Abstract
This paper reports the short-term creep behaviour at elevated temperatures of a MarBN steel variant. Creep tests were performed at three different temperatures (625°C, 650°C and 675°C) with applied stresses ranging from 160 MPa to 300 MPa, and failure times from 1 to 350 h. Analysis of the macroscopic creep data indicates that the steady-state creep exhibits a power-law stress dependence with an exponent of 7 and an activation energy of 307 kJ mol−1, suggesting that dislocation climb is the dominant rate-controlling creep mechanism for MarBN steel. Macroscopic plastic instability has also been observed, highlighted by an obvious necking at the rupture region. All the macroscopic predictions have been combined with microstructural data, inferred from an examination of creep ruptured samples, to build up relations between macroscopic features (necking, damage, etc.), and underlying microstructural mechanisms. Analysis of the rupture surfaces has revealed a ductile fracture mode. Electron Backscatter Diffraction (EBSD) analysis near to the rupture surface has indicated significant distortion and refinement of the original martensitic substructure, which is evidence of long-range plastic flow. Dislocation pile-ups and tangles from TEM were also observed near substructure boundaries and precipitate particles. All of these microstructural observations suggest that creep is influenced by a complex interaction between several elements of the microstructure, such as dislocations, precipitates and structure boundaries. The calculated stress exponent and activation energy have been found to agree quantitatively with the highlighted microstructural features, bearing some relationships to the true observed creep microstructures.
Highlights
The 9–12% Cr steels have a combination of good creep properties and sufficient oxidation resistance, with additional benefits from good thermal properties and cost effectiveness [1,2]
Creep occurs as a result of interaction of both the work hardening process caused by the applied force and annealing process due to elevated temperature [5], resulting in strong microstructural changes [6,7]
All data extracted from the creep curves exhibit the following main characteristics: i) the steady-state is of long duration and, in most cases, does not exceed 3% strain, ii) the accumulation of strain in the tertiary stage is much higher compared to that in both reduced and steady-state creep regimes, and iii) significant necking is observed during the final stage of creep
Summary
The 9–12% Cr steels have a combination of good creep properties and sufficient oxidation resistance, with additional benefits from good thermal properties and cost effectiveness [1,2]. Creep occurs as a result of interaction (or competition) of both the work hardening process caused by the applied force (experienced as a decrease of the strain rate) and annealing process due to elevated temperature [5], resulting in strong microstructural changes (grain boundary sliding, dislocation climb, etc.) [6,7]. These microstructural changes can significantly affect the mechanical features of the material. When these microstructural changes develop into large-scale structural damage, the material fails due to formation of micro-cracks within grains and cavitation at grain boundaries (accelerated stage)
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