(The) study on fatigue crack growth in welded part of SA106 Gr.C

Abstract

The main steam line piping material of nuclear power plant is damaged by the continuous cyclic loading, and fatigue cracks in the welded part may severely affect the integrity of safety. Therefore, SA106 Gr.C, which is used as main steam line piping material in nuclear power plant, was investigated in terms of fatigue crack propagation characteristics in welded part. In addition, post weld heat treatment(PWHT) effect, crack closure and stress ratio effect were studied in detail. Fatigue tests were conducted in welded part, which is composed of base metal, heat-affected zone(HAZ) and weld metal, with 10Hz sinusoidal cyclic loading form at load ratio, R=0.1, 0.5. The HAZ structure was simulated by optimal heat treatments which were at 950℃ for 1 hr and at 475℃ for 10 min and the following water quenching. Specimen were post-heated at 600℃ for 1 hr and cooled down to 400℃ in furnace. The present study shows that crack closure effects were pronounced at R=0.1 while almost not observed at R=0.5. And the principal mechanism of crack closure was plasticity-induced crack closure. Another factor influencing the fatigue crack growth rate was crack deflection effect. PWHT process led to different effect on base metal from HAZ. In case of base metal, it accelerated fatigue crack growth rate but was resistent to fatigue crack propagation.