Numerical Simulation of Welding Residual Stress in Small-Diameter Thick-Walled 12Cr1MoVG Steel Pipe

Literature Overview

The paper by Chen Guohong, Guo Bixiang, Wang Ruomin, Zhao Haoran, Miao Chunhui, Wang Jingjing, and Tang Wenming (published in Physical Testing and Chemical Analysis, 2020, Vol. 56, No. 7, pp. 1–5) presents a finite element analysis of welding residual stresses in a multi-layer multi-pass welded joint of a small-diameter, thick-walled 12Cr1MoVG pearlitic heat-resistant steel pipe. The study was conducted by researchers from State Grid Anhui Electric Power Research Institute, State Grid Fuyang Power Supply Company, State Grid Lu'an Power Supply Company, and Hefei University of Technology. The finite element model employed the birth-and-death element method to simulate the sequential deposition of weld passes, and the calculated axial residual stress distributions on both the inner and outer walls of the weld joint were compared with experimental measurements, showing excellent agreement.

Core Technical Content

12Cr1MoVG is a pearlitic heat-resistant steel widely used in power plant boiler tubes, superheater tubes, and high-pressure piping systems. The "12" indicates approximately 1.2 percent chromium, "1Mo" indicates approximately 1 percent molybdenum, and the "VG" suffix denotes a grade suitable for high-temperature service. This alloy combines good high-temperature strength, oxidation resistance, and thermal fatigue resistance, making it a preferred material for components operating at temperatures above 400 degrees Celsius.

The welding of 12Cr1MoVG pipe presents several challenges:

The multi-layer multi-pass welding process is necessary for thick-walled pipe because depositing the entire weld in a single pass would result in excessive heat input, poor penetration control, and unacceptable distortion. Instead, the weld is built up in successive layers, each of which is deposited and cooled before the next layer is applied. This creates a complex thermal history that results in a non-uniform distribution of residual stresses throughout the weld joint.

The birth-and-death element method used in the finite element model is a standard approach for simulating welding processes. In this method, elements representing the weld metal are initially "killed" (deactivated) and then "born" (activated) sequentially as each weld pass is deposited. When an element is born, it is subjected to a thermal load that simulates the heat input from the welding arc, and the resulting thermal expansion is calculated. As the element cools, it contracts, and the surrounding material constrains this contraction, resulting in residual stresses. The sequential activation of elements captures the cumulative effect of multiple weld passes on the residual stress distribution.

Residual Stress Distribution Results

The key findings from the numerical simulation are summarized below.

The fact that the outer wall near the weld exhibits compressive residual stress is a positive finding. Compressive stresses are generally beneficial for fatigue life because they reduce the effective tensile stress range experienced by the material during cyclic loading. This is particularly important for 12Cr1MoVG components that are subjected to thermal cycling in power plant service. The compressive stress on the outer wall may be attributed to the constraint imposed by the cooler outer layers of the pipe on the hotter inner layers during welding.

The tensile residual stress in the inner wall base metal is a more concerning finding. Tensile stresses in the base metal can reduce the material's resistance to creep and fatigue damage, particularly at elevated temperatures. In the context of power plant boiler tubes, where the internal surface is exposed to hot gases and may experience cyclic thermal loading, tensile residual stresses can accelerate the initiation and propagation of cracks. This finding underscores the importance of post-weld heat treatment (PWHT) to relieve residual stresses in 12Cr1MoVG weldments.

Process and Standards Analysis

The welding of 12Cr1MoVG pipe is governed by several standards and codes, including:

For 12Cr1MoVG pipe, the following welding parameters are typically specified:

• Preheat temperature: 200–300 degrees Celsius, depending on wall thickness and carbon equivalent.
• Interpass temperature: 250–350 degrees Celsius, to prevent excessive cooling rate and cold cracking.
• Post-weld heat treatment: Normalization and tempering at 720–760 degrees Celsius for 2–4 hours, depending on wall thickness.
• Welding process: Shielded metal arc welding (SMAW) or gas tungsten arc welding (GTAW) with appropriate filler metal, typically E911-T1 or equivalent.

The finite element simulation presented in the paper provides a valuable tool for optimizing these welding parameters. By varying the heat input, welding sequence, and interpass temperature in the model, engineers can predict the resulting residual stress distribution and identify process parameters that minimize tensile stresses in critical locations. This approach can reduce the need for extensive experimental testing and accelerate the welding procedure qualification process.

Integration with Engineering Practice

The practical value of this study lies in its demonstration that finite element simulation can accurately predict welding residual stresses in complex multi-layer multi-pass weldments. The agreement between calculated and measured axial residual stress distributions on both the inner and outer walls confirms the validity of the model and the birth-and-death element method. This validation is important because it gives engineers confidence in using the model to optimize welding procedures and predict the effects of different process parameters on residual stress.

In engineering practice, the following steps are recommended for managing welding residual stresses in 12Cr1MoVG pipe weldments:

1. Perform a finite element simulation of the welding process to predict the residual stress distribution for the proposed welding procedure.
2. Identify critical locations where tensile residual stresses are predicted to be high, particularly in the inner wall base metal.
3. Modify the welding procedure to minimize tensile stresses, such as by using a back-step welding sequence or reducing the heat input per pass.
4. Verify the effectiveness of the modified procedure through experimental measurement of residual stresses, using techniques such as hole drilling or neutron diffraction.
5. Apply post-weld heat treatment to relieve residual stresses to acceptable levels, typically below 30 percent of the yield strength.
6. Document the welding procedure, residual stress measurements, and PWHT parameters in the quality records for the component.

This systematic approach ensures that the weldment is free from excessive residual stresses that could compromise its long-term performance under high-temperature service conditions.

Key Questions and Reflections

One question that arises from this study is the effect of residual stresses on the long-term creep behavior of 12Cr1MoVG weldments. The paper focuses on the residual stress distribution immediately after welding and PWHT, but the long-term evolution of these stresses under sustained high-temperature loading is not addressed. Creep relaxation can reduce residual stresses over time, but it can also cause stress redistribution that may lead to localized stress concentrations. Future research should investigate the time-dependent behavior of residual stresses in 12Cr1MoVG weldments under creep conditions.

Another consideration is the effect of welding sequence on the residual stress distribution. The paper simulates a specific welding sequence, but different sequences can produce different stress patterns. For example, welding the passes in a radial direction from the outer wall to the inner wall may produce a different stress distribution than welding from the inner wall to the outer wall. The finite element model can be used to evaluate different welding sequences and identify the one that produces the most favorable residual stress distribution.

The study also does not address the effect of welding defects, such as porosity, incomplete fusion, or slag inclusions, on the residual stress distribution. Defects can act as stress concentrators and may interact with residual stresses to initiate cracks. The finite element model should be extended to include defect simulation to assess the combined effect of defects and residual stresses on the integrity of the weldment.

Study Insights and Implications

The numerical simulation of welding residual stresses in 12Cr1MoVG pipe weldments is a valuable tool for optimizing welding procedures and ensuring the long-term integrity of high-temperature components. The birth-and-death element method provides a practical and accurate approach for modeling the sequential deposition of weld passes, and the validation against experimental measurements confirms the reliability of the model.

For engineers involved in the design and fabrication of 12Cr1MoVG weldments, this paper provides a clear demonstration that finite element simulation can be used to predict and control residual stresses. The key insight is that the residual stress distribution is not uniform; it varies significantly between the inner and outer walls and between the weld, HAZ, and base metal. This non-uniformity must be considered in the design and qualification of welding procedures to ensure that critical locations are not subjected to excessive tensile stresses.

In summary, this study contributes to the body of knowledge on welding residual stress management in thick-walled heat-resistant steel pipe weldments. The finite element simulation approach is validated and recommended as a tool for welding procedure optimization. Engineers should integrate numerical simulation into their welding qualification processes to reduce the risk of residual stress-related failures and improve the long-term reliability of high-temperature components.