Abnormal mechanical behavior of a medium-carbon steel under strong ferrite-pearlite-martensite triple-phase microstructures

Abstract

Understanding the relationships between the structure and mechanical properties of steel, which can be difficult to determine, is important to design and develop various high-strength, multi-phase steels. In the present work, the structure-property relationships of a medium-carbon AISI 1045 steel with ferrite-pearlite-martensite triple-phase (TP) microstructures are investigated and compared to those with ferrite-martensite dual-phase (DP) microstructures. Various step-quenching heat treatment cycles were carried out at 650°C for different holding times, followed by subsequent water quenching after reaustenitization of the samples at 860°C for 30min. Optical microscopy and field-emission scanning electron microscopy equipped with X-ray energy dispersive spectroscopy were used along with macrohardness, tensile, and nanoindentation tests to perform a detailed investigation of the structure-property relationships of the heat-treated samples. An abnormal mechanical behavior due to pearlite formation was observed in the ferrite-pearlite-martensite TP samples in comparison to the ferrite-martensite DP samples. The ferrite-pearlite-martensite TP samples consisting of more than 71% martensite were associated with a higher level of mechanical properties compared to those of the ferrite-martensite DP samples. The results are rationalized in part by the increased hardening response of 50.7% of the constrained pearlite in the TP microstructures induced by the martensitic phase transformation in conjunction with the lower ferrite hardening response of 22.6% in the DP samples.