Mechanical properties and microstructure of martensitic stainless steels 08X14NDL and GX4CrNi13-4

Namen diplomskega dela je karakterizacija mikrostrukture in mehanskih lastnosti nerjavnih jekel 08X14NDL in GX4CrNi13-4. Karakterizacija mehanskih lastnosti je potekala s pomočjo nateznega preizkusa, merjenja trdote in udarne žilavosti, karakterizacija mikrostrukture pa s pomočjo svetlobne mikroskopije, rentgenske fazne analize (XRD), vrstične elektronske mikroskopije (SEM), energijsko disperzijske spektroskopije (EDS) in izračunov ravnotežnih faznih diagramov za preiskovana jekla. Mikrostruktura jekel 08X14NDL in GX4CrNi13-4 je sestavljena pretežno iz popuščenega martenzita in δ-ferita. Delež δ-ferita se med toplotno obdelavo ne spreminja in znaša 18,7 % za jeklo 08X14NDL in 2,6 % za jeklo GX4CrNi13-4. S pomočjo mikrokemijske analize EDS je bilo ugotovljeno, da δ-ferit v obeh jeklih vsebuje predvsem večji delež kroma in manjši delež niklja kakor martenzit. Mehanske lastnosti jekla GX4CrNi13-4 so dosegale zahteve standarda, pri jeklu 08X14NDL pa so bile trdote prenizke glede na zahteve standarda. Mikrotrdota δ-ferita je bistveno manjša od mikrotrdote martenzita. Z ustrezno kemično sestavo bi v jeklu 08X14NDL lahko zmanjšali delež mehkejšega δ-ferita.The main purpose of the diploma work was characterization of microstructure and mechanical properties of 08X14NDL and GX4CrNi13-4 martensitic stainless steels. Mechanical properties were characterized by tensile testing, hardness testing and impact toughness testing. Microstructure was characterized by using light microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and calculation of equilibrium phase diagrams. Microstructure of 08X14NDL and GX4CrNi13-4 steels is composed of martensite and δ-ferrite. The volume fraction of δ-ferrite is 18.7 % for the 08X14NDL steel and 2.6 % for the GX4CrNi13-4 steel and does not change markedly during the process of the heat treatment. EDS microanalysis shows that δ-ferrite in both alloys contains more chromium and less nickel than the martensite. The mechanical properties of the GX4CrNi13-4 steel have met the requirements of the standard while in the case of the 08X14NDL steel the hardness was lower than required. Microhardness of δ-ferrite is significantly lower than the microhardness of martensite. The content of δ-ferrite with lower hardness could be reduced by a proper chemical composition of the 08X14NDL steel