Austenitic stainless steels are widely used in a variety of industries, such as power plants, aerospace, automotive, construction and electronics, due to their excellent mechanical properties including ductility, toughness and corrosion resistance. For application in various industries, many parts and products require plastic processes such as cold rolling, cold forging and bending. These processes cause a phenomenon in which the strength is increased but the toughness is lowered by the plastic deformation. However, heat treatment of the plastic-deformed material can improve mechanical properties by grain refinement due to recrystallization and phase transformation. In this study, the effect of grain refinement and phase transformation by heat treatment of cold-worked austenitic stainless steels were evaluated in order to improve mechanical properties such as toughness and strength. After the cold working process, strength and hardness were increased by the phase transformation of martensite from austenite, and an increase in dislocations by plastic deformation. After heat treatment, the ductility and toughness of the austenitic stainless steels strengthened by cold working process were increased due to grain refinement caused by recrystallization of the remaining austenite, and the reverse transformation from martensite to austenite induced by heat treatment.
The spot welds of Transformation Induced Plasticity (TRIP) steels are prone to interfacial failure and narrow welding current range. Hard microstructures in weld metal and heat affected zone arenormally considered as one of the main reason to accelerate the interfacial failure mode. There fore, detailed observation of weld microstructure for TRIP steels should be made to ensure better weld quality. However, it is difficult to characterize the microstructure, which has similar color, size, and shape using the optical or electron microscopy. The atomic force microscope (AFM) can help to analyze microstructure by using different energy levels for different surface roughness. In this study, the microstructures of resistance spot welds for AHSS are analyzed by using AFM with measuring the differences in average surface roughness. It has been possible to identify the different phases and their topographic characteristics and to study their morphology using atomic force microscopy in resistance spot weld TRIP steels. The systematic topographic study for each region of weldments confirmed the presence of different microstructures with height of 350nmfor martensite, 250nm for bainite, and 150nm for ferrite, respectively.
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