2005 Volume 91 Issue 12 Pages 888-896
Long-term isothermal aging behaviors up to 30000 h has been investigated in V modified 2.25Cr-1 Mo 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 aging behavior. 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 aging behavior 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-1 Mo steel, it is found to be smaller in the modified steel at the same impurities level (J-factor). One of the reasons has been attributed to the increased Mo content in solid solution of the modified steel based on the chemical analysis of precipitates.
The hardness generally increases first and then decreases with aging time. The change of hardness is more pronounced for the conventional steel. The increase in hardness is due to new precipitation and the decrease due to enlargement of precipitates and recovery of dislocation structure. Behavior of recovery was investigated with SEM-EBSP. Image quality (IQ) and grain average misorientation (GAM) were analyzed. Based on these results, it has been suggested that the more stable behavior in hardness in the modified steel can be attributed to the presence of stable MX type precipitates containing V.
