CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Preheating Effects on Cooling Temperature Field and Stress Field of Overlay Welding on Medium-High Carbon Steel
Literature Overview
This 2013 publication in Journal of Thermal Engineering and Materials Processing by Zang Xinliang, Liu Ligang, Wang Yuhui, Qian Yu, and Yang Qingxiang from Yanshan University and Qinhuangdao Northern Pipe Industry Co., Ltd., presents a numerical simulation study of how preheating temperature affects the cooling temperature field and residual stress field during overlay welding on medium-high carbon steel substrates. Funded by the Hebei Provincial Natural Science Foundation (E2012203019) and the Hebei Provincial Hundred Talents Support Program (SPRC021), this work addresses a critical practical problem in the fabrication of overlay-clad pipes and pressure vessels for harsh service environments.
Core Technical Points
Numerical Simulation Methodology
The study employs finite element analysis (FEA) to model the transient thermal and mechanical behavior during overlay welding. The simulation approach involves:
| Modeling Aspect | Methodology | Key Parameters |
|---|---|---|
| Thermal analysis | Coupled thermo-mechanical FE | Heat input, preheat temperature, cooling rate |
| Material properties | Temperature-dependent | Thermal conductivity, specific heat, Young's modulus |
| Boundary conditions | Symmetry, convective cooling | Ambient temperature, heat transfer coefficient |
| Welding process | Moving heat source | Travel speed, wire feed rate, arc force |
| Residual stress | Elastic-plastic analysis | Yield strength, plastic strain, thermal expansion |
The simulation captures the complex thermal history experienced by the substrate and overlay during welding, including:
- Peak temperature: Reaching several thousand degrees in the molten pool region
- Cooling rate: Varying from rapid (near the weld) to gradual (far from the weld)
- Thermal cycling: Multiple heating and cooling cycles during multi-pass welding
Preheating Temperature Effects
The study systematically investigates the influence of different preheating temperatures on the thermal and stress fields:
Without preheating (0°C): The rapid cooling rate produces high residual stresses, with compressive stresses near the weld and tensile stresses in the surrounding region. The high stress gradients increase the risk of cracking in the heat-affected zone (HAZ) and the overlay layer.
Moderate preheating (150-250°C): The cooling rate is reduced, resulting in lower residual stresses and improved ductility of the HAZ microstructure. The stress distribution becomes more uniform, reducing stress concentration at the overlay-substrate interface.
Excessive preheating (>400°C): While residual stresses are further reduced, excessive preheating may lead to undesirable microstructural changes in the HAZ, such as grain coarsening or phase transformation to soft phases.
Residual Stress Analysis
The simulation reveals that residual stress distribution is governed by:
- Thermal contraction: The primary driver of residual stress, as the weld metal and HAZ cool and contract against the surrounding material
- Phase transformation: Martensitic transformation in the HAZ of medium-high carbon steel can introduce additional transformation strains
- Constraint effects: The geometry of the component and the boundary conditions influence the stress distribution
- Multi-pass interaction: Subsequent passes partially relieve stresses from previous passes but introduce new stress patterns
| Preheat Temperature | Peak Residual Stress (MPa) | Maximum Cooling Rate (°C/s) | Cracking Risk |
|---|---|---|---|
| 0°C | 550-650 | 80-120 | High |
| 150°C | 400-500 | 50-80 | Moderate |
| 250°C | 300-400 | 30-60 | Low |
| 350°C | 250-350 | 20-40 | Very Low |
Integration with Engineering Practice
The simulation results have direct implications for welding procedure specification (WPS) development and qualification:
- Preheat temperature selection: The study provides quantitative data to justify preheat temperatures in WPS for medium-high carbon steel overlay welding. Typical preheat temperatures of 150-250°C are recommended for most applications, with higher temperatures required for thick sections or high-constraint geometries.
- Stress relief requirements: The simulation identifies regions of high residual stress that may require post-weld heat treatment (PWHT) to reduce the risk of stress corrosion cracking or fatigue failure.
- Weld sequence optimization: For multi-pass overlay welding, the simulation can guide the selection of weld sequence to minimize overall residual stress and distortion.
- Quality assurance: Understanding the expected residual stress distribution enables engineers to design inspection procedures that target high-stress regions for NDT examination.
For pressure vessel fabrication, the residual stress analysis is particularly important because:
- Residual stresses superimpose on operational stresses, potentially exceeding the allowable stress limits
- Stress corrosion cracking (SCC) susceptibility is enhanced by tensile residual stresses in the presence of aggressive media
- Fatigue life is reduced by high residual stress gradients, especially at the overlay-substrate interface
Key Questions and Reflections
While numerical simulation provides valuable insights, several limitations must be acknowledged. The accuracy of the simulation depends on the quality of material property data, which may vary significantly between different heat treatments and production batches. The thermal-mechanical coupling model assumes idealized boundary conditions that may not fully represent the actual welding environment, including the presence of clamping fixtures, backing bars, and interpass temperature control.
Additionally, the simulation does not capture the complex metallurgical transformations that occur during welding, such as martensite formation in the HAZ of medium-high carbon steel. These transformations introduce additional strains that are not fully represented in the elastic-plastic analysis. Engineers must therefore use simulation results as a guide rather than a definitive prediction, supplementing with experimental validation through strain measurement, X-ray diffraction, or neutron diffraction.
Study Insights and Implications
The numerical simulation work by Zang et al. provides a powerful tool for optimizing preheat temperatures in overlay welding on medium-high carbon steel substrates. The quantitative relationship between preheat temperature, cooling rate, and residual stress distribution enables engineers to make informed decisions about welding procedure parameters. For pressure vessel and pipe fabrication, where residual stress control is critical for long-term service reliability, this type of analysis should be integrated into the design and fabrication documentation. The study underscores the importance of considering thermal-mechanical effects in welding procedure development, moving beyond empirical approaches toward physics-based optimization of welding parameters.