High-Nitrogen Steel

The properties of high-nitrogen steels are briefly reviewed, along with their production technologies. High-nitrogen steels are preferable to traditional steels in many respects: specifically, their yield point is four times higher, with a distinctive combination of other properties; expensive alloying elements such as Ni, Mo, Co, and W are required in smaller quantities, if at all; and effective alloying with nontraditional elements (such as Ca, Zn, and Pb) is possible. The influence of the nitrogen content on the properties of such steels is considered. The production technologies for ferritic–pearlitic, martensitic, and austenitic steels are described, along with their applications. For ferritic–pearlitic steels, more careful maintenance of the chemical composition is required in alloying with nitrogen, which is more soluble than carbon, so as to prevent the formation of nitrides, which are insoluble on heat treatment. On tempering martensitic steels, nitrides and carbonitrides may be formed. The influence of nitrogen in those alloys may be associated with decrease in size of the nitride particles relative to the carbide particles. The increased thermal stability of nitrides and carbonitrides improves the mechanical and physical properties of the steel. Because nitrogen is closely equivalent to nickel in terms of the formation of γ phase, it may be used to replace nickel in austenitic steels, in the following proportions: 1 kg nitrogen to around 6–39 kg of nickel. In austenitic–martensitic steels, the main role is played by thermal martensite; stable austenite is formed on aging at the working temperatures. Examples of the effective use of high-nitrogen steels in machine parts are presented.