DEGRADATION OF MECHANICAL-PROPERTIES OF CAST CR-MO-V AND CR-W-V STEAM-TURBINE CASINGS AFTER LONG-TERM SERVICE AT ELEVATED-TEMPERATURES .1. TENSILE PROPERTIES, BRITTLE-FRACTURE STRENGTH AND CHARPY IMPACT PROPERTIES

The effect of elevated service temperature on tensile properties, brittle fracture strength and on the Charpy V-notch transition curve of Cr-Mo-V and Cr-W-V cast steels is presented. A lowering of the yield stress and ultimate tensile stress (softening) was observed with both types of cast steels after long-term exposure at elevated temperatures. The brittle fracture strength of Cr-Mo-V steel established by testing both the smooth bars sigma(B)F and notch specimens sigma(BF)* at low temperatures was not influenced during exposure at elevated temperatures. The fracture appearance transition temperature (FATT) of this steel determined by impact Charpy test was also not affected by long-term exposure. Thus, it could be concluded that this type of cast steel was not embrittled during operation. On the contrary, a decrease in brittle fracture strength sigma(BF)* (250 MPa) and an increase in FATT (50 degrees C) were observed in the Cr-W-V steel after 2 x 10(5) h of service.