From structure to development

Abstract

The unusual combination of properties of nitrogen steels as discussed in the previous chapters (increased strength, toughness, creep and wear resistance, excellent corrosion properties etc.) allows one to characterise them as a promising class of engineering materials. In relation to mechanical properties, a superior combination of toughness and yield strength contrasting the conventional inverse relationship between these properties is particularly important. This cannot be achieved by carbon. Moreover, attempts to increase the strength of corrosionresistant steels using carbon were not successful because of its deteriorating influence on the corrosion properties. Although carbon dissolves easily in the liquid steel, its solubility in the austenitic state is limited by elements like Cr and Ni constituting the base of stainless steels. Precipitated chromium carbides are therefore the main reason preventing the development of stainless carbon steels. In contrast, nitrogen has a low solubility in liquid iron-based alloys, but its content in austenite can be rather high. Thus, it has to be underlined that alloying by nitrogen is expedient only at appropriate contents of elements decreasing the thermodynamic activity of nitrogen in iron, which allows to introduce nitrogen to the liquid while the alloyed nitrogen austenite is rather stable against precipitation.