Effect of tempering temperature on microstructure and work hardening behavior of a triple-phase AISI ۴۱۴۰ steel

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.

Keywords:

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