Papers by Author: Kazuya Hashimoto

Abstract: In this work, two SAE 52100 grades with different oxygen contents were repeatedly quenched and the influence of this cyclic heat treatment as well as the role of the oxygen content level on the mechanical properties of the material were investigated by rolling contact fatigue tests. The repeated quenching process increased the retained austenite content and had little influence on the material’s hardness. The prior austenite grain size was decreased and consequently, refinement of the martensitic phase in both types of materials occurred. The higher content of the retained austenite (higher fracture toughness) and the refinement of the microstructure accounted for the higher fatigue properties of the repeatedly quenched material. Flaking failure was present in the material quenched only once, after 4.3x107 cycles (higher oxygen content) and 108 cycles (lower oxygen content). Flaking failure did not occur in the samples treated by repeated quenching. The results show the importance of both optimized heat treatment and alloy cleanness for the lifetime of bearing steel.

Abstract: High strengths and impact values are required for carburizing steels used for automotive drive train parts. It is well known that the finer-grain microstructures are favorable for these properties. In this study, with a view to effective refining of grain size with controlling the alloying elements, influence of Mn and Cr content on prior austenite grain size after carburizing and repeated quenching was investigated on Ti-modified carburizing steels with ferrite and pearlite microstructure. It was found that the increase in Mn and Cr contents in steels were effective for the grain refining. These effects were attributed to fine ferrite and pearlite microstructure caused by the lowered Ar3 point due to higher Mn content, and retarding the grain growth of austenite by the fine dispersion of carbides in the carburized layer of specimen due to higher Cr content.

Abstract: Crack formation by a rolling contact fatigue in a high carbon chromium bearing steel has been discussed. Newly developed method for preparing specimens including pre-existing voids enabled one to observe the early stage of fatigue crack formation. Many fatigue cracks were formed around the voids. The positions of crack formation and the direction of the cracks were consistent with those forecasted by finite element analysis. Fatigue crack formation was followed by formation of the WEAs.

Abstract: It is important to suppress grain coarsening during the carburizing treatment of automobile components such as gears and shafts so that high strength is maintained and heat treatment distortion is minimized. Two manufacturing methods are often used for the cost reduction. One is the increasing carburizing temperature. The other is the adoption of cold forging instead of hot forging. These methods are likely to result in grain coarsening in the case of conventional steels. It is well known that the key factors to control the grain growth are the initial austenite grain size and the volume and mean size of precipitates. In this study, the grain growth property of Ti-modified steel was investigated to confirm the influence of precipitates, and compared with those of Nb-modified and conventional steels. The influence of forging temperature on the grain growth property in a Ti-modified steel was also investigated.