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

Abstract

In order to investigate the structural degradation process during creep, interrupted creep tests were conducted of a Mod.9Cr-1 Mo steel in the range of stress and temperature from 71 to 167 MPa and 873 to 923K. The change of hardness and tempered martensitic lath width were measured in the grip and gauge parts of interrupted specimens. The lath structure was thermally stable in static conditions, but was not stable during creep, and the structural evolution was enhanced by creep strain. The relation between the change in lath width and strain was described in the form, ΔW=α(W_S-W₀)·ε, where ε is the strain, W₀ is the initial lath width, W_S is the final lath width depending solely on stress, and α is the constant of the magnitude of 0.67μm/strain. The change in Vickers hardness was expressed by a one-valued function of creep life consumption ratio. A model describing the degradation process of tempered martensitic structures was proposed on the assumption that dislocations were eliminated by the migration of lath boundaries. The model could explain the relation between change in hardness and creep life consumption ratio satisfactorily. The influence of precipitates on subboundaries was discussed in connection with the abnormal subgrain growth appearing in low stress regime.