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Selection of Quench Media Based on Their Cooling CharacteristicsZhang Ke-Jian (Beijing HuaLi Fine Chemical Company Ltd. Beijing 102200, China) |
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Abstract: Based on the cooling characteristics of quench media, principles for selection of quench media to meet the quality requirements for the heat treated components were discussed in this paper. An area enclosed between the minimum and the maximum cooling rate distribution curves was introduced as the criterion for selection of suitable quench media. This criterion is in agree with the CRB idea illustrated in the previous paper. All the principles suggested in the paper were exercised and exemplified by case analyses which occurred in heat treating workshops in China. How to select a quench medium which can be used in common for a variety of workpieces was also discussed. Keywords: quench media; heat treatment; selection
For quenching of steel components, the following effects are expected: (1) high and uniform hardness and adequate hardened depth; (2) no quenching crack; (3) little or no distortion. Proper selection of quench media is a premise for achieving these goals. At present, the cooling characteristics of a quench medium are measured mostly by adopting the international standard (ISO9950), and are illustrated on the cooling rate curve. However, in order to quench a particular workpiece with definite shape, size and property requirements, how to decide an ideal quench medium based on its cooling characteristics? In a workshop one quench tank is usually used to treat a variety of workpieces made from different steels. How can we choose a common quench medium for such varieties? A few[1,2] research has been done on them, but no systematic practical solutions were proposed. In production, some workpieces were not sufficiently hardened when quenched in oil, while they were cracked when quenched in water. Based on the previous work[3,4,5], this paper will introduce the methods about how to choose proper quench media in accordance with the distribution of cooling rates in a particular workpiece. Further more, the principles of selecting a medium common in use for a variety of workpieces will be suggested. Minimum and maximum cooling rate for quenching a specified workpiece
Fig.1 shows the distribution of cooling rates for a mineral oil and tap water. It can be seen that the cooling rate of the mineral oil is too slow so that many workpieces can hardly be hardened, while that of tap water is too high so that many steels can not be quenched in it. There is a wide gap between the two curves for oil and tap water. Neither the mineral oil nor tap water is good for quenching. Apparently, if the cooling rate of the mineral oil is increased to the level shown by the serrated curve in Fig.2, the workpiece will be hardened in that oil. Cracking and distortion caused by water quenching are due to its rather high cooling rate. Similarly, if the cooling rate of tap water is reduced, especially in the relatively low temperature range, the risk of cracking will be minimized. Suppose the cooling rate of tap water can be decreased to the level shown by the serrated curve in Fig.3, the workpieces could be safely treated without cracking or distortion. By combining Fig.2 and Fig.3, the cooling rate distribution of the ideal medium is obtained (see Fig.4). The left curve in Fig.4 is the allowable minimum cooling rate, while the right one is called the allowable maximum cooling rate. As long as the cooling rate curves of the selected media fall into this area between the two curves, the desired three quenching effects can be achieved simultaneously.
For instance, there are two different quench media with a distinct difference in cooling rate distribution: one has a high cooling rate close to the maximum allowable cooling rate in Fig. 4; the other has a low cooling rate close to the minimum allowable curve. Because both of their cooling rates are in the allowable area, both quenching media can be adopted. In other words, the two media can result in the same satisfactory consequence when they are used to quench the specified workpieces. In our previous publication ¡°A New Method for Solving the Problem of Distortion in Quenching¡± (4), the end-quench curve is modified into a hardness-cooling rate curve (HCRC) as shown in Fig.5. For a given hardness, one can find its corresponding cooling rate from the curve. According to the effects after quenching, the cooling rate is divided into four zones. They are named the excessive cooling rate zone, adequate cooling zone, insufficient cooling rate zone and excessively slow cooling rate zone respectively. The effects of quenching carried out in those zones for the specified workpieces are summarized in the tables. It can be seen that the desired three ideal effects of quenching can only be achieved when the cooling rate is in zone II.
In the previous paper, the cooling rate band (CRB) is defined. The CRB is the cooling rate range from the maximum to the minimum in a quenched workpiece. The width of this band depends on the workpiece and the method of quenching. When the cooling rates are uniformly distributed over different portions of the workpiece, the CRB is relatively narrow, while a wider CRB means that the cooling rate distribution is not uniform. In the HCRC, a wide CRB can easily bestride different zones, while a narrow CRB frequently falls into one zone. If the CRB enters the 1st zone, cracking and distortion will occur. If the cooling rate enters zone III, the workpiece will have insufficient hardness and suffer severe distortion after quenching. Consequently, the desired three ideal quenching effects can only be achieved when the CRB falls entirely into zone II. Accordingly, all the measures proposed in this paper are to move or contract the CRB so that it falls entirely into zone II. When the cooling rate curve of a workpiece is located on the left of the minimum allowable one, the workpiece is not hardened sufficiently and suffers severe distortion. On the other hand, when the cooling rate curve is located on the right of the maximum allowable one, quench cracking occurs. Therefore, according to the different results of quenching, the diagram can be divided into three zones by the minimum and maximum cooling curves. They are zone III (insufficient cooling rate), zone II (adequate cooling rate) and zone I (excessive cooling rate), as summarized in table II and shown in Fig. 6. It is named ¡°quenching effect-cooling rate curve¡±. In practice, it is difficult to draw such a diagram for specific workpiece. Diagrams like Fig.6 are only used for qualitative analysis.
Case analysis and the five principles for selection of cooling rate distributions
Case 1: In a factory for the quenching of automobile gears, martempering oil was used . After more than one year¡¯s operation, they found the hardness of the quenched gears began to decrease, and the distortion increased. By excluding other factors, they found that there was apparent difference in cooling rate distributions between the used and the fresh oil as shown in Fig.7. As compared with the fresh oil, the vapor phase stage of the old oil after one year¡¯s use (though normal supplement was performed through the year) had shortened, the maximum cooling rate increased, and the temperature at which the maximum cooling rate occurs greatly increased. According to conventional experience, the old oil in Fig.7 is a more ideal quenching oil which could meet the principle of ¡°with higher cooling rate at high temperatures and lower cooling rate at low temperatures¡±. Actually, the used oil resulted in severe distortion and low hardness so that it is inferior to the fresh oil. Fig.8 shows that the cooling rates of the old oil at moderate temperatures are lower than the fresh oil, so that its cooling rate curve enters zone III. So the hardness is too low. Beijing Huali Fine Chemicals Ltd. had done modifications to the old oil and successfully increased the cooling rate distribution at moderate temperatures to a level slightly higher than the fresh oil. It was proved in practice that by using the modified old oil, the hardness of the quenched gears were apparently increased, and distortion was also reduced to an extent less than or equal to that quenched in the fresh oil.
Case 2: A spring manufacturer was once faced with the following problem: in their regular assembly line they suddenly found that the hardness of the quenched steel plates slightly decreased and distortion increased. During the following days when they were looking for the reasons, the hardness continued dropping, and the distortion increased further. They realized that there was something wrong with the quenching oil and contacted Beijing Huali Fine chemicals Ltd. immediately. After careful inspection, it was found that the reason for the insufficient hardness and excessive distortion was due to a leak in the pipeline of the oil cooling system. Cooling water had infiltrated into the quenching oil and emulsified it. Water content was found to be as high as 4%. Figure 9 shows the comparison of cooling rates between the water-emulsified old oil and the water free fresh oil. It can be seen that the GMT of the water containing old oil was much higher than that of the fresh oil. Since the vapor phase stage was too long, its cooling rate curve entered zone III, leading to a decrease in hardness and an increase in distortion. After having been dehydrated by Beijing Huali Fine Chemicals Ltd., the cooling ability of the oil had been successfully recovered. From the above two case studies, it can be seen that the hardening power of a quenching medium cannot be evaluated simply from its quenching severity (H) nor the time needed to cool a specified nickel ball from 850¡æto 300¡æ (GMT). The hardening power should be evaluated by observing the cooling rate distributions of the available quench medium in the workpiece, please refer to our previous publication (5). In summary, the following five principles are suggested: (1) Carbon content of the steels. In the case of low carbon steels, because proeutetoid ferrite may be precipitated first, and the ¡°nose¡± temperature as well as the Ms temperature is relatively high. As a result, in order to properly harden these kinds of steels, a quenching medium with a shorter vapor phase stage should be selected, and the temperature at which the maximum cooling rate occurs should also be higher. On the contrary, for higher carbon content steels, the vapor phase stage should not be longer and the temperature for maximum cooling rate should be lower. From the point of view of the allowable maximum cooling rate curves, for lower carbon content steels, a higher allowable cooling rate should be chosen, while for higher carbon content steels, a lower allowable cooling rate may be appropriate. (2) The hardenability of the steel. As can be seen from the minimum allowable cooling rate curve, for low-hardenability steels high cooling rate is required; while for high-hardenabilty steels, a slow cooling rate is required. At the same time, because the CCT curves tend to move downward with the increase of hardenability, for low-hardenability steels a high temperature for maximum cooling rate is required; while for high-hardenabilty steels, a lower temperature for maximum cooling rate should be required. With certain high-hardenabilty steels, the under cooling austenite tends to transform to bainite. In order to avoid bainite transformation, high cooling rates at low temperatures are required. From the point of view of maximum allowable cooling rate distributions, a high allowable cooling rate for low-hardenability steels and low allowable cooling rate for high-hardability steels are needed. (3) The effective thickness of the workpiece. When the surface temperature of the workpiece reaches the Ms temperature, the cooling rate of the medium immediately slows down, and subsequently the heat flux from the interior of the workpiece to the quenching medium also decreases greatly. Therefore, the undercooling austenite at a certain depth is difficult to cool down below the Ms temperature; consequently, only a thin layer of martensite formed after quenching. Owing to this reason, when the workpiece is large and thick, the quenching medium used should have relatively high cooling rates at low temperatures in order to obtain a sufficient depth of hardened layer. On the other hand, if the workpiece is thin and small, a medium with relatively low cooling rate at low temperatures is more desirable. From the point of view of the maximum allowable cooling rate curve, a high cooling rate should be used for thick and large workpieces while a lower allowable cooling rate should be used for thin and small workpieces. (4) The complexity of the workpiece. It can be seen from the minimum allowable cooling rate curve that for intricately shaped workpieces, particularly those with interior holes or deep hollow surfaces, a quenching medium with a shorter vapor phase stage should be selected to reduce distortion and harden the interior holes. The interior hole or its surroundings dissipate heat more slowly than the other parts of the workpiece. The other portions cool faster and so enter the boiling stage first and cool quickly. Meanwhile, the interior hole is still in the vapor phase stage with a much slower cooling rate. This apparent difference may cause severe distortion of the workpiece and insufficient hardness at the interior hole and its surroundings. The way to solve this kind of problem is to select a quench medium with a short vapor phase stage. The similar effect can be achieved by properly increasing the velocity of the quench medium inside the interior hole. Opposite to this, in the case of a simple shaped workpiece, a quenching medium with longer vapor phase stage can be selected. From the point of view of the maximum allowable cooling rate curve, the allowable cooling rate should be slow for intricately shaped workpieces while it could be quick for simply shaped workpieces. (5) The allowable distortion. It can be inferred [6] that a narrow CRB should be required for those workpieces with little allowable distortion, while for those with relatively greater allowable distortion the CRB can be wider. If the CRB is pretty wide, any ordinary quench medium that meets the hardness requirement can be chosen. Among the methods of shortening the CRB of the workpiece the simplest and most effective one is to perform martempering. A medium used for martempering should firstly have the characteristics of a short vapor phase stage and its cooling rate is insensitive to its temperature change. Secondly, for thick or large workpieces a medium with a high cooling rate should be selected, while for thin or small workpieces a medium with slow cooling rate is adequate. There are a great variety of workpieces. The requirements of different workpieces may or may not be consistent. Therefore, searching for ¡°an ideal quenching medium suitable for all workpieces¡±, just like searching for ¡°an all-purpose medicine that can cure any disease¡± is unrealistic. Selection of a quench medium to meet the requirements of a variety of workpieces As it was already explained above, every specific workpiece has its own favorite zone defined by the minimum and maximum cooling rate. However, when it is necessary to quench different workpieces in one medium, how to choose the right one suits all? Apparently, one of the prerequisites for such a common medium is that the overlapping of the second zones of these workpieces must exist and be continuous. Fig.10 schematically shows such a common second zone that could meet the requirements of two different workpieces. Undoubtedly, their common second zone must be smaller than or equal to that of the smallest among the workpieces. Because their common second zone is the narrowest, it is not easy to select a suitable quench medium in production. In fact, there are only a few couples of media to be chosen in most cases, and these media were not reasonably selected; consequently, the quality of the quenching for many workpieces is poor, particularly the hardness distribution in the cross section frequently could not meet the requirements.
By proper application of the principles discussed previously, a single quench medium suitable for a variety of workpieces can be selected. The following will discuss quench oils and aqueous solutions separately. (1). Principles for the selection of quench oils. Nearly all the quench oils are mineral oils with a relatively high flash point: their specific heat is approximately half of that of tap water, while their thermal conductivity is around 1/4 of that of tap water. In addition to the temperatures where convection starts are higher, their viscosity is also much higher than that of water. This makes the cooling rate (especially the cooling rates at low temperatures) much slow than that of water, which reduces the danger of quench cracking for most workpieces.
Because of the too low cooling rates of oils, there is a common worry that large and thick workpieces and workpieces made of poor-hardenability steels may not be hardened sufficiently or can¡¯t obtain adequate hardening depth, and severe distortion may occur. As a result, when selecting a common quenching oil, considerations should be made according to the minimum cooling rate distribution curves of different workpieces. Fig.11 shows the minimum cooling rate distributions of several workpieces. Apparently, the three ideal effects of quenching can be achieved only when the cooling rate distribution curve of the selected quenching oil encircles the minimum cooling rate distribution curves of these different workpieces from the right. Generally speaking, if the selected quench oil has a short vapor phase stage and low convection starting temperature and high maximum cooling rate, the cooling rate curve of such a kind of oil can encircle more minimum cooling rate distribution curves from the right. In other words, it can meet the requirements of more workpieces and steels. This is the principle of selecting one single quench oil for a variety of workpieces. (2). The principles of selecting aqueous quenching media. The main risk with water quenching is quench cracking, which can be reduced by lowering the cooling rate of the quenching liquid at 300¡æ. The lower the cooling rate of the aqueous quenching liquid at 300¡æ, the better its ability to prevent cracking, and the requirements of more workpieces and steel types could be met [4]. If the maximum cooling rate distribution curves of several different workpieces are drawn together in one diagram, the right boundary of their second cooling rate zone can be found. Conclusions (1). The present paper has clarified why the same workpiece can be treated with different quenching media with different cooling characteristics to meet its heat treatment requirements. (2). The present paper explained why a variety of workpieces can be treated in the same quenching medium to meet their respective heat treatment requirements. (3). For a specified workpiece, the selection of a quenching medium can be done by considering the following five factors: ¢Ù the carbon content of the steel, ¢Ú the hardenability of the steel, ¢Û the effective thickness of the work piece, ¢Ü the extent of the complexity of the workpiece, and ¢Ý the allowable distortion. (4). For a quenching oil, from the point of view of cooling rate distribution, if the oil has a relatively short vapor phase stage, high convection starting temperature and high maximum cooling rate, it can be selected for many workpieces and steel types. (5). For an aqueous quenching solution, from the point of view of its cooling rate distribution curve, the lower its cooling rate at 300¡æ the more workpieces and steel types can be treated to meet the heat treatment requirements. References [1] Karl- Erik Thelning. Proc. 5th International Congress on Heat Treatment of Metals, selected translation by the Editorial Board of the Chinese Journal ¡°Heat Treatment of Metals¡±, Machinery Workers Press, 1988,9:366¡«368. [2] S.O. Segerberg. Heat Treating, 1989, 12: 30¡«33. [3] Zhang Kejian. Heat Treating(Chinese), 1992,8: 50¡«53. [4] Zhang Kejian. Heat Treating(Chinese),1995, 3:23¡«24. [5] Zhang Kejian. Heat Treating(Chinese), 1997, 6:37. [6] Chartes E. Bates. ASM HandbookTM, Vol 4, 1990, 96-98. |
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