Toughening optimization on a low carbon steel by a novel Quenching-Partitioning-Cryogenic-Tempering treatment

A low-carbon Mn-Si-Cr alloyed steel was treated by two different heat treatment routes, quenching- partitioning-tempering (Q-P-T) and quenching- partitioning- cryogenic- tempering (Q-P-C-T). The strength and toughness were enhanced concurrently after Q-P-C-T treatment (i.e., ultimate tensile strength: 1236 MPa and the impact energy at 20 degrees C: similar to 113 J cm(-2), respectively). Compared to the toughness of Q-P-T, the toughness of Q-P-C-T was improved by 23%. Microstructural characterization was carried out by conducting scanning electron microscopy, X-ray diffraction, electron backscatter diffraction and transmission electron microscopy. The enhanced toughness was attributed to the increasing high-angle boundary and newly formed martensite, which was caused by cryogenic treatment of Q-P-C-T. At last, a rational model was built to explain the toughening mechanism by microstructural evolution during Q-P-C-T. The work provides a meaningful idea that cryogenic treatment can cooperate with advanced heat treatment scientifically to acquire optimized performance of conventional steels.