Effect of isothermal heat treatments under Ms temperature on the microstructures and mechanical properties of commercial high-silicon spring steel

Abstract

Abstract The effect of isothermal heat treatment temperatures under martensite start (Ms) temperature on the microstructure and mechanical properties of high-silicon commercial spring steel has been investigated. For this purpose, tensile specimens are prepared from AISI 9254 steel isothermally heat-treated under Ms temperatures (225 °C, 250 °C, and 275 °C) for 168 h after austenitizing at 870 °C for 30 min. Optical microscopy, scanning electron microscopy and X-ray diffraction analysis were used to characterize the microstructures of the specimens. Mechanical properties were determined by the tensile and hardness tests. Experimental results revealed that microstructure consists of carbide-free bainite, carbon enriched retained austenite, and martensite in high-silicon spring steel by the isothermal treatment under Ms temperature. The yield and tensile strength were increased by decreasing the isothermal temperature. However, uniform elongation and breaking energy were decreased by decreasing the isothermal temperature. The specimen which was isothermally heat-treated at 250 °C under Ms temperature showed a very good combination of tensile strength and total elongation as 2046 MPa and 8.5%, respectively. Dimples along with cap and cone formation which are evidence of a ductile fracture were observed in fractured surfaces of all isothermally heat-treated specimens.