Amir Talebi - Faculty of Materials & Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, Iran
R. Bakhtiari - Department of Materials Science and Engineering, Razi University, Kermanshah, Iran.
B. Abbasi-Khazaei - Department of Materials and Textile Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran.
Mehdi Ghobeiti-Hasab - Department of Metallurgy and Materials Engineering, Dezful Branch, Islamic Azad University, Dezful, Iran.

In this research, AISI ۴۱۴۰ steel samples were normalized at ۸۵۰ ºC for ۶۰ min, then, they were kept at ۷۲۰ ºC for ۳ min and were directly transferred to a salt bath with temperature of ۴۰۰ °C for ۴ min and after that, the samples were quenched in water (۲۵ °C). Finally, the triple-phased samples were tempered at ۲۵۰, ۴۵۰ and ۶۵۰ ºC for ۹۰ 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 ۲۵۰ °C was upper bainite and martensite along with carbide particles resulted from the decomposition of martensite, while the microstructure of sample tempered at ۴۵۰ °C was lower bainite and martensite along with carbide particles. Nevertheless, in the sample tempered at ۶۵۰ °C, bainite and martensite phases were decomposed completely to carbide. In comparison with not-tempered sample, the sample tempered at ۲۵۰ °C had higher yield strength, greater elongation and lower ultimate tensile strength. As the tempering temperature increased from ۲۵۰ to ۶۵۰ °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 ۲۵۰ °C, while in the second stage, it continuously decreased for all tempered samples.

Triple-phase steel, Tempering temperature, Microstructure, Work-hardening