INFLUENCE OF MARTENSITIC QUENCHING ON FATIGUE PROPERTY OF HIGH CARBON CHROMIUM BEARING STEEL UNDER ROTATING BENDING

摘要

In order to clarify the martensitic quenching dependence of the fatigue behavior, the rotating bending tests were carried out for high carbon chromium bearing steels. The quenching treatments were performed four kinds of following cooling conditions after heating at 1108K×40min. (1) oil quenching at 353K, (2) salt-bath quenching at 353K and (3) deep freezing at 77K. In addition, (4) heating at 1093K×40min + oil quenching at 353K (so called secondary quenching) after quenching (1). And all specimens were tempered at 453K×120min after each quenching. As a result of fatigue tests, the S-N curves of specimens quenched in oil and quenched in salt-bath showed the same characteristics. In contrast, fatigue life of the deep freezing specimens and the secondary quenching specimens in high stress level was longer, but the S-N curves showed the tendency that the fatigue limit did not appear even if stress amplitude was decreased at 800MPa. And the fatigue fracture of every deep freezing specimen and every secondary quenching specimen took place in a surface induced failure mode. It seemed that those specimens were sensitive to surface roughness because of the high hardness extremely, and the crystal slip at the specimen surface was not restrained due to a notch effect. In the case of surface polished specimens, interior inclusion induced failure took place with so-called fine granular area FGA in the vicinity around the interior inclusion. However, no difference of FGA forming state was found among specimens made by every heat treatment condition. Therefore, it seems that the fatigue life in the interior inclusion induced fracture is not improved by the martensitic quenching in this study.