Nitriding treatment
Nitrided gears
Nitriding treatment refers to a chemical heat treatment process in which nitrogen atoms infiltrate the surface of the workpiece in a certain medium at a certain temperature. Nitrided products have excellent wear resistance, fatigue resistance, corrosion resistance, and high-temperature resistance.
Here we take a look at Netrex's video, Netrex explains very well what nitriding is.
Introduction to Nitriding Treatment
The aluminum, chromium, vanadium, and molybdenum elements in traditional alloy steel materials are very helpful for nitriding. When these elements come into contact with the nascent nitrogen atoms at the nitriding temperature, stable nitrides are formed.
In particular, the molybdenum element not only acts as an element for generating nitrides but also acts as a reduction in brittleness that occurs at the nitriding temperature. The elements in other alloy steels, such as nickel, copper, silicon, manganese, etc., do not contribute much to the nitriding characteristics.
Generally speaking, if the steel contains one or more nitride-forming elements, the effect after nitriding is relatively good. Among them, aluminum is the strongest nitride element, and nitriding with 0.85 to 1.5% aluminum has the best results.
As far as chromium-containing chromium steel is concerned, if there is enough content, good results can also be obtained. But there is no carbon steel containing alloy because the nitrided layer is very brittle and easy to peel off, so it is not suitable for nitriding steel.
There are six commonly used nitriding steels as follows:
(1) Low alloy steel containing aluminum (standard nitrided steel)
(2) SAE 4100, 4300, 5100, 6100, 8600, 8700, 9800 series of medium carbon low alloy steel containing chromium.
(3) Hot work 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.
Standard nitrided steel containing aluminum can obtain a high hardness and high wear resistance surface layer after nitriding, but the hardened layer is also very brittle. On the contrary, chromium-containing low-alloy steel has lower hardness, but the hardened layer is tougher, and its surface also has considerable wear resistance and beam resistance. Therefore, when selecting materials, you should pay attention to the characteristics of the materials and make full use of their advantages to meet the functions of the parts. As for tool steels such as H11 (SKD61) D2 (SKD-11), they have high surface hardness and high core strength.
Effect
Increase the wear resistance, surface hardness, fatigue limit, and corrosion resistance of steel parts.
Technical process
Surface cleaning of parts before nitriding
Most parts can be nitrided immediately after degreasing by gas degreasing. Some parts also need to be cleaned with gasoline, but if the final processing method before nitriding uses polishing, grinding, polishing, etc., it may produce a surface layer that hinders nitriding, resulting in uneven or uneven nitriding after nitriding.
Defects such as bending occurred. At this time, one of the following two methods should be used to remove the surface layer. The first method first uses gas to remove oil before nitriding. Then use alumina powder to sandblast the surface (abrasive cleaning). The second method is to apply phosphate coating to the surface.
Exhaust air of nitriding furnace
Place the processed parts in the nitriding furnace and seal the furnace cover to heat up, but before heating to 150°C, the furnace must be exhausted. The main function of the furnace is to prevent explosive gas from contacting with air when ammonia is decomposed, and to prevent oxidation of the surface of the processed object and the support.
The gas used is ammonia and nitrogen. The essentials for removing the air in the furnace are as follows:
①After the parts to be processed are installed, the furnace cover is sealed, and anhydrous ammonia gas is started, and the flow rate is as high as possible.
②Set the automatic temperature control of the heating furnace to 150°C and start heating (note that the furnace temperature cannot be higher than 150°C).
③When the air in the furnace is removed to less than 10%, or the exhaust gas contains more than 90% of NH3, then the furnace temperature is increased to the nitriding temperature.
Ammonia decomposition rate
Nitriding is carried out by contacting other alloying elements with nascent nitrogen, but the production of nascent nitrogen is that the steel itself becomes a catalyst when ammonia gas comes into contact with the heated steel to promote the decomposition of ammonia.
Although nitriding can be performed under ammonia with various decomposition rates, the decomposition rate is generally 15-30%, and the thickness required for nitriding is maintained for at least 4-10 hours, and the treatment temperature is maintained at about 520℃ .
Cool down
Most industrial nitriding furnaces have heat exchangers to quickly cool the heating furnace and the processed parts after the nitriding work is completed. That is, after nitriding is completed, the heating power is turned off to reduce the furnace temperature by about 50°C, and then the ammonia flow rate is doubled and the heat exchanger is started.
At this time, pay attention to observe whether there are bubbles overflowing in the glass bottle connected to the exhaust pipe to confirm the positive pressure in the furnace. After the ammonia gas introduced into the furnace becomes stable, the flow rate of ammonia can be reduced until the positive pressure in the furnace is maintained.
When the furnace temperature drops below 150°C, the furnace cover can be opened after introducing air or nitrogen using the method of removing the gas in the furnace as described above.
Gas nitriding
Gas nitriding was published by German AF ry in 1923. The workpiece was placed in a furnace, and NH3 gas was directly fed into the nitriding furnace at 500-550°C and kept for 20-100 hours to decompose the NH3 gas into an atomic state.
Nitriding treatment with (N) gas and (H) gas is the main purpose of producing a wear-resistant and corrosion-resistant compound layer on the surface of the steel. Its thickness is about 0.02～0.02m/m, and its nature is extremely hard Hv 1000 ~1200, and extremely brittle. The decomposition rate of NH3 varies depending on the flow rate and temperature.
The larger the flow rate, the lower the decomposition rate, the smaller the flow rate, the higher the decomposition rate, and the higher the temperature, the higher the decomposition rate. The lower the temperature, the lower the decomposition rate. NH3 gas undergoes thermal decomposition at 570℃ as follows:
NH3 →〔N〕Fe + 3/2 H2
The decomposed N then diffuses into the surface of the steel to form. Phase Fe2-3N gas nitriding, the general disadvantage is that the hardened layer is thin and the nitriding time is long.
Gas nitriding has low efficiency due to the decomposition of NH3 for nitriding, so it is generally fixed to select steels suitable for nitriding, such as containing Al, Cr, Mo and other nitriding elements, otherwise nitriding will not be possible.
Generally, JIS and SACM1 are used. The new JIS, SACM645 and SKD61 are also called quenching and tempering with strengthening and toughening treatment. Because Al, Cr, Mo, etc. are all elements that increase the transformation point temperature, the quenching temperature is higher, and the tempering temperature is also higher than that of ordinary structural alloy steels. Tempering brittleness occurs during long-time heating at the nitriding temperature, so the quenching and tempering treatment is applied in advance.
NH3 gas nitriding, because the surface is rough, hard and brittle due to the long time, it is not easy to grind, and the long time is not economical. It is used for the nitriding of the feeding tube and screw rod of the plastic injection molding machine.
Liquid nitriding
The main difference of liquid nitrocarburizing is that there is Fe3Nε phase in the nitrided layer, Fe4Nr phase exists but not Fe2Nξ phase nitride. The ξ phase compound is hard and brittle in the nitriding process, which is poor in toughness, and liquid nitrocarburizing The method is to remove rust, degrease, preheat the workpiece and place it in a nitriding crucible.
The crucible is made of TF-1 as the main salt, and it is heated to 560～600℃ for several minutes to several hours. , The depth of the nitriding layer is determined according to the size of the external load on the workpiece. During processing, an air tube must be inserted at the bottom of the crucible to decompose a certain amount of air nitriding agent into CN or CNO, which will penetrate and diffuse to the working surface, So that the outermost compound of the surface of the workpiece is 8-9%wt of N and a small amount of C and the diffusion layer.
Nitrogen atoms diffuse into the α-Fe base to make the steel more fatigue resistant. During the nitriding period, due to the decomposition and consumption of CNO, Therefore, it is necessary to continuously test the composition of the salt in 6-8 hours of treatment in order to adjust the air volume or add new salt.
The material used for liquid soft nitriding treatment is iron metal. The surface hardness after nitriding is higher if the surface hardness contains Al, Cr, Mo, Ti, and the more gold content, the shallower the nitriding depth, such as carbon steel Hv 350 ～650, stainless steel Hv 1000～1200, nitrided steel Hv 800～1100.
Liquid nitrocarburizing is suitable for wear-resistant and fatigue-resistant automobile parts, sewing machines, cameras, etc., such as cylinder liner processing, valve processing, piston barrel processing, and non-deformable molds. Countries that use liquid nitrocarburizing include Western European countries, the United States, the Soviet Union, and Japan.
Ion nitriding
This method is to place a workpiece in a nitriding furnace, vacuum the furnace to 10-2～10-3 Torr (㎜Hg) in advance, then introduce N2 gas or N2 + H2 mixed gas, and adjust the furnace to reach 1～10 Torr, connect the furnace body to the anode, the workpiece to the cathode, and apply hundreds of volts of DC voltage between the two poles.
At this time, the N2 gas in the furnace will be brightly discharged into positive ions and move to the working surface. The voltage drops sharply, causing the positive ions to rush to the surface of the cathode at a high speed, transforming kinetic energy into gas energy, so that the surface temperature of the workpiece can rise, due to the impact of nitrogen ions, the surface of the workpiece is splashed with Fe.CO and other elements to combine with nitrogen ions. FeN, as a result, iron nitride is gradually adsorbed on the workpiece to produce nitriding.
Ion nitriding basically uses nitrogen, but if hydrocarbon gas is added, it can be used for ion soft nitriding, but it is generally called ion nitrogen Chemical treatment, the nitrogen concentration on the surface of the workpiece can be adjusted by changing the partial pressure ratio of the mixed gas (N2 + H2) filled in the furnace.
When pure ion nitriding, a single-phase r′ (Fe4N) structure on the working surface contains N content At 5.7 to 6.1%wt, the thickness of the layer is within 10μm. The compound layer is strong and not porous, and it is not easy to fall off. Because the iron nitride is constantly absorbed by the workpiece and diffuses into the interior, the structure from the surface to the interior is FeN → Fe2N → Fe3N→ Fe4N changes in sequence, the single-phase ε (Fe3N) contains 5.7-11.0%wt of N, and the single-phase ξ (Fe2N) contains 11.0-11.35%wt.
The ion nitriding first generates the r phase and then adds In the case of hydrogen carbide, the compound layer and the diffusion layer that change into the epsilon phase, the increase in the diffusion layer contributes a lot to the increase in fatigue strength.The erodibility is best in ε phase.
The degree of ion nitriding treatment can start from 350°C. The treatment time can be several minutes or even a long time considering the material and its related mechanical properties. This method is the same as the previous nitriding treatment using thermal decomposition method. The method is different. Because this method uses high ion energy, materials such as stainless steel, titanium, cobalt, etc., which were considered difficult to be treated in the past, can also be easily treated with excellent surface hardening.
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