Nitriding treatment refers to a chemical heat treatment process in which nitrogen atoms penetrate the surface of the workpiece in a certain medium at a certain temperature. Nitriding can make the surface of parts have higher hardness and wear resistance. For example, the surface hardness of parts made of 38CrMoAlA steel after nitriding treatment can reach HV=950-1200, equivalent to HRC=65-72, and the high strength and high wear resistance after nitriding can be maintained to 500-600 C without significant change. It can improve the anti-fatigue ability. Due to the formation of greater compressive stress in the nitriding layer, parts show higher fatigue limit and lower notch sensitivity under alternating load. The fatigue limit of nitrided parts can be increased by 15-35%. Nitriding improves the corrosion resistance of the workpiece too. Because nitriding makes the workpiece surface form a dense and chemically stable epsilon phase layer, which has high corrosion resistance in water vapor and alkaline solution. This nitriding method is simple and economical and can replace zinc plating, bluing, and other chemical coating treatment. Besides, some dies after nitriding can not only have better wear resistance and corrosion resistance, but also have less bonding phenomenon between dies and parts, and prolong the working life of dies.
The NH3 gas is directly transported into the nitriding furnace at 500-550 C for tens of hours. The NH3 gas is then decomposed into an ionic state for nitriding treatment. Active nitrogen atoms are absorbed by the surface of the workpiece and diffused into the interior to form a nitriding layer. The thickness of the nitriding layer can be accurately controlled by controlling the ammonia flow rate and residence time with modern computer technology. For the right kind of metal alloy, the thickness of the nitriding layer can reach as much as 0.6mm.
For nitriding aiming to improve hardness and wear resistance, alloy steels containing Mo, Al and V, such as 38CrMoAlV and 38CrMoAV, must be used in nitriding. After nitrogen treatment, these steels contain various alloy nitrides in the nitriding layer, such as AlN, CrN, MoN, VN, etc. These nitrides have high hardness and stability and are uniformly dispersed in steel, making the nitrided layer of steel have high hardness and wear resistance. Cr can also improve the hardenability of steel so that large parts can obtain uniform mechanical properties when quenched and tempered before nitriding. Mo can also refine grain size and reduce the second temper embrittlement of steel. If ordinary carbon steel becomes a nitride, it forms pure iron nitride in nitriding layer, when heated to a certain temperature, the nitriding layer is easy to decompose, aggregate and coarsen, and can not obtain high hardness and high wear resistance.
When nitriding for corrosion resistance purpose, nitriding temperature is generally controlled between 600-700 C, the decomposition rate is about 40-70%, the residence time is from 15 minutes to 4 hours, and the depth of nitride layer formed is generally less than 0.05 mm. For corrosion-resistant purpose nitriding, any kind of steel can be applied.
Liquid nitriding (also known as Salt bath nitriding )
In the process of liquid nitriding, the medium is no longer ammonia but molten salt containing nitrogen element. Liquid nitriding is a relatively new chemical heat treatment process, the temperature does not exceed 570 C, the treatment time is shorter than gas nitriding, only 1-3 hours. And no special steel is required. The test shows that the wear resistance of 40Cr treated by liquid nitriding is 50% higher than that of ordinary quenching and tempering, and the wear resistance of cast iron treated by liquid nitriding is much higher than that of ordinary quenching and tempering. Not only that, practice has proved that parts treated by liquid nitriding have different degrees of improvement in fatigue resistance, corrosion resistance and other aspects
However, liquid nitriding also has its drawbacks: for example, the nitrogen-iron compound layer in its nitriding surface is relatively thin, only 0.01-0.02 mm. The toxicity from salt dissolution reaction potentially affects the health of operators, and waste salt handling is also a problem.
Ion nitriding, also known as glow ion nitriding or Plasma nitriding, is a recently developed heat-treatment process. It has the advantages of short production cycle, high surface hardness of parts and brittleness control of the nitriding layer.
Basic Principle of Glow Ion Nitriding:
Glow ion nitriding is to place parts in the vacuum chamber of ion nitriding. The nitriding parts are connected with the cathode (negative electrode) of high voltage direct current power supply, and the shell of the electric furnace is connected with the anode (positive electrode) of high voltage direct current power supply. When ammonia is filled into the vacuum container, the pressure in the container keeps between 200-1000PA, and 800-1000 volt direct current is added between cathode and anode. The ammonia gas is then ionized, this ionized gas produce positively charged nitrogen cation [N+] and negatively charged anion [N-], forming a plasma region. In the plasma region, under the acceleration of high voltage electric field, the positive ion of nitrogen rushes rapidly to the cathode, bombards and cleans the surface of the workpiece, transforms kinetic energy into thermal energy. When nitrogen ion transforms into a nitrogen atom, it also emits a large amount of thermal energy and emits bright lilac light. In addition, when the voltage drops near the workpiece, it also generates heat. These three kinds of heat will heat the parts to the required nitriding temperature. At this temperature, the nitrogen ion reacts with the metal surface of the part, and the nitrogen atom penetrates the surface of the part and diffuses into the interior, forming a nitriding layer. Advantages of glow ion nitriding:
(1) The surface heating speed is fast, and the heating and cooling time can be shortened to one-tenth to one-twelfth of other nitriding processes. Only the surface of the workpiece is heated, the rest part are in low temperature (about 100 C), which not only saves heating power but also reduces the deformation of parts. (2) The diffusion process is fast. Under the action of a high voltage electric field, the nitriding atom moves many times faster than the gas nitriding, and the infiltration rate is faster. Generally, it only takes 3-10 hours. (3) The nitriding layer has good toughness, high fatigue resistance and high wear resistance. The brittle white epsilon phase (Fe2N) of the nitriding layer is controlled in the range of 0-0.2mm, thus eliminating the grinding of nitriding parts. The surface hardness can reach as high as HV900 (HRC64), and the depth of the nitriding layer can be controlled from 0.09 mm to 0.87 mm.
The trade-off of the glow ion nitriding is the high investment equipment it needed. Also, better-trained operators are needed too.
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