Quality issues in gear quenching cooling and their solutions

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Quality issues in gear quenching cooling and their solutions. Quality issues in gear quenching cooling and their solutions. Whether it is carburizing quenching, cyaniding quenching, induction heating quenching, or full heating quenching, the main heat treatment quality problems that can arise during the gear quenching cooling process include: 1. Insufficient hardness after quenching, uneven hardness in the quenched state, and insufficient depth of quenching hardening； 2. Excessively high hardness in the core after quenching ； 3. Quenching deformation out of tolerance ； 4. Quenching cracking ； 5. The surface brightness is insufficient after oil quenching. Such quality issues that occur in factories are often related to the material of the gears, pre-treatment, quenching heating, and quenching cooling. After ruling out issues related to the material, pretreatment, and heating, the role of quenching media and related technologies becomes particularly prominent. In fact, research on quenching cooling abroad in recent years has also shown that, in efforts to improve the quality of heat treatment, quenching cooling is precisely the most important factor to consider. Quenching cooling is mostly carried out in a liquid medium. Gear quenching usually uses quenching oil, water-soluble quenching media, and tap water. Therefore, below we will first analyze the relationship between the aforementioned quality issues that may arise during gear quenching cooling and the properties and usage of the quenching media used, and identify the characteristics of the cooling rate distribution required to address different problems. Next, the characteristics of the cooling rate distribution of common quenching media and the considerations for their selection are briefly introduced. I. Quality issues during quenching and cooling 1. Insufficient hardness and inadequate hardening depth A low quenching cooling rate is the cause of insufficient hardness, uneven hardness distribution, and inadequate hardening depth in gears. However, depending on the material, size, and heat treatment requirements of the gears being quenched, this can be further categorized into situations such as insufficient cooling rate during the high-temperature stage, insufficient cooling rate during the medium-low temperature stage, and insufficient cooling rate during the low-temperature stage. For example. For small and medium-sized gears, insufficient hardening after quenching is often caused by too slow cooling rates during the medium to high temperature stages; whereas for gears with a large module that require a deeper hardened layer, it is essential to increase the cooling rate at low temperatures. For quenching oils, generally speaking, those with a short vapor film stage, fast cooling rates at medium temperatures, and fast cooling rates at low temperatures tend to yield high and uniform quench hardness as well as sufficient hardening depth. The way the workpiece is mounted also has a significant impact on the quenching cooling effect. To ensure smooth flow of the quenching oil and to make proper use of the stirring device, better results can be achieved. Increasing the low-temperature cooling rate of the quenching medium used can often increase the depth of the hardened layer. When the carbon concentration distribution in the carburized layer is the same, using a quenching oil with a higher low-temperature cooling rate often results in a deeper hardened layer. Therefore, by employing a quenching oil with a fast cooling rate, it is possible to reduce the carburizing time of the workpiece while still achieving the desired depth of the hardened layer. The greater the required depth of the carburized and quenched layer, the more significant the effect of this method in reducing the carburization time. 2. Excessively high hardness in the core after quenching. Such problems may be related to an overly fast cooling rate of the chosen quenching medium, or to a high low-temperature cooling rate of that medium. One solution is to change the quenching oil to meet the requirements. The second approach is to contact the manufacturers of quenching media and add appropriate additives specifically to reduce the cooling rate of quenching oil at medium and low temperatures. The third approach is to use a steel grade with lower hardenability. 3. Quenching deformation problem: Quenching deformation has given many factories a real headache. As is common practice, resolving deformation problems usually involves multiple departments, and the solutions adopted are often comprehensive measures. Recently, articles on quenching deformation have been published, attributing the causes of such deformation mainly to insufficient cooling speed and uneven cooling. On this basis, solutions and principles for increasing the cooling speed and achieving uniform cooling are proposed, which can serve as a reference. The measures to increase the quenching cooling rate are also listed in this reference; when applying them, it is sufficient to select appropriately those measures that have the same effect and add them. This can solve most of the quenching deformation problems of gears. For example, deformation of the internal spline hole in gears is often caused by insufficient high-temperature cooling rate of the selected quenching oil, or by an excessively long vapor film stage. Increasing the high-temperature cooling rate of the oil as well as its cooling rate throughout the entire cooling process generally resolves the deformation problem of the internal spline holes. For small and medium gears, especially those with high precision, selecting and using appropriate isothermal gradient quenching oil is an essential measure to control deformation. 4. Gear quenching cracking issue: This problem mainly occurs during induction heating quenching. By selecting an appropriate water-based quenching medium, such as the PAG-type quenching media commonly used both domestically and internationally (e.g., JinYu 8-20 produced by Beijing Huali Fine Chemicals Company), to replace the tap water that was previously used, the problem is resolved. Induction heating quenching uses a PAG medium. It can achieve high and uniform hardening hardness as well as a deep and stable hardened layer, with an extremely low risk of cracking. 5. Brightness issue: In applications where this is a requirement, bright quenching oil or rapid bright quenching oil should be used. Generally, quenching oils with good brightness have a cooling rate that is not high enough, while quenching oils with a very high cooling rate do not have good brightness. Furthermore, hot oil generally has poor luster; it can be replaced with new oil or additives can be added to improve the luster. II. 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 following discusses separately the selection methods and precautions for using these media in gear quenching. 1. Tap water: Tap water is the most economical and clean quenching medium. For the quenching and tempering as well as induction heating quenching of some gears with low carbon content, poor hardenability, and simple shapes, tap water can often be used. As a quenching medium, tap water has the cooling property that it cools the workpiece very rapidly when it is at a high temperature, and it also cools it quickly when the workpiece is at a low temperature. A fast cooling rate enables the hardening of workpieces with poor hardenability and relatively large thicknesses. This is the advantage of tap water. However, quenching with tap water has three major disadvantages: first, the rapid cooling at low temperatures makes it easy for most steel grades and workpieces to suffer from quench cracking. Secondly, the workpiece cools too quickly during the high-temperature stage; slender and thin workpieces are prone to quenching deformation due to improper immersion into water. Third, and a drawback that is often overlooked, is that as the water temperature rises, the vapor film phase during quenching cooling gradually increases, and the cooling rate of the workpiece at moderate to low temperatures also decreases over time. For this reason, when workpieces are placed closely packed together for water quenching, water can only reach the inner workpieces by passing through the gaps between the workpieces on the outside. The water temperature gradually increases as it passes through the workpiece outside. In this way, the workpieces on the outside come into contact with water at a lower temperature, while those on the inside come into contact with water at a higher temperature. This results in different quenching and cooling effects for the workpieces stacked inside and outside. External workpieces cool quickly, achieve high hardness after quenching, and are prone to cracking. Workpieces stored inside experience slow cooling, resulting in low hardness after quenching. The denser the workpieces are stacked, the less smooth the flow of water during quenching, and the greater this difference becomes. This drawback makes tap water unsuitable for quenching small workpieces stacked closely together. When oil quenching is used, an increase in oil temperature slightly shortens the cooling vapor film phase. Moreover, a higher oil temperature reduces the viscosity of the oil, improving its fluidity, which helps to increase the cooling rate. This enables the internal and external areas of workpieces that are stacked closely together to be cooled to roughly the same degree. It can be said that this is an advantage of oil quenching. When using tap water as a quenching fluid, it is necessary to be aware of its advantages and disadvantages. Make use of its advantages and avoid its disadvantages. Make sure to control the water temperature properly. When quenching by stacking, it is necessary to arrange the workpieces in a loose manner and stir them to allow the quenching fluid to flow smoothly between them, thereby reducing the temperature difference of the water inside. 2. Water-soluble quenching media: The biggest drawback of tap water as a quenching medium is its extremely fast cooling rate at low temperatures, which causes various steel workpieces to crack easily. The main cause of quench cracking in steel parts is rapid cooling by water at the temperature of martensitic transformation (the Ms point) and in the temperature range below it. For this reason, the primary goal in researching and developing water-soluble quenching media is to reduce the cooling rate of water at low temperatures. Given that the Ms point of most structural steels is around 300°C, the cooling capacity of a water-soluble quenching fluid is typically expressed by the cooling rate when the workpiece is cooled to 300°C, namely the so-called 300°C cooling rate. Typically, the cooling rate at 300°C with an aqueous quenching medium is used to grade such a medium, so that heat treatment workers can choose from them. In short, the lower the cooling rate of water-based quenching fluids at 300°C, the greater their ability to prevent workpieces from cracking ； At 300°C, the cooling rate is high, resulting in a high hardening capacity; of course, the tendency for the workpiece to crack during quenching is also high. Therefore, when choosing a water-soluble quenching medium, one should first understand its cooling rate at 300°C. Among similar types of quenching media, the lower the concentration required to achieve the same cooling rate at 300°C, the lower the cost of using it. There are many types of water-based quenching media, and each type has different properties. PAG-type media have adjustable cooling properties, and their concentration is easy to measure and control. It is suitable for both overall quenching and various types of induction heating quenching, and can be used stably over a long period of time; as a result, it is widely popular and has become the most commonly used aqueous quenching medium in the heat treatment industry both domestically and internationally. Since the liquid temperature has a significant impact on cooling characteristics, a circulating cooling system should be installed when using water-soluble quenching media, in order to adjust the liquid temperature during use. Generally speaking, when quenching in water-soluble quenching fluids, workpieces should not be placed in the water in a dense pile, to avoid significant differences in the quenching results between workpieces that are close to each other. 3. General machine oils: The most commonly used general machine oils in factory heat treatment processes are N32 oil (formerly Grade 20 oil) and N15 machine oil (formerly Grade 10 oil). As quenching media, these oils are characterized by a long vapor film formation time during the high-temperature stage of the workpiece, a low quenching cooling rate, and slow cooling at low temperatures. The oil vapor film phase is long, and the cooling rate of the workpiece during the high-temperature stage is slow; a possible problem is that workpieces made of low-carbon steel are prone to pro-eutectoid ferrite transformation ； Workpieces with complex shapes, such as gears with spline holes, are particularly prone to deformation. During the medium and low-temperature stages, the cooling is slow, which makes it difficult to harden larger workpieces or results in an insufficient depth of the hardened layer, leading to quenching deformation. Conventional engine oil has poor antioxidant properties, and it tends to age and deteriorate during use. The main sign of this aging and deterioration is an increase in the oil’s viscosity and a decrease in its cooling rate at low temperatures. The effect of aging is that it improves the hardness and hardening depth of the workpiece after quenching, while increasing quenching deformation. Increased viscosity and the formation of sludge often make cleaning after quenching difficult, as well as increasing oil consumption. 4. Special quenching oils: Special quenching oils are generally classified into ordinary quenching oils, rapid quenching oils, isothermal gradient quenching oils (also known as hot oil), vacuum quenching oils, and brightening quenching oils, among others. Compared to ordinary machine oil, specialized quenching oil has better thermal stability, which enables better assurance of the quality of part quenching. Of course, the most important aspect by which dedicated quenching oils excel over ordinary machine oils is their cooling properties. Compared to ordinary machine oil, different specialized quenching oils have the characteristic of a shorter vapor film stage in the cooling rate profile, thereby allowing the workpiece to cool down more rapidly during the high-temperature phase. Among them, the highest cooling rate of rapid quenching oils is relatively high, while the cooling rate during the medium and low temperature stages varies significantly depending on the type of quenching oil used. The cooling characteristics of hot oil are such that the vapor film stage is shorter, while cooling is slower during the low-temperature phase of workpiece quenching. Quick quenching oil is mainly used for slightly thicker workpieces and steel grades with lower hardenability. Hot oil is mainly used for smaller workpieces and steel grades with good hardenability. It can be said that every quenching oil has workpieces suitable for it. However, with a few exceptions, each heat treatment furnace aims to process a wide range of steel grades and diverse types of workpieces; therefore, there is a tendency to use quenching oils with a broader range of applicability. Generally speaking, oils for quenching have a short vapor film stage, rapid cooling during the medium-temperature phase, and a high cooling rate during the low-temperature phase; such oils possess strong cooling capabilities, which gives them a wide range of applications. Deformation in many oil-quenched workpieces occurs simultaneously with insufficient quenching hardness and inadequate hardening depth. By switching to this widely applicable quenching oil, it is often possible to address issues such as workpiece deformation, insufficient hardness, and inadequate hardening depth at the same time. The vapor film phase of the quenching oil is short, meaning that the oil cools down rapidly during its high-temperature stage. This feature helps prevent the precipitation of pro-eutectoid ferrite, as well as preventing deformation in splined gears within the strip. In short, a higher overall cooling rate of the quenching oil helps to achieve a deeper quenched hardened layer. However, based on an analysis of the cooling rate distribution, aside from the requirement for rapid cooling during the medium and high temperature stages, the low-temperature cooling rate of the oil has a greater impact on the depth of the hardened layer obtained. The higher the cooling rate at low temperatures, the deeper the quenched and hardened layer tends to be. Stirring the quenching oil can increase the cooling rate of the oil. For oils with a relatively low cooling rate, stirring has a significant effect on enhancing their cooling capacity ； For specialized quenching oils with a high cooling rate, the effect of stirring is relatively minor. Another issue related to the quenching quality of gears is an excessively deep hardened layer. The hardened layer is too deep, often resulting in the use of interrupted teeth. One effective way to solve such problems is to reduce the low-temperature cooling rate of the quenching oil. In summary, when selecting quenching oil to ensure the quality of gear quenching, the choice should be made based on the steel type of the gears being treated, their geometric characteristics, and the requirements for heat treatment, taking into account the cooling rate profile of the oil. It is advisable to work together with the technical experts from the quenching oil manufacturers to determine the appropriate quenching oil and the proper methods of use through discussion and analysis. III. Changes in quenching media during use: Whether it is quenching oil or water-soluble quenching media, they come into contact with high-temperature workpieces during use and are subject to varying degrees of contamination. When exposed to air, the medium gets oxidized. High temperatures can cause reactions such as thermal decomposition, oxidation, and polymerization of organic media. Pollution may make the oxidation and other changes of the medium more complex. 1154973479l"> All these changes and their resulting products remaining in the medium will cause the deterioration of the medium. Changes in the color, transparency, and viscosity of quenching oil are all signs of deterioration. It can be said that there is no medium that does not deteriorate. There are only three things that concern us: first, the effect of changes on the cooling properties of the medium; second, the speed at which deterioration occurs; and third, the methods used to understand and correct the effects of this deterioration in order to ensure long-term stable quenching quality. As mentioned earlier, water-soluble quenching agents are primarily used to reduce the low-temperature cooling rate of water. The trend in the properties of water-soluble quenching fluids during use is exactly the opposite: these changes result in an increasing cooling rate at low temperatures, in an attempt to return to the conditions prior to the use of a quenching agent. The trend in the performance of regular engine oil over time is, in simple terms, that the cooling rate at low temperatures gradually decreases at first, the vapor film phase becomes shorter, while the cooling rate at medium and high temperatures increases slightly. As the usage time increases, the viscosity of the oil further rises, which in turn slows down its cooling rate at medium and high temperatures; as a result, the cooling effect on the workpiece becomes significantly worse. The changes that occur in specialized quenching oil during use are relatively complex; they consist of changes in the additives added to it as well as changes in the base oil, and represent the combined result of these two types of changes. Different quenching oils and various operating conditions can result in significant variations. It should be noted that, under conditions free from water contamination, the low-temperature cooling rate of almost all dedicated quenching oils gradually slows down over time with prolonged use. When used for a longer period of time, the quenching effect on the workpiece also deteriorates significantly. The oil has good stability and deteriorates slowly ； Oil has poor stability and deteriorates quickly. The higher the operating temperature of the oil, the faster it deteriorates. Measures such as equipping a circulating cooling system to keep the oil temperature within an appropriate range, and using circulation stirring in the tank to prevent localized overheating, can all slow down the rate of oil degradation and extend its service life. The rate of deterioration of different media is influenced not only by the variety and quality differences of those media, but also by one common factor: within the same period, the greater the quantity of quenched workpieces, the more severe the deterioration of the quenching fluid. In many cases, the amount of quenched workpieces referred to here should mean the total surface area of the quenched workpieces. The smaller the workpiece, the larger the total surface area per unit weight, and the more the quenching medium deteriorates. Management issues are also significant problems that cannot be ignored when it comes to the quality of gear quenching. In addition to following the manufacturing procedures strictly, the management of quenching media is crucial, especially to prevent contamination. When quenching oil mixes with water, resulting in water being emulsified in the oil, it often leads to insufficient quenching hardness or quenching cracks. On the contrary, the emulsification of oil in the PAG quenching solution can sometimes also cause quenching cracks. Changes in the color, transparency, and other aspects of the quenching medium can reflect its degree of degradation. Measuring changes in the viscosity, flash point, residue, acid value, etc., of the quenching oil can also determine the degree of deterioration. Among the effects caused by transformation, the change in cooling characteristics has the greatest impact on the heat treatment results of the workpiece. To ensure the effective quenching and cooling of gears, it is recommended to regularly test the cooling properties of the quenching oil and media used, as well as to manage these cooling properties. Generally, small factories do not need to have cooling property testers on hand; they can instead send samples to facilities equipped with such instruments for testing. The quenching medium manufacturer shall conduct this testing for the user’s factory. To determine the cooling characteristics of quenching media, it is advisable to use a cooling characteristic tester that complies with international standards (ISO9950). To analyze the cooling characteristics of a medium, one should consider the distribution of its cooling rates, rather than just looking at the highest cooling rate value. Media with the same maximum cooling rate, or the same cooling time to 300°C, can exhibit significantly different heat treatment results due to their varying cooling rate distributions. The relationship between the characteristics of the medium’s cooling rate distribution and its cooling efficiency was briefly discussed earlier; for more details, please refer to the references at the end of this article. By recording the quenching hardness, hardening depth, and quenching deformation of similar workpieces over a long period of production and analyzing their trends, we can understand the patterns of change in the cooling properties of the quenching medium. With these records, we can not only control the cooling characteristics of the medium but also analyze the causes of heat treatment accidents on site, thereby resolving such quality issues in a timely manner. As mentioned earlier, medium deterioration is inevitable. When it has been in use for a short time or when not a large amount of material has been quenched in the workpiece, the degree of degradation is low, and the problem of degradation of the medium is not reflected in the quenching results of the workpiece. As the amount of quenching is increased further and the degree of transformation rises to the point where the workpiece does not meet the required quenching standards, the problem of transformation truly arises. As the quenching amount increases further, more transformation occurs, and the quality of the quenched workpiece becomes worse. The main problem that may arise when a water-soluble quenching medium deteriorates is that the hardness of the workpiece after quenching becomes too high, resulting in cracking of the workpiece. Therefore, the degree of deterioration of water-soluble quenching fluids can be monitored by observing the trend in quenching hardness of similar workpieces, and it is best to take measures before quench cracking occurs. When specialized quenching oil deteriorates during use, the main problems that may arise are lower hardness of the workpiece after quenching, insufficient hardening depth, and significant deformation. Therefore, the degree of degradation of the quenching oil can be monitored based on the trends in the hardness and hardening depth of the workpiece. After the quenching medium deteriorates, can adding more of the original fresh medium restore the cooling properties of the quenching fluid? The answer to this question is: some media can, while others cannot. For water-based quenching media with adjustable concentration, such as PAG-type water-soluble quenchants, it is generally possible to restore the cooling properties to their original level by adding more of this medium, based on an understanding of its behavior changes. An aqueous quenching fluid with fixed formula proportions cannot be used. Ordinary quenching oil and specialized quenching oil also won’t work. After the quenching oil deteriorates, simply adding more of the original fresh quenching oil generally cannot restore the cooling properties of the oil to those of new oil. However, once the variation patterns of the oil are understood, its cooling properties can be restored through modified additives. IV. Quenching media and technical services provided by Beijing Huali Fine Chemicals Company. Beijing Huali Fine Chemicals Company not only produces various quenching oils and PAG-based water-soluble quenching agents that can serve as alternatives to foreign products, offers long-term after-sales service, and conducts tests on the cooling properties of quenching fluids used in customers’ factories, but it also provides technical expertise and related products to improve domestic and imported quenching oils whose performance has declined over time, helping to restore their cooling properties to those of new oils. All these technologies and products can be used to help factories address the quality issues in gear quenching and cooling listed at the beginning of this article.