High Temperature Microscope Observation of the Austenite Grain Size of Steels

In order to determine continuously the austenite grain size of steels with a high temperature microscope, observations of the austenite grain growth of 0.2% C steels (Si–Mn killed, or Al killed) in various atmospheres were carried out. The results obtained were as follows: (1) When 0.2% C steels were heated in 3×10−5 mmHg vacuum, the austenite grains on the surface were coarsened remarkably to ASTM No.2 at 950°C, and their grain size was markedly different from that in the inner zone. (2) When these steels were heated in extremely pure argon atmosphere in the pressure range of 0.6∼1.0 atm, the austenite grains on the surface maintained a grain size of ASTM No. 8 up to about 1150°C, and then coarsened. The value of their grain size was nearly equal to that of the inner grains. (3) According to an electron microscopic study, the degree of oxidation on the specimen surface in high vacuum was less than that in argon. Therefore a high vacuum was replaced with the argon atmosphere at 950°C or 1000°C, but no changes in grain size were observed. (4) As regards the surface grain growth of 0.2% C steels in various atmospheres, it was observed that the lower the degree of vacuum or the higher the argon pressure, the higher the grain coarsening temperature. (5) Remarkable grain growth which occurred at about 950°C in high vacuum was also observed at the specimen surface several microns in depth even after removing the mechanically polished layer by electrolytic polishing.