Itsuo Ishigami1, Mamoru Nakamura1 and Kyuhiko Yamanaka1
1Osaka Prefectural Industrial Research Institute, Osaka
When a polished section of high speed steel tempered after being vacuum oil-quenched is etched, a white layer about 15 to 40 μ thick is observed at the edge of the section. Some properties of the white layer produced on the specimen SKH-9 are clarified by means of an optical microscope a micro-vickers hardness tester, an electron probe microanalyzer, and an X-ray diffractometer.
The main results obtained are as follows:
(1) The white layer is formed only on the specimen tempered after being vacuum oil-quenched.
(2) Carburization is the direct cause for the formation of the white layer.
(3) This layer consists of a small amount of M6C-type carbide as well as coarse martensite and of a large amount of retained austenite. Consequently, the white layer has low hardness.
(4) The thickness of the layer increases as quenching temperature is raised, and is also affected by the size and form of the specimen. It is however independent of holding time at the austenitizing temperature.
(5) The resistance of the white layer to decomposition on tempering increases as the Cr content is increased.
(6) The maximum hardness zone composed of M6C-type carbide and coarse martensite of high carbon content exists in the inside of the boundary between the white layer and the matrix.
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