Quality problems in quenching and cooling of the h

2022-10-16
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Quality problems and solutions in gear quenching and cooling (Part 2)

4 Special quenching oil

special quenching oil is generally divided into ordinary quenching oil, rapid quenching oil, isothermal graded quenching oil (also referred to as hot oil), vacuum quenching oil and bright quenching oil. Compared with ordinary engine oil, special quenching oil has better thermal stability and can better ensure the quenching quality of workpieces. Of course, the most important aspect that special quenching oils are superior to ordinary engine oils is their cooling characteristics. Compared with ordinary engine oil, different special quenching oils have the characteristics of short vapor film stage in the cooling rate distribution, so that the workpiece can be cooled faster in the high temperature stage. Among them, the maximum cooling rate of rapid quenching oil is relatively high, and the cooling rate at medium and low temperature stage is greatly different due to different varieties of quenching oil. The cooling characteristic of hot oil is that the steam film stage is shorter, while the cooling is slower in the low temperature stage of workpiece quenching and cooling. Rapid quenching oil is mainly used for thicker and larger workpieces and steel grades with lower hardenability. Hot oil is mainly used for smaller workpieces and steel grades with better hardenability. It should be said that any quenching oil with a large workbench has a workpiece suitable for it. However, except for a few cases, each heat treatment furnace hopes to process more steel types and a variety of workpieces. Therefore, it tends to choose quenching oil with a wider range of applications. Generally speaking, quenching oil has short vapor film stage, fast cooling in medium temperature stage and high cooling speed in low temperature stage. This kind of oil has strong cooling capacity and wide application range. The deformation of many oil quenched workpieces occurs simultaneously with its insufficient quenching hardness and quenching depth. Using this kind of quenching oil with a wide range of adaptability can often solve the problems of workpiece deformation, insufficient hardness and insufficient hardening depth at the same time. The steam film stage of quenching oil is short, that is, the oil cools quickly in the high temperature stage. This feature is conducive to preventing the precipitation of proeutectoid ferrite and the deformation of gears with internal splines. In short, the overall cooling rate of quenching oil is high, which is conducive to obtaining a deeper quenching hardened layer. However, from the analysis of the distribution of cooling rate, in addition to the rapid cooling required in the medium and high temperature stages, the low temperature cooling rate of oil has a greater effect on the depth of the obtained hardened layer. The higher the cooling rate at low temperature, the deeper the hardening layer. Stirring the quenching oil can improve the cooling rate of the oil. The effect of agitation on improving the cooling capacity of oil with relatively low cooling rate is greater; For the special quenching oil with high cooling speed, the effect of agitation is relatively small. Among the quenching quality problems of gears, there is also a kind of quenching hardening layer that is too deep. The hardened layer is too deep, and breaking teeth are often used. One of the effective ways to solve this kind of problem is to reduce the low temperature cooling rate of quenching oil

in a word, when selecting quenching oil to ensure the quenching quality of gears, it should be selected from the cooling speed distribution characteristics of the oil according to the steel type, shape characteristics and heat treatment requirements of the gears to be treated. It is best to work with the relevant technicians of the quenching oil manufacturer to determine the appropriate quenching oil and reasonable use method through discussion and analysis

III. Changes of quenching medium in use no matter quenching oil or water-soluble quenching medium, they will contact high-temperature workpieces and will be polluted to varying degrees in use. When entering air, the medium will be oxidized. High temperature may cause thermal decomposition, oxidation and polymerization of organic media. Pollution may complicate the oxidation and other changes of the medium. All these changes and the change products left in the medium will cause the medium to deteriorate. The color change, transparency change and viscosity change of quenching oil are all manifestations of deterioration. It should be said that there is no medium that does not deteriorate. We are only concerned about three points: the first is the impact of changes on the cooling characteristics of the medium, the second is the speed of metamorphism, and the third is how to understand and correct the impact of metamorphism, so as to ensure long-term stable quenching quality

as mentioned earlier, water-soluble quenchants are mainly used to reduce the low-temperature cooling rate of water. The change trend of water-soluble quenchant in use is just the opposite. The change gradually increases the low-temperature cooling rate, trying to return to the level before no quenchant is added. The change trend of ordinary engine oil in use is simply that at the beginning, the low-temperature cooling rate gradually decreases, the vapor film stage gradually shortens, and the cooling rate at the medium and high temperature stage slightly increases. With the increase of service time, due to the further increase of the viscosity of the oil, the medium and high temperature cooling speed of the oil will also slow down, resulting in the obvious deterioration of the cooling effect of the workpiece

the change of special quenching oil in use is relatively complex, which includes the change of additives and the change of base oil, which is the comprehensive result of these two changes. There will be great differences in the changes of different quenching oils and different service conditions. It should be pointed out that under the condition of being free from water pollution, almost all special quenching oils will gradually slow down after long-term use. When the service time is longer, the quenching effect of the workpiece will also be significantly worse. The oil is stable and deteriorates slowly; The stability of oil is poor, and it deteriorates quickly. The higher the service temperature of oil, the faster the deterioration of oil. Equipped with circulating cooling system to stabilize the oil temperature in an appropriate range, and prevent local overheating through circulating mixing in the tank. 2. Measures such as changing the indenter or anvil can slow down the deterioration rate of the oil and prolong the service life of the oil. In addition to the influence of different types and quality of media, there is a common influencing factor for the deterioration speed of different media, that is, the more quenching workpieces are quenched in the same period, the more serious the deterioration of quenching liquid is. In many cases, the amount of quenched workpiece here should refer to the total surface area of the quenched workpiece. The smaller the workpiece, the larger the total surface area with the same weight, and the more the quenching medium deteriorates. Management is also a major problem that can not be ignored that affects the quality of gear quenching. In addition to strictly following the process operation, the management of quenching medium, especially the prevention of pollution, is of great importance. Quenching oil mixed with water and emulsified in oil often leads to insufficient quenching hardness or quenching cracking. On the contrary, oil emulsified in PAG quenchant sometimes causes quenching cracking. Changes in the color and transparency of quenching medium can reflect its deterioration. The degree of deterioration can also be determined by measuring the changes of viscosity, flash point, carbon residue, acid value, etc. of quenching oil. Among the effects caused by metamorphism, the change of cooling characteristics is the most related to the heat treatment effect of the workpiece. In order to ensure the quenching and cooling effect of gears, it is recommended to regularly test the cooling characteristics of the quenching oil and quenching medium used, and manage the cooling characteristics of the medium. Generally, small factories do not need to be equipped with a cooling characteristic tester, but can measure it in the unit equipped with this instrument. The quenching medium production unit shall carry out this test for the user's factory. To measure the cooling characteristics of quenching medium, it is best to use a cooling characteristic instrument that meets the international standard (iso9950). The analysis of the cooling characteristics of the medium should be compact, elegant and solid. The distribution of the cooling speed of the medium should be considered, rather than the maximum cooling speed. Media with the same maximum cooling rate or the same cooling time to 300 ℃ may have very different heat treatment effects because of their different cooling rate distribution. The relationship between the distribution characteristics of medium cooling rate and its cooling effect has been briefly discussed earlier. For details, please refer to the references later in this article

record the quenching hardness, hardening depth and quenching deformation of similar workpieces in long-term production, and analyze their change trend, so that we can understand the change law of quenching medium cooling characteristics. With these records, we can not only control the cooling characteristics of the medium, but also help analyze the causes of on-site heat treatment accidents, so as to solve the quality problem in time. As mentioned earlier, medium deterioration is inevitable. When the service time is not long, or the amount of quenched workpiece is not much, the degree of deterioration is small, and the deterioration of medium cannot be reflected from the quenching effect of workpiece. The amount of quenching is further increased, and the degree of modification is increased to the extent that the quenching of the workpiece cannot meet the requirements, the modification problem really occurs. The quenching quantity continues to increase, and the quenching quality of the workpiece will be even worse if it is deteriorated more. The main problem that may occur after the deterioration of water-soluble quenching medium is that the quenching hardness of the workpiece is too high, resulting in the quenching crack of the workpiece. Therefore, the modification degree of water-soluble quenching liquid can be monitored from the change trend of quenching hardness of similar workpieces, and it is best to take measures before quenching crack occurs. After the special quenching oil is deteriorated in use, the possible problems are low quenching hardness, insufficient hardening depth and large deformation of the workpiece. Therefore, the deterioration degree of quenching oil can be monitored according to the change trend of workpiece hardness and hardening depth. After the quenching medium is deteriorated, can the cooling characteristics of the quenching liquid be restored by adding the original new medium? The answer to this question is: some media can and some media cannot. For water-based quenchants with adjustable concentration, such as PAG type water-soluble quenchants, on the basis of understanding their change rules, generally, the cooling characteristics can be restored to the new time level by adding the original new medium. The water-based quenchant with fixed formula proportion cannot be used. Neither ordinary quenching oil nor special quenching oil can be used. After the quenching oil is deteriorated, only adding the original new quenching oil can not restore the cooling characteristics of the oil to the level of new oil. However, after understanding the change law of oil, the cooling characteristics of oil can be restored by modifying additives

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