Effect of Microstructural Characteristics on Mechanical Properties of Ferritic Stainless Steel

To improve the mechanical properties of hot-rolled ferritic stainless steel (SUS430), the microstructural characteristics of SUS430 were changed using a heating process under various conditions. The hardness of SUS430 decreased upon the increase in the heating temperature to 900 degrees C, and the hardness increased when the sample was heated to temperatures greater than 900 degrees C. The high hardness of the sample heated at 1000 degrees C (H-1000 degrees C) is attributed to the heating time: A high hardness was obtained for a H-1000 degrees C sample that was heated for 1 h (H1000 degrees C-1h), but this decreased when the heating time was increased to more than 1 h. The high hardness of H1000 degrees C-1h is caused by the fine Cr23C6 precipitates that are distributed in the sample around the grain boundaries. On the other hand, the large precipitates of Cr23C6 in H1000 degrees C-12h decrease the hardness. The hardness value of SUS430 is directly attributed to the mechanical properties and the ultimate tensile strength. The tensile strength of H1000 degrees C-1h was found to be about 200% and 20% higher than the as-received and H1000 degrees C-12h samples, respectively. Despite the increase in the tensile strength of the H1000 degrees C-1h sample, the ductility was not found to decrease significantly, for example, the fracture strain was approximately 25%. This occurrence is affected by a severe slip in the ferrite base grain, and the high strength of H1000 degrees C-1h is influenced by the interruption of the slip by the Cr23C6 precipitates. Unlike the tensile strength, similar fatigue properties were observed for both H1000 degrees C-1h and H1000 degrees C-12h, which is associated with the low crack driving force of H1000 degrees C-12h, caused by the roughness-induced crack closure arising from the large Cr23C6 precipitates.