Abstract
A high manganese austenitic steel with 12·74% Mn and 1·19%C, obtained by quenching in water from 1000°, was heated at several temperatures from 400° to 700°, for various lengths of time upto 50 hours. during which the change of microstructure, density, hardness and magnetization was studied. In microstructure, the following changes take place:-(1) separation of carbide, (2) formation of troostite (nodulus), (3) formation of martensite, (4) formation of troostite (acicular) from martensite. The separation of carbide firstly takes place; as the heating temperature rises, the rate of separation increases, though the quantity of the carbide decreases, on account of an increase of the solubility. The formation of troostite (nodulus) and martensite take place in temperature ranges, 420°-680°, and 420°-600°, respectively; in the early stage of heating, the changes proceed most readily in the vicinity of 550°, and as the heating time increases, the temperature at which the change is maximum shifts gradually to the lower temperature. The acicular troostite is formed in the temperatures above 500° from the martensite. The density increases with the separation of carbide and the formation of troostite, but decreases with that of martensite. The hardness increases with the formation of carbide and martensite, but decreases with that of troostite. Magnetization increases with the format on of martensite and troostite, though no change is observable with the separation of carbide. From these facts the process of tempering of the austenite at several temperatures was inferred. The troostites, nodulus and acicular, are formed by different mechanism, that is, the former is formed by the Ar1 change as the pearlite, while the latter is formed by tempering of martensite after long heating. According to Tammann's theory, the experimental results obtained were well explained. By means of the differential, dilatometer, the Ac1 range was determined to be 530°-680°.
