Austenitic stainless steels, including AISI 304 steel, exhibit good corrosion resistance, toughness, weldability, and creep resistance. Mechanically induced martensite transformation has proven itself to be one of the ways of increasing the relatively low strength exhibited by this type of steel. The martensite transformation can proceed according to one of the following reactions: γ ⒒ ε, γ ⒒ ε ⒒ α' in γ ⒒ α'. The course of the transformation depends on the deformation temperature, deformation degree, and the stacking fault energy.
In this master’s thesis the influence of deformation temperature and strain on the mechanically induced martensite transformation, the development of the microstructure, and hardening of AISI 304 steel, was investigated. For this purpose, light microscope (SM), scanning electron microscope (SEM) with backscatter electron diffraction (EBSD), X-ray diffraction (XRD), and ferritescope, were used.
It was concluded that when the deformation temperature is lowered, and the deformation degree is increased shear bands first appear in austenite grains. With further deformation shear bands began to cross. With the transformation of austenite into martensite the number of shear band cross sections decreased. The transformation reaction γ ⒒ ε ⒒ α' had been followed. The amount of martensite increased with a decrease in deformation temperature and increase in deformation degree. The rate of α-martensite formation was faster than that of ε-martensite.
Hardness increased with decreasing deformation temperature and increasing deformation degree. Until the formation of martensite, steel was strain hardened. The effect of strain-hardening was greater at lower temperatures. The formation of martensite and its increase in quantity further increased the hardness of the steel, however, as the quantity increased, the rate of hardening decreased.
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