著者

水野 湧太 熊谷 正芳 田邉 晃弘 新部 純三

出版者

一般社団法人 日本機械学会

雑誌

日本機械学会論文集 (ISSN:21879761)

巻号頁・発行日

pp.23-00242, (Released:2024-01-11)

参考文献数

11

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Since it is known that tensile strength and hardness are roughly proportional, a hardness test is often used as a simple and substitute test. However, it has been reported that the proportional relationship between tensile strength and hardness is broken above 595 HV (≈55 HRC). Besides, there are few examples of mechanical properties determined by compression tests despite the fact that high-hardness and high-strength materials subjected to significant compressive stress are used in machine element parts, e.g. bearings. Tensile strength is used instead of compressive strength for the design. Thus, the relationship between hardness and compressive strength in hard materials, quenched medium carbon steel, was revealed. The compressive strength at 8% plastic strain (compressive strength) was almost the same as the maximum compressive strength. The compressive strength and hardness were at their maximum in the as-quenched specimen and decreased with the increase in tempering temperature. The compressive strength and hardness had a linear relationship up to 2000 MPa similar to the relationship of tensile tests. However, the compressive strength increased slightly in relation to the hardness above 2000 MPa in contrast to tensile tests. Thus, the work-hardening index was introduced as a variation parameter to the function that expresses the relationship between compressive strength and hardness to obtain better estimation. The estimated compressive strengths using the work-hardening index are agreed well to the experimental results.

著者

新部 純三 熊谷 正芳 田邉 晃弘 水野 湧太

出版者

一般社団法人 日本機械学会

雑誌

日本機械学会論文集 (ISSN:21879761)

巻号頁・発行日

pp.21-00082, (Released:2021-08-05)

参考文献数

13

被引用文献数

2

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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.