Microstructure and Mechanical Properties of 0.18%-C Steel Samples Processed Through Five Different Heat Treatment Techniques

The present work critically investigates the effect of various heat treatments and continuous cooling rates on mechanical properties and microstructural attributes of 0.18%-C steel. During heat treatment, the heating temperature and holding time were kept at 925 °C and 1 h, respectively, for five different samples. Another sample was kept in ‘as received’ condition so that a comparative analysis could be done on its behalf. Mechanical properties like tensile strength, hardness, toughness, and shear strength were determined and analyzed for each sample. The microstructural changes were observed through an optical microscope. Coarse and fine pearlites with proeutectoid ferrite regions were mainly observed in microstructural images. The fracture behavior of the materials was observed by field emission scanning electron microscope. Both ductile and brittle fractures were observed in the broken specimens. Heat-treated samples showed a remarkable reduction in tensile strength and shear strength with a significant improvement in extension values prior to failure. The sand-cooled and furnace-cooled samples exhibited 58% and 46% enhancement in ductility. Similarly, the toughness of all the heat-treated products got increased except for air-cooled sample. Because of fine pearlite, the hardness of air-cooled, sand + brine cooled, and sand + oil-cooled specimens has increased by 4%, 20%, and 9%, respectively, in comparison with the original specimen. Also, the lamellar ferrite + cementite regions have made the sand-cooled and furnace-cooled specimens tougher than the original specimen.