Thermal fatigue of unirradiated and neutron-irradiated steel and alloys

The thermal and stress effects on materials of the first wall and the blanket of thermonuclear reactors are transient, and therefore the fatigue problem is important. Therefore thermal fatigue and crack development in unirradiated and neutron-irradiated steel and alloys was investigated experimentally in the interval from 50 to 350 degrees C. The tests were done on cylindrical samples 6 and 8 mm in diameter and 20 and 30 mm long which had longitudinal notches 1.5 and 0.5 mm deep with radii of 0.15, 0.25, and 0.5 mm. After the samples of austenite steel, ferrite steel, and high-nickel alloy, and titanium alloy were made, they were annealed in argon for 1 h at temperatures of 1050, 800, 1050, and 750 degrees C, respectively. The samples were thermal-fatigue loaded remotely for 15,000 cycles as follows: They were heated in an electric furnace to 300 or 350 degrees C for 8-10 min and then cooled in water to 50 degrees C over 1.5-2 min. The irradiated samples were thermal cycled at a temperature which did not exceed the temperature of the preliminary neutron irradiation. The length of a crack and the way it formed and developed were characterized by standard layer-by-layer metallography of sections of the thermal-cycled samples. Comparison of the thermal fatigue of the unirradiated materials shows that ferrite steel has a lower failure resistance to thermal cycling than does austenite steel and the alloy. Thus, for cycling strains of 0.22-0.23%, cracks 0.1 mm long were observed in the ferrite steel 01Kh13MT after 8,300 cycles, while they arose in samples of the austenite steel 03Kh16N15M3B and the alloy 03Kh20N45M4BCh only after 12,000 and 14,500 cycles. Samples of the 04Kh16N11M3T steel hardly failed after 15,000 cycles