Nitriding treatment refers to a chemical heat treatment process in which nitrogen atoms penetrate into the surface of the workpiece in a certain medium at a certain temperature. The nitrided products have excellent wear resistance, fatigue resistance, corrosion resistance and high temperature resistance.
Introduction to nitriding treatment
The elements of aluminum, chromium, vanadium and molybdenum in traditional alloy steel are very helpful for nitriding. These elements form stable nitrides when they come into contact with primitive nitrogen atoms at nitriding temperatures. Molybdenum, in particular, acts not only as a nitride forming element, but also as a reduction in brittleness that occurs at nitriding temperatures. The elements in other alloy steels, such as nickel, copper, silicon, manganese, etc., do not contribute much to the nitriding characteristics. In general, if the steel contains one or more nitride generating elements, the effect after nitriding is better. Among them, aluminum is the strongest nitride element, and the nitriding results containing 0.85 ~ 1.5% aluminum are the best. In the case of chromium containing chromium steel, if there is sufficient content, good results can also be obtained. However, carbon steel without alloy is not suitable for nitriding steel because the nitriding layer generated by it is very brittle and easy to peel off.
There are six types of nitriding steel commonly used as follows:
(1) Low alloy steel containing aluminum (standard nitriding steel)
(2) Medium carbon low alloy steel containing chromium element SAE 4100,4300,5100,6100,8600,8700,9800 series.
(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
The standard nitriding steel containing aluminum can obtain a high hardness and high wear-resistant surface after nitriding, but its hardened layer is also very brittle. On the contrary, low alloy steel containing chromium has a lower hardness, but the hardened layer is relatively ductile, and its surface also has considerable wear resistance and beam resistance. Therefore, when selecting materials, it is appropriate to pay attention to the characteristics of materials and make full use of their advantages to meet the function of parts. Tool steels such as H11(SKD61) and D2(SKD-11) have high surface hardness and high core strength effects.
Increase the wear resistance, surface hardness, fatigue limit, and corrosion resistance of steel parts.
Technical process
• Part surface cleaning before nitriding
Most parts can be nitrided immediately after gas degreasing. Some parts also need to be cleaned with gasoline, but if the final processing method before nitriding is used in polishing, grinding, polishing, etc., it may produce surface layers that hinder nitriding, resulting in uneven nitriding layers or bending defects after nitriding. At this time, one of the following two methods should be used to remove the surface layer. The first method is to remove oil with gas before nitriding. The surface is then abrasive cleaned with abrasive oxide powder. The second method is to treat the surface with a phosphate coating.
• Air removal from nitriding furnace
The treated parts are placed in the nitriding furnace and the furnace cover can be heated after sealing, but the air must be removed from the furnace before heating to 150 ° C.
The main function of the exhaust furnace is to prevent the explosive gas caused by the decomposition of ammonia gas and the contact with the air, and to prevent the surface oxidation of the treated material and the support. The gases used are ammonia and nitrogen.
The rules for removing air from the furnace are as follows:
① After the processed parts are installed, the furnace cover is sealed, and the anhydrous ammonia gas is started, and the flow rate is as much as possible.
② Set the automatic temperature control of the heating furnace at 150℃ and start heating (note that the furnace temperature cannot be higher than 150℃).
③ When the air in the furnace is removed to less than 10%, or the discharged gas contains more than 90% NH3, the furnace temperature is raised to the nitriding temperature.
Decomposition rate of ammonia
Nitriding is carried out in contact with other alloying elements and the primary nitrogen, but the production of the primary nitrogen, that is, the steel itself becomes the catalyst when the ammonia is in contact with the heating steel and promotes the decomposition of ammonia.
Although nitriding can be performed under various decomposition rates of ammonia, a decomposition rate of 15 to 30% is generally used, the required thickness of nitriding is maintained for at least 4 to 10 hours, and the treatment temperature is maintained at about 520 ° C.
cooling
Most industrial nitriding furnaces have heat switches to rapidly cool the furnace and process parts after nitriding is completed. That is, after nitriding is completed, the heating power supply is turned off, the furnace temperature is reduced by about 50 ° C, and then the ammonia flow rate is doubled and the heat switch is opened. At this time, it is necessary to observe whether there is a bubble overflow in the glass bottle connected to the exhaust pipe to confirm the positive pressure in the furnace. After the ammonia gas is settled in the furnace, the ammonia flow can be reduced until the positive pressure in the furnace is maintained. When the furnace temperature drops below 150 ° C, the furnace gas removal method described above is used, and the furnace cover can be opened after the air or nitrogen is introduced.
NH3 → [N] Fe + 3/2 H2
The decomposed N is diffused into the surface of the steel to form. Phase Fe2-3N gas nitriding, the general disadvantage of thin hardening layer and long nitriding treatment time.
Gas nitriding due to the decomposition of NH3 for nitriding efficiency is low, so generally a fixed selection of steel suitable for nitriding, such as Al, Cr, Mo, and other nitriding elements, otherwise nitriding can not be carried out, using JIS, SACM1 new JIS, SACM645 and SKD61 to strengthen and toughen treatment, also known as tempering Al, Cr, SKD61. Mo and other elements increase the temperature of the transformation point, so the quenching temperature is high, and the tempering temperature is higher than that of ordinary structural alloy steel, which is tempered brittleness between the nitriding temperature for a long time, so the tempering and toughening treatment is applied in advance. NH3 gas nitriding, because the surface is rough for a long time, hard and brittle is not easy to grind, and a long time is not economical, is used for the nitriding of the feed tube and screw of the plastic injection molding machine.
Liquid nitriding
The main difference of liquid soft nitriding is that there is a Fe3Nε phase in the nitriding layer, the Fe4Nr phase exists and does not contain Fe2Nξ phase nitride, and ξ phase compounds are hard and brittle nitrides that are poor in toughness in nitriding treatment. The method of liquid soft nitriding is to treat the workpiece, first remove rust, degreasing, preheating, and then place it in the nitriding crucible, which is the main salt agent TF-1. It is heated to 560 ~ 600℃ for a few minutes to a few hours, according to the external load of the workpiece, and determine the depth of the nitriding layer, in the treatment, an air tube must be passed into the bottom of the crucible to decompose into CN or CNO with a certain amount of air nitriding salt agent, permeating and diffusing to the working surface. The outermost compound on the surface of the workpiece is 8 ~ 9%wt N and a small amount of C and 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 of CNO consumption, so it is constantly tested in 6 to 8 hours of salt composition, to adjust the amount of air or add new salt.
The material used for liquid soft nitriding treatment is iron metal, and the surface hardness after nitriding is higher with Al, Cr, Mo, and Ti elements, and the more gold content, the shallower the nitriding depth, such as carbon steel Hv 350 ~ 650, stainless steel Hv 1000 ~ 1200, nitriding steel Hv 800 ~ 1100.
Liquid soft nitriding is suitable for wear-resistant and fatigue-resistant automotive parts, sewing machines, cameras, etc., such as cylinder liner treatment, valve treatment, piston cylinder treatment, and mold that is not easy to deform.
Ion nitriding
This method is to place a workpiece in the nitridation furnace, pump a vacuum in the furnace to 10-2 ~ 10-3 Torr(㎜Hg) in advance, and then introduce N2 gas or N2 + H2 mixture, adjust the furnace to 1-10 Torr, connect the furnace body to the anode, the workpiece to the cathode, and pass hundreds of volts DC voltage between the two poles. At this time, the N2 gas in the furnace will produce a brilliant discharge into positive ions, moving to the working surface, and the cathode voltage will drop sharply in an instant, so that the positive ions will rush to the cathode surface at a high speed, transforming kinetic energy into gas energy, making the surface temperature of the workpiece rise, and the workpiece surface will play Fe.C.O after the impact of nitrogen ions. Such elements splash out and combine with nitrogen ions into FeN, thus iron nitride is gradually adsorbed on the workpiece and produces nitriding, ion nitriding is basically the use of nitrogen, but if the addition of hydrogen carbide gas can be used for ion soft nitriding treatment, but generally referred to as ion nitriding 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. During the process of pure ion nitriding, the single-phase r '(Fe4N) tissue containing N content is 5.7 ~ 6.1%wt on the working surface, and the thick layer is less than 10μm. The compound layer is strong but not porous and is not easy to fall off. As iron nitride is continuously adsorbed by the workpiece and diffused to the interior, the structure from the surface to the interior is FeN → Fe2N → Fe3N→ Fe4N sequence change, single-phase ε(Fe3N) N content in 5.7 ~ 11.0%wt, single-phase ξ(Fe2N) N content in 11.0 ~ 11.35%wt. When ion nitridation first generates the r phase and then the hydrogen carbide series is added, it becomes the ε-phase compound layer and diffusion layer, because the increase of the diffusion layer has a lot of help to increase the fatigue strength. The ε phase is the best.
