(SS) and base Cr-Mo steel. Furthermore, when pressure vessel cooled down to ambient temperature, hydrogen atoms absorbed inside of the Cr-Mo steel rather accumulates between interface of SS and Cr-Mo steel than degassing outside of the wall which may cause disbonding or initiation of hydrogen embrittled cracking. Figure 1 shows the calculation example of shutdown procedure. It is shown from the analysis that the crack at welded structure driving force arises at stainless structure welded area. Therefore, special attention should be paid whether the crack at the interface of stainless steel and Cr-Mo base initiates and penetrates through wall, which may cause final collapse of the entire vessel. In this study, fracture mechanics tests were conducted to investigate how hydrogen assisted cracking of 2.25Cr-1Mo steels grows by I.H.E. mechanisms examining loading method, materials toughness level and steel strength. NOMENCLATURE RL :Rising load CD :Constant Displacement KIH :Stress intensity for onset of subcritical crack growth Kth :the threshold intensity factor for hydrogen charging environment. KIC-H:material toughness measured in the hydrogen charging environment. MATERIALS Table 1 shows chemical composition of steels tested. Impurities Si, P and Sn were intentionally added to simulate the old temper embrittled steel controlling J-factor=(Si+Mn)x(P+Sn)x104 wt% level. After hot rolling, those heats were subjected to the quenched and tempered heat treatment + PWHT at 690°C for 8hrs. followed by step cooling. Table 2 shows mechanical properties. Low J steel is the new generation made steel with high fracture toughness at room temperature. Mid J and High J steels were temper embrittled by step cooling. The compact tension specimen were machined and Ni/Au were plated to prevent hydrogen degassing from inside of the steel. Hydrogen were charged in autoclave at 420°C, 12MPa for 48hrs followed by water quenching to room temperature and preserved in liquid nitrogen container until fracture mechanics test in air environment. CRACK GROWTH BEHAVIOR Effect of loading condition Slow rising load (:RL) and constant displacement(:CD)loading method (Figure 2) were applied on hydrogen crack growth testing. Load was controlled by Crack Mouth Opening Displacement(CMOD) with a speed of 0.00003mm/sec. efficacious for hydrogen embrittlement 1). Crack was monitored by D.C. potential drop method. Figure 3 shows typical result of RL+CD test. Crack initiation was occurred after 3hrs of a rising load test. Continuous propagation was observed during rising load applied. After CMOD kept constant, the crack growth rate decreased and finally stopped after 12hrs.. Figure 4 shows the repetitional RL and CD test. Subsequent imposing of rising load obviously enhances crack growth, whereas crack growth was deactivated by keeping CMOD constant. Finally, a remarkable increase in crack growth observed in the later RL stage and resulted in fast fracture.
RkJQdWJsaXNoZXIy MjM0NDE=