Effects of grain size and heat treatment on strength and yield point phenomena of low carbon steel

Abstract

We examined the effect of grain size on flow stress and yield-point phenomena in variously heat-treated low-carbon steel specimens. We found that the experimental results are easily understood if the flow stress consists of two internal stresses, Yint and Yir. Yint is a common internal stress that is work hardened; Yir is the cause of yield-point phenomena, and its value is supposed to be rapidly decreased (work-softened) to zero with strain. Both Yint and Yir were affected by grain size. Furthermore, the effect of heat treatment was strong on Yint but weak on Yir. The Hall-Petch coefficient k was appreciably larger at lower yield points than in the large strain region. This comparison supports the hypothesis of the existence of two internal stresses because the grain size dependence of flow stress at lower yield points has two origins, Yint and Yir, whereas the latter case has only one origin, Yint. In the strain region exceeding yield-point elongation, the stress-strain curves of specimens with various grain sizes were perfectly superposed on each other by parallel translation, which means that if Yint is expressed as Yint (ε) = f (ε) for a specimen with a certain grain size, it is expressed as Yint (ε) = f (ε + Δ) - Δσ for arbitrary grain size specimens, where Δε and Δσ are the amounts of translation in strain and stress, respectively.