The Influence of Martensite Starting Structure Characteristics on the Distribution of Ferrite Grain Size in Low-Carbon Steel

In this study, the influence of martensite starting structure characteristics on the distribution of ferrite grain size in low-carbon steels after tempering was investigated. By varying austenitization times (30 min and 4 hr), martensite structures with different grain sizes of prior austenite were produced. The results indicated that the bimodality of ferrite grain size in the 30 min austenitized sample was higher than the 4 hr austenitized sample. Because of the smaller grain size of prior austenite, and so higher density of martensite laths per unit volume, the size of cementite particles formed during the first stage of tempering was smaller in the 30 min austenitized sample than the 4 hr austenitized sample. Consequently, Ostwald ripening was faster in the former sample compared to the latter sample. This resulted in faster ferrite grain growth, and the formation of a heterogeneous microstructure consisted of large and fine ferrite grains. Therefore, after second tempering, a bimodal microstructure was formed in the 30 min austenitized sample.