Abstract
In order to analyze the mechanism of the carburization that is the direct cause for the formation of the white layer produced on the high speed steel tempered after being vacuum oil-quenched, the relations between heat-treatment conditions and case depths were investigated and the decomposed gas produced from the quenching oil was analyzed using pure iron and S15C as quenching specimens. It was clarified that the specimen was carburized in the oil on quenching, and that the occurence of the carburizing on vacuum oil-quenching required the following conditions:
(1) Specimens are heated in vacuum to clean prior to quenching.
(2) Quenching temperature is sufficiently high to form a vapor film in quenchant after specimens are quenched in it.
(3) Quenchants decompose to produce the carburizing gas as a result of heating.
(4) The carbon content in austenite is under the solubility limit on quenching.
The present results give reasonable explanations for the facts that the white layer is formed only on the vacuum oil-quenched high speed steel and that the thickness of the white layer increases as the quenching temperature is raised and is affected by the size and form of specimens although it is independent of holding time at the austenitizing temperature. When the formation of the white layer is not desirable, oil-quenching of the specimens after gas-cooling to a certain temperature is one of the methods for suppressing its formation.
