Vpliv deformacijske hitrosti na mehanske lastnosti jekla PT955 - Dlopst, Primož (2017)
Namen diplomske naloge je preučiti, kako hitrost obremenjevanja vpliva na mehanske lastnosti jekla PT 955, namenjenega za turbinske lopatice v hidroelektrarnah. Toplotno obdelane vzorce premera 10 mm smo s strojem za natezni preizkus obremenili do preloma ter nato poslikali prelome s pomočjo svetlobnega mikroskopa. S svetlobnim mikroskopom smo pregledali in dokumentirali tudi mikrostrukture vzorcev ter ugotavljali deleže mikrostrukturnih sestavin, predvsem perlita in martenzita. Ugotovili smo, da je pri nižjih deformacijskih hitrostih čas trajanja deformacije daljši. Iz tega sklepamo, da imajo procesi poprave pri nižjih deformacijskih hitrostih na voljo več časa, da pride do zmehčanja materiala preko procesov poprave. Pri višjih hitrostih deformacije pa je časa na voljo precej manj, saj je tudi čas trajanja same deformacije krajši. Tako smo ugotovili, da procesi poprave, ki vodijo do zmehčanja materiala in omogočajo nadaljnjo deformacijo, pri višjih deformacijskih hitrostih ne zmorejo slediti sami deformaciji. Na koncu smo iz diagramov napetost-deformacija odčitali mehanske lastnosti in jih med seboj primerjali glede na hitrost, s katero smo preizkušance potrgali. Dokazali smo, da se z višanjem hitrosti obremenjevanja poveča natezna trdnost, pri bistvenem povišanju hitrosti obremenjevanja pa je opazen vpliv na Rp0,2 in sicer velja, da se napetost tečenja pri 0,2 % trajnega raztezka z višanjem hitrosti obremenjevanja povečuje. The purpose of the thesis is to examine how the rate of deformation affects the mechanical properties of the PT 955 steel intended for turbine blades in hydroelectric power plants. Thermally treated samples, with a diameter of 10 mm, were loaded with the tensile tester to the fracture and then the fractures were imaged using a light microscope. The light microscope examined and documented the microstructure of the samples and determined the proportions of the microstructural components, especially the pearlite and martensite. We found that, at lower deformation rates, the duration of deformation is longer. From this we conclude that the processes of recovery at lower deformation rates have more time to get the softening of the material through the recovery processes. At higher deformation rates, time is much less, since the duration of the same deformation is shorter. Thus, we found that the recovery processes leading to softening of the material and allowing for further deformation can not be followed by deformation at higher deformation rates. Finally, from the diagrams of the strain-stress, we read the mechanical properties and compared them with each other according to the rate at which we tested the test pieces. We have shown that the increase in the loading rate increases the tensile strength, while the significant increase in loading velocity has a noticeable effect on Rp0.2, although the yield stress at 0.2% of the permanent elongation increases with the increase in the loading rate.