the saturation of the surface of metal articles with nitrogen in order to increase their hardness, wear resistance, fatigue limit, and corrosion resistance. Nitriding is performed on steel, titanium, and some alloys, most frequently on alloyed (especially chrome-aluminum) steels, as well as on steel containing vanadium and molybdenum.
The nitriding of steel occurs at 500–650°C in an ammonia medium. Above 400°C ammonia begins to dissociate according to the reaction NH3 → 3H + N. The atomic nitrogen thus formed diffuses into the metal, forming nitrogenous phases. At nitrogen temperatures below 591°C the nitrided layer consists of three phases: є—the nitride Fe2N, γ’—the nitride Fe4N, and α—nitrogenous ferrite, which contains about 0.01 percent nitrogen at room temperature. At temperatures of 600–650°C the γ phase can also form; upon slow cooling it decomposes at 591°C into the eutectoid α + γ’. The hardness of the nitrided layer increases to HV = 1200 (HV is hardness according to Vikkers’ method; it corresponds to 12 giganew-tons [GN] per m2) and is maintained during repeated heating to 500–600°C; this ensures a high wear resistance of the articles at elevated temperatures. Nitrided steels considerably surpass case-hardened and tempered steels in wear resistance. Nitriding is a lengthy process; 20–50 hours are required to obtain a layer 0.2–0.4 mm thick. An increase in temperature accelerates the process but decreases the hardness of the layer. Areas not subject to nitriding are protected by tin plating in the case of structural steels and nickel plating in the case of stainless and heat-resistant steels. In order to decrease the brittleness of the nitrided layer, the nitriding of heat-resistant steels is sometimes carried out in a mixture of ammonia and nitrogen.
The nitriding of titanium alloys is carried out at 850–950°C in nitrogen of high purity. Nitriding in ammonia is not used because it increases the brittleness of the metal.
During nitriding, a thin upper layer of nitride and a solid solution of nitrogen in αtitanium are formed. The thickness of the layer after 30 hours is 0.08 mm with a surface hardness HV = 800–850 (which corresponds to 8–8.5 GN/m2). The introduction of certain alloying elements (up to 3 percent Al, 3–5 percent Zr, and others) into the alloy increases the rate of diffusion of nitrogen, thereby increasing the depth of the nitrided layer, while chromium decreases the rate of diffusion. The nitriding of titanium alloys in rarefied nitrogen
(100–10 N/m2 [1–0.1 mm of mercury]) makes it possible to obtain a deeper layer without a brittle nitride zone.
Nitriding is widely used in industry—for example, in components operating at up to 500–600°C, such as cylinder cases, crankshafts, drive gears, valve pairs, and components of heating equipment.
D. I. BRASLAVSKII