Heat treatment of parts with holes
The expansion and shrinkage of the aperture is one of the more complicated problems in heat treatment because it is difficult to be corrected mechanically. When the hole diameter expands beyond the tolerance, the grinding finishing of the parts will be powerless. The deformation of complex groove brings great difficulty to machining and even makes the parts which cost a lot of man-hours to be scrapped.
The basic rule of pore diameter deformation in quenching is that the thermal stress causes the pore diameter to shrink while the microstructure stress and the volume change during the microstructure transformation cause the pore diameter to swell.
Many factories in China have rich experience in utilizing thermal stress shrinkage cavity. When the hole size is smaller and the thickness is larger, the shrinking effect is better. When the cooling speed is faster, the shrinking effect is better. In addition, in order to strengthen the effect of shrinkage cavity, the clamping plate can clamp the workpiece to quench rapidly, so that the cooling speed of the outer surface is greatly faster than that of the inner surface, and the effect of shrinkage cavity is better. This method can be used to repair the hole has been expanded parts, can also be used to offset the parts of the quenching hole swell tendency. The reason why thermal stress causes pore diameter shrinkage is that in the first stage of cooling, the outer thermal shrinkage exerts pressure on the inner pore and causes the inner pore to shrink due to plastic deformation.
The swelling effect of tissue stress is caused by the following two aspects: in the case of martensite formation both inside and outside, when the outer layer is formed first, it will give the inner hole a tension, which will cause its plastic deformation and expansion. The formation of martensite in the outer layer without the formation of the inner hole part, or the formation of the inner hole without the formation of the outer layer, is due to the formation of part of the martensite caused by the expansion of the whole workpiece. The martensite is formed at about the same time inside and outside the thin ring workpiece, which is small in terms of the microstructure stress, but large in terms of the volume change before and after quenching (the surrounding length increases a lot). This type of workpiece tends to be bulged the most. So as long as the parts with holes have martensite formation, it will increase the factor of hole expansion.
Figure 8-30 is a bevel gear, 40Cr steel manufacturing, technical requirements: hardness 37~44HRC, hole deformation requirements:, that is, only shrinkage, no expansion. The process is: preheat to 450 ℃, 40 ~ 50 min, then salt heating furnace, 825 ℃, 40 ~ 50 min. Alkali bath cooling, 150 ℃, 10 min. The nitrate tempering furnace, 420 ℃, 20 min air cooling. Results: the hole diameters of the three specimens were -0.02mm, and the hardness after tempering was 39~43HRC. The deformation is well controlled and the quality is qualified.
The following test was conducted for this parts to try to further reduce the deformation, namely USES the 870 ℃ heating, 350 ℃ nitrate isothermal quenching, 28 min, the results of 30 HRC hardness, deformation + 0.04 mm hole. After the 880 ℃ heating, 880 ℃ nitrate isothermal 15 min. Results the hardness of 41HRC, hole deformation + 0.67mm, it is proved that the isothermal quenching of nitrate can not meet the requirements. Therefore, the bloated workpiece was repaired (that is, heated to the critical point A1 below with water cooling, after several times, the inner hole was restored to the original size). According to the above - mentioned alkaline bath classification, to meet the technical requirements. Oil quenching has also been tested, and the result is much larger than the inner hole of the nitro isothermal quenching.
FIG. 8-30 schematic diagram of bevel gears
Successful experience is:
40Cr steel diameter of 90mm gear, quenching hardness is easy to reach, but often occur in the hole swell. It is generally believed that the isothermal quenching (forming bainite) has a small deformation, and the treatment of this part indicates that the graded quenching of alkali bath (forming martensite) is effective to prevent the expansion of the inner hole, while the isothermal quenching is not an effective method to prevent the expansion of the inner hole for parts such as 40Cr steel. Contrary to the above gear shaft, alkali bath (quenching) is better than oil quenching (slow cooling) to prevent inner hole expansion.
Why does the hole in the caustic bath not swell? In terms of cooling speed in high temperature zone, alkali bath is the largest. In terms of low temperature zone, air cooling after alkali bath classification is smaller than oil cooling, that is, alkali bath quenching generates greater thermal stress than the other two media. It can be seen that alkali bath quenching overcomes the defect of swelling mainly by using larger thermal stress shrinkage to offset the hole expansion caused by the structure transformation. The basic method to avoid hole expansion in production is to increase the cooling inhomogeneity in the high temperature zone and increase the cooling speed difference between inner and outer layers. For thin annular parts, the clamping plate can be used to clamp into the coolant, which makes the original workpiece thin and the inner layer of the cooling difference is small outer layer of fast, slow cooling of the inner layer, as shown in figure 8-31. It should be noted that this method cannot guarantee the hardening of the inner hole when the hardenability of the steel is low and the inner hole requires high hardness.
FIG. 8-31 schematic diagram of clamping plate for inner hole of ring parts with shrinkage
In addition to the use of iron clamping quenching this method to promote the contraction, there are water pipes to flush water in the hole for local cooling method, than in water quenching shrinkage even more. Can also be hole parts before quenching with asbestos and iron coating, quenching the same can be larger shrinkage. In both of these methods, the external cooling is slow, and the middle hole is heated, and the external thermal stress is generated, which compresses the middle hole. When the middle hole turns into martensite, the volume is expanded, and the expansion direction is towards the center of the hole, thus causing shrinkage deformation. This method is very effective for repairing the holes in the worn-out and enlarged drawing die.
The shape of the part also has A significant impact on the expansion and contraction of the hole. For example, the workpiece shown in FIG. 8-32 has two dimensions, A and B, which have an impact on the deformation in the direction of A. Generally, when A<b, it="" tends="" to="" shrink,="" while="" when="" a="">B, it tends to swell.</b,>
FIG. 8-32 diagram of parts with holes