Effect of tempering temperature on microstructure and work hardening behavior of a triple-phase AISI 4140 steel

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 a 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 an FE-SEM equipped with an EDS detector and a universal tensile test machine, respectively. Results showed that the microstructure of sample tempered at 250 °C was upper bainite and martensite along with carbide particles resulting 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 the 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, the strength coefficient (k) decreased with increasing the tempering temperature. 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.