Effect of heat treatment on content of retained austenite in surface-hardened steels

Magistrsko delo je bilo izvedeno z namenom določitve vplivov različnih parametrov toplotne obdelave na delež zaostalega avstenita v jeklih za indukcijsko kaljene valje. Preizkuse smo izvajali na treh različnih jeklih: OTV1B, OTV1 in OHV5. Kaljenje smo izvajali na indukcijskem stroju BIKS 750, preizkušanci pa so bili v obliki valjev z dimenzijo O 193 mm × 540 mm. Po kaljenju smo izbrane preizkušance popuščali na temperaturah od 100 °C do 650 °C. Popuščanje je trajalo 30 ur pri temperaturah do 300 °C ter 2 uri pri višjih temperaturah. Preostali del preizkušancev pa smo po kaljenju podhladili na temperature –70 °C, –100 °C in –130 °C in jih nato popuščali 30 ur na 120 °C. Po končani toplotni obdelavi smo preizkušancem izmerili trdote na površini ter izrisali popuščne diagrame. Glavni del naloge predstavljajo rezultati zaostalega avstenita, ki smo ga merili z rentgensko difrakcijo, za določitev mikrostrukturnih sestavin pa so bile izvedene tudi metalografske analize. Rezultati rentgenske difrakcije so nam pokazali, da je začetni delež zaostalega avstenita po kaljenju pri vseh treh jeklih znašal okoli 22 vol. %. Pri popuščanju se je količina zaostalega avstenita obdržala do določene temperature. Ta se je rezlikovala glede na jeklo. Pri jeklu OTV1B se je zaostali avstenit obdržal do 350 °C, pri jeklih OTV1 in OHV5 pa je delež zaostalega avstenita začel padati že pri temperaturah popuščanja med 150 in 250 °C. Pri višjih temperaturah se je delež zaostalega avstenita pri vseh jeklih znižal pod 1 vol. %. Pri podhlajenih preizkušancih so vrednosti zaostalega avstenita med 9 in 12 vol. % in se ne razlikujejo veliko glede na temperaturo podhladitve.The master\u27s thesis was carried out with the aim of determining the influence of different heat treatment parameters on the retained austenite content in induction hardened steels. The experiments were performed on three different steels: OTV1B, OTV1 and OHV5. Hardening was performed on a BIKS 750 induction machine, and the samples were in the form of cylinders with dimensions Ø 193 mm × 540 mm. After hardening, the selected samples were tempered at temperatures from 100 °C to 650 °C. Tempering lasted 30 hours at temperatures up to 300 °C and 2 hours at higher temperatures. After completion of the heat treatment, we measured the hardness on the surface of the samples and drew hardness-temperature diagrams. Another set of samples was cooled after hardening to temperatures of –70 °C, –100 °C and –130 °C, and then tempered at 120 °C for 30 hours. X-ray diffraction was used to determine the retained austenite content for all three steel grades, while the metallographic analyses were performed for the microstructure characterisation. The results of X-ray diffraction showed us that the initial percentage of retained austenite after hardening for all three steels was around 22 vol. %. During tempering, the amount of retained austenite remained the same up to a certain temperature. This was different for individual steels. In the case of steel OTV1B, the retained austenite remained up to 350 °C, while in the case of steels OTV1 and OHV5, the retained austenite content started to decrease at tempering temperatures between 150 and 250 °C. At higher temperatures, the proportion of retained austenite dropped below 1 vol. % in all steels. For cooled samples, the values for retained austenite ranged between 9 and 12 vol. % and varied only slightly as a function of the cooling temperature