Advanced High Nitrogen Steels

The significant application of nitrogen as an alloying element commenced in the 80s of the past century. The steels produced then, which contained 0.5–1.0% of nitrogen, were called High-Nitrogen Steels (HNS) or nitrogen “hyperequilibrium” steels. At such its contents, nitrogen imparts unique properties to the steel; for example, stainless high-nitrogen steels are characterized by high strength and high corrosion resistance at the same time, and therefore the high-nitrogen steels have initiated a new branch in physical metallurgy.

In the case of 9-12 % chromium steels higher nitrogen content supports the precipitation of particles of vanadium nitride, VN, which leads to increase the creep resistance of these steels with increasing nitrogen content. In the case of two-phase, austenitic-ferritic steel, nitrogen affects corrosion resistance of these steels, mechanical properties and has significant influence on the phase composition, i.e. the ratio of austenite and ferrite. Most often the ferrite content in these steels is between 40 and 60 percent. An increase of nitrogen in the duplex microstructure has several significant effects on the phase diagram. Nitrogen additions have a strong stabilization effect on austenite when considering the high temperature ferritic transformation.

Considering the fact that nitrogen is element, which is able to stabilized austenite, it can be used as an inexpensive substitute for other more expensive elements.

Nitrogen alloyed different steel grades can be used in various fields such as:

• Transportation (cables, blades of reactors, landing parts of aircrafts, wheels for trains, body of cars, double shell for fuel tankers, etc.)
• Environment technologies, (safely in oil pipeline, petrol prospection, etc.)
• Industrial plants and equipments (mechanical industry, car industry, nuclear reactors, control devices, cutting machine, paper industry, etc.)
• Civil engineering
• Leisure and sport industry (high demand for extreme mechanical resistance and lightness)
• Defense and space industry

Nitrogen steel can be produced in open air (electric arc and induction furnace, electroslag remelting), under nitrogen pressure (pressurized induction furnace, pressure electroslag remelting-PESR-), by powder metallurgy and surface alloying methods.

The research staff of Steel Technology Department, CMRDI, has many contributions in the field of high nitrogen steels. In their studies, they used both open air and controlled atmosphere technologies for production of several grades of steels containing nitrogen.

The effect of vanadium and nitrogen in low carbon (0.1%) manganese steel showed that vanadium microalloying increases the strength of steel through solely precipitation strengthening or both precipitation strengthening and grain refining effect. The strengthening effect of vanadium seems to have no negative effect on elongation. The effectiveness of vanadium is greatly enhances by increasing the nitrogen content. Increment of 194 and 110 N/mm² in the yield and ultimate tensile strengths, respectively, are attained by increasing the nitrogen content from 0.015 to 0.025 % in a steel with a base composition of 1.8%Mn and 0.15%V. The grain refinement of vanadium/nitrogen microalloying seems to be due to inhibition of austenite grain growth as a result of the precipitation of vanadium nitride in austenite during forging. Precipitation strengthening of these steels is achieved by the precipitation of vanadium carbide and nitride in ferrite or bainite. Up to 70% of the total nitrogen content of steel precipitates as vanadium nitride which could be achieved with V/N ratio of about 6-7. By microalloying of low carbon-manganese steels with vanadium and nitrogen, high yield strength up to 835 N/mm² can be attained in the forging condition.

However, stainless steels, which have nickel content less than 4%, exhibit lower strength compared with stainless steels containing higher nickel content (>5% Ni). It can be concluded that partial and total replacement of nickel by nitrogen produce stainless steels with stable phase as well as it improves the mechanical properties of austenitic stainless steels at room temperatures.

Three steels with the same base composition were used for recent study, having different nickel and nitrogen contents with nickel in the range 0.089 – 14.42 % and nitrogen 0.452 – 0.012%. The effect of soluble and insoluble nitrogen on mechanical properties of stainless steels was studied. Reference steel containing 0.039%C, 1.64%Si, 1.62%Mn, 0.021%P, 0.0115%S, 20.24%Cr, 2.39%Mo, 14.42%Ni, and 0.012%N, and two developed steels having the same base chemical composition of the reference steel except Ni and N contents were used. One of the developed steels has 6.54%Ni and 0.232%N and the other has 0.452%N and nearly nickel free (0.089%Ni).

The results showed that microstructures of the reference and the developed stainless steels were mainly austenitic phase. It was concluded that nitrogen content has significant effect on grain refinement; insoluble nitrogen has more significant effect than soluble nitrogen on grain refinement. Partial replacement or total replacement of nickel improves yield and ultimate tensile strength. Elongation decreases with increasing reciprocal of square root of grain size (1/D0.5). The hardness–of tempered stainless steel after solution treatment–increases by increasing nitrogen content at temperatures of 1373, 1423, and 1473K. However, it decreases by increasing solution treatment temperature due to enlargement of grain size as soluble nitrogen increases accompanied by decrease in insoluble nitrogen (nitrides) with increasing solution treatment temperatures. The actual factors, which control the mechanical properties of nitrogen containing austenitic stainless steels may be summarized in the grain boundary hardening, matrix strengthening, the solid solution strengthening and the precipitation hardening. The difference between predicted and actual strength increases by increasing insoluble nitrogen as a result of increasing the precipitation hardening. Insoluble nitrogen is more significant than soluble nitrogen on grain boundary hardening.

The effect of nitrogen content of two types of stainless steels with the same base composition except in nickel and nitrogen contents on the mechanical properties was investigated.

Reference and high nitrogen stainless steels were produced in open air in induction furnace. The reference steel has 14.37%Ni and 0.010%N and the nitrogen stainless steel has 11.22%Ni and 0.122%N. Other alloying elements are 0.042 – 0.046%C, 0.69 – 0.95%Si, 1.37 – 1.38%Mn, 0.0172 - 0.0182%P, 0.007 – 0.010%S, 18.65 – 18.80%Cr, 2.32 – 2.34%Mo. The results showed that the partial replacement of nickel by nitrogen increase both yield and ultimate tensile strength from 244 MPa and 517 MPa to 353 MPa and 683MPa with insignificant change in elongation (changed from 55.4% to 54.5% for free and nitrogen stainless steel respectively).