Abstract
The effect of alloying elements on the structure and the strength-toughness relationship of tempered martensite and bainite have been studied in 0.15%C-0.5%Mo and 0.3%C-0.5%Mo steels by the addition of 1.5-2.5% to Mn, Cr, and Ni.
Differences in the substructure and the strength-toughness relationship are not large in the tempered martensite when the alloying element is changed. The difference, however, becomes remarkable in the bainite with the increase of the formation temperature, especially in the lower C steels. The results of the comparison among these alloying elements made on the bainite formed at higher ltemperatures can be summarized as follows. The bainite in Ni steels shows the coarsest structure with the coarsest particles of carbide. The bainite in Mn steels is similar to that in Ni steels. The structure of the bainite in Cr steels is the finest and quite different from those in the other steels. As for the toughness of bainite, Ni steels show the best followed by Mn steels, and Cr steel the worst. The size of unit fracture facet of these bainite is almost the same. The hardness of the bainite is rather dependent on the formation temperature than the alloying element.
It is concluded from these results that the intrinsic toughness affected by the alloying element plays more important role in these transformation products formed at higher temperatures than the factors such as the size of unit fracture facet and carbides which have been considered to be important if the alloying element is the same.
