Effect of carbon distribution range in mixed bainite / martensite / retained austenite microstructure on mechanical properties

Carbide-free bainitic steels with the bainitic ferrite (BF) as matrix constituent have been widely used and manufactured. The mixed microstructure of pre-formed martensite (M), BF, and retained austenite (RA) was obtained by quenching for a short time below the martensitic transformation temperature followed by austempering. A similar amount of fresh martensite was obtained by interrupted austempering at the same temperature. Results show that the carbon distribution among these constituents in the mixed microstructure plays a major role in controlling mechanical properties. The carbon content of M and BF in the M-BF-RA microstructure is lower than that of BF-M-RA microstructure, while that of RA was reversed. Mechanical properties of the M-BF-RA microstructure exhibit higher toughness, higher product of strength and elongation as well as higher yield ratio, rather than strength, compared to that of the BF-M-RA microstructure. It can be attributed to toughened pre-formed martensite with low carbon content and mechanical stable RA with high carbon content. In addition, the pre-formed martensite decreases the block size, leading to the increase of high angle grain boundaries, which further improves toughness. The high carbon fresh martensite BF increased the strength of the BF-M-RA microstructure, but sharply decreases ductility and toughness.