Understanding the Effect of Austempering Temperature on the Crystallographic Features and Mechanical Properties of Low-Carbon Bainitic Steel

The effect of austempering temperature on crystallographic features and mechanical properties is investigated in low-carbon bainitic steel, focusing on the relationship between microstructure and mechanical properties. After isothermal holding at, above, and below martensite start (MS) temperatures and tempering, a mixed microstructure of martensite/bainite and martensite/austenite (M/A) constituents is obtained. The fraction of M/A constituents increases as the austempering temperature increases, while the density of block boundaries decreases. The instantaneous work hardening rate exhibits continuous decay without a notable transition because of the retained austenite in the M/A constituents. The toughness decreases with increasing austempering temperature, which is related not only to the fraction of M/A constituents but also to the density of block boundaries. Isothermal treatment below the MS temperature enables the formation of structures with fewer M/A constituents and high-density block boundaries, through which excellent toughness can be achieved.