The effect of heat treatment on the microstructure and mechanical properties of leaf spring steels

This study investigated the impact of tempering temperature on the mechanical properties and microstructure of 51CrV4 and 55Cr3 spring steels. These steels are commonly used in the production of leaf springs, which are crucial components of suspension systems in vehicles used for freight transportation on roads and railways. In order to achieve this objective, the normalizing procedure was initially implemented on all steel samples, resulting in the attainment of identical starting structures in the steels. In the quenched-tempered procedure, the samples underwent hardening by being heated to 870 ºC for 30 minutes and then rapidly cooled in oil. Subsequently, the tempering procedure was conducted at temperatures of 300, 375, 450, and 525 ºC. The mechanical properties were assessed by conducting tensile and hardness tests (Vickers and Rockwell methods) in order to ascertain the alterations resulting from heat treatments. In addition, Charpy V-notch impact tests were conducted on several steels within a temperature range of -40 to +80 ºC in order to ascertain the influence of tempering temperature on the transition temperature for impact. The microstructures resulting from heat treatments were analyzed using a metallurgical microscope, the phases were analyzed using X-ray diffraction (XRD) technology, and the fracture surfaces were investigated using a scanning electron microscope (SEM). With an increase in the tempering temperature, the strength (yield and tensile), hardness values of both steels declined, but the ductility (percent elongation and cross-section reduction), toughness and impact energy values rose. At low temperatures, the impact energy values of both steels declined under all tempering levels, and no noticeable transition from ductile to brittle behavior was seen in the tempered steels. Furthermore, normalized steels exhibited superior ductility compared to all tempering temperatures. Additionally, the impact energies of these steels experienced significant reductions below 10 J at low impact temperatures of 0 and -40 ºC.