2021 年 87 巻 900 号 p. 21-00082
Although elastic limit and tensile strength are essential for mechanical strength design, the measurement of high hardness materials is a challenge due to their sensitivity on notches. As an alternative, the tensile strength can be easily assumed from hardness using the conversion table (e.g. SAE J 417); however, the table does not include the data for high hardness region, namely tempered materials at low temperature. We performed tensile tests and obtained elastic limits and tensile strengths of quenched martensitic medium-carbon steel with tempering at several temperatures. The hardness was monotonically decreased with the increase of tempering temperature. However, the tensile strength and elastic limit improved as the increase of temperature at low tempering temperature. In contrast, at high tempering temperature, the tensile strength and elastic limit were decreased with increase of tempering temperature. Consequently, the ratios of hardness to tensile strength or elastic limit were changed below or above the transition point. The ratios were constant below a tempering temperature (290 ℃ for elastic limit and 200 ℃-300 ℃ for tensile strength in the present experiments) and increased with the increase of the tempering temperature above the temperature. The variation of the ratio, hardness to tensile strength, is due to the brittle fracture on high hardness specimens before reaching the threshold to start plastic instability. In addition to, it is considered that the variation of the ratio, hardness to elastic limit, is caused by another mechanism, a decrease in elastic limit due to mobile dislocations.
