Let s discuss how to reduce the amount of deformation after carburizing of thin walled small workpie

Prevention method of deformation First of all, in the heat treatment of forgings, reduce the deformation of parts in the way of placement, one is to hang as vertically as possible, the other is to put it vertically at the bottom of the furnace, the third is to use two points of horizontal support, the fulcrum position is between one-third and one-quarter of the total length, and the fourth is to lay flat on the heat-resistant steel tooling. Secondly, in the cooling process of parts, the type of quenching medium, cooling performance, hardenability, etc. are related to deformation. Changes in cooling performance can be adjusted by changing the viscosity, temperature, level pressure, use of additives, stirring, etc.

The higher the viscosity and the higher the temperature of the quenching oil, the smaller the oval deformation. In the stationary state, the deformation is smaller. There are several ways to reduce distortion:

salt bath quenching; high-temperature oil quenching; QSQ method; Vacuum quenching; One groove and three stages quenching. Salt bath quenching is similar to high-temperature oil quenching, both are quenched at the martensitic transition temperature, so that the uniformity of the martensite phase transformation increases. QSQ is two-liquid quenching.

Vacuum quenching is to reduce the liquid surface pressure of the quenching medium, thereby prolonging the vapor film stage, and the cooling rate in the high temperature zone decreases, so that the cooling rate of all parts of the part is uniform. The structure of one tank and three stages of quenching is simple, first the parts are oil-cooled from the quenching temperature to a temperature slightly higher than the ms point, and then the parts are boiled and kept in the atmosphere to make the overall temperature of the parts uniform, and then oil-cooled, so that the martensitic phase transformation is carried out uniformly, and the irregularity of deformation is greatly improved.

Practical summary 1. Try to control the seepage layer in the required downline from the process; Keep the quenching temperature as low as possible; Maximize the temperature of quenching oil 2. Tooling: design appropriate tooling to change and improve deformation, such as adding mandrel, etc. 3. Improve from the way of furnace installation.

First of all, it is necessary to control the hardenability, purity and uniformity of raw materials, and the processing route is best not to change frequently, and under the premise of ensuring internal quality, a lower quenching temperature and quenching oil temperature should be used to be above 150.

Heat treatment deformation There are two types of heat treatment deformation: one is the change in size, and the other is the change in the geometry of the part. Depending on the heat treatment technology, the deformation and anti-deformation methods of the part size and geometry are also different.

In the process of austenitization heated by heat treatment, the longer the holding time and the higher the temperature, the more carbon dissolves into the austenite, and the greater the expansion of the martensite transformation. When cooled, the martensitic expands the most, followed by the upper bainite, and the volume of the lower bainite and the drosinites change very little. When tempered at low temperatures, the martensitic shrinks, and the amount of shrinkage is proportional to the carbon content of the supersaturation.

When heated at room temperature-200, some of the residual austenite will be transformed into martensite, which expands. However, the swelling does not change much due to the decomposition of the martensite around 200. In conventional heat treatment, the main reason for the shape change of the part is the thermal stress and phase transformation stress that occur during heat treatment heating and quenching.

Heating too fast, parts that are too large relative to a furnace, and different temperatures in each part of the part can lead to thermal distortion. When holding heat, the residual stress of the processing is released and deformation, and the weight of the part also causes deformation. During cooling, thermal stress will form and deform the part due to the different cooling rates of different parts of the part.

Even if the cooling rate is the same, the cooling is always fast on the surface and slow on the heart. As a result, the surface of the pre-phase transformation causes the plastic deformation of the non-phase transformed core. If there is segregation of the alloy composition in the material, or if the surface is decarburized, the phase transformation stress is more uneven and more likely to cause deformation of the part.

In addition, if the thickness of the parts is uneven, the cooling rate will be different.

A: Inner hole spline quenching process:

1. For gears with low hardness requirements for spline holes, a margin of 2mm can be left on the inner hole and the end face of the orifice before carburizing, and the carburizing layer allowance on the inner hole and end face can be removed after carburizing, so that the inner hole and end face can reach the final or process size. The hardness of the inner hole and the end face is lower than the hardness of the tool, and the internal spline can be broached directly with the broach. These gears can also be finished with a conventional process prior to carburizing.

During carburizing, anti-seepage paint is applied to the inner hole and both ends of the orifice, and the internal spline is broached after carburizing. Because the effect of the impermeable coating is not very good in practical application, the inner hole should be stuffed to delay the cooling rate of the inner hole and reduce the quenching hardness of the inner hole to facilitate the dressing of the keyway after quenching.

2. Pull out the internal spline before carburizing, quench it directly after carburizing, and push and correct the internal spline with a spline pusher on the press after heat treatment. This processing method must control the shrinkage deformation of the carburizing and quenching of the spline hole in the gear, so as to facilitate the trimming of the internal spline in the next process.

3. In order to stabilize the deformation of the spline hole in the gear after carburizing and quenching, some measures were first taken in the gear material and thermal processing technology. The loose internal structure of the steel is one of the reasons for the large shrinkage of the inner hole. The material is selected according to the standard and the forging ratio is increased to make the tissue compact to reduce the amount of inner hole shrinkage.

If there are defects such as Wechsler structure and ribbon structure in forgings, it is difficult to eliminate normalizing at room temperature, and the uneven structure increases the residual stress after cold working. After gear carburizing and quenching, the deformation of the inner hole increases. Therefore, strict control of the forging process is an important part of reducing the deformation of the inner bore of the gear.

For the normalizing temperature, it is controlled at 940-950, which is higher than the carburizing temperature, which is more in line with the actual requirements. After the tooth blank is fully normalized, uniform pearlite and ferrite are obtained, the grain size is 7-8 grades, and the deformation of the inner hole of the gear becomes smaller.

4. The shape of the gear is different, and the plastic deformation resistance of each section is different when heating and cooling. After the gear of the same material is carburized and quenched, the radial shrinkage of the inner diameter with the same spline size is also different. The shrinkage deformation law and deformation amount of heat treatment spline hole are related to the structural shape of the gear.

In order to obtain a small amount of deformation, the structure of the two ends of the splined hole should be symmetrical and the wall thickness should be uniform, and the wall thickness of the hole should be increased as much as possible.

5. For gears with a large amount of internal spline hole shrinkage, appropriately increase the radial size of the spline broach according to the deformation law of the spline hole, and try to make the spline hole size within the qualified range after carburizing and quenching, which is both economical and effective.

6. For gears with high precision requirements, the quenching process of the mandrel is adopted, and the carburizing is followed by the quenching of the mandrel. It not only ensures the depth of the carburizing layer on the tooth surface of the spline, but also reduces the heat treatment deformation of the spline with large diameter. The large diameter of the quenching mandrel should be smaller than the large diameter of the spline hole, and the theoretical reduction value should be equal to the shrinkage hole deformation of the part, but the actual value should be slightly larger.

The diameter of the quenching mandrel should not be larger than the diameter of the spline shaft with the matching spline hole of the part to avoid interference. The tooth thickness should be smaller than the width of the tooth groove of the spline hole, and the tooth thickness should be reduced to the left and right to prevent the interference of the tooth side, and at the same time increase the oil flow and improve the spline hardness.

It is necessary to mill the keyway first, and then quench, otherwise the material will become hard after quenching, and it will not be easy to process.

No. Since the carburizing temperature is generally 920-930, it needs to be kept warm for several hours or even more; Compared with the general quenching process parameters, not only the temperature is high, but also the holding time is long, which will inevitably cause the coarse grain of the workpiece. At the same time, the carbon concentration in the surface layer is obviously segregated after carburizing, and the segregation of components will also lead to tissue segregation.

Therefore, direct quenching after carburizing will inevitably increase the tendency to crack.

After the carburizing is completed, it is cooled and then heated and quenched at room temperature, because the grains will be refined, and the carbon concentration will also slow down the concentration gradient due to diffusion, so that the operation will reduce the quenching cracking tendency.

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Measuring surface hardness is to hit the hardness on the most surface of the workpiece.

The carburized layer depth is the vertical distance from the most surface of the workpiece to the HV550 position.