Abstract
In this paper results of microhardness testing of plasmatically nitrided layers after implantation and following isothermal annealing of five types of structural steels and one type of tool steel are presented. Plasmatically nitrided steel samples were isothermally annealed at temperatures of 500, 550, and 600 °C in time periods of 5, 10, 20, and 40 hours. The results of microhardness measurements on surfaces and cross-sections of the specimens are quantitatively evaluated. It is established that: 1. the temperature of isothermal annealing of nitrided steel has no effect on microhardness of the samples; 2. the time period of isothermal annealing has a weak effect on microhardness of the samples; 3. the effect of above-mentioned parameters, i.e. of T and t, is superimposed on the effect of chemical composition of the nitrided steel, which is the main factor influencing the microhardness of the samples after plasma nitridation and the following isothermal annealing.

Key words
plasma-nitrided layers, structural steels, tool steels, microhardness testing

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