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

Literature Overview

This study presents a finite element analysis (FEA) approach to understanding how preheating influences the transient temperature field and residual stress distribution during weld overlay of medium-high carbon steel substrates. The topic addresses a fundamental concern in overlay welding practice: the competition between minimizing cracking susceptibility (which favors higher preheat) and controlling residual stress and distortion (which may be adversely affected by excessive preheat). For engineers working with materials such as C45, C55, 42CrMo4, and similar grades where carbon equivalent (CE) values typically range from 0.35 to 0.55, understanding the preheat-temperature-stress relationship is essential for developing sound welding procedures.

Core Technical Methodology

The numerical model employs a coupled thermo-mechanical approach using a sequential coupling strategy, where the thermal analysis is solved first and the results are mapped onto a mechanical model. The heat source is modeled using a double-ellipsoidal Goldak model, which accounts for the asymmetric heat distribution in moving arc processes. The material properties are defined as temperature-dependent functions, incorporating density, thermal conductivity, specific heat, Young's modulus, and coefficient of thermal expansion across the relevant temperature range (room temperature to 1500°C).

The simulation considers a multi-pass overlay deposit (typically 3–5 passes) on a plate substrate of 25–40 mm thickness, which is representative of common engineering scenarios such as repair of worn shafts, cylinder liners, and pressure vessel heads.

Temperature Field Analysis

The results demonstrate that preheating significantly modifies the thermal cycle parameters at the weld. At a preheat of 0°C, the peak cooling rate (at 800°C) in the heat-affected zone reaches 8–12°C/s, while at 300°C preheat, this rate drops to 3–5°C/s. This reduction in cooling rate is the primary mechanism by which preheat mitigates cracking susceptibility in high-carbon steels, as it extends the time the microstructure spends in the critical transformation temperature range (550–700°C), allowing for diffusive transformation rather than martensitic transformation.

An important observation is that the temperature field distribution becomes increasingly symmetric with higher preheat, as the substrate no longer acts as a dominant heat sink. This symmetry has implications for residual stress patterns, as discussed below.

Residual Stress Field Analysis

The residual stress distribution reveals a complex interplay between thermal contraction, plastic deformation, and microstructural transformation. At low preheat (0–100°C), the longitudinal residual stress in the weld overlay exhibits high tensile values (280–350 MPa) concentrated in the weld centerline, with compressive stresses developing in the adjacent heat-affected zone. At higher preheat levels (300–400°C), the peak longitudinal stress decreases to 200–260 MPa due to the reduced thermal gradients and increased material ductility during solidification.

The results indicate that while preheat reduces peak residual stresses, it simultaneously increases the affected zone (the region experiencing stresses above 100 MPa), which may have implications for fatigue life assessment. The transverse stress is consistently lower than the longitudinal stress, consistent with the constraint conditions imposed by the weld geometry and deposition sequence.

Cracking Susceptibility Assessment

Using the calculated thermal cycles as input to a cracking susceptibility model based on the modified Kolhoff criterion, the study quantifies the cracking risk at different preheat levels. The Kolhoff criterion compares the calculated cooling rate with a critical cooling rate derived from the material's carbon equivalent and alloying content.

For a C45 steel (CE ≈ 0.42), the critical cooling rate is approximately 3.5°C/s at 800°C. At 0°C preheat, the calculated cooling rate significantly exceeds this critical value, indicating a high probability of martensitic transformation and subsequent cracking. At 200°C preheat, the cooling rate approaches the critical value, suggesting marginal safety. At 300°C and above, the cooling rate falls well below the critical value, indicating low cracking susceptibility.

These findings align with the empirical preheat recommendations in welding procedure specifications, where C45 steels typically require 150–250°C preheat for overlay welding operations. The numerical simulation provides a more rigorous justification for these empirical values and allows for extrapolation to thicker sections or different geometries where empirical data may be unavailable.

Engineering Practice Integration

In practice, the simulation results can be applied through several approaches. For routine production welding, the preheat recommendations derived from this type of analysis can be incorporated into welding procedure specifications (WPS) with confidence, knowing that the thermal and mechanical consequences have been quantitatively evaluated. For repair operations on existing equipment where preheat may be limited by adjacent components or operational constraints, the simulation can be used to assess the risk associated with reduced preheat and to determine whether compensating measures (such as post-weld heat treatment or modified deposition sequences) are necessary.

The study also highlights the importance of interpass temperature control. Maintaining interpass temperatures within the preheat range (±50°C) ensures consistent thermal cycles across multiple passes and prevents localized overheating that could lead to grain coarsening or excessive dilution in the overlay layer.

Key Questions and Reflections

The study raises several important considerations for practice. First, the accuracy of the simulation depends critically on the material property inputs, particularly the temperature-dependent transformation behavior, which is difficult to characterize experimentally for complex alloys. Engineers should be aware that the simulation results represent a best estimate and should be validated against experimental measurements when possible.

Second, the model assumes uniform preheat across the entire component surface, which may not be achievable in practice for large or complex geometries. Localized preheat using induction heating or resistance heating may produce non-uniform temperature fields that deviate from the simulated conditions. Third, the model does not account for the effects of hydrogen diffusion, which is a significant contributor to delayed cracking in high-carbon steels and may require additional post-weld dwell time at preheat temperature before cooling.

Study Insights and Outlook

The numerical simulation approach provides a powerful tool for optimizing preheat parameters in overlay welding of medium-high carbon steels. The key insight is that preheat serves a dual purpose: it reduces cracking susceptibility by slowing the cooling rate below the critical transformation rate, and it reduces residual stress by lowering thermal gradients. However, excessive preheat beyond 300°C for most medium-carbon grades offers diminishing returns in terms of stress reduction while increasing the risk of grain coarsening and reduced mechanical properties in the heat-affected zone. The optimal preheat range for most medium-high carbon steel overlay operations is 200–300°C, balancing all competing factors. Future work should incorporate hydrogen diffusion modeling and microstructural transformation kinetics to provide more comprehensive predictions of weld quality.