Effect of Quenching Process on Microstructure and Residual Stress of Large Thick-Walled Steel Pipes

Literature Overview

This paper by Hu Zhiyu, Han Jinfeng, Meng Jiahong, Bi Xiangrong, Sun Wenbin, Wang Haiyan, and Gao Xueyun from Inner Mongolia University of Science and Technology, Beijing Mechanical and Electrical Research Institute, and the School of Science at Inner Mongolia University of Science and Technology was published in Journal of Heat Treatment of Metals (Vol. 44, No. 6, 2023, pp. 167-172). Supported by the Inner Mongolia Natural Science Foundation (2020BS05006) and the Inner Mongolia University of Science and Technology Innovation Fund (2019QDL-B09), the study investigates the effect of different quenching processes on the microstructure and residual stress distribution in large thick-walled steel pipes made of 30Cr2NiMo steel using numerical simulation.

Core Technical Points

30Cr2NiMo is a low-alloy steel commonly used in large thick-walled steel pipes for applications requiring high strength and toughness, such as pressure vessels, pipelines, and structural components. The quenching process is critical for achieving the desired microstructure and mechanical properties, but it also introduces residual stresses that can affect the dimensional stability, fatigue life, and crack resistance of the component.

Numerical Simulation Results

The numerical simulation reveals several important findings regarding the quenching of large thick-walled steel pipes:

The Step Stress Concentration Problem

The most critical finding is that the residual stress in large thick-walled steel pipes concentrates at the step, which is the geometric transition between the larger diameter section and the smaller diameter section. This stress concentration can reach approximately 600 MPa in the traditional quenching process, which is close to the yield strength of 30Cr2NiMo steel and creates a high risk of quenching cracks. The step geometry acts as a stress concentrator, and the differential cooling rates between the thick and thin sections exacerbate the problem.

The Optimized Quenching Process

The optimized quenching process involves a three-stage approach: oil quenching for 420 seconds, followed by air cooling for 480 seconds, and then returning to oil quenching. The air cooling stage allows the temperature gradient between the thick and thin sections to decrease, reducing the thermal stresses that drive crack initiation. The subsequent oil quenching ensures that the final microstructure achieves the required hardness and strength. The key insight is that the air cooling stage is sufficient to reduce the stress concentration at the step without significantly altering the final microstructure.

Microstructure Analysis

The microstructure analysis shows that the fine tube section achieves a fully martensitic structure, which provides the highest strength. The coarse tube surface has a martensite plus retained austenite structure, while the core has a martensite, bainite, and retained austenite structure. This microstructural variation is a result of the different cooling rates experienced by different regions of the thick-walled pipe. The presence of retained austenite in the coarse tube sections is beneficial as it provides some toughness and can transform to martensite under stress, providing strain-induced hardening.

Engineering Practice Integration

For the steel pipe manufacturing industry, this study has direct practical implications. Large thick-walled steel pipes with stepped geometries are commonly used in oil and gas pipelines, pressure vessels, and power plant components. The quenching process must be carefully controlled to achieve the required mechanical properties without introducing excessive residual stresses. The optimized quenching process presented in this study provides a practical approach to reducing quenching cracks while maintaining the required microstructure.

From a welding perspective, the residual stresses introduced during quenching can interact with welding residual stresses if the pipe is subsequently welded to other components. The combination of quenching and welding residual stresses can significantly increase the risk of cracking, particularly in the heat-affected zone. Therefore, stress relief treatment may be required after welding, and the initial residual stress state from quenching should be taken into account in the welding procedure specification.

Key Questions and Reflections

Several aspects of this study merit further consideration. The numerical simulation, while powerful, must be validated against experimental measurements to ensure accuracy. The study does not appear to include experimental verification of the residual stress predictions, which would be essential for practical application. Additionally, the optimized quenching process parameters (420 seconds oil, 480 seconds air, then oil) are specific to the particular pipe geometry and material studied, and would need to be adjusted for different pipe sizes and materials.

The study also does not address the effect of quenching on the weldability of the pipe material. The retained austenite content, which varies with position in the pipe, can affect the welding behavior, particularly the susceptibility to hydrogen-induced cracking. Furthermore, the long-term stability of the retained austenite under service conditions, particularly at elevated temperatures, should be considered.

Study Insights and Implications

This paper provides a valuable contribution to the understanding of quenching processes for large thick-walled steel pipes. The identification of the step as the critical location for stress concentration and the development of an optimized quenching process that reduces this stress while maintaining the required microstructure are significant practical achievements. The use of numerical simulation to optimize quenching parameters is an efficient and cost-effective approach that can be applied to other pipe geometries and materials. For engineers in the steel pipe industry, this study reinforces the importance of understanding the interaction between process parameters, microstructure, and residual stress, and demonstrates how numerical methods can be used to optimize manufacturing processes for improved quality and reliability.