Abstract
The change in microstructure on quenching and tempering, and the precipitation process of austenite in 9% nickel steels with 0.001, 0.01 and 0.1% carbon, which were carefully prepared by vacuum melting, have been investigated by transmission electron microscopy and the X-ray diffraction method and from the transformation characteristics on continuous cooling. The main results are as follows: (1) The typical structure of a 0.1%C 9Ni steel, when cooled at rates above 2.5°C/min, is the mixed structure of martensite and bainite which contain dislocations of high density. The internal twins were also observed in the martensite and the oriented carbide-precipitates in the bainite. In steels with the carbon content less than 0.01%, when quenched from the austenite field, the massive structure with dislocations of comparatively low density was observed. (2) The volume fractions of the austenite retained on cooling or precipitated on tempering were determined by the X-ray method, and it was found that the retained austenite played an important role in the enhancement of the precipitation of austenite particularly in the early stage of tempering. (3) On tempering a 0.1%C 9Ni steel at about 525°C, a considerable amount of austenite is formed at the expense of carbides precipitated there. The austenite thus formed is extremely stable even at −196°C. However, the austenite precipitated on tempering at higher temperatures such as above 600°C becomes less stable, thereby it is transformed into martensite on cooling from the tempering temperature. The stability of the precipitated austenite is also much reduced as the carbon content of specimen is lowered.
