Change in Vickers Hardness and Substructure during Creep of a Mod.9Cr-1Mo Steel

Abstract

In order to investigate the structural degradation during creep, interrupted creep tests were conducted of a Mod.9Cr–1Mo steel in the range of stress and temperature from 71 to 167 MPa and 873 to 923 K. The change of hardness and tempered martensitic lath width were measured in grip and gauge parts of interrupted specimens. The lath structure was thermally stable in static conditions, however, it was not stable during creep and the structural change was enhanced by creep strain. The relation between the change in lath width and strain was described quantitatively. The change in Vickers hardness was expressed by a single valued function of creep life consumption ratio. Based on the empirical relation between strain and lath width, a model was proposed to describe the relation between change in hardness and creep life consumption ratio. The comparison of the model with the empirical relation suggests that about 65% of hardness loss is due to the decrease of dislocation density accompanied by the movement of lath boundaries. The role of precipitates on subboundaries was discussed in connection with the abnormal subgrain growth appearing in low stress regime.