A study of blocky retained austenite and properties under variously heat-treated ultra-fine bainitic steel

Abstract

The effect of four processes on the morphology of high chromium ultra-fine bainitic steel, especially blocky retained austenite (BRA), and properties is investigated in this paper. In all samples, samples under conventional austempering at 330 °C (CA-330) has more contend of retained austenite (32.5%), but ultimate tensile strength and total elongation are the lowest, namely 1487 MPa and 3.0%, because of BRA ( average dimension, 6.16 μm) rapidly transforming into hard and brittle martensite during tensile testing. Compared with mechanical properties of the CA-330 sample, the ultimate tensile strength and total elongation of other samples increases owing to the reduction in BRA dimension to 1.1 μm∼1.9 μm by reducing isothermal temperature or two-step austempering step or adding quenching during two-step austempering or pre-deformation austempering. The optimal property combined with tensile strength of 1754 MPa and elongation of 16.4% is achieved at 240 °C under two-step austempering (TA-240) with the average BRA dimension of 1.1 μm.