Evolution of lath substructure and internal stresses in a 9% Cr steel during creep

Abstract

The changes in the dislocation substructure and internal stresses in a tempered 9% Cr heat resistant steel during creep at 923 K were studied. The mean lath size gradually increased from 330 nm for the initial tempered state to 740 nm once the specimen had crept to failure under a nominal stress of 118 MPa after 1 271 hours. Correspondingly, the dislocation density within the lath decreased from 6.2 × 1014 m-2 to ∼1014 m-2. The tempered structure of the martensite lath was characterised by large lattice curvatures, which were attributed to a high density of dislocations with like signs and the long-range stress fields originating from the martensite lath boundaries. An internal stress of 49 MPa, as evaluated by measuring the lattice curvature within individual laths in specimens crept to 1% (just after transient creep stage), is comparable to the threshold stress of 51 MPa estimated from the creep rate and stress relationship. The improved creep resistance of advanced 9% Cr martensitic steel results from both dispersion strengthening and the internal stresses of the martensite lath substructure.