Middle-carbon martensite steels are vital materials for mechanical components and their mechanical properties have attracted significant interest. However, the decrease in the elastic limit of the as-quenched materials is one of the remaining puzzles. Herein, we quantitatively characterized the dislocation density and its structure in the as-quenched and tempered martensite steel by neutron diffraction line profile analysis and discussed their impact on the yield stress. The dislocation density in the as-quenched specimen was the highest at 9.7 × 1015 m−2, while it decreased with an increase in the tempering temperature. In addition, the component ratios of edge and screw dislocations decreased and increased, respectively, depending on the increase in the tempering temperature. The dislocation arrangement parameter (M) varied between the tempering temperatures of 220 and 290°C. Although there was a large difference between the yield stress obtained from the tensile test and that estimated from the dislocation density, the experimental results could be explained by correcting them with the inverse of M value as an index showing the effective dislocation density ratio.

In recent years, there has been a demand for lightweight shaft products to improve the tact time of equipment. Shaft products are stepped in order to use bearings at the shaft end. Therefore, drilling a hole in the center of a shaft product is not enough to reduce the weight of the product to ensure the wall thickness. Therefore, focusing on swaging technology, a type of drawing, we devised a weight reduction method in which both ends of a thin-walled steel pipe are plastically formed to form the shaft end and a structure with a large space inside is created. The thin-walled steel pipe is swaged to form the shaft ends, grooved with threads, heat-treated, and finished to produce a lightweight product. Calculation results show that the weight is 54% of that of the conventional product, a significant weight reduction. The allowable speed can be increased to 125% of the conventional product, which may contribute to an improvement in tact time. The prototypes manufactured by the drawing process showed no difference in appearance from the conventional product, and the measured weight was 55% of the conventional product, almost exactly as calculated. As a result of devising a weight reduction method using the drawing process and manufacturing a prototype, the possibility of significantly reducing the weight of a shaft product was seen. We will continue to evaluate the performance and life of the product in order to commercialize it.

Carbon steel with a medium or high carbon content can realizes high hardness, and is widely used for bearing parts, which are the main mechanical elements, because of its high strength and wear resistance. The authors investigated changes in the tensile strength and elastic limit of medium carbon steel by tempering. As the result, the tensile strength and elastic limit of the as-quenched steel were lower than those of the tempered. In this study, to reveal the mechanism of the decrease in the elastic limit of quenched medium carbon steel was studied. Relaxation test and electron backscattering diffraction analysis were performed for characterization of inhomogeneous strains. The stress of 80% Young's modulus was taken as the elastic limit, and the specimen was held for 15 minutes at the stroke when the stress was reached elastic limit. The kernel average misorientation values were decreased after the relaxation test, suggesting that the relaxation test reduced the non-uniform strain in the crystal grains.

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%

Carbon steels realize high strength by quenching, which are used for mechanical elements such as gears and bearings. Martensitic transformation makes harder due to high dislocation density and fine crystal grains. The mechanical performance increase in proportion to their hardness. However, in the case of medium carbon steels, steels quenched and tempered at 290℃ have better mechanical properties than as quenched steels. There are some factors for decrease of mechanical properties such as dislocation density and retained austenite, this study focused on dislocation microstructure. To obtain their dislocation microstructure, line profiles from neutron diffraction were analyzed using convolution multiple whole profile (CMWP) fitting method. It is known that there are as proportional relationships between the square root of dislocation density and yield stress, but the analysis results show no proportional relationship. Therefore, we employed dislocation arrangement parameter to take into account mobile and immobile dislocations. Thus, effective dislocation density was introduced and may be able to explain the relationship with yield stress.