Long-Term Isothermal Aging Behavior of V-Modified 2.25Cr-1Mo Steels

Long-term isothermal aging behavior up to 30,000h has been investigated in V-modified 2.25Cr-1Mo steels. When impurity elements were added, FATT in Charpy impact test vs. aging time curves showed basically of parabolic type, indicating the importance of segregation of impurities as the critical factor in controlling temper embrittlement. On the other hand, when the amount of impurities was small, FATT showed complex behavior with aging time. This suggests the importance of other factors in controlling temper embrittlement such as recovery of dislocations and enlargement of various carbide precipitates. When the degree of temper embrittlement after aging is compared in terms of the increase in FATT due to aging between the modified steels and the conventional 2.25Cr-1Mo steels, it is found to be small in the modified steel at the same impurity level (J-factor). It was also found that the modified steels showed no hydrogen embrittlement. No change was observed in FATT after hydrogen charge on the contrast to the conventional steel, which showed significant increase in FATT. This effect has been attributed to the small amount of diffusible hydrogen content in the modified steel by the presence of hydrogen trapping sites, which is present even after long-term isothermal aging.