BEHAVIOR IN FATIGUE-RELAXATION OF A CREEP RESISTANT 12-PERCENT CR STEEL AFTER HELIUM IMPLANTATION

Push-pull fatigue specimens of the tempered fully martensitic steel DIN 1.4914 (heat MANET) were irradiated at the High Energy Dual Beam Facility of KfK. The influence of helium dose (less-than-or-equal-to 200 appm) and He dose/damage ratio (10-170) have been studied in strain controlled fatigue experiments with hold times at an irradiation and test temperature of 450-degrees-C. Compared with continuously cycled specimens, hold times of 2 min reduce the number of cycles to failure N(f) by 18-20% in the compressive and by 35-40% in the tensile phase. Under all irradiation conditions the reduction in N(f) was found to be less than 30%. Both tensile and compressive strain holds lead to a stress relaxation of 70-80 MPa. In all tests with hold times, fatigue life was found to be dominated by fatigue damage and not by creep damage. The observed microstructural changes are correlated with the fatigue properties.