Quenching gear quenching cooling is mostly carried out in a liquid medium. For example, gear quenching is usually quenching oil, water-soluble quenching medium and tap water. Therefore, the following will first analyze the relationship between the above-mentioned quality problems that may occur in gear quenching and the characteristics and usage of the quenching medium used, and point out the distribution characteristics of the cooling rate of the quenching liquid required to solve different problems. Then briefly introduce the cooling speed distribution characteristics of common quenching media and the precautions when selecting.
Selection of quenching media for gears
The media currently used for gear quenching are mainly various quenching oils, water-soluble quenching media and ordinary tap water. The selection methods and precautions for these media in gear quenching are discussed separately below. Worm reducer
1. Water-soluble quenching medium
The main cause of the quenching of steel is that the steel begins to undergo martensite transformation (Ms point) and cools too quickly in the following temperature range. For this reason, the first goal of research and development of water-soluble quenching media is to reduce the low-temperature cooling rate of water. Considering that the Ms point of most structural steels is around 300 ° C, the cooling characteristics of the quenching liquid are usually expressed by the cooling rate of the water-soluble quenching liquid when the workpiece is cooled to 300 ° C. The medium can be graded with a 300 ° C cooling rate of the water-soluble quenching medium for use by the heat treatment worker.
There are many kinds of water-soluble quenching media, among which PAG media has adjustable cooling characteristics and easy concentration measurement and control. It is suitable for both integral quenching and various types of induction heating quenching, and can be used stably for a long time. Therefore, it is widely welcomed and has become the most widely used water-soluble quenching medium in the heat treatment industry at home and abroad.
2. Special quenching oil
Special quenching oil is generally divided into ordinary quenching oil, rapid quenching oil, isothermal grading quenching oil (also referred to as hot oil), vacuum quenching oil and bright quenching oil. Compared with ordinary engine oil, the special quenching oil has better thermal stability and can better ensure the quenching quality of the workpiece. Of course, the most important aspect of special quenching oils over ordinary engine oils is their cooling characteristics. Compared with ordinary oil, different special quenching oils have a short steam 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 the rapid quenching oil is relatively high, and the cooling speed of the medium and low temperature stage is greatly different due to the difference of the quenching oil. The characteristic of the hot oil in the cooling characteristics is that the vapor film stage is shorter, and the cooling is slower in the low temperature stage of the workpiece quenching and cooling.
3. Ordinary mechanical oil
The common mechanical oils used in the heat treatment of the factory are N32 engine oil (formerly No. 20 engine oil) and N15 engine oil (formerly No. 10 engine oil). As a quenching medium, the characteristics of this type of oil are that the steam film takes a long time at the high temperature stage of the workpiece, the quenching cooling rate is not high, and the low temperature cooling is slow.
The steam film phase of the oil is long and the cooling of the workpiece is slow in the high temperature stage. The problem may be that the low carbon steel workpiece is prone to proeutectoid ferrite transformation, and the complex shape of the workpiece, such as the gear with the spline hole, etc. Particularly easy to deform. The cooling is slow in the middle and low temperature stages, so that relatively large workpieces are not hardened or the depth of the hardened layer is insufficient and thus quenching deformation occurs.
4. Tap water
Some quenching and quenching and induction heating quenching with low carbon content, poor hardenability and simple shape gears can often use tap water. As a quenching medium, the cooling characteristic of tap water is that the workpiece cools rapidly when it is in a high temperature stage, and it cools quickly when the workpiece is in a low temperature stage. The fast cooling rate can harden the hardenability and thicker workpiece and obtain a deeper hardened layer. This is the advantage of tap water. However, quenching with tap water has three major drawbacks. The first is that cryogenic cooling is too fast, making most steel grades and workpieces susceptible to quenching. The second is that the workpiece is cooled too fast in the high temperature stage, and the relatively slender and thin workpiece is prone to quenching deformation due to improper water inlet. Thirdly, it is also a shortcoming that many people tend to overlook. As the water temperature rises, the stage of quenching and cooling of the vapor film will gradually increase, and the cooling rate of the workpiece in the middle and low temperature stage will gradually decrease. For this reason, when the workpiece to be quenched is relatively small, and the water is quenched by the densely packed stacking method in the basket, the temperature of the water contacted by the workpiece outside the pile is low, and the temperature of the water contacted by the workpiece stacked inside is high, thereby The external workpiece is subjected to rapid cooling, high hardness after quenching, and easy to be quenched, and the workpiece stacked inside is subjected to slow cooling and low hardness after quenching.
When tap water is used as the quenching liquid, it should be used to control the temperature of the water. When quenching by stacking, try to make the workpiece pile loose, and stir the quenching liquid to flow smoothly from the workpiece to reduce the temperature difference between the inner and outer water.
It should be said that any quenching oil has a workpiece suitable for it. However, except for a few cases, each heat treatment furnace is expected to handle a relatively large number of steels and a variety of workpieces. Therefore, it is preferred to use a wider range of quenching oils. Generally, the quenching oil has a short vapor film stage, a fast cooling in the middle temperature stage, and a large low-temperature cooling rate, and the oil has a strong cooling capacity, and its application range is wide. The deformation of many oil-hardened workpieces is accompanied by insufficient hardening hardness and insufficient hardening depth. The use of such a wide range of quenching oils can often solve problems such as deformation of the workpiece, insufficient hardness and insufficient hardening depth. The quenching oil has a short vapor film stage, that is, the high temperature stage of the oil cools down quickly. This feature is advantageous for preventing the precipitation of the pro-eutectoid ferrite and also for preventing the deformation of the spline gear in the belt. Simply put, the total cooling rate of the quenching oil is high, which is beneficial to obtain a deep quench hardened layer. However, from the analysis of the cooling rate distribution, in addition to the rapid cooling required in the middle and high temperature stages, the low-temperature cooling rate of the oil has a greater effect on the obtained hardened layer. The higher the cryogenic cooling rate, the deeper the quench hardened layer tends to be. Screw lifter envelope worm reducer
Quality problems in quenching and cooling
1. Insufficient hardness and insufficient hardening depth
The low quenching cooling rate is responsible for insufficient hardening of the gear quenching, uneven hardness and insufficient hardening depth. However, according to the material, shape and heat treatment requirements of the actual quenching gear, it can be divided into different situations such as insufficient cooling speed in the high temperature stage, insufficient cooling speed in the low temperature stage, and insufficient cooling speed in the low temperature stage.
For the quenching oil, in general, the oil has a short vapor film stage, a medium temperature cooling rate, and a low temperature cooling rate, and a high and uniform quenching hardness and a large hardening depth are often obtained.
Increasing the low temperature cooling rate of the quenching medium used can often increase the depth of the hardened layer. In the case where the carbon concentration distribution of the infiltrated layer is the same, a quenching oil having a higher cryogenic cooling rate is used, and a deeper quench hardened layer is often obtained. Therefore, after the quenching oil with a fast cooling speed is used, the required carburizing time can still be obtained by shortening the carburizing time of the workpiece. The greater the depth of the required carburized hardened layer, the more obvious the effect of this method for shortening the carburizing time.
2. The hardness of the heart is too high after quenching
It is related to the fact that the selected medium is too fast or the medium is cooled too fast. One solution is to change the quenching oil to meet the requirements. The second method is to contact the manufacturer of the quenching medium and add appropriate additives to reduce the medium and low temperature cooling rate of the existing quenching oil. The third method is to switch to a steel with lower hardenability.
3. Quenching deformation problem
The cause of the deformation is attributed to insufficient quenching cooling speed and uneven speed. Based on this, a solution to improve the cooling rate and try to achieve uniform cooling is proposed. For example, the internal spline hole of the gear is deformed, which is often due to insufficient high-temperature cooling rate of the selected quenching oil, or the steam film stage of the oil is too long. Increasing the high-temperature cooling rate of the oil and simultaneously increasing the cooling rate of the oil throughout the cooling process generally solves the problem of deformation of the internal spline holes. For medium and small gears, especially for relatively precise gears, it is an indispensable measure to control the deformation by selecting the isothermal quenching oil.
4. Brightness problem
Generally, the brightness of the bright quenching oil is not high enough, and the brightening speed of the quenching oil is not good enough. In addition, the gloss of hot oil is generally poor. The longer-lasting oil is slightly less bright, and it can be replaced with new oil or added with additives to improve the brightness.
Gear quenching cooling technology