Tensile Test of High-Strength Medium Carbon Steel and Reduction of Elastic Limit

Steels are useful for mechanical elements because of their mechanical property. When designing the strength of a machine, the tensile strength is an important factor, but it is not easy to measure. On the other hand, hardness measurement is easier than that and can be measured in the shape of the product, thus it is frequently used. Hardness and tensile strength are in a proportional relationship, and there are conversion tables which are published. However, it is not showed for hardness higher than 55HRC. The surface hardness of bearings and gears which are subjected to high surface pressure is higher than 55HRC, so the hardness cannot be converted to tensile strength. Therefore, we performed experiments using medium carbon steel and confirmed the hardness and tensile strength in the range of 25 to 63HRC, including hardness above 55HRC. The results showed that the tensile strength and elastic limit peaked at about 55HRC, and when the hardness is at 63HRC, the tensile strength decreased by 25% and the elastic limit decreased by approximately 75%