How to control the deformation of thin wall parts during carbonitriding and quenching?
The inner ring of elastic bearing seat (hereinafter referred to as “inner ring”) is a part of an aircraft engine. In the working process, there is constant sliding, rolling and other relative motion process, and there is friction between the inner ring and other parts. At the same time, it also bears a certain alternating bending stress and contact fatigue stress, sometimes there is a certain impact force, and the working conditions are very bad. Corresponding to the service conditions, the common failure forms of the parts are excessive wear, surface spalling, fracture and so on. Therefore, the technical requirements of the parts are high hardness, high wear resistance and high contact fatigue resistance. Because the carbonitriding layer has higher hardness, wear resistance, corrosion resistance, bending strength and fatigue contact strength than the carburizing layer, but generally the carbonitriding layer is shallower than the carburizing layer, so it is generally used for parts with light load and high wear resistance. Therefore, the inner ring is carbonitriding quenched to obtain high surface hardness and high dimensional accuracy.
The material of inner ring is 12cr2ni4a steel, which has high hardenability. After carbonitriding quenching and low temperature tempering, not only the surface has high hardness, but also the core strength matches well with toughness and plasticity. This kind of material can be widely used to manufacture various kinds of engine important wear-resistant parts, such as gas separation double gear, elastic transmission gear, driving gear, driven gear, as well as various shafts, pins, pistons and rolling shafts. There are strict requirements for deformation control of the part during carbonitriding quenching, but the part is easy to deform due to its own characteristics (thin wall) and some difficulties in quenching operation of the press.
How to control the deformation of thin wall parts during carbonitriding and quenching?
In this paper, aiming at the deformation of the inner ring after carbonitriding and quenching and the low hardness, the quality of carbonitriding and quenching of the inner ring is improved by adopting the methods of testing the mandrel before quenching, changing the charging method of parts, and correcting after processing.
Heat treatment process and existing problems
Heat treatment process and technical requirements
The overall processing route of the inner ring: blanking → rough machining, cutting sample → mechanical finishing → carbonitriding quenching → mechanical finishing → final inspection. The structure of the inner ring is shown in Figure 1.
Fig. 1 Structure of inner ring of bearing pedestal (Note: final dimension of parts in brackets)
The main process parameters of inner ring heat treatment are as follows.
- 1) Carbonitriding: 830 ℃± 10 ℃, holding for 6 – 8h, acetone 50 – 100 drops / min, ammonia 2.0 – 3.0l/min, carbon potential 0.85% ± 0.05% (carried out in drop type well type carburizing furnace).
- 2) Quenching: the parts are bound and put into the solid carburizing agent protection box, heated to 810 ℃± 10 ℃ and kept for 70 – 100min (in the box furnace), then put into the mandrel and quenched by press.
- 3) Cold treatment: – 60 ℃ for 2.5 ~ 3 h, air cooling (in low temperature box).
- 4) Tempering: tempering temperature is 160 ℃± 10 ℃, holding for 2.5 ~ 3 h, air cooling (in well tempering furnace).
Technical requirements of inner ring after heat treatment: surface hardness ≥ 90hr15n, core hardness 32 ~ 45.5hrc,
Technical requirements of inner ring after heat treatment: surface hardness ≥ 90hr15n, core hardness 32 ~ 45.5hrc, average value of mm after heat treatment is mm, flatness ≤ 0.15mm.
At present, after carbonitriding and quenching, the inner ring usually has the following quality problems:
1) The inner ring was deformed seriously after carbonitriding, so that the inner diameter of the inner ring was deformed from mm to mm after heat treatment, which exceeded the required range.
2) When the elastic bearing seat is quenched by press, it is difficult for the parts to fit into the mandrel, which affects the quenching quality and makes the hardness of the parts unqualified.
Because the above two kinds of unqualified phenomenon will occur frequently, not in line with the production requirements, resulting in parts repair, or even scrap, seriously affecting the progress of scientific research and production, so it is necessary to analyze the causes of heat treatment defects of this part, in order to find a reasonable solution.
Cause analysis of deformation
The essence of deformation is to release the internal stress of the part. If the released stress is greater than the yield strength of the material, permanent deformation will occur. The heat treatment deformation of parts is mainly caused by heat treatment stress, which is mainly reflected in the following aspects.
- 1) The temperature gradient caused by different heating and cooling rates results in thermal stress.
- 2) The microstructure stress caused by different phase transformation sequence between the surface and the core and different cross-section areas.
- 3) The phase transformation stress caused by different specific volume of different phases. However, the structural shape of the parts, the quality of raw materials, the processing state before heat treatment, the self weight of the parts, and the improper support or clamping of the parts during heating and cooling in the furnace also lead to deformation.
Cause analysis of inner ring deformation
The deformation of inner ring during carbonitriding is caused by the following reasons.
- 1) In the process of carbonitriding, the inner ring is directly placed on the flat plate without any fixed measures.
- 2) In the process of carbonitriding, the motor drives the fan to rotate, so that the carburizing agent is evenly distributed in the furnace, which is convenient for the parts to absorb. But at the same time, it will also cause the vibration of the crucible, which will inevitably cause slight collision and mutual extrusion between the parts, resulting in the deformation of the parts.
- 3) When the inner ring is carbonitriding, the thermal stress and structural stress will also cause some deformation of the parts.
Cause analysis of low inner ring hardness
The reasons for the low hardness of the inner ring during quenching are as follows.
1) Due to the deformation of the part during carbonitriding, it is difficult for the part to fit into the mandrel during quenching (it is required that the two earring shaped sections of the end face of the part must face the two wide faces of the mandrel), as shown in Figure 2.
Fig. 2 parts press quenching
2) The thickness of the part is relatively thin, only 2mm, which is easy to cool and shrink. If the quenching transfer speed is slow or the oil cooling system of the press can not be quickly opened, the quenching speed will be affected, which will eventually affect the hardness of the part, resulting in the low hardness of the part.
Control of part deformation
- 1) Correct the carbonitriding plate, so that the parts can be placed on the plate smoothly, and after the parts are placed on the plate, take fixed measures, that is, fix the two ends of the inner ring of the elastic bearing seat with a small iron wire, and keep a certain distance between the two parts, so as to avoid the collision or extrusion between the parts caused by the vibration of the motor during the carbonitriding process, so as to reduce the deformation.
- 2) For the inevitable deformation of elastic bearing seat due to thermal stress and structural stress during carbonitriding, we adopt the method of correcting it immediately after carbonitriding to ensure the qualified size of parts.
Control of part hardness
- 1) According to the deformation of the parts after carbonitriding, which leads to the parts can not be put into the mandrel during quenching, the parts should be corrected before quenching, and then the quenching mandrel should be set to ensure the correction effect, so that the parts can be quickly put into the mandrel during quenching, so as to ensure that the size of the parts is qualified and the hardness also meets the requirements.
- 2) When the part is quenched, not only the transfer speed is required to be fast (due to the small batch of parts, the parts are also relatively small, in order to ensure the quenching quality of parts, manual transfer is often used, and the transfer can be completed within 5S under normal circumstances), but also the two earrings of the part must be inserted into the two wide faces of the mandrel. In the past, the parts were randomly placed in the protection box when charging the furnace, so that the position of the earrings had to be adjusted when clamping out the parts during quenching, which delayed the quenching time. Now, when charging the furnace, the two earrings of the parts are fixed in the same direction, and the two wide faces of the mandrel are also placed according to the direction of the parts, which greatly speeds up the transfer speed during quenching and ensures the hardness of the parts.
Results and discussion
Through the implementation and control of quality improvement measures for carbonitriding quenching, three batches of Carbonitriding quenching parts were counted, and the hardness, flatness and inner diameter of the parts were tested. The results are shown in Table 1.
Table 1 test results of improved inner ring
|Batch number||Surface hardness HR15N||Core hardness HRC||Flatness / mm||Inner diameter (average) / mm|
It can be seen from table 1 that the surface hardness, core hardness, flatness and deformation of the three batches of parts fully meet the technical requirements, which completely solves the problems of low hardness and inner hole deformation after carbonitriding and quenching of the original parts, and eliminates serious quality problems such as frequent repair and even scrap of parts.
Authors: Lu Rui, Ke Wenmin, Zhang chaoming, Wu Chao, Tang Junli
Source: China Pipe Sleeve Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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