Development of deep ion nitriding process and equipment
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China is one of the countries where the research and application of ion nitriding technology is very early. The gear nitriding process has also been successfully applied in China. The industrial fields of application include steel, chemical, machine tools, automobiles and energy, especially at some high speeds. , heavy duty, precision gear applications. In recent years, due to its small size, small mass, compact structure and high transmission efficiency, planetary transmissions are being used more and more, including internal ring gears, and yaw teeth in wind power of new clean energy industry. Nitriding heat treatment is required for the ring and the like. Other processing methods are difficult to meet the requirements of the inner ring gear for deformation. The nitriding process is generally preferred for surface hardening of worms. In particular, the deep nitriding process of the gear can replace the carburizing and quenching process in a certain range, thereby eliminating the process of grinding the teeth, saving manufacturing costs and construction period. With the development of the economy, the industrial application of the gear nitriding process has great potential for development.
The early nitriding process is limited in practical applications due to shallow infiltration layer, low hardness of the core, and low bearing capacity of the tooth surface. Especially due to the influence of the concept of "shell effect", the application of nitriding in gears is relatively conservative. For example, the contact fatigue limit and bending fatigue limit of nitriding gears in product design are determined by a foreign institution. 70% and 75%. Therefore, how to further improve the bearing capacity of the nitriding gear has become the direction of the relevant engineering and technical personnel.
Through a large number of experimental studies, the key to determine the bearing capacity of the hardened gear is the ratio of the orthogonal shear stress to the microhardness of the transition zone of the hardened layer. The size depends on the load, the radius of curvature of the tooth surface, and the hardened layer. Depth, core hardness and residual stress in the transition zone. In order to prevent contact fatigue failure of the gear, the relevant conditions must be met from the tooth surface to the core. Therefore, increasing the effective hardened layer depth and improving the distribution of the hardness gradient of the transition zone can effectively increase the fatigue strength of the gear. The contact fatigue test in the literature shows that under the test conditions, the hardness of the core is increased from 240-260 HBW to 310-330 HBW, which can increase the contact fatigue strength by about 30%; the thickness of the layer increases from 0.5 mm to 0.8-1.0 mm, which can be improved. The contact fatigue strength is about 25%; the compound layer with the γ' phase as the main surface can increase the contact fatigue strength by nearly 40% compared with the ε+γ' dual phase layer. Therefore, the use of medium hard tempering + tough deep nitriding is an important way to improve the bearing capacity of nitriding gears.
In the past ten years, with the development of related technologies, ion nitriding equipment has been greatly developed, new technologies and components are constantly being adopted, and the furnace body tends to be low in energy consumption and multifunctional. The marketing application of pulsed power supply has also achieved great results, and the level of automation control has been continuously improved, so that the implementation of the gear nitriding process has a solid equipment guarantee. This paper discusses the technical points of the deep ion nitriding process, and introduces the characteristics and application effects of the device through an example of equipment development.
1. Selection and process of deep controlled ion nitriding
The deep nitriding process is adopted on the gears, and both domestic and foreign have been explored and studied. Examples of successful early use are: Philadelphia Gear Company of the United States, which conducts gas nitriding up to 1 mm in depth, with a process cycle of 150 h, and then removes the white bright layer on the surface; the Zhengzhou Machinery Research Institute of China has carried out research on the deep ion nitriding process. Only 60 ~ 70h can make the nitriding layer depth of 0.8 ~ 1.2mm, the surface obtained γ' single-phase structure, no need to wear off the white bright layer processing, and obtained the national patent. Domestic scholars have carried out in-depth and extensive research on the process, organization and mechanism of deep rapid nitriding, and carried out cyclic temperature-changing ion nitriding, thermal cycle ion nitriding, pressurized nitriding, low-pressure pulse nitriding, and rare earth osmosis. Process tests such as deformation-induced osmosis and high-temperature nitriding, together with research and test on the bearing capacity and mechanical properties of nitriding gears, promoted the application of deep nitriding process on gears.
1. Materials and pretreatment
The selection of suitable materials is a prerequisite for deep controlled ion nitriding. The basic requirements for deep controlled ion nitriding steel are:
(1) It has high hardenability and ensures that the gear of a certain section size can be hardened.
(2) It has high tempering stability and ensures that the hardness of the core after nitriding for a long time does not decrease.
(3) The nitriding speed is fast, and it is not easy to produce defective tissue.
(4) The matrix has good toughness and hardness distribution.
(5) Low impurity content and few defects.
It is generally required that the raw materials contain alloying elements which are liable to form nitrides, such as: Cr, Mo, V, Al, etc. In the domestically used steel materials, the following materials are screened. The strengthening level of the base material of the nitriding gear and the thickness of the hardened layer are all key factors related to the bearing capacity and service life of the gear. The deep nitriding of the gear should be comparable to the bearing level of carburizing and quenching. The gear blank must be pre-hardened and quenched to obtain uniform and fine tempered sorbite structure.
Due to the high sensitivity of 38CrMoAl material, and the tendency of nitriding defects and nitriding rate, it is not recommended as a deep nitriding material for gears.
Among them, 25Cr2MoV is the recommended gear nitriding material with good performance. After pretreatment, the conventional mechanical properties are tested and compared with the quenched and tempered state of carburized steel 20CrMnMo and 20Cr2Ni4. All indexes meet or exceed the carburizing steel. Quenched and tempered state level.
2. Control of nitriding layer structure
The microstructure of the surface of the nitrided layer directly affects the brittleness of the tooth surface and the mechanical properties such as the fatigue strength of the gear. One of the main advantages of the ion nitriding process is that different phase components can be obtained by controlling the ratio of nitrogen to hydrogen in the furnace atmosphere. According to a document, under the experimental conditions, when ion nitriding is carried out in a medium atmosphere of wNi=8% to 12%, a compound layer structure of γ' single phase can be obtained, and when the nitrogen content is 5%, no Pure diffusion layer structure of the compound layer. For the 42CrMo and 40CrNiMo materials with 0.4% wC≈, a certain amount of ε phase is formed due to the high carbon content, and the single-phase structure of complete γ′ is not easily obtained.
3. Deep nitriding layer acquisition
The thickness of the conventional nitrided layer is generally below 0.6 mm, and deep nitriding of 0.8 to 1.2 mm is carried out, which tends to greatly prolong the treatment time. In order to make the deep nitriding process easy to be applied in industry, it is necessary to ensure that the process cycle is not too long, and an effective measure is to increase the nitriding temperature, but this is constrained by the hardening effect of the medium hard matrix and the good nitriding layer. The three-stage process is processed.
The first stage of strong infiltration: the purpose of this stage of treatment is to make the outer surface of the gear rich in higher nitrogen concentration in the shortest possible time to obtain a larger gradient of nitrogen concentration for later diffusion at higher temperatures. A higher barrier increases the diffusion rate of nitrogen. The temperature is generally 520 ~ 530 ° C, time 12 ~ 15h, the surface nitrogen concentration can be saturated, the time is extended, the significance is not significant.
The second stage of expansion: on the basis of the first stage, it is necessary to strengthen the diffusion of nitrogen atoms inside the steel, it is necessary to appropriately increase the treatment temperature to increase the diffusion coefficient of nitrogen, and obtain a hard layer with a deeper and gentle hardness gradient. The increase of temperature can not affect the strength of the medium-hard matrix, but also take into account the side effects of temperature increase on the gear deformation, generally take 570 ~ 580 ° C, the expansion time can be about 40h.
The third stage of percolation: after the expansion, the microhardness decreased in different extents within the depth of 0.3mm. This is because the diffusion of nitrogen at a certain depth is reduced by prolonged diffusion at a higher temperature, and the nitride has a certain degree of aggregation and the coherent relationship with the parent phase is destroyed, and the elastic distortion stress field acts. Weakened, the obstacle to the movement of dislocations is reduced. For this reason, the same process as the first stage of strong infiltration is used to make the seepage to improve the hardening effect of the layer.
Microhardness gradient after completion of each stage in the three-stage nitriding process. The depth of the layer can reach 0.8-1.2mm, the surface hardness can reach 550-570HV, and the surface layer of the compound layer with γ' phase as the main or single phase can be obtained.
2. Development of computer-controlled ion nitriding equipment for deep nitriding of gears
The smooth implementation of the process requires advanced equipment to guarantee, but at present, most of the ion nitriding furnaces used in industrial production in China are simple in structure, low in automation, and the structure is not reasonable enough. In recent years, with the development of power electronics technology and industrial automation control technology, the technology of domestic pulse power supply has matured. Due to its series of advantages, it has been accepted by the market and has been widely used, computer controlled ion nitriding equipment. It has also been gradually developed and its functions have been improved.
This paper describes the ion nitriding equipment system designed and developed for a company's gear products (ring gear, worm and small modulus gears, etc.), which integrates the current advanced domestic configuration and is equipped with a computer control system. About the software.
In addition, the furnace auxiliary heating system is added, which is beneficial to increase the heating temperature, improve the furnace temperature uniformity, reduce the arc cleaning and heating time, especially the plasma-based ion source and the heat source. Separate control of the system helps to reduce the occurrence of arc discharges to more widely adjust and control process parameters.
The main technical parameters and configuration of the equipment are as follows: effective working area is 1600mm × 1500mm, ion heating power is 200kW, auxiliary heating power is 75kW, one with 2X-70 and 2X-30 rotary vane vacuum pumps, ion heating transformer and One auxiliary heating transformer, one power control cabinet, one PC computer, system configuration and connection. The main features are discussed below.
Auxiliary heating system
The auxiliary heating resistance wire is made of 8mm 0Cr25Al5 iron-chromium aluminum alloy wire vertically fixed on the heat insulation screen of the furnace. The ceramic insulation is used between the insulation screen and the heat insulation screen. The auxiliary heating line voltage is 90V, and the current output is 0~480A. A total of 30 uniform distributions in the circumferential direction, 5 sets of heaters in each group, and then parallel star connection. The heating power is regulated by a bidirectional thyristor and can be continuously given.
2. Selection of main instruments and meters
In the past, the vacuum measurement of the ion nitriding furnace mostly uses a resistance vacuum gauge. The resistance vacuum gauge is sensitive to the measured medium composition. It is calibrated according to the air medium, and can be used when the furnace is pre-vacuum, and the furnace is ventilated during the nitriding process. There will be no reference value for future readings. Therefore, we use the more advanced CPCA-100Z capacitive membrane absolute pressure transmitter to accurately measure and control the furnace pressure during the actual nitriding process.
Ordinary glass rotor flowmeters can only be manually adjusted, and automation and control cannot be achieved due to poor accuracy and stability. The D07-7B mass flowmeter selected in this system can accurately measure and control the flow without being affected by the pressure and type of the medium. A total of 4 mass flowmeters are respectively connected with nitrogen, hydrogen, ammonia and propane. Gas to meet the needs of different processes.
The vacuum butterfly valve is driven by Nucom-Z drive device. It can get 4~20mA current signal through the positioner on the power cabinet, and it can be added to the drive device. The butterfly valve opening angle can be controlled at any position from 0 to 90° instead of manual. The adjustment is made to achieve the purpose of controlling the pumping rate of the vacuum pump.
3. Computer Control System
The system uses Mitsubishi programmable control system and PC, coupled with Beijing Kunlun state-of-the-art intelligent Chinese human-machine interface, system connection.
The software main menu includes five sub-menus, in which the temperature setting of 10 segments and the pressure setting of 10 segments can be performed in the sub-menu of “warming and warming parameter settingâ€. Voltage, time and auxiliary heating power can also be performed at this interface. And the setting of each parameter such as media flow. Input and store the relevant parameters of the process, and enter the operation interface to realize the fully automatic operation of the process. The temperature and pressure parameters during the process execution can be recorded in real time, and can be adjusted and printed.
This equipment is used for deep nitriding production of gears.
Third, the application examples of deep nitriding of gears and recommended application range
For many years, Zhengzhou Machinery Research Institute has carried out many fruitful work in the industrial application of deep nitriding of gears. The gears have a modulus of 2.5 to 10 mm, an accuracy of 5 to 7 and a high line speed of 118 m/s. The shape has straight teeth, helical teeth, double arcs, spiral bevel teeth, inner ring gears, non-circular gears and worms. The following are two practical applications of deep nitriding. Example 1: Zhenhai Petrochemical Plant with an annual output of 2.5 million tons of main fan high-speed double arc gear, gear module 4.5mm, line speed 118m / s, load factor 1.63kN / m2, material 34Cr Ni3Mo, surface hardness 620 ~ 650HV, effective The hardened layer has a depth of 0.8 mm and a brittleness level of 1. Example 2: Baosteel Coking Plant main winch reducer input shaft gear, gear module 9mm, precision 7 grade, material 25Cr2MoV, surface hardness 730 ~ 760HV, effective hardened layer depth 0.85mm, brittle 1 level.
Based on years of research in the field of gear nitriding and the application experience of actual engineering, it is recommended to select the gear nitriding process under the following working conditions.
Fourth, the conclusion
Although the deep nitriding process has been applied to the gear to a certain extent, there are still many problems to be solved. In addition to the influence of the traditional concept, the cooperation of hot and cold processing is required. Due to the improvement of the hardness of the matrix, The tooth making process and the knife have new requirements; the process of grinding the teeth is omitted, and the deformation should be minimized, not only in the process of nitriding process, but also in the previous process, such as before the finishing process. Stress tempering and other measures. In order to master and apply the deep nitriding process more skillfully, it is recommended to work hard from the following points:
(1) Selection of raw materials At present, domestically available steel materials are limited, and some steel types are not easy to purchase. It is necessary to develop some special steel materials for gear nitriding, such as precipitation hardening steel, which can further improve the hardness of the matrix during nitriding. .
(2) Process research further increases the seepage speed, increases the depth, reduces the deformation, and explores new processes.
(3) Data accumulation Explore the relationship between tissue and fatigue performance, accumulate the mechanical performance data of nitriding gears, and constantly improve relevant standards.
(4) Equipment development Establish an expert intelligent system for gear nitriding, improve the control level of equipment, and solve the problem of low automation under mass production.
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