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

Literature Overview and Research Context

The study by Zang Xinliang and colleagues (2013), published in Materials Heat Treatment and supported by the Hebei Natural Science Foundation (E2012203019) and the Hebei Provincial Hundred Talents Support Program (SPRC021), addresses a critical engineering challenge in overlay welding: the influence of preheating temperature on the cooling temperature field and residual stress distribution in medium to high carbon steel substrates. This work originates from Yanshan University's School of Mechanical Engineering in collaboration with Qinhuangdao Northern Pipe Co., Ltd., reflecting a strong industry-academia linkage that is particularly valuable for practitioners.

Medium and high carbon steels, such as those in the C45 to C70 range or equivalent grades like 45#, 50#, and 65Mn, are widely used in wear-resistant applications including mining equipment, mining machinery, and heavy-duty structural components. However, their high carbon content creates a significant susceptibility to cold cracking during welding operations. The preheating step is therefore not merely a procedural formality but a fundamental metallurgical control measure that directly determines the integrity of the overlay joint.

Core Technical Content and Key Findings

The numerical simulation approach employed in this study utilizes finite element analysis (FEA) to model the transient thermal and mechanical fields during overlay welding. The simulation typically incorporates the following key process parameters:

The central finding of this research is that preheating temperature exerts a decisive influence on both the peak temperature distribution and the subsequent cooling rate in the heat-affected zone (HAZ). At lower preheating temperatures (below 150°C), the cooling rate at the 800°C threshold can exceed 20°C/s, promoting the formation of hard, brittle martensite in the HAZ. As preheating temperature increases to 250–300°C, the cooling rate drops significantly to below 10°C/s, allowing for more favorable transformation products such as bainite and tempered martensite.

The residual stress analysis reveals that the maximum longitudinal tensile stress in the overlay weld and HAZ decreases substantially with increasing preheating temperature. At a preheating temperature of 100°C, peak residual stresses may reach 450–520 MPa, while at 300°C, they drop to approximately 300–380 MPa. This reduction is attributed to the elevated starting temperature reducing thermal gradients and allowing greater plastic deformation to accommodate thermal contraction.

Interpretation of Thermal Field Behavior

The thermal field simulation demonstrates that the cooling temperature field is characterized by a steep gradient near the fusion line, with the highest cooling rates occurring at the interface between the overlay layer and the base material. This interface region is particularly critical because it is where the highest carbon concentration from the base metal can dilute into the overlay weld, creating a zone of high hardness and potential cracking. The simulation results confirm that preheating effectively flattens the thermal gradient, reducing the cooling rate at this critical interface region.

From a metallurgical perspective, the cooling rate directly governs the phase transformation kinetics. For medium carbon steels with a carbon equivalent of approximately 0.5%, a cooling rate below 15°C/s at 800°C is generally required to avoid untempered martensite formation. The simulation data provides quantitative support for setting minimum preheating temperatures based on the specific carbon content and geometry of the component being overlaid.

Interpretation of Stress Field Behavior

The residual stress field is analyzed in three directions: longitudinal (along the weld), transverse (perpendicular to the weld), and through-thickness. The longitudinal stress is typically the highest and most detrimental, as it promotes cracking along the weld axis. The simulation shows that the stress distribution is asymmetric, with higher tensile stresses developing in the region closer to the start of the weld bead and lower stresses near the end.

An important insight from this work is the identification of an optimal preheating temperature window. Below 150°C, the stress reduction is marginal and cold cracking risk remains high. Between 200°C and 300°C, there is a pronounced reduction in both residual stress and cooling rate. Beyond 300°C, however, the benefits plateau, and excessive preheating may lead to grain coarsening in the HAZ, reduced hardness in the base material, and increased distortion. This suggests that for most medium carbon steel overlay welding applications, a preheating temperature of 250–300°C represents the optimal balance.

Engineering Practice Integration

Preheating Temperature Selection Guidelines

Based on the simulation findings, the following practical guidelines can be derived for field applications:

These recommendations align with the requirements of NB/T 47014 for weld procedure qualification and GB/T 150 for pressure vessel fabrication, which mandate preheating for materials with carbon equivalent exceeding 0.45%.

Practical Considerations for Pipe Overlay Welding

Given that the industry collaborator is Qinhuangdao Northern Pipe Co., Ltd., it is worth noting the specific challenges of overlay welding on pipe components. The cylindrical geometry introduces additional complexity compared to flat plate welding, as the curvature affects heat dissipation and residual stress distribution. The simulation results for flat plate conditions should be applied to pipe welding with appropriate corrections for geometry. For small-diameter pipes (below DN100), the effective preheating area should extend at least 50 mm from the weld zone on each side to ensure adequate thermal mass and uniform temperature distribution.

Non-Destructive Testing Implications

The residual stress levels predicted by the simulation have direct implications for non-destructive testing (NDT) strategy. High residual stresses (above 400 MPa) in the overlay weld can mask or simulate crack indications during ultrasonic testing (UT). Therefore, for components welded with insufficient preheating, the UT signal interpretation should account for potential stress-related artifacts. Conversely, components welded with adequate preheating (250–300°C) will exhibit lower residual stresses, leading to cleaner UT signals and more reliable defect detection.

Key Questions and Reflections

One question that arises from studying this literature is the extent to which numerical simulation results can be directly translated to production welding conditions. The simulation assumes idealized boundary conditions, uniform material properties, and constant process parameters. In actual production environments, factors such as wind, ambient temperature, welder technique variation, and base material condition can significantly alter the thermal and stress fields. The simulation results should therefore be treated as a baseline reference, with production parameters validated through coupon testing and actual weld inspection.

Another reflection concerns the interaction between preheating and the choice of overlay material. The simulation focuses on thermal and stress fields, but the dilution behavior at the interface is equally important. For medium carbon steel substrates, using a low-carbon or austenitic stainless steel overlay (such as 309L or 309Cb) can reduce dilution effects and improve the mechanical properties of the transition zone. The preheating temperature must be optimized in conjunction with the overlay material selection to achieve the best combination of wear resistance, toughness, and crack resistance.

The study also raises the question of whether alternative crack prevention measures, such as low-hydrogen filler metals, reduced heat input, or post-weld stress relief, could complement or partially replace the need for high preheating temperatures. In practice, a combination of measures is often the most effective strategy, with preheating serving as the primary control while filler metal selection and post-weld treatment providing additional safety margins.

Study Insights and Implications

This literature provides a valuable quantitative foundation for preheating temperature selection in medium to high carbon steel overlay welding. The numerical simulation approach offers a cost-effective alternative to extensive coupon testing, enabling rapid evaluation of different preheating scenarios without the need for physical weld preparation and destructive testing. For engineering practice, the key takeaway is that preheating is not simply a "warm-up" step but a fundamental metallurgical control that must be carefully calibrated to the base material composition, component geometry, and desired overlay properties.

The findings reinforce the importance of a systematic approach to overlay welding procedure development, where thermal modeling, metallurgical analysis, and mechanical testing are integrated into a cohesive qualification program. For pressure vessel fabrication governed by GB/T 150 or ASME VIII Div.1, the simulation results can support the justification of preheating requirements in welding procedure specifications (WPS) and provide engineering rationale for deviations from standard practice when necessary.

In conclusion, this study demonstrates that a preheating temperature of 250–300°C is optimal for most medium to high carbon steel overlay welding applications, effectively reducing cooling rates below critical thresholds and lowering residual stresses to acceptable levels. Engineers should use these findings as a starting point for procedure development, always validating simulation predictions through practical weld testing and rigorous non-destructive examination before approving production procedures.