High nitrogen steels (HNS) are a new class of high alloy martensitic, austenitic or duplex grades with up to 0.9 mass% of N in solid solution. They are applied e.g. to stainless tools and bearings, in chemical engineering and for high strength non-magnetic components.
Dissolving Nitrogen into Steels
In contrast to carbon, nitrogen is a volatile element and therefore requires special measures to be dissolved in the melt. The first one consists of alloying. The Nitrogen solubility increases in the following order of elements Xi = N, C, Si, Al, Ni, Co, Cu/W, Mo, Mn, Cr, Nb, V, Ti, the oblique indicating the change from repulsing to attracting elements. As the solubility product of Nb, V and Ti in austenite is rather low, the Nitrogen solubility of HNS is mainly based on Cr, shifting the majority of alloys into the stainless range. The second measure is to further raise the Nintrogen content of the melt by applying nitrogen pressure. Pressurised electroslag remelting (PESR) is commercially available producing ingots of up to 20 tons in weight. The third measure makes use of the high uptake of Nitrogen in austenite. Respective steel powder is subjected to solid state nitriding and than compacted by hot isostatic pressing (HIP). This allows for the highest Nitrogen content, e.g. 3 mass% in stainless powder metallurgical (PM) tool steels containing e.g. wear resistant NbN nitrides embedded in a martensitic matrix. Measures two and three may lead to Nitrogen contents in the steel, which are above the solubility at temperatures of hardening or solution annealing, if carried out at the partial pressure of nitrogen in air.
Interactions of Alloy Nitrogen
Thus the interaction of alloy nitrogen with the furnace atmosphere or vacuum may differ considerably from carbon and require specific precautions. At first one has to look at the thermodynamic equilibrium between the steel surface and the surrounding atmosphere. Next the kinetics of phase transformations in the whole cross section are of interest to avoid e.g. embrittling precipitates. This is especially important for stainless austenitic steels, which are usually low in carbon but may contain up to 0.9 mass% nitrogen. Finally the expected changes in volume and the resistance to oxidation of HNS during heat treatment have to be considered in comparison to respective carbon grades.