31CrMoV9 ring gear ultra-deep deep gas nitriding process

Nitriding is one of the commonly used final heat treatment processes for the inner ring gear and other high-speed heavy-duty wear-resistant parts of the core components of wind power transmission boxes. It has high surface hardness and wear resistance, high fatigue strength and high resistance after nitriding. Corrosive, at the same time also has the ability to resist seizure and scratch, and the workpiece distortion is relatively small. At present, the European brand steel 31CrMoV9 is widely used as a ring gear material in foreign countries. This material has good hardenability, high hardness after surface nitriding, and excellent wear resistance. The ring gear in domestic wind power speed increasing box is also increasing. Use this material.
Due to the high content of 31CrMoV9 alloy (see Table 1 for chemical composition), especially the affinity of vanadium and nitrogen is strong. In addition, the inner ring gear of wind power transmission box has a depth of more than 0.6mm. The technical requirements of the internal ring gear nitriding are shown in Table 2). The depth of the layer has broken through the depth of the traditional nitriding layer. The nitriding heating and holding time is long, and the network nitride is easily formed during nitriding, which may cause the brittleness of the permeable layer. A series of quality defects such as increase, wear resistance and fatigue strength, these quality defects have fatal consequences for wind power transmission boxes.
Table 1 Chemical composition (mass fraction) of 31CrMoV9 (%)

Table 2 Process requirements for ring ring nitriding in 31CrMoV9 wind power speed increase box

Note: The above items are all in accordance with GB/T 11354-2005 steel parts nitriding layer depth measurement and metallographic examination.
Table 3 31CrMoV9 nitriding process test plan

Note: KN is the nitrogen potential value, and the nitrogen potential value is a parameter for characterizing the nitriding ability of the atmosphere, and has a corresponding relationship with the commonly used ammonia gas decomposition rate.
At present, there are few studies on 31CrMoV9 ultra-deep deep gas nitriding process reported at home and abroad. In this paper, the 31CrMoV9 ring gear ultra-deep deep gas nitriding process was studied and the 31CrMoV9 ultra-deep deep gas nitriding was developed. Craft.
1. Process test plan
The test equipment is Aichelin controlled atmosphere nitriding furnace. Three sets of samples are used for nitriding heat treatment. The sample size is 10mm×10mm×10mm square sample, and the surface roughness of the sample is strictly controlled. The sample is the same furnace material. And the workpiece body is heightened and tempered, and then removed. The tempering process is 870 °C for 4h quenching, 620 °C tempering for 4h, the sample hardness is 310-320HBW, and the structure is uniform tempering cable. Clan. The gas nitriding process adopts three process schemes. The two schemes are three-stage method, which consists of three sections of strong seepage, diffusion and denitrification. The other process scheme is two-stage method, which consists of only two sections of strong seepage and diffusion. See Table 3.
Table 4 Test results of 31CrMoV9 nitriding process test plan

2. Test results
Each process plan is tested three times. According to the "GB/T11354-2005 steel parts nitriding layer depth measurement and metallographic examination", the test items are deep nitride layer, nitride, white bright layer depth, loose and brittleness. The test results of the three process schemes are shown in Table 4. Nitride, porosity and map comparison are shown in Figure 1 and Figure 2.

Figure 1 Metallographic organization of each process plan (500×)

Figure 2 Brittleness comparison of each process plan (100×)
3. Analysis and discussion
The medium of gas nitriding is ammonia (NH3). The result of gas nitriding is mainly determined by the behavior of ammonia in the furnace, and the behavior of ammonia in the furnace is mainly caused by nitriding temperature, nitriding time and ammonia decomposition rate ( The influence of furnace gas nitrogen potential). Therefore, the 31CrMoV9 nitriding process test program mainly adjusts the nitriding temperature, nitriding time and furnace gas nitrogen potential, in order to develop a high-quality and high-efficiency nitriding process.
The nitriding effective time of process scheme 1 is 104h, which is a three-stage nitriding method. The first section is nitriding at 515 °C, so that the surface forms a diffuse nitride and a high ammonia concentration, and the second section rises to 525 °C. And reduce the nitrogen potential, so that the nitrogen atom diffuses rapidly inward, the third stage temperature is still 525 ° C, further reduce the nitrogen potential, and carry out nitrogen withdrawal. From the test results, the layer depth is deeper, and a coarser nitride similar to veins is formed, and the white bright layer is thicker, the brittleness level is also 2, and fine cracks appear in the specimen indentation.
Process scheme 2 nitriding effective time is 200h, also three-stage nitriding method, the nitriding temperature is reduced, the first period is shortened, the third period is increased, the nitride strip is thin, almost no white bright layer and loose phenomenon, The brittleness grade is grade 1, and the sample indentation has no cracks and sag.
Sample process scheme 3 is a two-stage nitriding method, first nitriding at 505 °C, the nitrogen potential is between process plan 1 and process plan 2, the second stage is raised to 510 ° C, and the nitrogen potential is also between process plan 1 and Between process plan 2.
The content of 31CrMoV9 alloy is relatively high, especially the affinity of vanadium and nitrogen is strong, and it is easy to form nitride. The three process schemes all have vein nitride. The process scheme 1 has large diffusion and diffusion, and the diffusion time is too short. There are more vein-like nitrides in the diffusion layer; in process scheme 2 and process scheme 3, due to the lower nitrogen potential, the strong expansion is relatively small, and only a small amount of vein nitride exists in the diffusion layer.
Among the three process schemes, the surface hardness of the sample of process scheme 1 is the highest, and the surface hardness of process scheme 2 and process scheme 3 is similar, but the nitriding temperature of process scheme 1 is the highest, being 515-525 ° C, mainly due to the process scheme. 1 The nitrogen potential is the highest, and the nitride formed on the surface is the most, and the surface hardness of the 31CrMoV9 nitride is not only related to the nitriding temperature, but also related to the quantity and state of the alloy nitride.
The surface white bright layer produced by the nitriding of 31CrMoV9 is one of the main problems of gas nitriding. It is generally considered that the white bright layer is ε+γ, and the content of nitride on the surface must be controlled. Process scheme 2 obtained the least white bright layer, the main reason is that the nitriding time is long enough, and the nitrogen potential is low enough to make the nitrogen concentration inside and outside the sample uniform. Process Scheme 3 due to the relatively low nitrogen potential and long-term diffusion, the white bright layer formed on the surface is also shallow. In process scheme 1, the nitrogen potential is high, the strong osmosis time is long, and the temperature is high. The concentration of nitrogen inside and outside the sample is not balanced, and it is easier to form a thick white bright layer and a poor looseness level. In addition, when the white bright layer appears to be loose, its brittleness will also increase, and the brittleness of the process schemes 2 and 3 is significantly better than that of the process 1.
4. Conclusion
(1) 31CrMoV9 ultra-deep deep nitriding can achieve better nitriding effect by adjusting the strong osmosis and diffusion nitrogen potential and time.
(2) 31CrMoV9 ultra-deep deep nitriding in a certain temperature range, its surface hardness is related to the nitrogen nitride potential, but can obtain a higher surface hardness (about 800 HV).
(3) Process scheme 2 can obtain good quality and has no white bright layer, but it takes a long time and has poor economy. Process Scheme 3 achieves better quality and is less time consuming than Process 2, taking into account quality and efficiency. At present, our company has produced hundreds of sets of 31CrMoV9 ring gear according to process plan 3, with reliable quality and stable process.
About the author: Ma Liang, Xu Aifei, Ding Sheng, Gu Xiaoming, Huang Xing, Changzhou Tianshan Heavy Industry Machinery Co., Ltd., R&D Department of Thermal Processing Technology Room.

Camlock Type DP

Camlock Type Dp,Camlock Fitting Type Dp,Plastic Camlock Fittings Type Dp,Pp Camlock Quick Coupling Type Dp

Cixi Chengtuo Hardware Parts Factory , https://www.ibccoupling.com