It is a variant of the low-pressure carburizing process, primarily designed for thicker layers. Thanks to its technological solutions, it enables processing at higher temperatures without the occurrence of grain growth.

Ammonia is dosed in the temperature range from 400 to 700°C. This temperature range is optimal both for limiting the amount of residual austenite and from an economic perspective, as dosing occurs during the heating stage, thus not extending the overall process time. The nitrogen generated from the catalytic decomposition of ammonia saturates the steel’s surface layer, initially causing grain refinement and subsequently limiting the growth of austenite grains during carburizing temperature. The restriction of grain growth is influenced by nanoprecipitates in the form of nitrides and carbo-nitrides of alloying elements, which act as heterogeneous nuclei for the forming austenite grains and as barriers to their growth.

The amount of nitrogen absorbed in the surface layer during the pre-nitriding phase is relatively small, but the depth of its penetration is comparable to the thickness of the carburized layer, which ensures that grain growth restriction occurs throughout the entire carburized layer. This amount is sufficient to form an adequate number of nanoprecipitates in the entire carburized area, while on the other hand, this amount of nitrogen does not stabilize austenite, thus not increasing the amount of residual austenite in the surface layer, which would otherwise reduce the mechanical properties of that layer. This is confirmed by comparative tests, including fatigue strength and pitting resistance.