The overall focus of investigations was to identify the influence of zirconium and titanium upon development of the microstructure and mechanical properties of X11CrNiMo12 steel. Special emphasis was laid on investigations of the influence of zirconium and titanium upon non metallic inclusions. For this purpose we made two series of laboratory heats with an addition of zirconium and titanium. Samples from these heats were subjected to forging and to a proper heat treatment. We examined chemical composition of samples and tested them with the use of optical and scanning electron microscopes. We also defined their mechanical properties. The zirconium and titanium additions have a very positive influence upon the size of prior austenitic crystal grains of X11CrNiMo12 steel and upon the achieved values of Charpy notched bar impact test where strength properties do not worsen. Added titanium and zirconium result in the development of new types of non-metallic inclusions. Zirconium-based oxides, which have at the same time a favourable spherical form, are the smallest in size. Zirconium addition also has the largest positive influence upon toughness improvement. With excessive share of titanium and/or zirconium, nitrides develop in the microstructure whereby titanium and zirconium-based oxides serve as a heterogeneous nucleus for the formation of nitrides which has a negative impact upon mechanical properties.
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