Document Type : Technical Note
Faculty of Materials & Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, Iran
Department of Materials Science and Engineering, Razi University, Kermanshah, Iran.
Department of Materials and Textile Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran.
Department of Metallurgy and Materials Engineering, Dezful Branch, Islamic Azad University, Dezful, Iran.
In this research, AISI 4140 steel samples were normalized at 850 ºC for 60 min, then, they were kept at 720 ºC for 3 min and were directly transferred to a salt bath with temperature of 400 °C for 4 min and after that, the samples were quenched in water (25 °C). Finally, the triple-phased samples were tempered at 250, 450 and 650 ºC for 90 min. The microstructure and tensile properties were examined by a FE-SEM equipped with an EDS detector and a universal tensile test machine, respectively. Results showed that the microstructureکار سختی of sample tempered at 250 °C was upper bainite and martensite along with carbide particles resulted from the decomposition of martensite, while the microstructure of sample tempered at 450 °C was lower bainite and martensite along with carbide particles. Nevertheless, in the sample tempered at 650 °C, bainite and martensite phases were decomposed completely to carbide. In comparison with not-tempered sample, the sample tempered at 250 °C had higher yield strength, greater elongation and lower ultimate tensile strength. As the tempering temperature increased from 250 to 650 °C, the yield strength and ultimate tensile strength decreased while the elongation increased. Tensile test results of the tempered samples showed a two-stage work-hardening behavior. In both stages, with increasing the tempering temperature the strength coefficient (k) decreased. The work-hardening exponent (n) in the first stage decreased only for the samples tempered at higher than 250 °C, while in the second stage, it continuously decreased for all tempered samples.