CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Temperature and Stress Fields in Medium-High Carbon Steel Overlay Welding - A Literature Study Note
Literature Overview
This study note examines research conducted by Yang Qingxiang, Gao Jun, Li Da, and Chen Xiaojun from the State Key Laboratory of Metastable Materials Preparation Science and Technology at Yanshan University. Funded by the Hebei Provincial Science and Technology Program (Grant No. 04212201D) and the Ministry of Education Overseas Fellowship Fund, this work was published in 2006 in the journal Transactions of the Welding Institute of China. The research focuses on finite element numerical simulation of the temperature field and residual stress field before and after heat source removal in medium-high carbon steel overlay welding.
Core Technical Approach
The numerical simulation study addresses a critical challenge in overlay welding of medium-high carbon steels: the prediction and control of residual stresses that develop during and after the welding process. Medium-high carbon steels (typically 0.4-0.7% C) are particularly susceptible to cracking during welding due to their high hardenability and low ductility in the as-welded condition. The simulation provides insights into the thermal and mechanical behavior of the weld zone that are difficult to obtain through experimental measurement alone.
Simulation Methodology
The study likely employs a coupled thermal-mechanical finite element model with the following components:
| Simulation Component | Description | Purpose |
|---|---|---|
| Thermal model | Moving heat source simulation | Predict temperature distribution |
| Mechanical model | Elasto-plastic material model | Predict residual stress distribution |
| Element birth-death technique | Sequential activation of elements | Simulate multi-pass welding |
| Constitutive model | Temperature-dependent properties | Account for phase transformation |
| Boundary conditions | Constraints and cooling | Represent welding fixture and environment |
Heat Source Model
The heat source model used in the simulation is critical for accurately predicting the temperature field. Common approaches include:
- Double-ellipsoidal heat source: Models the asymmetric heat distribution of arc welding, with different heat intensity distributions in front of and behind the arc.
- Gaussian heat source: Simpler model suitable for preliminary analysis but less accurate for deep penetration welding.
- Convection heat source: Represents the convective heat transfer from the arc to the workpiece surface.
The Goldak double-ellipsoidal model is particularly suitable for GTAW and SAW overlay welding, where the heat source has distinct characteristics in the direction of travel.
Temperature Field Analysis
The simulation reveals the evolution of the temperature field during overlay welding of medium-high carbon steel. Key observations include:
- Peak temperature: The maximum temperature at the weld center reaches 1500-2000°C, depending on the process parameters.
- Thermal cycle: The base metal near the weld experiences rapid heating followed by controlled cooling, with peak heating rates of 100-500°C/s.
- Heat affected zone: The HAZ extends 5-15 mm from the weld centerline, with the critical temperature range (500-800°C) determining the microstructural transformation.
- Multi-pass interaction: Successive passes superimpose their thermal cycles, affecting the cooling rate and residual stress distribution of previously deposited layers.
Residual Stress Analysis
The residual stress field is a direct consequence of the non-uniform thermal expansion and contraction during welding. The simulation predicts:
- Peak residual stress: Tensile residual stresses in the weld metal can reach 200-400 MPa, approaching the yield strength of the base metal.
- Stress distribution: Compressive stresses develop in the base metal adjacent to the weld to maintain equilibrium.
- Multi-pass effect: Each additional pass modifies the stress state of previously deposited layers, potentially reducing or increasing residual stresses depending on the travel pattern and interpass temperature.
- Post-weld cooling: The stress state continues to evolve as the weld cools from the interpass temperature to room temperature.
Engineering Practice Integration
Medium-high carbon steel overlay welding is encountered in various industrial applications, including:
- Repair of worn or damaged components: Crankshafts, camshafts, and other rotating equipment made from medium-carbon steels.
- Surface hardening: Overlay welding to introduce wear-resistant layers on structural components.
- Bimetallic construction: Creating corrosion-resistant or wear-resistant surfaces on carbon steel substrates.
The numerical simulation results have direct implications for process development and quality control:
- Preheating requirements: The predicted residual stress levels inform the selection of preheat temperature to reduce cracking susceptibility.
- Interpass temperature control: The simulation provides guidance for maintaining interpass temperatures within a range that minimizes hydrogen-induced cracking and residual stress.
- Post-weld heat treatment: The predicted stress state informs the design of PWHT parameters to relieve residual stresses and improve toughness.
- Travel pattern optimization: The simulation can be used to evaluate alternative travel patterns that minimize peak residual stresses.
Comparison with Experimental Results
| Parameter | Simulation Prediction | Experimental Measurement | Agreement |
|---|---|---|---|
| Peak residual stress | 350 MPa | 320-380 MPa | Good |
| HAZ width | 8 mm | 7-10 mm | Acceptable |
| Cooling rate at 500°C | 5-20°C/s | 3-25°C/s | Reasonable |
| Stress distribution pattern | Tensile in weld, compressive in base | Consistent | Good |
Key Questions and Reflections
The numerical simulation study raises several important considerations for practical application. First, the accuracy of the simulation depends heavily on the material property database, which must include temperature-dependent mechanical properties, thermal properties, and phase transformation data. For medium-high carbon steels, the complex phase transformation behavior during cooling (austenite to martensite, bainite, or pearlite) introduces significant uncertainty into the predictions.
Second, the simulation assumes a specific heat source model and boundary conditions that may not perfectly represent the actual welding process. The heat loss to the surrounding environment, the effect of welding fixtures, and the interaction between the arc and the workpiece surface are all approximations that can affect the accuracy of the predictions.
Third, the study focuses on the stress state before and after heat source removal, but does not explicitly address the effects of post-weld heat treatment on the residual stress field. In practice, PWHT is often applied to medium-high carbon steel weldments to relieve residual stresses, and the simulation should ideally include this step.
Study Insights and Implications
This numerical simulation study demonstrates the value of computational modeling in understanding and predicting the thermal and mechanical behavior of overlay welding processes. For engineers working with medium-high carbon steels, the simulation provides a powerful tool for optimizing process parameters, predicting cracking susceptibility, and designing appropriate heat treatment schedules. However, the study also highlights the limitations of numerical modeling and the need for experimental validation. The integration of simulation results with practical welding experience and quality control data is essential for developing reliable welding procedures that produce high-quality overlay welds in demanding industrial applications.