2.47 GPa grade ultra-strong 15Co-12Ni secondary hardening steel with superior ductility and fracture toughness

초록

This study investigated the effect of multi-step heat treatment on the microstructure, mechanical properties and fracture behavior of thick 15Co-12Ni secondary hardening steel. As-quenched sample was found to have elongated prior austenite grain (PAG) and coarse lenticular martensitic structure. On the other hand, heat-treated sample was observed to have fine lenticular martensitic structure due to fine PAG size and a lot of nano-sized carbides. Also, after heat treatment, nano-scale reverted austenite film was formed at the martensite interfaces. The heat-treated sample showed 2.47 GPa superior tensile strength and superior elongation of about 12 %. The high strength was mainly due to fine block size and high number density of nano-sized carbides. The average value of plane strain fracture toughness (K-IC) was 29.3 MPa ,root m, which indicated a good fracture toughness even with the high tensile strength. The tensile fracture surface was observed to have ductile fracture mode (cup-and-cone) and the formation of about similar to 1 mu m ultra-fine dimples. In addition to this, nano-sized carbides were observed within the dimples. The findings suggested that the nano-sized carbide had a positive effect not only on the strength but also on the ductility of the alloy. The fractured surface after toughness test, also showed ductile fracture mode with a lot of dimples. Based on the above results, correlation among microstructural evolution, deformation and fracture mechanisms along the heat-treatment was also discussed. (C) 2021 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

키워드