Abstract
	The paper is devoted to the study of initiation and propagation stages of thermal fatigue cracks in austenitic stainless steels subjected to repeated thermal shocks. This degradation process is typical very often for high temperature pressured water components. Two types of specimens were tested. The first one was equipped by the central hole and the second by the hole and two notches acting as stress concentrators. The surfaces of tested specimens were investigated after 1000, 3000 and 6000 thermal cycles. The temperature cycle has been chosen in the range from 100°C to 350°C, and 300°C, respectively. It was detected that partial fatigue cracks were spontaneously initiated in notches bottoms. Cracks were visible on free surface of the specimen even after 1000 cycles under loading. Thermal cracks were fractographically investigated and the mechanism of main crack formation has been revealed. Stress-strain analysis proves that stresses around notch root are sufficient to cause mechanical fatigue damage. For further improvement of stress-strain behavior of test specimens the numerical analysis of finite elements method is also needed. Comparing three tested steels no significant differences have been detected in their rezistivity to thermal fatigue. The circumstances that can cause thermal cracks initiation should be avoided in design of power plant station components. In existing plant components the risk of thermal fatigue cracking should be assessed and conditions necessary for the prevention of this degradation process should be clarified.