Analysis of grain growth and morphology of bainite in medium-carbon spring steel

Abstract

This work analyses grain growth and the statistics of this process in 55Si7 commercial spring steel with a chemical composition that makes it possible to produce nanostructured (NB) carbide-free (CFB) bainite. It was found that the tested steel was coarse-grained, while high austenitization temperatures were conducive to the formation of abnormal grains with unstable growth. The size of the prior austenite grains significantly affected the morphology of bainite and the portion of individual phases. Despite the high concentration of silicon (1.89%), cementite was found in the bainite ferrite laths. The amount of lower bainite with carbides was higher in larger grains, which contained more close-packed bainite ferrite groups. In addition to an austenite film-like morphology, the obtained microstructures contained unstable block austenite, and the lowest amount was observed in the smallest prior austenite grain size. The grain size also affected the impact strength, while the presence of abnormal grains significantly reduced it. After impact tests, all analysed grain sizes showed a quasi-cleavage nature due to prior austenite grains. Deformation at the top of the main crack induced a rapid martensitic transformation of the blocky austenite, which was the main reason for the overall poor impact strength.