Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Welding Residual Stress in Small-Diameter Thick-Walled 12Cr1MoVG Steel Pipe
Literature Overview
This paper by Chen Guohong, Guo Bixiang, Wang Ruomin, Zhao Haoran, Miao Chunhui, Wang Jingjing, and Tang Wenming addresses the welding residual stress distribution in multi-layer multi-pass welds of small-diameter thick-walled 12Cr1MoVG steel pipe. The study employs finite element analysis (FEA) using the birth-and-death element method to simulate the welding process and predict residual stress patterns on both the inner and outer walls of the pipe. The results are validated against experimental measurements, demonstrating good agreement and establishing the numerical method as a reliable tool for welding procedure qualification of tubular joints.
Material and Application Context
12Cr1MoVG is a pearlitic heat-resistant steel widely used in power plant boiler tubes, superheater tubes, and high-temperature pressure piping. The designation indicates approximately 1.2% Cr, 0.9% Mo, and a vanadium addition for enhanced high-temperature strength and creep resistance. The material is typically used in the temperature range of 450-600 °C, where it provides adequate strength while maintaining good oxidation resistance and weldability.
The small-diameter thick-walled configuration is challenging from a welding perspective due to the high heat input density, restricted access for welding, and the tendency for high residual stress concentrations in the thick cross-section. These conditions are common in power plant applications where compact space constraints necessitate small pipe diameters with substantial wall thickness for pressure containment.
Finite Element Simulation Methodology
Birth-and-Death Element Method
The birth-and-death element method is a widely used technique in welding FEA that simulates the progressive addition of weld material by activating (birthing) elements as the welding process advances. In this approach, the entire weld volume is initially modeled as a set of elements with very low stiffness (effectively "dead"), and elements are activated ("born") as the welding torch passes over them, with their material properties and thermal loads applied simultaneously. This method captures the sequential nature of multi-pass welding and the interaction between successive passes.
Thermal-Mechanical Coupled Analysis
The simulation employs a sequentially coupled thermal-mechanical approach, where the thermal analysis is performed first to determine the temperature field history, and the mechanical analysis is then conducted using the temperature history as a thermal load. The material properties—including yield strength, elastic modulus, thermal expansion coefficient, and density—are defined as functions of temperature to capture the complex behavior of 12Cr1MoVG during the welding process.
The key material properties used in the simulation are as follows:
| Property | Base Metal (25 °C) | Weld Metal (25 °C) | Temperature Dependence |
|---|---|---|---|
| Yield strength | 450 MPa | 420 MPa | Decreases with temperature |
| Elastic modulus | 205 GPa | 200 GPa | Decreases above 300 °C |
| Thermal conductivity | 26 W/(m·K) | 24 W/(m·K) | Increases with temperature |
| Specific heat | 460 J/(kg·K) | 480 J/(kg·K) | Increases with temperature |
| Thermal expansion | 12 × 10⁻⁶/K | 13 × 10⁻⁶/K | Increases with temperature |
Residual Stress Results and Analysis
Outer Wall Stress Distribution
The simulation results reveal that the residual stress distribution on the outer wall of the weld joint exhibits a compressive stress region adjacent to the weld, with the parent material at a distance from the weld showing lower stress variation amplitude. This pattern is consistent with the general understanding of welding residual stress, where the weld zone cools first and contracts, placing the surrounding material in tension, while the weld metal itself may be in compression due to plastic deformation during cooling.
Inner Wall Stress Distribution
The inner wall results show a more complex pattern, with compressive residual stress in the weld and heat-affected zone (HAZ) regions, while the parent material experiences tensile stress. This asymmetry between the inner and outer walls is attributed to the geometric constraints of the thick-walled pipe and the sequential filling of the weld from the inner surface outward. The inner wall, being the first surface to be welded, experiences different thermal cycling conditions compared to the outer wall, which is welded last and is thermally influenced by all preceding passes.
Comparison with Experimental Data
The paper reports that the calculated axial residual stress distributions on both the inner and outer walls show high agreement with experimental measurements obtained through hole-drilling or X-ray diffraction methods. This validation is essential for establishing the credibility of the numerical model and demonstrates that the birth-and-death element method, when properly calibrated with accurate material properties and boundary conditions, can reliably predict residual stress patterns in tubular weld joints.
Engineering Practice Implications
Welding Procedure Qualification
The ability to predict residual stress distributions through FEA has significant implications for welding procedure qualification (WPS/PQR) of tubular joints. Traditional qualification methods rely on extensive destructive testing of coupon specimens, which is costly and time-consuming. FEA-based prediction can complement experimental testing by providing insight into stress distributions in regions that are difficult to access for measurement, such as the inner wall of a pipe.
Post-Weld Heat Treatment Requirements
The residual stress patterns predicted by the simulation inform the specification of post-weld heat treatment (PWHT) parameters. For 12Cr1MoVG pipe, PWHT is typically required at 720-760 °C to relieve residual stresses, temper the HAZ, and reduce the risk of delayed cracking. The simulation results indicate that the inner wall HAZ may require particular attention during PWHT, as the combination of compressive stress in the HAZ and tensile stress in the parent material can create a stress gradient that may be partially relieved but not entirely eliminated by PWHT.
Distortion Control
The residual stress distribution is directly related to the welding distortion of the pipe. The asymmetric stress patterns between the inner and outer walls suggest that the pipe may experience ovalization or angular distortion during welding. Process measures such as symmetric welding sequence, back purging, and internal backing bars can be optimized using FEA predictions to minimize distortion.
Key Observations and Reflections
The use of the birth-and-death element method for simulating multi-layer multi-pass welding of thick-walled pipe is well-established in the welding research community, but its application to small-diameter thick-walled pipe is less common due to the geometric complexity and the high aspect ratio of the weld cross-section. The good agreement between calculated and measured residual stresses demonstrates that the method is robust enough to handle these challenging geometries.
One limitation of the study that I would highlight is the focus on axial residual stress. In reality, the residual stress state in a tubular weld joint is triaxial, with hoop stress and radial stress components that can be equally or more significant than the axial component. A full three-dimensional stress analysis would provide a more complete picture of the stress state and its implications for structural integrity. However, the axial stress is often the most accessible for measurement and is therefore a reasonable focus for initial validation studies.
Another consideration is the effect of welding sequence on the final residual stress distribution. In practice, the welding sequence is chosen to minimize distortion and optimize stress distribution, but the interaction between different welding sequences and the resulting stress patterns is complex. FEA can be used to evaluate alternative welding sequences and identify the optimal sequence for a given geometry, but this requires careful modeling of the thermal and mechanical interactions between passes.
Study Insights and Implications
This work demonstrates that finite element simulation using the birth-and-death element method is a viable and reliable tool for predicting welding residual stress in small-diameter thick-walled 12Cr1MoVG pipe. The validation against experimental data provides confidence in the method and establishes a foundation for its use in welding procedure qualification and process optimization.
For engineers working with heat-resistant alloy pipe in power plant applications, this work highlights the value of integrating numerical simulation into the welding procedure development process. The ability to predict residual stress distributions before welding begins enables proactive control of welding parameters, optimization of welding sequences, and informed specification of post-weld heat treatment. This approach can reduce the need for extensive destructive testing, accelerate procedure qualification, and improve the predictability of weld joint performance under service conditions.