Microstructure evolution and fracture mechanism of a novel 9Cr tempered martensite ferritic steel during short-term creep

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 707; pp. 466 - 477
• Main Authors: Xiao, Bo, Xu, Lianyong, Zhao, Lei, Jing, Hongyang, Han, Yongdian, Tang, Zhengxin
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 07.11.2017; Elsevier BV
• Subjects: Chemical precipitation; Chromium steels; Creep rate; Creep strength; Creep tests; Deformation mechanisms; Dimpling; Ductile fracture; Dynamic recrystallization; Evolution; Ferritic stainless steels; Fracture mechanics; Fracture mechanism; G115 steel; Grain boundaries; Laves phase; Martensitic stainless steel; Martensitic stainless steels; Microstructure; Microstructure evolution; Necking; Niobium; Precipitates; Tempered martensite; G115 steel; Fracture mechanism; Precipitates; Microstructure evolution
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2017.09.086

In this work, the microstructure evolution and fracture mechanism of a novel 9% chromium tempered martensite ferritic steel G115 were investigated over the temperature range of 625–675°C using uniaxial creep tests. The creep curves consist of a primary transient stage followed by an apparent secondary stage, and an accelerated tertiary creep regime. The relationship between the minimum creep rate and the applied stress followed Norton's power law. Based on the EBSD analysis, there were no obvious textural features formed after creep deformation, and with the increase in creep time, the number of subgrains slightly increased, and then sharply increased, indicating dynamic recrystallization (DRX) occurs after creep deformation. In addition, three types of precipitates can be observed after creep deformation: W-rich Laves phase, Nb-rich MX, and Cu-rich precipitates. The Nb-rich MX with a square shape and Cu-rich precipitates with an ellipsoidal shape remain very stable. However, the W-rich Laves phases distributed mainly on the grain boundaries have rod-like, chain-like, and bulky shape, which are coarsened significantly. Representative fractographs of the G115 steel after creep deformation exhibit significant necking with an elliptical shape. A dense array of deep and equiaxed dimples appear in the central region under the tested creep conditions. Ductile fracturing is the dominant fracture mechanism during short-term creep deformation.