The Relation between the Alloying Elements and the Mechanical Properties of Low-carbon Low-alloy Bainite

Abstract

The effect of alloying elements on the mechanical properties of bainite in some low carbon Ni-Cr-Mo ste-els was investigated, and was compared with that in tempered martensite. The bainite formation was achieved in a salt bath held at 350°C after the austenitization.
Although the heat treatment was carried out in the salt bath using the plates as thin as 7mm thick, the cooling rate was not fast enough to completely avoid the austenite decomposition prior to the isothermal trans-formation. Therefore, the variation of the mechanical properties of the bainite with the addition of alloying elements could mainly be interpreted in terms of the transformation characteristics. For instance, nickel improved both the strength and toughness of the bainite by lowering the formation temperature of the ba-inite, resulting in the fine bainite laths with cementite laths on a specific ferrite plane (B-III type bainite). This provided a fine unit crack path, which consisted of a bundle of the ferrite laths approximately sharing a common {100}α cleavage plane. It was shown that the results could be explained in terms of the unit crack path, the dislocation density, and the morphology of the precipitates.