Microstructural degradation mechanisms during creep in strength enhanced high Cr ferritic steels and their evaluation by hardness measurement

• Published in: Journal of nuclear materials Vol. 416; no. 3; pp. 273 - 279
• Main Authors: Ghassemi Armaki, Hassan, Chen, Ruiping, Kano, Satoshi, Maruyama, Kouichi, Hasegawa, Yasushi, Igarashi, Masaaki
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 30.09.2011; Elsevier
• Subjects: Applied sciences; Controled nuclear fusion plants; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Fission nuclear power plants; Fuels; Installations for energy generation and conversion: thermal and electrical energy; Nuclear fuels; Evaluation; Creep; Thermal ageing; Ferritic steel; Creep strength; Plastic deformation; Microstructure; Nuclear reactor
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2011.06.007

Effect of static recovery on the acceleration of subgrain coarsening during creep plastic deformation. [Display omitted] ► Short-term “H” and long-term “L” creep regions have different creep characteristics. ► Strain-induced recovery of subgrains proceeds in the both creep regions “H” and “L”. ► In region ‘‘L”, two additional degradation mechanisms accelerate creep failure. ► Thermal coarsening of precipitates and subgrains appear during long-term creep ‘‘L”. ► In region “L”, strain-induced coarsening of precipitates accelerates creep failure. There are two creep regions with different creep characteristics: short-term creep region “H”, where precipitates and subgrains are thermally stable, and long-term creep region “L”, where thermal coarsening of precipitates and subgrains appear. In region “H”, the normalized subgrain size ( λ - λ 0 ) / ( λ ∗ - λ 0 ) has a linear relation with creep strain and its slope is 10 ε −1. But, region L is the time range in which the static recovery and the strain-induced recovery progress simultaneously. In this region, the static recovery accelerates the strain-induced recovery, and subgrain size is larger than that line which neglects the contribution of the static recovery. In region “L”, the Δ λ / Δ λ ∗ -strain present a linear relation with a slope 35 ε −1. There is a linear relation between hardness and subgrain size. Hardness drop, H 0 − H, as a function of Larson–Miller parameter can be a good measure method for assessment of hardness drop and consequently degradation of microstructure. Hardness drop shows an identical slope in creep region “H”, whereas hardness drop due to thermal aging and creep in region “L” show together a similar slope. In region “H”, degradation of microstructure is mainly due to recovery of subgrains controlled by creep plastic deformation, and precipitates do not have a major role. However, in creep region “L”, there are three degradation mechanisms that accelerate creep failure; (1) strain-induced recovery of subgrains due to creep plastic deformation, (2) static-recovery of subgrains and precipitates and (3) strain-induced coarsening of precipitates due to the appearance of static-recovery.