The Microstructure and Toughness of High Tensile Strength Steels

Abstract

The impact properties of some high tensile strength steels were investigated in relation to the austenite grain size and the cooling rate.
The results showed the existence of optimum cooling rates for the transition temperature. The optimum cooling rate depended on the hardenability of steels, and as the austenitizing temperature was raised it shifted to the lower cooling rate.
The electron microscopic observation revealed that the structure with superior impact properties was the duplex martensite-bainite structure. The morphology of carbide in this bainite was similar to that in the lower bainite, but the trace analysis for such bainite revealed that it had the same lath-like morphology with a ‹111› _α {110} _α as that in the upper bainite and the lath martensite.(This bainite in a low-carbon low-alloy steel was named as Bainite-III type in a previous paper.)
The orientation measurements of facet pit and fracture surface clarified that the cleavage plane was (100). Based on the direct observation of fracture surface by scanning electron microscope, the unit crack path was defined. The unit crack path showed experimentally a better correlation to the transition temperature than the austenite grain size.
These results suggested that the role of bainite in the duplex structure was the partitioning of austenite grain prior to the martensite transformation.