EFFECT OF AUSTENITE GRAIN SIZE AND C CONTENT ON THE SUBSTRUCTURE AND TOUGHNESS OF TEMPERED MARTENSITE AND BAINITE.

Abstract

Specimens of two 5%Ni-0. 5%Mo steels with 0. 15 and 0. 30% C, respectively, were used in the investigation. The results indicate that increase of austenite grain size coarsens the lath and packet substructures of tempered martensite but does not affect the bainite substructure. In general, carbon increase refines the substructure in both martensite and bainite. it also increases the thickness and number of rod-like carbides in the lath structure and the carbide size in massive bainite. The toughness of tempered martensite is greatly affected by austenite grain size, especially in the lower C steel, but not much by C content when compared at the same strength level. The toughness of bainite shows little austenite grain size dependency and is much deteriorated with the increase of C content. From the overall results it is concluded that the role of carbides to impair toughness should be emphasized especially in bainite, although the concept of effective grain size is valid to explain to some extent the toughness behavior in a single steel. (This paper was previously published in Japanese in Tetsu-To-Hagane v 60 1974 p 372. )