Carbide-free bainitic (CFB) steels with a matrix of bainitic ferrite and thin layers of retained austenite, to reduce the manufacturing costs, usually do not contain alloying elements. However, a few reports were presented regarding the effect of alloying elements on the properties of these steels. Thus, this study evaluates the effects of vanadium and rare earth (Ce-La) microalloying elements on the structure, phase transformation kinetics, and mechanical properties of carbide-free bainite steel containing silicon fabricated by the casting and austempering procedure. Optical and scanning electron microscopy (OM and SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were used to study the microstructure and phase structure. The transformation kinetics were examined by a dilatometry test. Hardness, tensile, and impact tests evaluated the mechanical properties. Due to adding alloying elements, the fracture toughness and change in matrix phases relation was studied by the crack tip opening displacement (CTOD) test and SEM fractography. The microstructure of the silicon added sample was completely carbide-free bainite. The test results showed vanadium helped CFB formation, even in continuous cooling. The primary austenite grain (PAG) size grew by vanadium addition. The EBSD phase map illustrates an increment in the percentage of retained austenite by vanadium. In contrast, the addition of 0.03 wt% rare earth reduced the primary austenite grain size and reduced the retained austenite content. The results of the dilatometry test confirmed that vanadium and rare earth addition both reduced the critical cooling rate of the bainite transformation. Vanadium leads to an earlier cessation of bainite transformation, while rare earth elements postpone this transformation. Mechanical tests showed that the tensile strength of carbide-free bainite steels was strongly influenced by the morphology and volume fraction of austenite. Retained austenite, when transformed to martensite during the transformation-induced plasticity (TRIP) phenomenon, leads to increased tensile strength and fracture toughness, or retained austenite with a film-like shape prevents the growth of cracks by blinding the crack tip. The result of the CTOD test exhibited that retained austenite plays the leading role in increasing crack resistance when TRIP occurs.
In this study, the effects of the austempering temperature on the microstructure and mechanical properties of carbide-free nano bainitic steel (CFB) were discussed. For the achievement of CFB 2 wt. %, silicon was added to DIN 36CrNiMo4 steel, austempered at 450°C and 300°C for 1.5 hours and 10 hours, respectively, and then quenched in oil. The microstructure was studied by optical microscope, scanning electron microscope, and X-ray crystallography. The results showed that the 300°C austempered samples contained the ferrite phase to a greater degree than the martensitic phase. The higher amounts of martensite in the 450°C austempered samples were due to the higher amounts of initial retaining austenite that transformed to martensite in oil quenching. Also, in the samples where carbide participated, there were lower degrees of the martensitic phase, and no carbides afford higher impact toughness and lower hardness for the 300°C austempered sample. As a result of the higher impact toughness in the ball mill wear test, the 300°C austempered sample has a lower weight loss than the 450°C austempered sample.
In this study, the microstructure and mechanical properties of chromium-molybdenum steel, which is used in mining mills as a liner, were compared after quench and tempering (QT) and quenching and partitioning (QP). Quasi QP was performed by austenitizing and quenching to the exact martensite start temperature (Ms), with the partitioning procedure at an upper temperature Ms in a furnace (common QP is done in a salt bath, so this process is called quasi QP). The procedure was characterized by optical microscopy, scanning electron microscopy, and X-ray diffraction. During the QP procedure, the bainitic ferrite is shaped, and carbon diffuses from the martensite and ferrite to the remaining austenite, but the microstructure of QT is a mixture of martensite and lower bainite. The mechanical properties were measured by a tensile test and Charpy impact test. Samples treated by QP had much higher strength and ductility than those treated by QT. The hardness and wearing weight loss of the QP process was less than QT, but the difference was not great.
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