| الوصف: |
Avstenitna nerjavna jekla, med katere spada tudi jeklo AISI 304, izkazujejo dobro korozijsko obstojnost, žilavost, varivost in odpornost na lezenje. Mehansko inducirana martenzitna transformacija se je izkazala za enega od načinov povečanja relativno nizke trdnosti, ki jo izkazujejo jekla te vrste. Martenzitna transformacija lahko poteka po naslednjih reakcijah: ? ? ?, ? ? ? ? ?' in ? ? ?'. Potek transformacije je odvisen od temperature, stopnje deformacije in energije napake zloga. V okviru magistrske naloge smo preučevali vpliv temperature in stopnje deformacije na mehansko inducirano martenzitno transformacijo, razvoj mikrostrukture in utrjevanje jekla AISI 304. V ta namen smo za raziskave uporabili svetlobni (SM) in vrstični elektronski mikroskop (SEM) z uklonom povratno sipanih elektronov (EBSD), rentgensko difrakcijo (XRD) in feritoskop. Ugotovili smo, da se z nižanjem temperature deformacije in višanjem stopnje deformacije v avstenitnih kristalnih zrnih najprej pojavijo strižni pasovi. Z nadaljnjo deformacijo je sledilo njihovo križanje. Z nastankom martenzita pa se je število strižnih pasov in njihovih presečišč zmanjšalo. Transformacija avstenita v martenzit je potekala po reakciji ? ? ? ? ?'. Deleža martenzita ? in ?' sta se z nižanjem temperature deformacije in višanjem stopnje deformacije povečevala, s tem da se je delež martenzita ?' povečeval hitreje kot delež martenzita ?. Trdota se je višala z nižanjem temperature in višanjem stopnje deformacije. Do nastanka martenzita se je jeklo deformacijsko utrjevalo. Nastanek martenzita in povečanje njegovega deleža sta še dodatno zvišala trdoto jekla, vendar se je z večanjem deleža martenzita hitrost utrjevanja nižala. 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. |
