Microstructure and Mechanical Properties of Intercritically Treated Grade 91 Steel

Wang, Yiyu; Zhang, Wei; Lim, Yong Chae; Wang, Yanli; Feng, Zhili

doi:10.3390/ma13183985

Title: Microstructure and Mechanical Properties of Intercritically Treated Grade 91 Steel

Journal Article · Thu Sep 10 00:00:00 EDT 2020 · Materials

DOI:https://doi.org/10.3390/ma13183985· OSTI ID:1659354

; Zhang, Wei;

; Wang, Yanli;

Premature creep failures at the intercritical heat affected zone (ICHAZ) of creep-resistant steel weldments have been frequently reported. However, the creep degradation mechanism of different microstructure constituents in ICHAZ is complicated and needs further clarification. In this work, Grade 91 steel was intercritically heat-treated at a temperature (860 °C) between the critical temperatures AC1 and AC3, and a correlation between microstructure and mechanical properties of the heat-treated specimen was built. The effects of austenitization and tempering resulting from the intercritical treatment (IT) differentiated the local strain energies between the two microstructure constituents: newly transformed martensite (NTM) and over-tempered martensite (OTM). The formation of NTM grains led to a hardness increase from 247 HV0.5 in the base metal to 332 HV0.5 in the IT specimen. The ultimate tensile strength (UTS) increased from 739 MPa in the base metal to 1054 MPa in the IT specimen. Extensive growth of the OTM grains and rapid recovery of NTM grains took place simultaneously in the IT specimen during a typical tempering at 760 °C. These microstructure degradations led to a lowered hardness of 178 HV0.5, a reduced UTS of 596 MPa, and a poor creep resistance with a minimum creep strain rate of 0.49 %/h at 650 °C in an IT + tempering (ITT) specimen.

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Research Organization:: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sponsoring Organization:: USDOE Office of Fossil Energy and Carbon Management (FECM); USDOE Office of Fossil Energy (FE); USDOE Crosscutting Research Program

Grant/Contract Number:: FWP-FEAA118; AC05-00OR22725

OSTI ID:: 1659354

Alternate ID(s):: OSTI ID: 1661224

Journal Information:: Materials, Journal Name: Materials Vol. 13 Journal Issue: 18; ISSN 1996-1944

Publisher:: MDPI AGCopyright Statement

Country of Publication:: Switzerland

Language:: English

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Grade 91 steel
creep strength
intercritical treatment
microstructure
mechanical properties

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