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.

Data-based selection of creep constitutive models for high-Cr heat-resistant steel

“…Then, we separately discuss the possible mechanism in the two regions in terms of the applied stress below and above~120 MPa. In the lower stress region, the work by Endo et al [14] suggests that the coarsening mechanism by the applied stress was not activated and we thus consider that the martensitic structure should be stabilized by the precipitates, leading to the steady state of creep deformation. While the coarsening of the fine precipitates progressively occurs, there is a certain duration of the steady state; this is because there should be a sufficient amount of fine precipitates in the initial microstructure.…”

“…The Gr.91 steel exhibits long-term stability of the martensitic structure owing to fine precipitates introduced by tempering [11]. According to previous works [12][13][14], however, the martensitic structure is not necessarily stable and the lath boundary of the martensitic structure migrates under an applied stress in creep deformation. For example, Endo et al [14] claimed that the dislocations composing the subgrain boundary like the lath boundary, which are pinned by the fine precipitates, can migrate by unpinning under an applied stress above a certain level and estimated that the stress necessary for unpinning is approximately 100 MPa.…”

Effect of tempering temperature on the stress rupture properties of Grade 92 steel

“…Major variation in lath width and precipitate size was noted in samples tempered at 740°C as compared to 760°C and 780°C. This difference can be attributed to the creep stress assisted boundary migration [30] which is a precursor to the observed changes in the initial microstructure, resulting in high rupture life. However, in case of 780°C tempering, the initial microstructure did not allow any significant variation in lath width and precipitate size resulting in less rupture times.…”

Advanced technology in creep life prediction and damage evaluation for creep strength enhanced ferritic steels

“…It was suggested that the changes in decomposed profiles due to ageing might reflect the dislocation density conditions and precipitation of M 23 C 6 , etc. [48].…”