Thermal fatigue of tool steels for die casting

Tema magistrske naloge je preiskava odpornosti proti termičnemu (temperaturnemu) utrujanju orodnih jekel za delo v vročem. Med seboj smo primerjali različna orodna jekla: SITHERM S361R, HTCS130, DIEVAR in na novo razvito jeklo SITHERM S140R, katerega razvoj je temeljil predvsem na povečanju toplotne prevodnosti. Izbrana jekla smo primerjali glede na kemijsko sestavo, mikrostrukturo, mehanske in fizikalne lastnosti, toplotne lastnosti ter s tem povezanim vplivom na odpornost na termično utrujanje. Cilj naloge je bilo določiti katero jeklo s pripadajočimi lastnostmi ima največjo odpornost na termično utrujanje in je tako najbolj primerno za izdelavo orodja za tlačno litje aluminija. Preiskovane vzorce smo toplotno obdelali na enako trdoto 44 – 46 HRc, tako da so na rezultate vplivale predvsem samo kemijska sestava, mikrostruktura in toplotna prevodnost. V okviru eksperimentalnega dela smo preiskovana jekla preizkušali z nateznim preizkusom, pregledali mikrostrukturo s svetlobnim mikroskopom, relavantne lastnosti (delež, velikost, oblika, velikostna porazdelitev ...) v povezavi s karbidi z vrstičnim elektronskim mikroskopom ter izvedli test odpornosti proti termičnemu utrujanju na termo-mehanskem simulatorju metalurških stanj Gleeble 1500D. Ugotovili smo, da trdnost jekla ni bistveno vplivala, najboljšo odpornost proti termičnemu utrujanju pa imajo tista jekla, ki imajo z višanjem temperature tudi najvišjo toplotno prevodnost ter najboljše karakteristike povezane s karbidi.The topic of the master´s thesis is to investigate the thermal fatigue resistance of selected tool steels for hot working. Four different tool steels were compared among each other: SITHERM S361R, HTCS130, DIEVAR and the newly developed SITHERM S140R. Development of new steel SITHERM S140R was primarily based on increasing thermal conductivity. The selected steel was compared regarding to the chemical composition, microstructure, mechanical, physical and thermal properties in relation to associated effect on thermal fatigue. The aim of the task was to determine which steel with the associated properties has the highest resistance to the thermal fatigue, and is therefore most suitable for the production of tool for the die- casting of aluminium. Investigated samples were heat treated to the same hardness of 44–46 HRc, so that the results were primarily influenced only by chemical composition, microstructure and thermal conductivity. Therefore, in the experimental part the tensile tests on tested steels were carried out, microstructures were observed using an optical microscope, relevant properties related to carbides were investigated using linear electron microscope while a thermal fatigue tests on the thermal-mechanical simulator of metallurgical states Gleeble 1500D were carried out. We found that the strength of the steel was not significantly affected, and the best resistance to thermal fatigue have those, which have the highest thermal conductivity with increasing temperature and most appropriate characteristics related to carbides