Nanobainite Generated in Low- and Medium-Carbon Steels via an Economical Alloying Strategy

Abstract

A low-cost strategy for generating fast transforming nanobainitic (nB) steels in low- and medium-carbon alloys is investigated. Accelerating the bainite transformation relies on adding 0.7–3 wt% Al. Alloys and heat treatments are designed via thermodynamic calculations and dilatometry. nB microstructures are generated via isothermal holding and continuous cooling. The microstructures generated are investigated via microscopy and mechanical characterization. Incubation periods less than 150 s as well as isothermal transformation times ranging from 2000 to 4000 s are recorded for all conditions. Increasing the Al content from 0.7 to 2.8 wt% lowers the incubation and transformation times from 150 to 15 s and 3000 to 2000 s, respectively, at a cost of a reduction in tensile strength and elongation % (EL%) from 1330 to 1270 MPa and from 13.5 to 7.7%, respectively. The introduction of δ-ferrite to the microstructure of the high Al alloy increased EL% up to 16% and reduced the tensile strength to 1105 MPa. Continuous cooling at a rate of 0.03 K s−1 increased the tensile strength by 100 MPa at similar EL%. Lowering the cooling rate to 0.003 K s−1 yielded similar properties as isothermal treatment because most of the transformation is concluded near the starting temperature.