Optimisation of heat treatment process of 30CrNiMo8 steel for achieving adequate cross section mechanical properties

Tema magistrskega dela je optimizacija toplotne obdelave jekla 30CrNiMo8 za doseganje ustreznih mehanskih lastnosti po preseku. Za preiskavo je bil izbran del odkovka jekla 30CrNiMo8 z dimenzijami prečnega prereza 500 mm × 500 mm in dolžine 1910 mm. Iz odkovka v izhodnem stanju (po kovanju) smo odrezali ploščo debeline 50 mm. Iz te plošče smo izrezali manjše vzorce, ki so bili velikosti 60 mm × 50 mm × 180 mm. Odrezali smo jih na razdaljo 30, 80, 125 in 250 mm od površine. Vse vzorce, predhodno izrezane iz plošče, smo poboljšali v laboratorijskem okolju, ostali del odkovka pa v industrijskem okolju po enakem postopku. Raziskali smo predvsem vpliv toplotne obdelave na mikrostrukturne spremembe in spremembe mehanskih lastnosti po globini. V okviru eksperimentalnega dela smo opravili kemijsko analizo, Jominy preizkus prekaljivosti, preizkušanje jekla z nateznim preizkusom, merili udarno žilavost, trdoto, opazovali mikrostrukture s svetlobnim in vrstičnim elektronskim mikroskopom ter naredili simulacije CCT diagramov. Z nateznim preizkusom smo ugotovili, da sta natezna trdnost in napetost tečenja pri vzorcih, poboljšanih v industrijskem okolju, za okoli 100 N/mm2 manjši kot pri vzorcih, poboljšanih v laboratorijskem okolju. Pri preizkusu udarne žilavosti po Charpyju smo ugotovili, da ima globina preizkušanja pri jeklu, poboljšanem v laboratorijskem okolju, veliko večji vpliv pri nižjih temperaturah, saj se žilavost od površine proti jedru hitreje zmanjšuje kot pri sobni temperaturi. Pri vzorcih, poboljšanih v industrijskem okolju, smo na globini 80 in 125 mm od površine ter v jedru poleg popuščenega martenzita opazili trakove iz popuščenega bainita in martenzita, kar je posledica makro in mikro izcej. Delež bainita v trakovih se je proti jedru povečeval. Po programu Dr. Sommer Heat Treatment Database je za tvorbo martenzitne mikrostrukture po celotnem preseku potrebna hitrost ohlajanja > 402 °C/min, kar bi zagotavljalo bolj enakomerne mehanske lastnosti po preseku jeklene plošče.The master\u27s thesis is the optimization of the heat treatment of 30CrNiMo8 steel to achieve the appropriate mechanical properties throughout cross-section. For the investigation a part of the 30CrNiMo8 steel forging with the cross sectional dimensions 500 mm × 500 mm and the length of 1910 mm was selected. From the forging at the outset (after the process of forging) a plate of 50 mm of thickness was cut. From that plate we cut out smaller samples, which were 60 mm × 50 mm × 180 mm in size. The cutting distances were 30, 80, 125 and 250 mm from the surface. All samples, which were previously cut from the plate, were quenched and tempered in the laboratory environment, while the rest of the forgings were quenched and tempered in the industrial environment by the same procedure. The examination focused on the influence of the heat treatment on microstructural changes and changes in mechanical properties according to the depth. In the experimental work a chemical analysis, a Jominy test of hardenability, a tensile test of steel were carried out as well as the impact strength and the hardness of steel were measured. In addition, microstructures were observed with the light and scanning electron microscope and the CCT diagrams were simulated. The tensile test has showed that both tensile strength and yield strength are in industrially quenched and tempered samples for about 100 N/mm2 lower than in samples quenched and tempered in the laboratory environment. Furthermore, through Charpy impact test it was found out that the depth of steel testing is much more important for the samples quenched and tempered in the laboratory environment at lower temperatures, since the toughness of the surface against the core is decreasing faster than at room temperature. In the industrially quenched and tempered samples we have noticed at the depth of 80 and 125 mm from the surface and in the core, besides the tempered martensite, also the bands from the tempered bainite and martensite, which is the consequence of macro and micro segregations. The proportion of bainite bands has increased to the core. According to Dr. Sommer Heat Treatment Database software the formation of a martensitic microstructure throughout the cross section requires a cooling rate of > 402 ° C/min, which would provide more uniform mechanical properties throughout the cross-section of the steel plate