Tensile properties of a 0.34C3NiCrMoV steel with mixed lower-bainitemartensite structures

Abstract

The primary objective of this work is to study the influence of microstructure on the tensile properties of a 0.34C3NiCrMoV steel with mixed lower-bainitemartensite microstructures. In general, the tensile yield strength and ultimate strength of the steel with mixed structures were found to decrease with an increase in the lower-bainite content, while a few exceptions were observed on mixed structures containing 10–25% lower bainite which showed higher yield strength than fully martensitic structure. The introduction of lower bainite in prior austenite grains was found to cause a refinement of the martensite packet size. However, the improvement of the strength of martensite due to the refinement of its substructure can only account for part of the strengthening in a mixed lower-bainitemartensite structure. The tensile ductility (RA%) of the mixed lower-bainitemartensite structure in general is quite close to that of the fully martensitic structure except for the fine austenite grain condition (8 μm), for which the mixed structures show inferior RA% as compared with the fully martensitic structure. On the other hand, the refinement of prior austenite grain size was found to cause beneficial effects on both tensile strength and ductility for the steel with mixed lower-bainitemartensite structure.