Superior Improvement of Microstructure and Mechanical Properties of a Medium Silicon Low Alloy 35CHGSA Steel Under Quenching and Partitioning Heat Treatment in Comparison to Conventional Quenching and Tempering Condition

In this experimental work, the improvement of microstructure and mechanical behavior of medium silicon low alloy 35CHGSA steel have been investigated under quenching and partitioning heat treatment in comparison to conventional direct water quenching and tempering condition. For this purpose, the samples of medium silicon low alloy 35CHGSA steel were first austenitized at 900˚C for 15 min and then cooled to room temperature. By the following two heat treatment methods. In the first method, the austenitized, samples were directly quenched in water and then tempered at 500°C for 60 min (Q&T). In the second method, the austenitized samples were quenched in the molten salt bath at 230°C for 1 min in order to form some martensite and then heated to 400°C for 12 min for carbon partitioning from martensite to the adjacent remaining austenite areas and finally water quenching (Q&P). Hardness and tensile tests were performed with phase analysis by XRD and microstructural observations under optical microscopy, field emission scanning electron microscopy (FESEM) equipped with energy-dispersive spectrometry (EDS), followed with electron diffraction via transmission electron microscope (TEM). The results show that the Q&P heat treatment process has been associated with the significant improvement in microstructure and mechanical properties of medium silicon low alloy steel compared to conventional water quenching and tempering condition. The product of ultimate tensile strength multiple elongation, which is a good criterion for design and development of advanced high strength steels, is significantly increased from 16.4 (Q&T samples) to 25.5% GPa (Q&P samples).