| Popis: |
Magistrsko delo je bilo izdelano v sklopu razvojnega programa podjetja Sorbit Valji, d. o. o. Podjetje se ukvarja s proizvodnjo konvencionalno kaljenih valjev za hladno valjanje, v zadnjih letih pa so proizvodni program razširili še na področje površinsko kaljenih valjev. Namen magistrskega dela je bil določiti, kakšen vpliv ima frekvenca induktorja pri indukcijskem kaljenju valjev na globino kaljene plasti za srednjelegirano jeklo s 5 mas. % kroma. Toplotna obdelava se je najprej izvajala na indukcijskem stroju BIKS 750, na cilindričnih preizkušancih iz jekla OTV1B in premera 400 mm. Preizkušanci so bili segreti z elektromagnetno indukcijo različnih frekvenc na temperaturo avstenitizacije in nato kaljeni v vodi. Uporaba različne frekvence vodi do spremembe temperaturnega profila po preseku preizkušanca, kar posledično spremeni globino kaljenja. Ostali parametri indukcijskega kaljenja so bili pri vseh preizkušancih enaki. Preizkušanci so bili po kaljenju podhlajeni (kriogensko ohlajeni) do –130 °C za odpravo zaostalega avstenita, nato pa še popuščeni pri 160 °C za sprostitev notranjih napetosti. Na podlagi merjenja trdot smo določil globino minimalne uporabne trdote valjev za hladno valjanje. Za določitev mikrostrukturnih sestavin so bile izvedene metalografske analize in rentgenska difrakcija za določitev deleža zaostalega avstenita. Iz rezultatov lahko zaključimo, da se pri višjih frekvencah (350–650 Hz) globina kaljene plasti bistveno ne spreminja s frekvenco, medtem ko pri nižjih frekvencah (80–200 Hz) ta globina z manjšanjem frekvence močno naraste. Z visoko frekvenco lahko dosežemo globino kaljenja do 15 mm, z nižjimi frekvencami pa med 20 in 30 mm. This master thesis was written as part of the development program of the company Sorbit Valji d.o.o. The company produces conventional hardened rolls for cold rolling. In recent years, the production program has been expanded into the production of surface hardened rolls. The goal of this master’s work was to determine the influence of inductor frequency during induction hardening on hardening depth for medium alloyed steel with 5 wt. % chromium. The heat treatment was performed with a BIKS 750 induction machine on cylindrical OTV1B samples, measuring 400 mm in diameter. The test pieces were heated to the austenitization temperature using electromagnetic induction of different frequencies. After being heated to the set temperature, the test pieces were quenched in water to achieve the desired hardening effect. The use of different frequencies leads to a change in the temperature profile across the cross-section of the specimen, which subsequently changes the hardening depth. Afterwards, the test pieces were cooled (cryogenically treated) to –130 °C to eliminate residual austenite and then tempered at 160 °C to release internal stresses. Based on the hardness tests, we determined the depth of the minimal useful hardness of cold rolling mills. We performed metallographic analyzes and X-ray diffraction to determine the proportion of retained austenite. From the results, we can conclude that at higher frequencies (350–650 Hz), the depth of the hardened layer does not change significantly with frequency, while at lower frequencies (80–200 Hz) this depth increases exponentially. A hardening depth of 15 mm can be achieved with higher frequencies, while lower frequencies must be used to achieve a deeper hardening depth between 20 and 30 mm. |
