Vliv mikročistoty na mechanické vlastnosti korozivzdorných materiálů

Abstract

The degree of purity of stainless steels is a parameter which is never put into connection with their mechanical properties or their corrosion resistance properties. In spite of this fact it is necessary to know how much the mentioned properties are affected by non-metallic oxides, as it is impossible to produce steels with zero levels of these oxides. If the presence of the oxides in stainless steels is admitted we also have to know their highest bearable rate to be able to produce components of the desired quality from the mentioned steels while still maintaining acceptable costs. As this dissertation deals with cast stainless steels, its aim is to describe the possibilities of affecting the level of their pollution by the oxides during the foundry production, particularly then to determine production operations, which significantly contribute to the formation of the oxides. That is the reason why the greatest emphasis is placed on both the purity of feedstock and the metallurgy of the observed steels. One of the production operations, decisive for the final purity, was found to be the deoxidation of the steel. The way of deoxidation, as far as the types of deoxidation agents and their dosage are concerned, considerably affects morphology, size and presence of the oxides. Parameter K4 according to DIN 50 602, used in the long run in the concern where the author of this dissertation is employed, was chosen as a purity indicator. Samples with known mechanical values were put to purity tests using the method K according to the above mentioned norm (with sensitivity 4) and were also tested to determine their corrosion characteristics. Above all, it is necessary to mention here the electrochemical polarization method DL-EPR (based on the method of prof. ing. Vladimír Číhal DrSc.), which has only recently been put into practice as an ISO norm. Specific austenitic stainless steel and superduplex stainless steel, whose mechanical properties are not affected by heat treatment, were chosen for determining the impact of purity on mechanical properties of stainless steels and the impact of purity on their corrosion characteristics. This heat treatment, specifically solution heat treatment followed by quick quench in water, does not lead to different values of mechanical parameters, if carried out correctly. The whole research concerning every type of steel was carried out within long-term production of castings of the same kind.