Abstract
An anomalous expansion in the dilatation curve of cold-rolled 18-8 stainless steel was incidentally found at about 530°C.The present experiments were made in order to clarify the cause of the anomalous expansion observed in the dilatation curve of some meta-stable austenitic Cr-Ni steels coldworked at various temperature, and also to clarify the behaviour of the reverse transformation of martensite to austenite in these steels, mainly by the thermal dilatation method and the micro-structural observations.
When a cold worked 18-8 steel was heated at constant velocity of heating, the martensite in the steel transformed to austenite reversely step by step at the temperature range of 400-900°C.The first transformation began at 400°C, and the greater part of martensite transformed to austenite martensitically at 500-650°C, and the second at 680-760°C, the third at 800-900°C, respectively.However, the amount of transforming martensite decreased less and less as temperature raised.In such reverse transformation, the diffusionless process occurred at early stage and the diffusional process continued in after the first stage.The lath type martensite was separated along {111} plane, by growing austenite nucleated at that plane, and gradually thinned off.The second anomalous expansion was newly found in this experiment at the second reverse transformation temperature range, in addition to the anomalous first expansion previously found at the first reverse transformation range.The anomaly appeared in two phase steels consisting of the predominant martensite and retained austenite in proper phase ratio, regardless the steels were cold-worked or not.
It was considered that the anomalous expansion is related to the martensite which transformed diffusionally and to the reverse transformation character of textured martensite to the certain oriented austenite.
