33 KB1663 KB32 KB199219991997Liščić, Božidarletnik:31številka:68 stranistr. 521-528ISSN:1318-0010COBISSID:70826URN:URN:NBN:SI:doc-YBAXRT8BenInštitut za kovinske materiale in tehnologijeŽelezarna JeseniceŽelezarna RavneŽelezarna ŠtoreKovine zlitine tehnologijejeklakaljenjeprenos toplotetrdotaInfluence of heat transfer dynamics on hardness distribution after quenching| Vpliv dinamike prenosa toplote na porazdelitev trdote po kaljenju|The pattern of hardness distribution on round bars' cross-section after quenching was studied in relation to the change of heat transfer on the workpiece surface. It was found that a 'delayer quenching', producing a discontinuous change of cooling rate, may result in higher hardness in the core, than aat the surface. This phenomenon called 'inverse hardening' has been theoretically explained by Shimizu and Tamura. It depends on: hardenability of the steel, cross-section size of the workpiece and on quenching condition, and is related to the incubation period consumed before the cooling rate was changed. Own experiments using cylindrical specimens of 50 mm Dia, made of AISI-4140 steel, have shown that Controllable Delayed Quenching (CQD) technology has a great potential to increase the depth of hardening, compared to conventional quenching practice. Bending fatigue tests with inverse hardened and tempered specimens have shown a significant increase of the fatigue life compared to specimens having normal haardness distribution after quenching. CDQ-technology and 'inverse haardening' can reproducibly be realized using adequate steel hardenability and cross-section size of the workpiece, by quenching in PAG polymer-solution of high concentrtion, or in high pressure-circulated gassesRaziskane so bile značilnosti porazdelitve trdote na preseku kaljene okrogle palice v odvisnosti od spremembe prenosa toplote na površini palice. Kaljenje z zadržanjem, ki povzroča diskontinuirno spremembo hitrosti ohlajanja, tako ustvari večjo trdoto v jedru kot na površini. Ta pojav imenovan "inverzna utrditev" sta teoretično razložila Shimizu in Tamura. Odvisen je od kaljivosti jekla, preseka kaljenca in od pogojev kaljenja ter je povezan z inkubacijsko dobo, ki je bila porabljena pred spremembo hitrosti ohlajanja. Naši poizkusi na valjastih vzorcih ?$\phi$? 50 mm iz jekla AISI-4140 so pokazali, da ima "kontrolirano kaljenje z zadržanjem" velik potencial za povečanje globine kaljenja v primerjavi s klasičnim kaljenjem. Upogibni utrujenostni preizkusi na inverzno kaljenih in popuščenih vzorcih so pokazali pomembno povečanje življenske dobe v primerjavi z vzorci z normalno porazdelitvijo trdote po kaljenju. CDQ-tehnologija in "kaljenje z zadržanjem" sta lahko reproduktibilna, če ima jeklo primerno kaljivost in je presek kaljenca ustrezen s kaljenjem v polimerni PAG raztopini velike koncentracije ali v visokotlačni krožeči atmosferiTEXTznanstveno časopisjejournalsSlovenian National E-content AggregatorNational and University Library of Slovenia