UP-quenched SAE 4130 steel: Mechanical properties assessment and bainite formation

This work focuses on the effect of Up-Quenching experimentation on mechanical properties of SAE 4130 steel. In that sense, the effect of time and temperature of Up-quenching on hardness, yield strength (YS), ultimate tensile strength (UTS), elongation, and toughness of this automotive steel were investigated. Up-quenching of the austenitized specimens were performed in the temperatures range between 350 to 500 °C for the times period between 20 to 60 min. Mechanical properties were evaluated by means of tensile and hardness tests. In addition, Charpy impact tests were done to measure the fracture energy of the specimens. Microstructural analyses, including texture development and fracture surfaces analyses, were done using Scanning Electron Microscope (SEM) equipped with Electron backscattered diffraction (EBSD) and Transmission Electron Microscope (TEM). Results showed that UP-quenching has positive contribution on hardness, UTS, Elongation% (EL%) and toughness. The results of the present work are useful to control the mechanical properties of SAE 4130 steel via changing the heat treatment parameters. Electron microscopy results showed that the maximum amount of bainite was observed at 450 °C. And the optimum microstructure, that provides the best mechanical properties, is formed in the case of Up-quenching at 450 °C for 20 min {100} pole figures for the Up-quenching in 20 min were constructed and showed that at different Up-quenching temperatures, textured structures with the main components of S, Brass, Copper and Cube are attainable. Texture components were substantially weak in the cases of non-optimum temperatures of Up-quenching. Texture intensity was quite high for the Up-quenching case situation of 450 °C. As well, non-cube textures of S, Brass, and to some extent Copper, were developed for the Up-quenching condition of 450 °C, which clarified the acceptable combination of mechanical properties in this condition. Finally, fracture surfaces examination showed that increasing the temperature and time of Up-quenching leads to lower of porosity that is better distributed in the microstructure.