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Plunger Nitriding Process

Iron and steel parts in the active nitrogen medium, a certain temperature and holding time, so that the surface infiltration of nitrogen elements of the process, called nitriding. Although nitriding and carburizing are chemical heat treatments to strengthen the surface of parts, there are significant differences between them. Compared with carburizing, nitriding has the following advantages:

1) Nitriding surface has high hardness and high wear resistance.

2) with high residual compressive stress.

3) Nitriding process temperature is low, workpiece deformation is small.

4) Good corrosion resistance.

Because nitriding has the above characteristics, it has been widely used. The plunger described in this paper is a kind of part requiring high hardness, high wear resistance and high dimensional stability.

The parts require nitriding treatment, hardness ≥700HV, nitriding layer depth 0.3~0.6mm. In the production process, the depth of nitriding layer is set at 0.4~0.6mm. Then, how to formulate process parameters to ensure that the drawing requirements, is particularly important.

Nitriding process:

According to the parts material, the nitriding process is preliminarily determined to select the two-stage gas nitriding method: That is, the parts slowly rise with the furnace, keep warm at 200℃ for 2h, then rise to 525℃, keep warm for 18~25h, slowly rise to 540℃, keep warm for 22~30h, then slowly rise with the furnace to 560℃, keep warm for 3~4h for nitrogen removal treatment, and finally cool with the furnace to < 200℃ for air cooling. When the furnace temperature drops to 200℃, close the intake valve of ammonia and open the intake valve of nitrogen. According to the technical requirements of parts and production practice, the equipment used is SIMUWU gas nitriding furnace is vertical structure, with large load capacity, simple structure and easy to operate. It is mainly used for nitration heat treatment of die steel, carbon steel, alloy steel and powder metallurgy materials.

(1) Heating temperature

The heating temperature is respectively set as 525℃, 540℃, 560℃, the reason is to determine the nitriding temperature, should comprehensively consider the impact of temperature on the surface hardness of nitriding layer, depth of nitriding layer and deformation, but also to consider does not affect the mechanical properties of the core parts, nitriding temperature must be lower than tempering temperature. If the nitriding temperature is too low, the nitriding speed is slow, in order to reach a certain depth of nitriding, it is necessary to extend the holding time, at the same time, the surface of the parts can not absorb enough active nitrogen atoms, hardness is not high, so the nitriding temperature is selected in 500~560℃.

(2) Heating method

Choose heating with furnace temperature, heating medium choose nitrogen.

(3) Holding time

The holding time is 18~25h, 22~30h and 3~4h, respectively. The reason is that when the nitriding temperature is fixed, the determination of the holding time mainly depends on the depth of the nitriding layer required. With the prolongation of nitriding time, the depth of nitriding layer increases continuously and shows a parabolic law, that is, it starts to increase quickly, and with the prolongation of time, the depth of nitriding layer increases more and more slowly. Too long nitriding time has no obvious effect on improving the depth of nitriding layer. Therefore, at a lower nitriding temperature, it is impossible to get a deeper nitriding layer, only by increasing the nitriding temperature, can get a deeper nitriding layer. Nitriding heat preservation time is a process parameter affected by many factors, which can be determined correctly only through production practice. 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%.

(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. It should be inspected 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.

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 found out. 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.