Study on the Quenched Structure of a Plain Carbon Steel : (3rd Report) Effect of Grain Size on the Formation of Widmanstatten Structure

In preceding report, the present investigators studied on the quenched structure of a plain carbon steel. The results, it were truly plain that the drop of hardness was effected by the thickness of the oxide layer covering the carbon steel, which grew while heating the steel in the air, and the longer heating time was, the lower quenching hardness for low carbon steel was. But, low carbon steels formed the Widmanstatten structure. In this paper, the present investigators studied the effect or relationship of the thickness of the oxide layer covering the carbon steels and coarseness of grain size on the formation of Widmanstatten structure of carbon steels with carbon 0.105,0.208 and 0.32 preceded by normalizing at appropriate temperature through means of microstructure. The results obtained may be summarised as follows : (1) When specimen B heats for 8 hours at 880℃, the specimen B becomes duplexed structure. (2) Formation of Widmanstatten structure is very effected by the austenite grain size. (3) In order to produce the typical Widmanstatten structure it is necessary to determine cooling rate from A_3 arrest line. (4) Formation of Widmanstatten structure is differed by carbon content.