Nitriding refers to a chemical heat treatment process that allows nitrogen atoms to penetrate into surface of workpiece at a certain temperature and in a certain medium. Products treated with nitriding have excellent wear resistance, fatigue resistance, corrosion resistance and high temperature resistance.

 

Aluminum, chromium, vanadium and molybdenum elements in traditional alloy steels are very helpful for nitriding. When these elements come into contact with primary nitrogen atoms at nitriding temperature, they form stable nitrides. In particular, molybdenum is not only an element that generates nitrides, but also can be used to reduce brittleness that occurs at nitriding temperature.
Other elements in alloy steels, such as nickel, copper, silicon, manganese, etc., are not very helpful for nitriding characteristics. Generally speaking, if steel contains one or more nitride-generating elements, effect after nitriding is relatively good. Among them, aluminum is the strongest nitride element, and the best nitriding results are obtained with 0.85~1.5% aluminum. For chromium steel containing chromium, if content is sufficient, good results can also be obtained. However, carbon steel without alloy is not suitable as nitriding steel because nitriding layer generated by it is very brittle and easy to peel off.
There are six commonly used nitriding steels as follows:
(1) Low alloy steel containing aluminum (standard nitriding steel).
(2) Medium carbon low alloy steel containing chromium SAE 4100, 4300, 5100, 6100, 8600, 8700, 9800 series.
(3) Hot working die steel (containing about 5% chromium) SAE H11 (SKD-61) H12, H13.
(4) Ferritic and martensitic stainless steel SAE 400 series.
(5) Austenitic stainless steel SAE 300 series.
(6) Precipitation hardening stainless steel 17-4PH, 17-7PH, A-286, etc.
Although standard aluminum-containing nitriding steel can obtain a very high hardness and high wear resistance surface after nitriding, its hardened layer is also very brittle. On the contrary, low-alloy steel containing chromium has a lower hardness, but hardened layer is relatively tough, its surface also has quite good wear resistance and beam resistance. Therefore, when selecting materials, attention should be paid to characteristics of materials and their advantages should be fully utilized to meet functions of parts. As for tool steel, such as H11 (SKD61) D2 (SKD-11), it has high surface hardness and high core strength.
Role of nitriding treatment is to increase wear resistance, surface hardness, fatigue limit and corrosion resistance of steel parts.

Part surface cleaning before nitriding
Most parts can be nitrided immediately after degreasing using gas degreasing method. Some parts also need to be cleaned with gasoline, but in final processing method before nitriding, if polishing, grinding, buffing, etc. are used, a surface layer that hinders nitriding may be generated, resulting in uneven nitriding layer or bending defects after nitriding. At this time, one of following two methods should be used to remove surface layer.
The first method is to first use gas to remove oil before nitriding, and then use aluminum oxide powder to sandblast surface (abrasive cleaning).
The second method is to treat surface with a phosphate coating.
Exhausting air in nitriding furnace
Place treated parts in nitriding furnace and seal furnace cover before heating, but air in furnace must be exhausted before heating to 150℃.
Main function of exhausting air in furnace is to prevent generation of explosive gas when ammonia decomposes and contacts with air, to prevent surface oxidation of treated object and bracket. Gases used are ammonia and nitrogen.
Key points of exhausting air in furnace are as follows:
① After treated parts are installed, seal furnace cover and start to pass anhydrous ammonia, and flow rate is as large as possible.
② Set automatic temperature control of heating furnace to 150℃ and start heating (note that furnace temperature cannot be higher than 150℃).
③ When air in furnace is exhausted to less than 10%, or exhausted gas contains more than 90% NH3, then raise furnace temperature to nitriding temperature.
Ammonia decomposition rate
Nitriding is carried out by contacting with other alloying elements and primary nitrogen, but generation of primary nitrogen is that when ammonia gas contacts heated steel material, steel material itself becomes a catalyst and promotes decomposition of ammonia.
Although nitriding can be performed under various decomposition rates of ammonia, a decomposition rate of 15~30% is generally used, and required thickness of nitriding is maintained for at least 4~10 hours, and treatment temperature is maintained at about 520℃.

 

Cooling
Most industrial nitriding furnaces have heat exchangers in order to rapidly cool heating furnace and treated parts after nitriding work is completed. That is, after nitriding is completed, heating power is turned off to reduce furnace temperature by about 50℃, then ammonia flow is doubled and heat exchanger is started. At this time, it is necessary to observe whether there are bubbles overflowing from glass bottle connected to exhaust pipe to confirm positive pressure in furnace. After waiting for ammonia to be introduced into furnace, flow of ammonia can be reduced until positive pressure in furnace is maintained. When furnace temperature drops below 150℃, furnace cover can be opened only after air or nitrogen is introduced using method of removing gas in furnace as described above.
Gas nitriding was discovered by AFry in Germany in 1923. Workpiece is placed in furnace and NH3 is directly introduced into nitriding furnace at 500-550℃ for 20-100 hours to decompose NH3 into atomic nitrogen and hydrogen, thereby performing nitriding treatment. Main purpose is to produce a wear-resistant and corrosion-resistant compound layer on the surface of steel. Its thickness is about 0.02mm, and its properties are Hv1000-1200. It is extremely hard and brittle. Decomposition rate of NH3 varies depending on flow rate and temperature. The larger flow rate, the lower decomposition degree, and the smaller flow rate, the higher decomposition rate. The higher temperature, the higher decomposition rate, and the lower temperature, the lower decomposition rate. NH3 gas is thermally decomposed at 570℃ as follows:
NH3 →N+ 3/2H2
Decomposed N diffuses into surface of steel to form a thin hardening layer and a long nitriding treatment time.
Gas nitriding is generally fixed to a fixed type of steel suitable for nitriding, such as containing nitriding elements such as Al, Cr, and Mo, otherwise nitriding cannot be carried out. Generally, JIS, SACM1, new JIS, SACM645 and SKD61 toughening treatment, also known as quenching and tempering, are used. Because Al, Cr, Mo, etc. are elements that increase quenching temperature, quenching temperature is high, and tempering temperature is higher than that of structural alloy steel. After long-term heating at nitriding temperature, temper brittleness occurs. Therefore, quenching and toughening treatment is applied in advance. NH3 gas nitriding, because of long time, surface is rough, hard and brittle, not easy to grind, and long time is not economical, it is used for nitriding of feed pipe and screw rod of plastic injection molding machine.

Main difference of liquid nitriding is that there are Fe3Nε phase and Fe4Nr phase in nitriding layer, but no Fe2Nξ phase nitride. ξ phase compound is hard and brittle, which is not good for toughness in nitriding treatment. Method of liquid nitriding is to remove rust and degrease workpiece to be treated first, and then place it in nitriding crucible after preheating. Crucible is mainly TF-1 salt agent, which is heated to 560-600℃ for several hours. Depth of nitriding layer is determined by external force load on workpiece. During treatment, an air pipe must be passed through the bottom of crucible to decompose a certain amount of air nitriding salt agent into CN or CNO, which penetrates and diffuses to working surface, so that outermost compound on the surface of workpiece is 8-9%wt of N and a small amount of C and diffusion layer, and nitrogen atoms diffuse into αFe to make steel more fatigue-resistant. During nitriding, due to decomposition and consumption of CNO, salt composition must be continuously tested during 6-8 hour treatment in order to adjust air volume or add new salt.
Material used for liquid nitriding treatment is iron metal. Surface hardness after nitriding is higher when it contains Al, Cr, Mo, and Ti elements. The more gold content, the shallower nitriding depth, such as carbon steel Hv350~650, stainless steel Hv1000~1200, and nitrided steel Hv800~1100.
Liquid nitriding is suitable for wear-resistant and fatigue-resistant automotive parts, cylinder liner treatment, valve valve treatment, piston barrel treatment, and molds that are not easy to deform. Countries that use liquid nitriding include Western European countries, United States, Japan, etc.

 

This method is to place a workpiece in a nitriding furnace, evacuate furnace to 10-2~10-3 Torr (mmHg) in advance, then introduce N2 gas or N2+H2 mixed gas, adjust furnace to 1~10Torr, connect furnace body to anode, connect workpiece to cathode, and pass hundreds of volts of DC voltage between two electrodes. At this time, N2 in furnace will produce a brilliant discharge into positive ions, which will move to work surface. In an instant, cathode voltage drops sharply, causing positive ions to rush to cathode surface at high speed, converting kinetic energy into gas energy, so that surface temperature of workpiece can be increased. Due to impact of nitrogen ions, Fe, C, O and other elements on workpiece surface are splashed out and combined with nitrogen ions to form FeN. As a result, iron nitride is gradually adsorbed on workpiece to produce nitriding. Ion nitriding basically uses nitrogen, but if hydrocarbon gas is added, ion nitriding can be performed. Nitrogen concentration on workpiece surface can be adjusted by changing partial pressure ratio of mixed gas (N2+H2) filled in furnace. During pure ion nitriding, a single-phase r′ (Fe4N) structure is obtained on working surface, with a nitrogen content of 5.7~6.1%wt and a thickness of less than 10μm. This compound layer is strong and tough but not porous, and is not easy to fall off. Since iron nitride is continuously adsorbed and diffused into interior of workpiece, structure from surface to interior changes in the order of FeN→Fe2N→Fe3N→Fe4N. Single-phase ε (Fe3N) contains 5.7~11.0%wt of nitrogen, and single-phase ε (Fe2N) contains 11.0~11.35%wt of nitrogen. Ion nitriding first generates r phase, then adds hydrocarbon gas system to turn it into a compound layer and diffusion layer of ε phase. Increase in diffusion layer is very helpful for increase in fatigue strength, and corrosion resistance is best in ε phase.
Ion nitriding treatment can start from 350℃. Considering selection of materials and their related mechanical properties, treatment time can be from a few minutes to a long time. This method is different from thermal decomposition chemical reaction nitriding treatment method used in the past. This method uses high ion energy. Materials such as stainless steel, titanium, and cobalt that were previously considered difficult to treat can also be easily given excellent surface hardening treatment.