Views: 236 Author: ANEBON Publish Time: 2024-12-13 Origin: Site
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● 1. Introduction to nitriding
● 2. Application of nitriding principle
● 4. Nitriding treatment process
● 5. Preparation work before nitriding production
Nitriding: It is a chemical heat treatment process that allows nitrogen atoms to penetrate into the surface of a workpiece in a certain medium at a certain temperature. Common methods include liquid nitriding, gas nitriding, and ion nitriding. Traditional gas nitriding is to put the workpiece into a sealed container, pass it through flowing ammonia and heat it. After keeping it warm for a long time, the ammonia will thermally decompose to produce active nitrogen atoms, which will continuously adsorb onto the surface of the workpiece and diffuse into the surface of the workpiece, thereby changing the chemical composition and organization of the surface and obtaining excellent surface properties. If carbon is infiltrated at the same time during the nitriding process to promote the diffusion of nitrogen, it is called nitrocarburizing. Commonly used are gas nitriding and ion nitriding.
The nitrogen infiltrated into the steel forms iron nitride with different nitrogen contents from the surface to the inside with iron, and combines with alloy elements in the steel to form various alloy nitrides, especially aluminum nitride and chromium nitride. These nitrides have high hardness, thermal stability and high dispersion, so the nitrided steel parts can obtain high surface hardness, wear resistance, fatigue strength, seizure resistance, atmospheric and superheated steam corrosion resistance, tempering softening resistance, and reduce notch sensitivity. Compared with the carburizing process, the nitriding temperature is relatively low, so the distortion is small, but due to the low hardness of the core and the shallow nitriding layer, it can generally only meet the wear resistance and fatigue resistance requirements of light and medium loads, or machine parts with certain heat resistance and corrosion resistance requirements, as well as various cutting tools, cold and hot working molds, etc.
Common ones are gas nitriding, ion nitriding, and nitrogen and carbon co-nitriding.
Generally, the main purpose is to improve the wear resistance of the metal, so a high surface hardness is required. It is suitable for nitriding steels such as 38CrMOAI. The surface hardness of the workpiece after nitriding can reach HV850~1200. The nitriding temperature is low and the workpiece distortion is small. It can be used for parts with high precision requirements and wear resistance requirements, such as boring machine boring bars and spindles, grinding machine spindles, cylinder sleeves, etc. However, due to the thin nitriding layer, it is not suitable for heavy-load wear-resistant parts.
Gas nitriding can be carried out by general nitriding method (i.e. isothermal nitriding) or multi-stage (two-stage, three-stage) nitriding method. The former is that the ammonia nitriding temperature and ammonia decomposition rate remain unchanged during the entire nitriding process. The temperature is generally between 480~520℃, the ammonia decomposition rate is 15~30%, and the insulation time is nearly 80 hours. This process is suitable for parts with shallow nitriding layer, strict distortion requirements, and high hardness requirements, but the processing time is too long. Multi-stage ammonia nitriding is to use different temperatures, different ammonia decomposition rates, and different times for nitriding and diffusion in different stages of the entire ammonia nitriding process. The entire nitriding time can be shortened to nearly 50 hours, and a deeper nitriding layer can be obtained, but the nitriding temperature is higher and the distortion is larger.
The surface of a normal gas nitrided workpiece is silver-gray. Sometimes, it may also appear blue or yellow due to oxidation, but it generally does not affect the use.
The commonly used gas nitriding processes for nitriding process are isothermal nitriding, two-stage nitriding and three-stage nitriding.
(1) Isothermal nitriding: also known as one-stage nitriding. It is an ammonia nitriding process that is kept warm for a long time at a constant temperature. The nitriding temperature is 510~530℃, and its ammonia nitriding process curve is shown in Figure 1. The first stage is kept warm for 15~20h, which is the nitrogen absorption stage. This stage adopts a lower ammonia decomposition rate (18%~25%). The surface of the part forms a nitrogen concentration difference with the core of the part due to a large number of nitrogen atoms after washing. The second stage is the diffusion stage. In this stage, the ammonia decomposition rate is increased to 30%~40% to reduce the number of active nitrogen atoms, and the holding time is about 60h.
Figure 1 38CrMoA1A steel one-stage nitriding process
In order to reduce the brittleness of the nitrided layer, denitriding treatment is carried out 2~4h before the end of nitriding, the ammonia decomposition rate is increased to more than 70%, and the denitriding temperature is increased to 560~570℃. The isothermal nitriding process is simple, with low nitriding temperature, shallow nitriding layer, small part deformation, and high surface hardness, but the nitriding speed is slow and the production cycle is long. It is suitable for parts with shallow nitriding depth and high dimensional accuracy and hardness requirements.
(2) Two-stage nitriding: The two-stage nitriding process curve is shown in Figure 2. The process parameters of the first stage (except for the holding time) are the same as those of isothermal nitriding. In the second stage, the nitriding temperature is increased to 550~560℃ to accelerate the diffusion of nitrogen atoms and shorten the nitriding cycle. The helium decomposition rate is increased to 40%~60%. According to the requirements for the brittleness of the nitrided layer, the ammonia decomposition rate and temperature should be increased 2h before the rapid denitriding for denitriding.
Figure 2 Two-stage nitriding process of 38CrMoA1A steel
The time of two-stage nitriding is shorter than that of isothermal nitriding, the surface hardness is slightly lower, and the deformation is slightly increased. It is suitable for parts with deeper nitriding layers and larger batches.
(3) Three-stage nitriding: The process curve of three-stage nitriding is shown in Figure 3. It is developed on the basis of two-stage nitriding. This process is to appropriately increase the temperature of the second stage to accelerate the nitriding process, and at the same time add a third stage with a lower temperature to compensate for the low surface ammonia concentration due to the rapid diffusion of ammonia in the second stage, so as to ensure the surface nitrogen content and increase the surface hardness.
Figure 3 Three-stage nitriding process of 38CrMoA1A steel
Three-stage nitriding can further increase the nitriding speed, but the hardness is lower than that of general nitriding process, and the brittleness and deformation are slightly greater than those of general nitriding process.
Ion nitriding, also known as glow nitriding, is carried out based on the principle of glow discharge. The metal workpiece is placed as the cathode in a negative pressure container with a nitrogen-containing medium. After power is applied, the nitrogen and hydrogen atoms in the medium are ionized, and a plasma zone is formed between the cathode and cathode. Under the action of the strong electric field in the plasma zone, the positive ions of nitrogen and hydrogen bombard the surface of the workpiece at high speed. The high kinetic energy of the ions is converted into thermal energy, heating the surface of the workpiece to the required temperature. Due to the bombardment of ions, atomic sputtering occurs on the surface of the workpiece, which is purified. At the same time, due to adsorption and diffusion, nitrogen penetrates into the surface of the workpiece.
Compared with general gas nitriding, the characteristics of ion nitriding are: ① The nitriding cycle can be appropriately shortened; ② The nitriding layer is less brittle; ③ The consumption of energy nitrogen and hydrogen can be saved; ④ The parts that do not need nitriding can be shielded to achieve local nitriding; ⑤ Ion bombardment has the effect of purifying the surface, which can remove the passivation film on the surface of the workpiece, and can directly nitridize stainless steel and heat-resistant steel workpieces; ⑥ The thickness and organization of the nitriding layer can be controlled.
Advantages and disadvantages of ion ammonia infiltration:
Advantages: short nitriding time, easy quality control, fatigue resistance and high strength of nitrided layer. Since the nitriding temperature is 520~540℃, the workpiece has small deformation and high surface antimagnetism.
Disadvantages: complex equipment control and poor furnace temperature uniformity.
Low-temperature nitrocarburizing is also called soft nitriding, that is, below the iron-nitrogen eutectic transformation temperature, the workpiece surface is infiltrated with carbon while mainly infiltrating nitrogen. The fine carbides formed after carbon infiltration can promote the diffusion of nitrogen and accelerate the formation of high nitrogen compounds. These high nitrogen compounds can in turn increase the solubility of carbon. The mutual promotion of carbon and nitrogen atoms accelerates the infiltration rate. In addition, carbon in nitrides can also reduce brittleness. The compound layer obtained after nitrocarburizing has good toughness, high hardness, wear resistance, corrosion resistance and bite resistance.
Common nitrocarburizing methods include liquid method and gas method. The treatment temperature is 530~570℃ and the holding time is 1~3h. Early liquid salt baths used cyanide salts, and later a variety of salt bath formulas appeared. There are two commonly used ones: neutral salt with ammonia and salts mainly composed of urea and carbonate, but these reaction products are still toxic. The main gas media are: endothermic or exothermic gas (see controlled atmosphere) plus ammonia; urea thermal decomposition gas: dripping carbon and nitrogen-containing organic solvents, such as formamide, triethanolamine, etc.
Nitrocarburizing can not only improve the fatigue life, wear resistance, corrosion resistance and bite resistance of the workpiece, but also use simple equipment, low investment, easy operation, short time and small workpiece distortion, and sometimes give the workpiece a beautiful appearance.
In the entire manufacturing process of nitrided parts, nitriding is often the last step, and at most fine grinding or grinding is performed. The process flow of nitrided parts is generally: forging → normalizing (annealing) → rough machining → quenching and tempering → fine machining → stress relief → rough grinding → nitriding → fine grinding → assembly. Preheating treatment before nitriding includes normalizing (annealing), tempering treatment, and stress relief.
(1) Normalizing (annealing): Its purpose is to refine the grain, reduce hardness, and eliminate forging stress.
(2) Tempering treatment: It can improve the processing performance of steel and obtain a uniform tempered martensite structure to ensure that the core of the part has sufficient strength and toughness, while also making the nitriding layer and the base firmly bonded.
(3) Stress relief treatment: For precision parts with complex shapes, stress relief should be performed 1 to 2 times before nitriding to reduce deformation during the nitriding process.
(1) Decontamination treatment: Before loading the parts into the furnace, gasoline or alcohol should be used for degreasing and decontamination. Rust and dirt are not allowed on the surface of the parts.
(2) Anti-seepage treatment: For the non-nitriding parts of the parts, anti-nitriding treatment can be performed by electroplating or coating.
(3) The surface quality of the nitrided parts should be good, and no decarburized layer is allowed. Therefore, sufficient machining allowance should be left for parts before pre-heat treatment so that the decarburized layer can be completely removed during machining before nitriding to ensure the quality of the nitriding layer.
(4) Before loading the furnace, the inspection equipment and nitriding fixtures, electrical systems, pipelines, ammonia decomposition testers, etc. should be ensured to be in normal use; nitriding fixtures are not allowed to have dirt or oxide scale, if any, they should be removed.
(5) Furnace samples: Furnace samples should be made of the same material as nitrided parts and undergo the same pre-treatment.
