Effect of multiple-direction forging on microstructure and mechanical properties of ultra-high strength stainless steel
LENG Huan-hui1, LIU Zhen-bao2, LIANG Jian-xiong2, WANG Xiao-hui2, YANG Zhi-yong2, LUO Hai-wen1
1. School of Metallurgical and Ecological Engineering,University of Science and Technology Beijing, Beijing 100083, China; 2. Institute for Special Steels,Central Iron and Steel Research Institute, Beijing 100081, China
Abstract：The microstructure of martensitic ultra-high strength stainless steel after multiple-direction forging was quantitatively characterized by optical microscope (OM), scanning electron microscope (SEM), back scattering electron diffraction (EBSD) and transmission electron microscope (TEM). The effect of multiple-direction hot forging on the mechanical properties of experimental steel was analyzed. The results showed that with the increase of forging passes, the original austenite grain, martensite lath and lath block size of the experimental steel were gradually refined. After the five cycles of hot-forging, the sizes of prior austenite grains, martensite packets and blocks were refined from 226.1 to 3.2 μm, from 106.1 to 2.9 μm and from 2.3 to 1.5 μm, respectively. Consequently, the mechanical properties of the experimental steel were significantly improved, the yield strength, impact toughness and elongation increased from 1 030 to 1 175 MPa, from 140 to 194 J and from 9.3% to 11.6%, respectively. The strength improvement of experimental steel was attributed to both grain refinement and dislocation strengthening with the later as the main contributor. Moreover, it can be concluded that the two strengthening mechanisms cannot be added linearly but by the square-root rule according to dislocation theory.
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