In this study, the effects of temperature and time on the hardness of heat-resistant cast steel HK30 Nb obtained with centrifugal casting were analysed using the Taguchi method. On the basis of the analysis of the obtained results, we reached reliable conclusions on the improvement of the exploitation properties. The tests showed that precipitation annealing of a HK30 Nb casting can improve the exploitation properties at high temperatures. The defined temperature and holding time comply with

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... results of the mathematical analysis of the achieved properties, and the presumed research theory from this study can hence be confirmed. The studies showed that the mechanical properties improved during the precipitation-annealing process. During this process, significant transformations of the carbide phase occur in the microstructure, thereby changing its morphology, distribution, size and composition, which was confirmed by testing the microstructure with a scanning electron microscope. Keywords: Taguchi analysis, designed experiment, heat-resistant cast steel, precipitation annealing, microstructure, product quality V pri~ujo~i {tudiji avtorja s pomo~jo Taguchijeve metode analizirata vpliv temperature in~asa izlo~evalnega'arjenja na trdoto termi~no obstojne jeklene litine HK30 Nb izdelane s centrifugalnim litjem. Na osnovi dobljenih rezultatov analize sta pri{la do zanesljivih zaklju~kov, ki omogo~ajo izbolj{anje uporabnih lastnosti izbrane litine. Preizkusi so pokazali, da izlo~evalnò arjenje HK30 Nb litine lahko izbolj{a uporabne lastnosti litine pri povi{anih temperaturah. Definirane temperature in~asi zadr'evanja se ujemajo z rezultati dose'enih lastnosti na osnovi matemati~ne analize. Na osnovi predpostavljene raziskovalne teorije v tej {tudiji sta avtorja potrdila prakti~no dose'ene lastnosti. [tudija je pokazala, da se mehanske lastnosti izbrane litine izbolj{ajo s postopkom izlo~evalnega'arjenja. Med postopkom precipitacijskega'arjenja je pri{lo v mikrostrukturi do pomembne pretvorbe karbidne faze, to je spremembe njene morfologije, velikostne porazdelitve in sestave, kar je bilo ugotovljeno z metalografskimi preiskavami na vrsti~nem elektronskem mikroskopu. Klju~ne besede: Taguchijeva analiza, oblikovanje eksperimenta, termi~no obstojna jeklena litina, izlo~evalno'arjenje,