Vacuum nitriding furnace technology
Mechanism of nitrification
Nitrification is a heat treatment in which the workpiece is put into the medium of a large number of active nitrogen atoms and, at a certain temperature and pressure, infiltrates nitrogen atoms into the surface of the steel to form a nitrogen-rich hardening layer.
The function of nitrification
1. Nitriding can make the surface of the part more hard and wear resistant. Such as made of 38 crmoala steel hardness on the surface of the parts after nitriding treatment of HV = 950-1200, the equivalent of HRC = 65-72, and the high strength and high wear resistance after nitriding to 500-600 ℃, not significantly change.
2. Can improve fatigue resistance. As more compressive stress is formed in the nitrification layer, under the action of alternating load, the parts show higher fatigue limit and lower notch sensitivity, and the fatigue limit of the workpiece after nitrification can be increased by 15-35%.
3. Improve the anti-corrosion ability of workpiece. Due to nitriding, the workpiece surface forms a thin layer of phosphate phase with high chemical stability, which is highly resistant to corrosion in water vapor and alkaline solution. In addition, some of the molds can be nitrided to not only improve the wear resistance and corrosion resistance, but also reduce the adhesion between the molds and parts and extend the working life of the molds.
The realization method of nitrification
1. Gas nitriding
Gas nitriding is the workpiece in a sealed space, ventilation with ammonia, heated to 500-580 ℃ heat preservation for several to dozens of hours. Ammonia in above 400 ℃ will occur following decomposition reaction: nh3-2-3 h2 + 2 [N], thus furnace has a large number of reactive nitrogen atoms, reactive nitrogen atoms [N] is absorbed by the surface of the steel, and the internal diffusion, thus forming the nitride layer.
In order to improve the hardness and wear resistance of the nitriding usually nitriding temperature is 500-520 ℃. The residence time depends on the thickness required by the nitriding layer, which is generally calculated at 0.01mm/h. Therefore, it takes about 20-60h to obtain a thickness of 0.25-0.65mm. Raising the nitriding temperature can accelerate the nitriding process, but it will make the nitride aggregate and coarsening, thus reducing the hardness of the surface layer of the part.
For nitrification to improve hardness and wear resistance, alloy steel containing Mo, A, V and other elements must be used in nitrification, such as 38CrMoAlA and 38CrMoAA. These steels are highly nitrogenous and contain various alloy nitride such as AlN, CrN, MoN, VN, etc. These nitride have high hardness and stability, and distribute evenly in the steel, so that the nitride layer of steel has high hardness and wear resistance. Cr can also improve the hardenability of steel, so that large parts can get uniform mechanical properties before nitrification. Mo can also refine grain and reduce secondary temper brittleness of steel. If ordinary carbon steel is used, pure ferric nitride will be formed in the nitrification layer. When heated to a higher temperature, it will be easy to decompose, gather and coarsen, and no high hardness and wear resistance can be obtained.
Corrosion nitriding temperatures generally between 600-700 ℃, the decomposition rate is roughly in 40-70% range, residence time by 15 minutes to 4 hours, depth is generally not more than 0.05 m m. For corrosion-resistant nitrided steel, any kind of steel can be applied, and good results can be obtained.
2. Liquid nitriding
Liquid nitrogen is a relatively new chemical heat treatment process, the temperature does not exceed 570 ℃, the processing time is short, only 1-3 h; And do not use special steel, test shows: 40Cr after liquid nitrification treatment than ordinary quenching and tempering anti-wear capacity improved by 50%; The anti - wear ability of cast iron was improved by liquid nitriding. Not only that, practice has proved that the parts treated with liquid nitrification have different degrees of improvement in fatigue and corrosion resistance. The high - speed steel cutter is treated with liquid nitriding, which can increase the service life by 20 ~ 200%. After liquid nitriding, 3Cr2W8V die casting can increase the service life by 3-5 times. The liquid nitriding surface is hard but not brittle, and has a certain toughness, not easy to spalling phenomenon.
However, liquid nitrification also has disadvantages: for example, its nitrification surface layer is relatively thin, only 0.01 -- 0.02mm. Cyanide salt is used as raw material for liquid nitriding abroad, and non-toxic raw material for liquid nitriding has been used in China. Our non-toxic liquid nitriding formula is: urea 40%, sodium carbonate, potassium chloride, 30% and 20%, 10% potassium hydroxide (mixed salt melting point is 340 ℃ or so). Although liquid nitriding has many advantages, because of the toxic reaction of salt solution, it affects the health of operators and the waste salt is not easy to deal with. Therefore, with the use of increasingly restricted.
3. Ion nitriding
Ion nitrification, also known as "glow ion nitrification", is a recently developed heat treatment process, which has the advantages of short production cycle, high surface hardness of parts, and the control of nitrification layer brittleness. As a result, in recent years domestic development is rapid, use range is very wide.
Basic principle of ion nitriding:
Ion nitriding is put the parts in vacuum ion nitriding indoor, nitriding parts of the cathode (negative) of high voltage dc power supply, electric furnace shell to the anode (the anode) of dc high voltage power supply, when the vacuum container filling the ammonia, but keep container pressure between 200-1000 pa, between cathode and anode plus 800-1000 v dc voltage, ammonia gas ionization and the gas after ionization effect, produce positive nitrogen cationic [N +] and negatively charged anion [N -], forming a plasma. Nitrogen positive ions in the plasma zone, under the high voltage electric field acceleration, quickly rushed to the cathode, the bombardment cleaning surface of parts to nitride, the kinetic energy into heat energy, also because of the nitrogen ions into a nitrogen atom, and release large amounts of heat energy and emit bright violet color, voltage when landing near the workpiece also generate heat, heat of these three parts were heated to need nitriding temperatures.
At this temperature, nitrogen ions react with the metal surface of the part, and nitrogen atoms infiltrate into the surface and diffuse into the interior, forming nitrification layers.
Characteristics of ion nitriding:
(1) rapid surface heating can shorten the heating and cooling time to one-tenth to one-twelfth. And the rest in addition to the processing surface heating at low temperature (100 ℃) condition, both saved the heating power and reduce the deformation of the parts.
(2) the diffusion process is fast. Under the action of high pressure electric field, since the movement speed of nitriding atoms is many times faster than that of gas nitriding and the infiltration speed is faster, it generally only takes 3-10 hours.
(3) the nitrided layer has good toughness, high fatigue resistance and wear resistance, and the brittle white cement phase (Fe2N) of the nitrided layer is controlled in the range of 0-0.2mm, thus eliminating the grinding of the nitrified parts.
The surface hardness is as high as HV900(HRC64), and the depth of nitrification layer can be mastered at 0.09 -- 0.87mm.
Cost analysis of various nitriding methods
1. The salt bath nitrification furnace is simple in structure, low in price, easy to master in operation process and low in nitrification cost. However, the quality of nitrification is not high.
2. The gas nitriding furnace is complicated in construction, slightly expensive, slightly difficult in operation, but the nitriding quality is good, which can achieve deep infiltration layer and high hardness, but it takes a long time, and the amount of ammonia gas is also high
3. Ion nitrification furnace production and manufacturing process require a lot, the materials used are also very particular, the electrical control technology content is very high, and the overall requirements for operators are high, but the nitrification quality is the best, the infiltration speed is fast, and the nitrification cost is lower than the gas nitrification, which is a good development trend.
Take the one-time charging capacity of 400 kg as an example: the initial investment is as follows
Salt bath nitriding furnace investment in about 20,000 yuan
The gas nitriding furnace is around 40,000 yuan
Ion nitrification is around nine hundred thousand yuan
To achieve the same nitriding layer, the cost of ion nitriding is about 60% of the cost of gas nitriding (since nitriding in salt bath is difficult to reach the nitriding layer of gas nitriding and ion nitriding, so their operating costs cannot be compared).