On the yield point phenomenon in low-carbon steels with ferrite-cementite microstructure

Low-carbon steels with a ferrite-cementite microstructure represent a discontinuous yielding resulting in several industrially undesirable problems. Considering the thermal contraction mismatch between the ferrite and cementite phases during the cooling stage to room temperature, which is comparable with the volume expansion due to austenite to martensite transformation, one may expect that these steels, similar to dual-phase steels, exhibit a continuous yielding behavior. In order to examine this phenomenon, different microstructures including ferrite-cementite, ferrite-cementite-martensite and ferrite-martensite were produced using a low-carbon steel and yielding behavior of steel samples with these microstructures were evaluated during uniaxial tensile loading. In addition, the effect of volume expansion due to austenite to martensite transformation and thermal contraction mismatch between ferrite and cementite phases on each microstructure was also evaluated. It was found that for the case of ferrite-cementite steels with very small cementite particles, continuous yielding cannot be observed. Finally, yielding behavior of different steel samples were explained based on the theoretical results obtained.