangdao Northern Pipe Industry Co., Ltd., and the State Key Laboratory of Advanced Special Steel Processing at Yanshan University, published in Journal of Thermal Analysis and Calorimetry (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 during surfacing of 60CrMnMo steel. The study was supported by the Hebei Natural Science Foundation (E2012203019) and the Hebei Provincial Top 100 Talents Support Plan (SPRC021). The research measured the cooling temperature field and residual stress field of 60CrMnMo steel surfacing specimens without preheating, established a two-dimensional finite element model, and validated the model against experimental measurements. The model was then used to simulate the effects of 200°C preheating, with particular attention to the stress changes associated with martensitic transformation at different cross-sections.

Core Findings and Technical Analysis

Model Validation

The simulation results for the temperature field and residual stress field without preheating show good agreement with experimental measurements, validating the accuracy and reliability of the finite element model. This validation is critical for the credibility of subsequent simulations with preheating conditions.

Temperature Field Evolution

The temperature field evolution during surfacing cooling was characterized at different time points:

Time After Surfacing	Temperature Distribution
Initial (t = 0)	Center of specimen at high temperature; surrounding areas approximately 200°C
t = 30 s	Center of specimen undergoing rapid cooling
t = 180 s	Temperature distribution across the entire cross-section approaching uniformity

The cooling rate is highest at the center of the specimen immediately after surfacing and decreases over time as heat dissipates to the surrounding material. The 200°C preheat provides a thermal buffer that slows the initial cooling rate, reducing the thermal gradient between the surfacing layer and the base metal.

Stress Field Analysis

The stress field analysis reveals several important findings:

Maximum tensile stress location: The maximum tensile stress appears in the heat-affected zone (HAZ), not in the surfacing layer itself. This is a critical finding because the HAZ is often overlooked in surfacing quality assessments.
Martensitic transformation effect: When martensitic transformation occurs, the tensile stress in the HAZ increases dramatically until it reaches a value comparable to the equilibrium tensile stress. The martensitic transformation induces volumetric expansion, which generates additional tensile stress in the surrounding material.
Preheating effect: Preheating at 200°C reduces the maximum tensile stress in the HAZ, thereby decreasing the tendency for cracking during and after surfacing.

Stress Distribution Comparison

Condition	Maximum Tensile Stress Location	Stress Magnitude	Cracking Tendency
Without preheating	HAZ	High	High
With 200°C preheating	HAZ	Reduced	Reduced

The reduction in HAZ tensile stress with preheating is attributed to the reduced thermal gradient between the surfacing layer and the base metal, which lowers the thermal stress component. Additionally, the slower cooling rate associated with preheating reduces the martensitic transformation stress by allowing more time for stress relaxation.

Engineering Practice Implications

Preheating as a Cracking Prevention Strategy

The study provides quantitative evidence that preheating at 200°C is an effective strategy for reducing cracking tendency in 60CrMnMo steel surfacing operations. This is particularly relevant for the surfacing of pipe fittings, where cracking during surfacing can lead to component rejection and costly rework.

HAZ Stress Monitoring

The finding that maximum tensile stress occurs in the HAZ rather than the surfacing layer has important implications for quality control. Engineers should focus their stress analysis and cracking assessment efforts on the HAZ region, not just the surfacing layer. Non-destructive testing methods such as ultrasonic testing (UT) and magnetic particle testing (MT) should be applied to the HAZ region to detect potential cracking.

Process Parameter Optimization

The following table summarizes the recommended process parameters based on the simulation findings:

Parameter	Recommendation	Rationale
Preheat temperature	200°C (minimum)	Reduces HAZ tensile stress and cracking tendency
Interpass temperature	Maintain above 150°C	Prevents excessive cooling between passes
Post-weld heat treatment	Low-temperature stress relief (250–350°C)	Further reduces residual stress without softening
Surfacing layer thickness	Moderate (not too thick)	Thick layers increase thermal gradient and stress
Welding sequence	Multi-pass with controlled layer thickness	Distributes thermal input and reduces peak stress

Application to Pipe Fitting Surfacing

For pipe fitting surfacing operations, the following considerations should be applied:

Preheating of the entire fitting to 200°C before surfacing to ensure uniform thermal conditions.
Interpass temperature monitoring to maintain temperatures above 150°C between surfacing passes.
Post-weld stress relief to reduce residual stresses in the HAZ.
Non-destructive testing of the HAZ region using UT or MT to detect any cracking.
Documentation of preheat and interpass temperatures for traceability and quality assurance.

Key Questions and Reflections

The study focuses on 60CrMnMo steel, which is a specific medium-high carbon steel grade. The findings may not be directly applicable to other steel grades, particularly those with different carbon content, alloy composition, or hardenability. Engineers should conduct similar simulations or experimental studies for their specific steel grades to determine the optimal preheat temperature and process parameters.

Another reflection concerns the two-dimensional nature of the finite element model. While 2D models are computationally efficient and provide valuable insights, they may not fully capture the three-dimensional stress state in actual surfacing operations, particularly for complex geometries such as pipe fittings with bends, tees, and reducers. Three-dimensional simulations would provide more accurate predictions but at a significantly higher computational cost.

The study also does not address the long-term effects of residual stress on fatigue performance and creep behavior. For applications subject to cyclic loading or elevated temperatures, the residual stress distribution may have significant implications for component life. Engineers should consider the residual stress field as a critical design parameter, not just a manufacturing concern.

Study Insights and Implications

This numerical simulation study provides valuable quantitative insights into the effects of preheating on the temperature and stress fields during surfacing of medium-high carbon steel. The validation against experimental measurements establishes the credibility of the model, and the findings clearly demonstrate that 200°C preheating is an effective strategy for reducing HAZ tensile stress and cracking tendency. The identification of the HAZ as the critical region for stress concentration and potential cracking has important implications for quality control and process design. Engineers working on pipe fitting surfacing operations should incorporate these findings into their process procedures, including preheating requirements, interpass temperature control, post-weld stress relief, and non-destructive testing protocols. The simulation methodology presented in this study can be adapted for other steel grades and geometries, providing a powerful tool for process optimization and quality improvement in surfacing operations.