Comparative examination of the effects of different heat treatments applied to spring steels

In this thesis, round specimens with a thickness of 5 mm and a length of 125 mm were prepared from 38Si7 medium carbon Si-Mn steel as spring steel according to the EN ISO 6892-1:2016 standard. The investigation encompassed the mechanical properties and microstructural alterations of the material, with a focus on the application of noval multi-stage quenching-partitioning (Q&P) heat treatment and classical quenching-tempering (Q-T) heat treatments, which are regular approaches within the existing body of literature. Following austenitization at 860°C for 10 minutes, 12 specimens were subjected to Q-P heat treatment with first quenching at 250°C, different partition temperatures (350,400 and 450°C), and different time variables (300 and 400 seconds). The remaining five specimens were austenitized at 860°C for 10 minutes. Following this process, three of the specimens were quenched to room temperature and tempered at 430°C for 1 hour. The remaining two specimens were tempered at 430°C for 30 minutes. After the application of heat treatment, the specimens were subjected to tensile testing, and the resulting values were meticulously documented. One specimen was exposed to a tensile test without undergoing a heat treatment, and its corresponding value was documented. The values are presented in tabular form and later interpreted. Furthermore, the fracture surfaces were photographed from different distances using a Scanning Electron Microscope (SEM). The effects of the heat treatments on the microstructure of the samples were analyzed by taking images from the surface that had been cleaned with sandpaper and etched with an optical microscope. The hardness values of the samples were determined by means of the Rockwell Hardness method. The findings, as indicated by the results obtained, demonstrated that the hardness values of the samples subjected to conventional heat treatment were higher than those subjected to Q-P heat treatment. Furthermore, the effects of the partition times in the Q&P heat treatment applied to 38Si7 spring steel were lower than expected.