How to solve the problem of carburizing and quenching deformation of thin walled gears
Electric vehicle gear (hereinafter referred to as gear) is a new product developed by our company, because of its large diameter and thin wall. In the initial stage of trial production, a series of quality problems appeared after carburizing, quenching and tempering: the hardness of the tooth end face of the same gear was not uniform; The effective hardening depth of the same gear is not uniform; Because of the serious ellipse of the inner hole, the tooth surface can not be ground when the gear is grinded, and the unqualified rate of the product is up to 40%. After careful analysis and summary, through process verification, the combination of special carburizing tooling and reasonable carburizing placement mode has fundamentally solved the above problems and met the technical requirements of the product.
Gear technical requirements
As shown in Figure 1, the gear is made of 20CrMnTi, the effective hardening depth is 0.5-0.8mm, the surface hardness is 58-63hrc, and the core hardness is 30-43hrc. The microstructure of the gear was inspected according to the standard of metallographic structure of carburized gears for automobiles. Carbide ≤ 5 grade, retained austenite and martensite ≤ 5 grade; The accuracy grade of gear grinding is ≤ 5.
Figure 1 electric vehicle gear
Gear processing technology and inspection results
Gear production process
Blanking → forging → isothermal normalizing → rough turning → finish turning → gear hobbing → chamfering → small hole drilling → deburring → cleaning → carburizing → tempering → inner hole grinding → gear grinding → inspection → cleaning → installation.
Gear heat treatment equipment and carburizing process
The carburizing, quenching and tempering equipment of gear adopts the automatic production line of gas carburizing and nitriding furnace.
Test results
After carburizing, quenching and tempering, dissect the waste products machined in the furnace, inspect the metallographic structure, DC value of effective hardening layer, the hardness of the center and the hardness of the end face of the gear teeth. The test results are shown in the attached table. It can be seen from the table that the metallographic microstructure is grade 1-2, which meets the technical requirements of the product. The hardness of gear end face is even and qualified; Some hardness is uneven, high and low, and some hardness is qualified, some is unqualified; The core hardness and effective hardening depth are similar. In addition, the inner diameter dial gauge and the common normal micrometer are used to measure the inner hole and the common normal respectively. It is found that the roundness of the inner hole of the gear is 0.024-0.16mm, and the deformation of the inner hole is large; The variation of common normal length is 0.004-0.085mm.
Test results of metallographic structure, effective hardening layer DC value, core hardness and tooth end hardness of electric vehicle gear:
| Sample number | Carbide / grade | Retained austenite / grade | Martensite / grade | Ferrite / grade | Dc（550HV）/mm | Core hardness HRC | Hardness of tooth end face HRC |
| 1 | 1 | 1.5 | 1.5 | 1.5 | 0.30～0.38 | 26.1～27.5 | 52.3～53.5 |
| 2 | 1 | 2 | 2 | 1.5 | 0.72～0.73 | 39.6～40.1 | 60.2～61.7 |
| 3 | 1 | 1.5 | 1.5 | 1.5 | 0.42～0.43 | 29.5～30.6 | 54.5～55.4 |
| 4 | 1 | 2 | 2 | 1.5 | 0.48～0.70 | 32.0～38.7 | 56.8～60.6 |
| 5 | 1 | 2 | 2 | 1.5 | 0.71～0.72 | 38.9～39.6 | 60.4～61.3 |
Cause analysis and improvement measures
Cause analysis
There are a series of quality problems for products.
First of all, the whole carburizing process of the product was checked from the monitoring recorder. The carburizing temperature rise and drop in each heating zone were normal, the carbon potential change was stable, the quenching oil temperature, stirring speed and oil quantity were normal, and the tempering temperature and time were normal.
Secondly, we found that there was something wrong with the loading method. The main reason is that the loading workers put the loading materials unreasonably and the gears are stacked too tightly. Unreasonable charging method has an important influence on heat treatment distortion. The layout of carburizing is shown in Figure 2. The gear is placed flat on the honeycomb shaped square heat-resistant steel plate. After the gear is placed in one layer, the heat-resistant steel wire mesh is laid on it, and then two layers of gears are placed. After analysis, in the process of carburizing and quenching for the gear end face close to the heat-resistant steel plate and steel wire mesh, the temperature of the upper and lower ends of the gear is not uniform, the diffusion cycle of carburizing atmosphere is not smooth, the carburizing speed is not consistent, and there are carburizing dead angle, the flow of quenching oil is not smooth, and the cooling speed is not consistent, which leads to the effective hardening layer depth and the hardness of the end face The main factors of uneven hardness in the core; In addition, when carburizing at high temperature, the parts are stacked too tightly, and the machining stress, thermal stress, organizational stress, gear weight and tooling weight are superimposed on each other, which is another reason for inducing gear end face creep and warping deformation.
During carburizing and quenching, due to the uneven temperature of the upper and lower end faces of the gear, the diameter of the inner hole of the gear is large, the wall is thin, and there are eight small holes drilled there. The machining stress is large, and the strength of the gear at high temperature is low. This kind of stress and structural stress will aggravate the heat treatment distortion of the inner hole and the common normal length of the gear.
Fig. 2 electric vehicle gear carburizing layout
Improvement measures
In order to ensure the heat treatment quality of gears, we must consider carburizing and quenching. The key distortion control measures are reasonable tooling and gear clamping mode, quenching medium and reasonable stirring.
In this regard, we designed a ring washer as a special tool for gear carburizing. The schematic diagram of the ring washer is shown in Figure 3. The tool is easy to manufacture and has low production cost. Another carburizing placing method is used in combination with ring washer, and the schematic diagram of this placing method is shown in Figure 4.
In order to verify the reliability of the carburizing method, we continue to use the original carburizing process, so that the new carburizing method can be compared with the original method.
Specific loading method: first from the bottom to the top of the frame, each layer is a square heat-resistant steel plate, each frame is equipped with 8 honeycomb heat-resistant steel plates, the gap between the heat-resistant steel plates is uniform, and each steel plate is evenly placed with 11 ring washers, and the groove at the thin wall of each gear is downward and just falls on the ring washer. The features of the carburizing arrangement are as follows: due to the thin wall of the ring washer, the heat transfer of the gear is not affected by the temperature rise and drop during carburizing and quenching; The upper and lower surfaces of the gear do not directly contact the tooling, which has less influence on the tooth shape and tooth direction deformation of the gear; The gap between the upper and lower end faces, front and back, left and right sides of the gear is uniform, and the heating and cooling rates are uniform; The circulation of carburizing atmosphere is smooth without dead angle; The quenching cooling rate is uniform and the flow of quenching medium is smooth; In this way, the uniform heating and cooling of all parts of the gear are ensured; The carburizing atmosphere and quenching medium flow evenly, so that the gear can obtain smaller deformation and more uniform deformation law. In addition, the production operation method is safe, simple and convenient.
Figure 3 Schematic diagram of ring washer
Fig. 4 Schematic diagram of carburizing arrangement of electric vehicle
In order to verify the feasibility of the carburizing method. After carburizing, quenching and tempering, the test results are as follows: the hardness of gear end face is 60.2 ~ 61.7hrc; The effective depth of hardened layer detected by WEDM: 0.70 ~ 0.73mm; Core hardness: 38.8-39.2hrc; Microstructure: carbide grade 1, retained austenite and martensite grade 2, ferrite grade 1. Inner hole roundness: 0.003 ~ 0.06mm, common normal length variation: 0.002 ~ 0.020mm, gear precision grade 3 ~ 5 after grinding. It can be seen from the test results that through the design of ring washer and the use of new carburizing placement, the test results of the gear meet the technical requirements of the product. The qualified rate of products is 100%. After several small batch trial production, the product quality is stable and reliable. This process has been used for batch production.
Conclusion
There are a series of quality problems in the process of carburizing and quenching of gears. Through the design of ring washer and the combination of reasonable carburizing arrangement, the problems are effectively solved, the product quality is ensured, the production cost is reduced, the production efficiency is improved, and good economic benefits are obtained.
Authors: Huang Jincai, Lai Xianggong and Liao Xiaowen
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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