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 pre ssure 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 degrees C. If the coolant system is exposed to these temperature ranges for a long period, it may be possible to experience degr adation of the material. The present investigation is concerned with the de gradation characteristics of CF8M (cast duplex stainless steel), exposed to the thermal and sigma-phase degradation temperatures, 430 and 700 degrees C, respectively. After the CF8M specimens are held 100-3600 h at 430 degree s C for the thermally degraded specimens and maintained 20 min to 150 h at 700 degrees C for the sigma-phase degraded specimens, respectively, all spe cimens are water quenched. Each specimen of the thermally and sigma-phase d egraded 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 te sts, and fatigue crack growth tests are performed for each class of the spe cimens. From the present investigation the following results were obtained: (1) the difference between the thermally and sigma-phase degraded specimen s can be distinguished through their microstructures, (2) hardness and tens ile strength are increased with degradation, while elongation, reduction ar ea, and impact energy are decreased by increasing the degradation, (3) the fatigue crack growth rate (FCG) of the sigma-phase degradation at 700 degre es C is larger than that of the thermally degraded specimens, and (4) the F CG for both thermally and sigma-phase degraded specimens are larger than th ose of the virgin (nondegraded) specimens. (C) 2000 Elsevier Science S.A. A ll rights reserved.