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Diplomska naloga z naslovom Vpliv gašenja in stabilizacijskega žarjenja na mehanske lastnosti jekla PK11SP mikrolegiranega s titanom, je bila izdelana z namenom ugotoviti vpliv gašenja in stabilizacijskega žarjenja na mehanske lastnosti omenjenega jekla pri sobni temperaturi 23 °C in pri povišani temperaturi 350 °C. Osredotočili smo se na natezno trdnost, napetost tečenja, kontrakcijo in raztezek ter trdoto. Po toplotni obdelavi in izvedbi mehanskih preizkusov so bili metalografsko pripravljeni vzorci karakterizirani z vrstičnim elektronskim mikroskopom. Jeklo PK11SP spada med avstenitna nerjavna jekla stabilizirana s titanom. Titan preprečuje vezavo kroma v karbide, saj ima višjo afiniteto do ogljika kot krom. S tem se prepreči tvorba kromovih karbidov po mejah kristalnih zrn, kar posledično poveča odpornost jekla proti interkristalni koroziji. Poleg tega ima to jeklo zaradi dodatka titana boljše mehanske lastnosti pri povišanih temperaturah. Z namenom ugotavljanja mehanskih lastnosti je bilo jeklo najprej gašeno, sledilo je stabilizacijsko žarjenje pri 680 °C in 720 °C. Po toplotni obdelavi smo izvedli natezni preizkus na trgalnem stroju pri sobni in povišani temperaturi. Trdota je bila merjena po vsaki operaciji toplotne obdelave. Glede na dobljene rezultate smo ugotovili, da dobimo najvišje trdote, natezno trednost in napetost tečenja po gašenju iz 1050 °C in daljših časih žarjenja, kontrakcijo in raztezek pa po gašenju iz 1100 °C. Z vrstično elektronsko mikroskopijo smo analizirali dva metalografsko pripravljena vzorca. Potrdili smo prisotnost precipitatov, in sicer titan niobijevih karbidov (TiNb)C in titan niobijevih karbo-nitridov (TiNb)(CN), zaradi tvorbe katerih krom ostaja raztopljen v trdni raztopini in tako ohranja korozijsko odpornost jekla. The diploma thesis titled Effects of quenching and stabilization annealing on mechanical properties of PK11SP steel microalloyed with titanium was designed to determine the effect of quenching and stabilization annealing on mechanical properties of the steel mentioned above, at room temperature 23 °C and at elevated temperature 350 °C. We focused on ultimate tensile strength (Rm), yield strength (Rp0,2), contraction (Z), elongation (A5) and hardness (HB). After thermal treatment and mechanical tests, metallographically prepared samples were examined with a new scanning electron microscope FEG-SEM. The steel we used is austenitic stainless steel stabilized with titanium. Titanium prevents the binding of chromium to carbon since titanium has higher affinity to carbon than chromium. This prevents the formation of chromium carbides along the grain boundaries and consequently increases resistance to intergranular corrosion. Moreover, this steel is expected to have better mechanical properties at elevated temperatures because of the addition of titanium,. In order to determine the mechanical properties, the material was first quenched from 1050 °C and 1100 °C in water. Stabilization annealing at 680 °C and 720 °C followed. Mechanical tests were then carried out at room and elevated temperatures. Hardness was measured after each operation. Based on the results, we found that the best quenching hardness is obtained when quenching from 1050 °C and annealing for a longer period of time. The best tensile strength and the plasticity limit are also obtained under those conditions. Contraction and elongation are highest when the material is quenched from 1100 °C and annealed for a longer period of time. After SEM analysis we confirmed the presence of (TiNb)C and (TiNb)(CN) precipitates in the microstructure. Their presence is essential for maintaining chromium in solid solution and, consequently, preventing corrosion. |
