RESEARCH PAPER |
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Effect of Heat Treatment on Delta-ferrite and Impact Toughness of P91 Heat-resistant Steel |
ZHANG Jianbin1, LIU Fan1, XUE Fei2
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1 State Key Laboratory of Advanced Processing and Recyling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050; 2 Suzhou Nuclear Power Research Institute, Suzhou 215004 |
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Abstract Delta-ferrite, whose morphology, content and distribution depended on hot processing temperature, readily generated in P91 heat-resistant steel during hot processing, such as hot rolling, welding and heat treatment. The mixed microstructure, including martensite and delta-ferrite, was obtained by normalizing at different temperatures (1 150 ℃,1 250 ℃,1 300 ℃) in accor-dance with the designed heat treatment processing, and the normalizing microstructure (1 300 ℃) undertook oil cooling (at 1 050 ℃)+air cooling (at 760 ℃). The content, morphology and distribution of delta-ferrite were observed by OM (optical microscope), SEM (scanning electron microscope) with EDS (energy-dispersive spectrometry), and the microhardness, impact toughness and fracture failure mode of that were tested. The results demonstrated that delta-ferrite content of P91 steel increased along with the increase of normalizing temperature, and its morphology presented thin strip, strip and bulk, and bulk polygon. The delta-ferrite generated due normalizing at 1 300 ℃ could not be eliminated by normalizing at 1 050 ℃ and tempering at 760 ℃, while the content decreased, morphology and distribution of delta-ferrite altered. The impact energy reduced with increase in delta-ferrite content, and the impact fracture failure mode converted to brittle fracture from ductile-brittle fracture. In contrast with strip morphology, the bulk polygon of delta-ferrite at boundaries was not conducive to impact toughness.
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Published: 25 April 2018
Online: 2018-05-11
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