Effect of thermal exposure in helium on mechanical properties and microstructure of 316L and P91

In this paper, the effects of high temperature exposure in air as well as in impure He on mechanical properties of 316L and P91 steels were investigated. The experimental programme was part of material design of new experimental facility – high temperature helium loop. Some of the specimens were exposed in air at 750 °C for up to 1000 h. Another set of specimens were exposed in impure helium containing 1 ppmv CO₂, 2 ppmv O₂, 35 ppmv CH₄, 250 ppmv CO and 400 ppmv H₂ at 750 °C for up to 1000 h. Metalographical analysis, tensile tests, fracture toughness and hardness tests of exposed and non-exposed specimens were carried out. After the exposure both in air and He, the ultimate tensile strength of P91 decreased significantly more than that of 316L. After the exposure in He, the fracture toughness of 316L was reduced to 60% while fracture toughness of P91 showed no significant changes. The hardness of P91 decreased with exposure time in air. The measurement of the hardness of 316L was very scattered the most probably due to the heterogeneities in microstructure, the trend was not possible to evaluate. - Highlights: • Thermal exposure of 316L and P91 steels in air and impure helium. • Investigation of microstructure changes and hardness measurements. • Evaluation of tensile properties and fracture toughness. • Fractographic analysis and comparison of exposed and non-exposed specimens.