جستجوی مقالات مرتبط با کلیدواژه "retained austenite" در نشریات گروه "مواد و متالورژی"

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).

The Austempering process that leads to formation of a bainitic ‎microstructure in cast iron is significantly affected by any variation in ‎austempering time and temperature as well as chemical composition of ‎samples. The present work is aimed to reveal the effects introduced by ‎aluminum substitution for silicon in the alloy and austempering ‎treatment on bainite transformation kinetics. Samples of the same size ‎were initially austenitizing at 900°C for 2h in order to achieve a uniform ‎austenitic microstructure, followed by austempering at four upper ‎bainitic temperatures of (375, 400, 425 and 450°C) at various ‎times (1 to 512 min.). Microstructural analyses were carried out by ‎means of optical and electron microscopes and observed that by ‎increasing austempering temperature the thickness of ferritic bainitic ‎plates increases from 0.31 μm in 375°C to 0.63 μm in 450°C. It was ‎concluded that austempering transformation in aluminum ductile cast ‎iron is somehow similar Si cast iron. The difference is that the rate of ‎the first stage of austempering was higher while other stages went ‎through the same rates as the other types of cast iron.‎

The quenching and partitioning (Q&P) process is a new heat treatment cycle to produce the third generation of advanced high strength steels based on diffusion of carbon from martensite to retained austenite phase. The application of this process for various steels leads to a remarkable combination of mechanical properties including high strength and good ductility. Using this process in automotive industry causes better mechanical performance, greater car safety, weight saving and cost reduction. In this study, a low alloy high carbon steel has been subjected to Q&P process and its microstructure and mechanical properties have been investigated. The results showed that the microstructure of the quenched and partitioned steel contains carbon-enriched stabilized austenite and carbon-depleted martensite resulting in increasing the strength of steel with an acceptable ductility. Conducting Q&P heat treatment caused the retained austenite content and the hardness of steel to increase. Fracture in specimens treated by Q&P process was of brittle type and their fracture surface was considered to be quasi-cleavage.