Microstructural Characterization of Eutectoid Steel Strips

Investigations of microstructural differences of tempered eutectoid steel strips are presented. The constituent phases – fine mixture of ferrite and cementite and a small amount of retained austenite – considerably affect the mechanical properties, especially the resistance to high cycle fatigue. The amount of retained austenite was determined by XRD and EBSD analysis. Thermoelectric power (TEP) measurements were performed for 270 different batches of C75S type eutectoid steel strips. TEP measurement shows characteristically the fine microstructural differences. The TEP of the investigated samples varied between 5650 and 7030 nV/°C. The amount of retained austenite can be significantly higher at the surface (20 %) than at the internal part (0-5 %). The sensitivity of XRD analysis was reduced because of the presence of coarse cementite particles. Using Cu anode, the measurements were more successful than in case of using Co anode. The EBSD analysis showed that also samples immeasurable with XRD contain retained austenite, but its detectability with XRD analysis declines with the coarsening of cementite.