Advanced IHSI technology

The residual tensile stress in pipe welds is a factor in stress corrosion cracking and offers opportunity for improvement. IHSI (induction heating stress improvement) is a technique of applying a temperature difference to the inner surface of a welded pipe to change the residual tensile stress to compression stress. We have developed techniques for accurate analysis of residual stress by numerical simulation, thus contributing to IHSI.

A1: In welding, heat is applied locally to only the parts being joined. The expansion of the heated part is constrained by the surrounding material, so thermal stress arises. If that thermal stress produces a compressive plastic strain, it remains in the material after welding as residual tensile stress. The residual stress can be measured directly in actual structural members or it can be obtained analytically by numerical simulation.

A2: IHSI is a measure applied to pipe welds in nuclear power generation plants that have already begun operation. The pipe is cooled by the water flowing through it while at the same time it is heated from the outside by high-frequency induction to create a temperature differential. The temperature differential produces a thermal stress that creates residual compressive stress in the heated area. During IHSI treatment, a tensile thermal stress is created on the inner surface of the pipe, and the compressive plastic strain from the welding is converted to tensile plastic strain. The tensile plastic strain results in a residual compressive stress after the treatment.

A3: We are performing a temperature analysis to determine the temperature distribution within the weld, a thermal plasticity analysis to determine the stress distribution, and a magnetic field analysis to determine the amount of heating form high-frequency inductive heating. There are pipe welds of various shapes and sizes in nuclear power generation plants. In the development of IHSI, stress evaluation for different conditions can be done easily with many numerical simulations, saving time and expense. Numerical simulation is an important technique for accelerating the development process.