Mechanical and microstructural evaluation of a low cast steel under carbon isotermally heat treatment

Abstract

Carbo-austemepered thermal treatment consists in having a low carbon steel under sequential carburizing and austempering processes in order to get a carbon-rich layer and a nominal carbon composition at the steel nucleus. The improvement of the core strength, ductility and hardening by deformation in carbo-austempered cast steel could be achieved with a multiphase microstructure. High ductility is associated with its ferritic matrix and the ability of retained austenite to transform into martensite when subjected to external stress/deformation is found to increase the coefficient of hardening by deformation. The high mechanical strength will be derived from the presence of the austenitic transformation phases within the microstructure (bainite and martensite). This study focuses on the fabrication of a multiphase structure in the core of a carbo-austempered steel (0.3C-1.9Si-0.5Mn-0.9Cr %wt.) and the effect of the volumetric fraction of the proeutectoid ferrite on the levels of resistance and ductility. The samples were austenized in the intercritical range and then isothermally transformed at 250 °C and 300 °C. Subsequently, microstructural, phases, and mechanical characteristics were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and tensile tests. The results showed that the specific conditions of austenization in the intercritical range, time and temperature, determined the microstructure/properties ratio from the austenite/ferrite contents and the carbon contents reached in the high temperature austenite.