Influence of cooling methods on the microstructure evolution of the carburized layer in 18CNiMo7-6 steel after high-temperature carburizing

The microstructure and mechanical properties of carburized gears vary significantly if treated with different cooling methods after high-temperature carburizing.In this study, the microstructure and hardness distribution of the carburized layer in the 18CrNiMo7-6 steel after different quenching and cryogenic treatments were compared using a scanning electron microscope, Vickers hardness tester and X-ray diffractomer.The results indicate that both cooling methods, quenching and cryogenic treatment, had a significant influence on the amount of carbides and retained austenite in the carburized layer of the 18CrNiMo7-6 steel.Fine carbides gradually dissolved with the increase in the quenching temperature.When the quenching temperature was higher than 850 °C, fine carbides were completely dissolved.The amount of the retained austenite decreased with the cryogenic temperature.When the cryogenic temperature was lower than -120 °C, there was no significant decrease in the amount of the retained austenite.In addition, after different quenching and cryogenic treatments, there was no remarkable change in the effective case depth of the carburized 18CrNiMo7-6 steel.Therefore, from the perspective of the dissolution of carbides and the amount of retained austenite, 850 °C and -120 °C were the optimum quenching and cryogenic temperatures of the 18CrNiMo7-6 steel after high-temperature carburizing.Keywords: 18CNiMo7-6 steel, quenching, cryogenic treatment, microstructure of the carburized layer Mikrostrukture in mehanske lastnosti cementiranih mehanskih sklopov se lahko mo~no razlikujejo med seboj zaradi razli~nih pogojev ohlajevanja po visoko temperaturni cementaciji.V ~lanku avtorji opisujejo {tudijo mikrostrukture in porazdelitve trdote po preseku cementirane plasti cementiranega jekla 18CrNiMo7-6 po razli~nih na~inih kaljenja in postopkih podhlajevanja.Nastale mikrostrukture so med seboj primerjali s pomo~jo vrsti~nega elektronskega mikroskopa in rentgenskega difraktometra.Trdoto nastalih plasti so merili z Vickersovim merilnikom mikrotrdote.Rezultati raziskav so pokazali, da imata tako metoda ohlajevanja kot podhlajevanja pomemben vpliv na vsebnost karbidov in zaostalega austenita v plasti cementiranega jekla 18CrNiMo7-6.Drobni karbidi se postopoma raztapljajo z nara{~anjem temperature austenitizacije.Ko je le-ta vi{ja od 850 °C se drobni karbidi popolnoma raztopijo v austenitni osnovi.Vsebnost zaostalega austenita se zmanj{uje z zni`evanjem temperature podhlajevanja.Ko je temperatura podhlajevanja ni`ja od -120 °C ni ve~pomembne spremembe v vsebnosti zaostalega austenita.Dodatno avtorji ugotavljajo, da pri razli~nih na~inih kaljenja in razli~nih metodah podhlajevanja ne pride do bistvene spremembe debeline (globine) cementirane plasti cementiranega jekla 18CrNiMo7-6.Zato avtorji raziskave ocenjujejo da je, glede na raztapljanje karbidov in vsebnost zaostalega austenita, optimalna temperatura austenitizacije za visokotemperaturno cementacijo jekla 18CrNiMo7-6 enaka 850 °C in optimalna temperatura podhlajevanja enaka -120 °C.