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- Preface
The process of infiltrating the surface of steel parts with nitrogen elements in an active nitrogen medium at a certain temperature and holding time is called nitriding. Although nitriding and carburizing are both chemical heat treatments to strengthen the surface of parts, they have significant differences. Compared with carburizing, nitriding has the following advantages: Professional high precision CNC haas
1) The nitrided surface has high hardness and high wear resistance.
2) Has high residual compressive stress.
3) The nitriding process temperature is low and the workpiece deformation is small.
4) Has good corrosion resistance.
Because nitriding has the above characteristics, it has been widely used. The plunger discussed in this article (see Figure 1) is a part that requires high hardness, high wear resistance and high dimensional stability.
The plunger is a key component in a certain mechanism. During the processing of the product, the choice of the heat treatment process scheme directly affects the heat treatment quality of the product, and the quality of the heat treatment also affects the stability of the mechanism’s operating conditions, which will eventually affect the operating system. service life. Professional high precision CNC haas
- Asking questions
The material of the plunger parts is 38CrMoAlA steel, and its chemical composition is shown in Table 1.
This part requires nitriding treatment, the hardness is ≥700HV, and the depth of the nitriding layer is 0.3~0.6mm. In the production process, the depth of the nitriding layer is set at 0.4~0.6mm. Then, how to formulate process parameters to ensure that the drawing requirements are met is particularly important.
- Heat treatment plan
According to the material of the parts, the nitriding process is preliminarily determined to select the two-stage gas nitriding method: that is, the parts are slowly heated up with the furnace, kept at 200 °C for 2 hours, then heated to 525 °C, kept at 18~25 hours, slowly heated to 540 °C, and kept at a temperature of 22~ After 30h, the temperature was slowly raised to 560°C with the furnace, kept for 3~4h for denitrification treatment, and finally cooled to <200°C with the furnace for air cooling. When the furnace temperature drops to 200°C, close the inlet valve of ammonia gas and open the inlet valve of nitrogen gas. According to the technical requirements of the parts and the actual production, the equipment used is RN-75-6 gas nitriding furnace. Professional high precision CNC haas
(1) Heating temperature
The heating temperature is set to 525°C, 540°C, and 560°C respectively. The reason is that when determining the nitriding temperature, the influence of temperature on the surface hardness, depth and deformation of the nitriding layer should be comprehensively considered. For the mechanical properties of the core, the nitriding temperature must be lower than the quenching and tempering temperature. If the nitriding temperature is too low and the nitriding speed is slow, in order to achieve a certain nitriding depth, the holding time needs to be extended, and at the same time, the surface of the part cannot absorb enough active nitrogen atoms, and the hardness is not high, so the nitriding temperature is selected at 500~560 °C.
(2) Heating method
Choose to heat with the furnace temperature, and choose nitrogen as the heating medium. Professional high precision CNC haas
(3) Holding time
The holding time is selected as 18~25h, 22~30h, and 3~4h respectively. The reason is that when the nitriding temperature is constant, the determination of the nitriding holding time mainly depends on the required depth of the nitriding layer. With the prolongation of nitriding time, the depth of the nitriding layer increases continuously and changes in a parabolic law, that is, it starts to increase rapidly, and as the time prolongs, the depth of the nitriding layer increases more and more slowly, and the excessively long nitriding The effect of time on increasing the depth of the nitriding layer is not obvious. Therefore, at a lower nitriding temperature, it is impossible to obtain a deeper nitriding layer. Only by increasing the nitriding temperature can a deeper nitriding layer be obtained. The nitriding holding time is a process parameter affected by many factors. Generally, it is necessary to explore the production practice before it can be correctly determined. According to experience, when the depth of nitriding layer is less than 0.4mm, the average nitriding speed is 0.015~0.02mm/h; when the depth of nitriding layer is 0.4~0.7mm, the average nitriding speed is 0.005~0.015mm/h; The deeper the infiltration layer, the slower the infiltration rate.
(4) Ammonia decomposition rate
The ammonia decomposition rate is selected as 18%~35%, 45%~65%, and ≥75%, respectively. The reason is that the ammonia decomposition rate directly affects the speed of nitrogen absorption on the surface of the parts. The decomposition rate is too high, the volume of nitrogen and hydrogen in the furnace gas is large, and the adsorption of a large amount of hydrogen on the surface of the parts will hinder the absorption of nitrogen on the surface of the parts, reduce the surface nitrogen concentration, and reduce the hardness and depth of the nitriding layer; if decomposed If the rate is too low, a large amount of ammonia cannot be decomposed in time, and too few active nitrogen atoms are provided, which will also reduce the nitriding speed. Therefore, in order to make the hardness gradient of the nitriding layer as gentle as possible, the depth of the nitriding layer should be increased, and a lower ammonia decomposition rate (18%~35%) should be used in the early stage of nitriding. At this time, the surface of the part quickly absorbs a large number of nitrogen atoms, which increases the nitrogen concentration on the nitriding surface of the part, thereby forming a nitride with a large dispersion to improve the hardness of the surface of the part; the second stage of nitriding can improve the decomposition of ammonia. The nitrogen concentration in the surface layer will not be further increased, and the nitrogen atoms will continue to diffuse into the inner layer, increasing the thickness of the nitriding layer. In order to reduce the brittleness of the nitriding layer, before the end of nitriding, the furnace temperature is raised to 560~570℃, and the temperature is kept for 3~4 hours for denitrification treatment, so that nitrogen atoms can continue to diffuse into the inner layer to reduce the nitrogen concentration on the surface. At this time, the ammonia decomposition rate can be controlled above 75%. The suitable ranges of nitriding temperature and ammonia decomposition rate are shown in Table 2. Professional high precision CNC haas
(5) Cooling method
Choose furnace cooling to <200℃ and air cooling. During the cooling stage of the furnace, it is still necessary to continue to supply ammonia gas and maintain a certain positive pressure in the nitriding furnace to prevent air from entering and oxidizing the surface of the parts.
(6) Cooling medium
Choose nitrogen, air.
(7) Inspection method after heat treatment
The inspection items are mainly the depth of nitriding layer, compound layer and diffusion layer. The inspection methods are divided into metallographic method, hardness method and fracture method. We chose the metallographic method, and carried out the inspection according to the provisions of GB/T 11354-2005 “Determination of the depth of nitriding layer of iron and steel parts and inspection of metallographic structure”.
Test of the hardness of the nitriding layer: Due to the thin and high hardness of the nitriding layer, the Vickers hardness tester is generally used to test the surface hardness.
Brittleness test of nitriding layer: divided into 5 grades according to the degree of cracking of edges and corners of Vickers hardness indentation. Inspection should be carried out on the surface of the sample with the furnace, and the inspection should be carried out according to the provisions of GB/T 11354-2005 “Determination of the depth of nitriding layer of iron and steel parts and metallographic structure inspection” for important parts ≤ grade 3. Professional high precision CNC haas
- Process curve
Through the actual production, the nitriding process parameters were re-adjusted, and a new nitriding process was formulated, as shown in Figure 2. The production is carried out according to the set process parameters, and the various indicators of the parts can meet the requirements of the drawing.
In operation, you also need to pay attention to:
1) Parts that do not need nitriding should be protected, and tin plating on the surface is a good protective measure. Tin does not dissolve nitrogen atoms, and the melting point of tin is low (232°C). At the nitriding temperature, although the tin layer will melt, tin can form a thin protective film on the surface of the part by the effect of surface tension, which is adsorbed on the part. On the surface, the tin-plated layer can prevent the infiltration of nitrogen atoms. It has been proved by actual production that the tin plating layer should be controlled at 0.004~0.008mm. If the tin-plated layer is less than 0.004mm, the protective layer will not work; if the tin-plated layer is greater than 0.008mm, the tin will easily flow to the surface of the parts to be nitrided, resulting in soft spots on the nitrided surface of the parts.
2) Before the parts are installed in the furnace, the surface should be carefully cleaned (wiping with gasoline or alcohol) to ensure that the surface of the parts is free from oxide scale, grease and other dirt, so as not to affect the nitriding quality; the nitriding process must strictly follow the safety rules. Special attention: Do not ignite before the air in the furnace is exhausted after nitrogen is passed, otherwise it will easily cause an explosion. Professional high precision CNC haas
3) Select a reasonable fixture according to the shape of the part. The galvanized iron wire of the tooling and binding parts must be free of rust and oil. The zinc layer of the galvanized iron wire should be removed by pickling or burning at 800℃. When binding the parts, leave enough space between the parts to ensure the smooth atmosphere in the furnace.
- Effect of heat treatment process
Through the analysis of nitriding temperature, holding time and ammonia decomposition rate, the nitriding process parameters were determined and applied to actual production, and the optimum parameters of nitriding process were explored. The product samples have undergone metallographic inspection, and the nitriding structure level fully meets the requirements of GB/T 11354-2005 “Determination of Nitriding Layer Depth of Iron and Steel Parts and Metallographic Structure Inspection”, which can reach the second level, the brittleness test level 1, and the parts The nitriding hardness is controlled at 780~840HV, and the depth of the nitriding layer is 0.45mm, which fully meets the technical requirements in the drawing, as shown in Figures 3 and 5.
- Conclusion
In the nitriding production process, after the heat treatment technicians formulate the process, they also need to go deep into the production site for real-time monitoring. At the same time, operators are required to strictly control the nitriding temperature, ammonia decomposition rate and other process parameters of the equipment, and adjust various process parameters in a timely manner according to the actual production situation to ensure the normal operation of the nitriding process. Only by mastering the actual production situation, the heat treatment technician can compile a reasonable and practical nitriding process specification. Professional high precision CNC haas
