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The operating lifetime of the components made of a large-scale forged material, such as the power generating components in a thermal power plant, is influenced by the initiation of defects such as cracks. Peening is known to be effective for the improvement of fatigue strength and the prevention of the stress corrosion cracking initiation. Shot peening that generates compressive residual stress on the material surface is applied to the stress concentration region of the high-strength large-scale forged material in order to relax the tensile stress, prevents crack initiation and extends operation lifetime. To confirm the durability of the compressive residual stress generated by peening, the influence of thermal aging treatment on the compressive residual stress is evaluated. As a result, it is reported that compression is maintained though compressive residual stress relaxes. The purpose of this study is to develop evaluation method of the relaxation behavior of residual stress generated by shot peening and laser peening during actual plant operations. Shot peening and laser peening were performed on the specimens of Ni-base superalloy Alloy 706, which is used as the material of the discs of gas turbines, and the thermal aging treatment test was performed. It shows that the compressive residual stress decreases to an elastic limit after heating and then relaxes due to creep strain generation. It is found that residual stress was almost determined by the residual stress just after heating.

Evaluation on Relaxation Behaviors of Compressive Residual Stress of Peened Ni-Base Superalloy Alloy 706 Test Specimens after Thermal Aging Treatment