Atomic scale characterization of carbon partitioning and transition carbide precipitation in a medium carbon steel during quenching and partitioning process

Abstract

Transmission electron microscopy (TEM) and 3D Atom probe tomography (APT) were used to investigate the elemental partitioning, carbon (C) redistribution and carbide precipitation associated with quenching and partitioning (Q&P) treatment of medium C steels (0.4 wt% C) alloyed with three levels of silicon (Si) (0.25, 0.75 and 1.5 wt%). Different types of carbides resulting mainly from tempering of martensite (M) and/or partial decomposition of C-enriched austenite have been characterized. The results reveal formation of transition carbides η (Fe2C) in High (H)-Si (1.5 wt% Si) steel that are stable even at high partitioning temperatures (300 °C). However, these η carbides formed in Medium (M)-Si (0.75 wt% Si) steel were only partially stable, where a fraction of η carbides decomposed to cementite. In Low (L)-Si (0.25 wt% Si) steel, only the presence of cementite precipitates was evident. In addition, the cementite precipitates were found to be relatively coarse in size compared to η carbides, suggesting that cementite grew during the partitioning process at 300 °C. Apart from carbides, segregation of C clusters in M lath boundaries was clearly revealed by APT. In addition, the possible formation of a metastable, hexagonal (ω) phase between the thin nano-twinned high C martensite has been explored.