An investigation of the degradation characteristics for casting stainless steel, CF8M, under high temperatures

Stainless steel castings are used in pipes and valves subjected to high pressure and temperatures. The primary coolant system of a nuclear power plant is made of a stainless steel casting and the operating temperatures are in the range of 290–330°C. If the coolant system is exposed to these temperature ranges for a long period, it may be possible to experience degradation of the material. The present investigation is concerned with the degradation characteristics of CF8M (cast duplex stainless steel), exposed to the thermal and σ-phase degradation temperatures, 430 and 700°C, respectively. After the CF8M specimens are held 100–3600 h at 430°C for the thermally degraded specimens and maintained 20 min to 150 h at 700°C for the σ-phase degraded specimens, respectively, all specimens are water quenched. Each specimen of the thermally and σ-phase degraded materials is classified into five classes depending on the holding time at the given temperatures. In order to investigate the characteristics of degradation, microstructure, micro Vickers hardness, tensile, impact tests, and fatigue crack growth tests are performed for each class of the specimens. From the present investigation the following results were obtained: (1) the difference between the thermally and σ-phase degraded specimens can be distinguished through their microstructures, (2) hardness and tensile strength are increased with degradation, while elongation, reduction area, and impact energy are decreased by increasing the degradation, (3) the fatigue crack growth rate (FCG) of the σ-phase degradation at 700°C is larger than that of the thermally degraded specimens, and (4) the FCG for both thermally and σ-phase degraded specimens are larger than those of the virgin (nondegraded) specimens.