The aim of the master's thesis was to reveal the degradation mechanisms in thermal fatigue of a new hot work steel with high thermal conductivity (54W/mK), whereby the tested samples were heat treated only, ion nitrided and externally cooled with nitrogen, as well as without cooling. To test the resistance to thermal fatigue, we used a new test developed at IMT, in which the test pieces have the shape of a hexagonal prism and are internally cooled by water. The test pieces were made of Fastcool-55 material, manufactured by Rovalma, which is mainly used for aluminum die casting. Due to the poor hardenability, the test pieces were quenched in oil and then tempered to a hardness of 48 HRc. The poor hardenability is mainly due to the lower amount of alloying elements, since the alloying (carbide-forming) elements differ in content from the alloying elements in classic hot work tool steels, which on the other hand, has a beneficial effect on improving thermal conductivity. The main carbide-forming element is molybdenum, whose carbides have high thermal conductivity. On the nitrided samples, only a slight increase in microhardness was achieved compared to the heat-treated state, i.e. an increase of about 70 HV, and consequently no significant difference in behavior during crack growth and other degradation processes was observed. On the test specimens, cracks first formed at the edges of the hexagon at a distance of about 1.2 mm from the edge, and behind this area a classic network of cracks formed. The tool steel used shows a high tendency to oxidation and therefore peeling of the surface layer. Visual assessment of resistance to thermal fatigue indicates a high sensitivity to crack formation in relation to cooling. This leads us to the fact that the cooling process of tools in industrial practice must be planned very carefully.
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