Explosive Elimination of Welding Residual Stress in High-Strength Steel Pressure Pipes for the Three Gorges Project

Literature Overview

This paper by Chen Huaining, Liu Hequan, Lin Quanhong, and Chen Jing, published in Welding (2000, No. 12, pp. 20-22), reports on the application of explosive stress relief treatment to 60 mm thick NK-HITEN 610U2 quenched and tempered high-strength steel weld plates used in the Three Gorges Project pressure steel pipes. The study examined both butt welds and fillet welds, and measured the three-dimensional residual stress distribution before and after explosive treatment. The results demonstrated that explosive treatment not only significantly eliminated surface tensile residual stresses but also achieved a uniform and substantially reduced residual stress distribution through the thickness of the weld plate.

Technical Analysis of the Explosive Stress Relief Process

The explosive stress relief method utilizes the shock wave generated by detonating a controlled amount of explosives to impose a high-strain-rate compressive stress on the welded structure. The shock wave propagates through the material at the speed of sound in steel (approximately 5,900 m/s) and induces plastic deformation in the regions where the residual tensile stress is highest, thereby relieving the residual stress through reverse yielding.

For the NK-HITEN 610U2 steel used in this study, the material properties are particularly challenging for residual stress relief. This quenched and tempered high-strength steel has a yield strength of approximately 610 MPa and a tensile strength of approximately 690 MPa, with a relatively low ductility compared to conventional carbon steels. The high yield strength means that conventional thermal stress relief methods require very high temperatures (typically above 600°C), which may compromise the microstructure and mechanical properties of the quenched and tempered condition. The explosive method offers a solution that achieves stress relief at room temperature without affecting the material's microstructure.

The key parameters of the explosive treatment are summarized below:

The three-dimensional residual stress measurements revealed that before treatment, the surface residual stress was predominantly tensile, reaching values of 200 to 350 MPa in the weld region, with a complex distribution through the thickness. After explosive treatment, the surface residual stress was reduced to near-zero or slightly compressive values, and the through-thickness distribution became remarkably uniform, with maximum variations of less than 50 MPa across the 60 mm thickness.

Engineering Practice and Quality Control Implications

The application of explosive stress relief to the Three Gorges Project pressure steel pipes represents a landmark case in large-scale engineering practice. The pressure steel pipes in this project are critical safety components that must withstand extremely high internal water pressures, and the welding residual stress is a major concern for the initiation and propagation of stress corrosion cracking and fatigue cracks. The reduction of residual stress from tensile to near-zero or compressive values significantly improves the resistance of the welded joints to these failure modes.

For quality control purposes, the explosive stress relief process requires careful management of several factors. The explosive charge must be precisely calibrated to ensure adequate shock wave intensity without causing structural damage or excessive deformation. The detonation sequence must be controlled to prevent asymmetric loading that could induce new residual stresses. Post-treatment non-destructive testing, including ultrasonic testing and magnetic particle testing, is essential to verify that no new defects have been introduced. Additionally, the mechanical properties of the steel should be verified after treatment to confirm that the quenched and tempered condition has been maintained.

The choice of explosive stress relief over conventional thermal stress relief for this application was driven by several practical considerations. The large size of the pressure steel pipe sections made conventional furnace-based stress relief impractical. The high-strength quenched and tempered steel could not tolerate the elevated temperatures required for effective thermal stress relief without risk of microstructural degradation. The explosive method provided a rapid, effective, and non-thermal solution that was compatible with the material properties and the project schedule.

Study Insights and Reflections

The most significant insight from this study is the demonstration that explosive stress relief can achieve superior results compared to conventional methods for high-strength steel welded structures of large dimensions. The three-dimensional residual stress measurements provide quantitative evidence that the method is effective not only at the surface but throughout the entire thickness, which is critical for the long-term integrity of thick-walled pressure components. For engineers working on similar projects involving high-strength steel pressure vessels or pipes, this study provides a proven methodology that can be adapted to different geometries and material specifications. The key challenge lies in the precise calibration of the explosive charge and the rigorous quality control of the post-treatment condition, both of which require specialized expertise and instrumentation. This work stands as a testament to the innovative solutions that can be developed when conventional methods are inadequate for the demands of large-scale engineering projects.