Control and application of cooling characteristics

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Control and application of cooling characteristics of water-soluble quenching medium

the heat treatment process is mainly composed of heating and cooling. Once the product state is determined, the cooling medium will play a decisive role in the cooling process. The traditional quenching medium is mainly water and oil. With the continuous improvement of mechanical properties of mechanical products, the cooling characteristics of the traditional quenching medium can not meet the needs of heat treatment. The cooling speed of water at high and low temperature stage is fast, which is easy to produce quenching cracks, while the cooling speed of oil at high and low temperature stage is slow, which makes the performance unable to meet the product technical requirements. The development of ideal quenching medium, that is, the medium with fast cooling rate in high temperature stage and slow cooling rate in low temperature stage, has become a new subject and has been widely studied

in the mid-1990s, the new quenching medium represented by AQ251 introduced by Houghton company of the United States was gradually popularized and applied. It not only has good cooling characteristics, but also has the characteristics of safety, reliability and no fire hazards compared with oil, so it is welcomed by many enterprises. At the initial stage of application, AQ251 showed very good effect, but with the passage of time, the shortcomings of this water-soluble quenching medium began to be exposed, that is, its aging problem. The quality problems caused by the aging of water-soluble quenchants have occurred frequently, resulting in many irreparable losses. In 2005, the material blank loss caused by the aging of quenching medium in a locomotive factory was more than 500000 yuan, and a large number of surface quenching cracks also appeared in medium frequency quenching

therefore, it is urgent to study the use and maintenance of water-soluble quenching medium and the change law of cooling characteristics in order to find control methods

I. cooling characteristics of quenching medium

the cooling capacity of quenching medium is generally compared with the test results of cooling characteristics. The cooling characteristics consist of cooling characteristic curve and characteristic data. Cooling characteristic curve includes cooling process curve and cooling rate curve. The characteristic data consists of the maximum cooling rate (V max), the temperature at which the maximum cooling rate occurs (Tvmax) and the 300 ℃ cooling rate (V300). The cooling characteristics of quenching medium are generally tested under the same conditions by using the cooling characteristics tester, that is, the quenching temperature, medium temperature and testing time are the same. The cooling rate curve can be simply drawn by using the characteristic data

II. Cooling characteristic control

the process of controlling the cooling characteristic of water-soluble quenching medium is to find out the change law of its cooling characteristic through test and use, and then formulate control objectives and implement them according to the material and structural characteristics of the product. Due to the reverse solubility of water-soluble quenching medium, the production site also consumes a lot, which needs to be adjusted and supplemented frequently. For economic reasons, it cannot be replaced as a whole frequently. Therefore, it is more economical and scientific to detect and control the cooling characteristics

1. Effect of concentration on cooling characteristics

we used the khr-01 cooling characteristic tester produced by Zhengzhou Kehui high tech Development Co., Ltd. to carry out standard test on the newly prepared 5% ~ 20% series AQ251 solution, and the results are shown in Figure 1

as can be seen from Figure 1, the Vmax of AQ251 decreases with the increase of concentration, and Tvmax first increases and then decreases with the increase of concentration. When the concentration drops below 8%, Tvmax drops to about 600 ℃, and when the concentration is 5%, V300 increases to 87 ℃/s, which is close to the cooling characteristics of clean water. At this time, the low-temperature cooling capacity of quenching medium increases sharply. When the concentration is more than 20%, Tvmax shows a downward trend, but its cooling capacity is still low because its Vmax and V300 are very low

from the above, it can be seen that the best use concentration range of AQ251 is 10% ~ 20%. When it is lower than 10%, V Max and V300 increase sharply. In addition, PAG quenchants have an aging trend, which makes TV Max decrease. As a result, the cooling rate of the product in the low temperature zone increases and the quenching cracking tendency increases. Therefore, the product should not be used as much as possible

2. Effect of temperature on cooling characteristics

using the same testing equipment, we tested the cooling characteristics of the 13% crankshaft quenching and tempering AQ251 used on site at 20 ~ 50 ℃. With the increase of temperature, the Vmax of the medium increases, and Tvmax and V300 remain basically unchanged. However, it can be seen from the curve that although the changes of Tvmax and V300 are very small and there is no obvious change in cooling at high temperature stage, the cooling capacity at low temperature stage is still increased, so the influence of temperature on its cooling characteristics can not be ignored. For Quenched and tempered parts, especially some products with obvious quenching cracking tendency, the service temperature range of quenching medium should be appropriately limited; For induction hardened workpieces, since the hardenability is not the main factor affecting the quality, it can be considered to appropriately increase the AQ251 concentration and reduce the cooling rate at the low temperature stage without limiting its temperature

3. Effect of aging on cooling characteristics

with the passage of time, the water-soluble quenching medium is aging gradually, although its concentration has not changed, its cooling characteristics are getting worse. This is a common feature of water-soluble quenchants, because polymer chain breaking will occur in the process of long-term storage or continuous heating and cooling. Oil quenchants also have the same problems. They are not valued because of their slow cooling speed and relatively long aging cycle, which will not cause quality problems. The aging characteristics of AQ251 are that TV Max decreases and V300 increases. This change is very unfavorable to heat treatment. Due to the combination of Tvmax and V300 changes, the cooling rate of the medium at low temperature stage increases sharply, and the quenching crack tendency of the product is more obvious

4. Control method

before recognizing the aging phenomenon of water-soluble quenching medium, we generally control it according to the influence of concentration and temperature on cooling characteristics. The specific method is to select the appropriate concentration range according to the product material, structural characteristics and heat treatment requirements, and determine that the temperature before quenching is 20 ~ 30 ℃ lower than the reverse melting point according to the reverse solubility of water-soluble quenching medium. The concentration is generally obtained by multiplying the refractive reading of the handheld sugar meter by the refractive coefficient. With the understanding of the aging phenomenon of the medium, we believe that the refractive index is affected by the kinematic viscosity, resulting in the inaccurate locking data of the concentration detection mixer switch. The refractive index must be obtained through the viscosity detection regularly. This method can make the concentration detection more accurate, but it ignores the influence of aging on the cooling characteristics. As the loss caused by aging gradually increases, many enterprises also realize that the concentration detection can not meet the control requirements of cooling characteristics, so there is a method to detect the periodic cooling characteristics of quenching medium

the aging of water-soluble quenching medium is a gradual process, and the change of cooling characteristics will not be quickly reflected in one cycle. In addition, many products have a wide range of adaptability to cooling characteristics, so there is no qualified standard to judge the periodic test results, let alone adjust the cooling characteristics. However, at this time, it should be said that the control cooling feature has become a consensus and accepted by users, but the form of control needs to be further explored

as the requirements of heat treatment products for the change of cooling characteristics of quenching medium become narrower and narrower, the quality problems caused by medium aging will be fully exposed. In one case, it is necessary to approach the cooling characteristic value of the quenched crack edge according to the material and mechanical property requirements of the product; Another situation around the major strategic needs of the country and the needs of industrial upgrading in Zhejiang Province is the aging of quenching medium, which makes the cooling characteristics reach the edge of quenching cracking tendency. At this time, more strict control measures should be taken to prevent the ethane in the Middle East from meeting the local demand, so as to prevent the cooling characteristics from developing to the edge of aging. In fact, at this time, we have found the boundary conditions for the control of cooling characteristics. The following takes AQ251 used for crankshaft quenching and tempering in our factory as an example to introduce the control method of cooling characteristics

when our factory first used AQ251 for crankshaft quenching and tempering, the main material used for processing was 42CrMoA railway locomotive crankshaft. The mechanical properties of the crankshaft are required to be at a medium level. Since 1996, there has been no locomotive crankshaft quenching and tempering crack accident. The cooling characteristic value V300 of 13% AQ251 has increased from 49 ℃/s to 64 ℃/s. In 2003, our factory entered the marine crankshaft Market and began to produce 42CrMo4 marine crankshaft. The material is basically the same as that of 42CrMoA, but the mechanical property requirements have been greatly improved, especially the impact energy requirements, which cannot meet the technical requirements according to the conventional heat treatment. When selecting quenching medium, 6% ~ 8% AQ251 is considered. After process test and small batch production, the mechanical properties meet the product technical requirements. However, in june2005, quenching cracks occurred on 7 continuously quenched and tempered crankshafts. Through analysis, it is mainly due to the aging of AQ251. However, according to the analysis of cooling characteristic cycle detection, the change range of characteristic value is not as large as that of 13%aq251 used on site, but its absolute value has reached the quenching crack boundary. Subsequently, we developed control methods based on various tests and combined with the use of other water-soluble media. We believe that for Quenched and tempered products, the lower limit value of V300 should be selected according to the product technical requirements and its materials, and then the upper limit value of V300 should be determined according to the on-site production experience (such as the inspection of cooling characteristics when the crankshaft cracks). Select a cooling characteristic test cycle (e.g. 1 month) to record the usage within this cycle, and test the cooling characteristic when the usage times exceed a certain limit

in view of the aging characteristics of water-soluble quenching medium, the qualification standards of the two test results should be different. For the test V300 within the cycle, it can be judged whether it is qualified according to the upper and lower limits; For periodic inspection, V300 should be judged as qualified by a certain amount less than the upper limit. For induction quenching, we should control not only V300 but also Tvmax. In general, because induction hardening only produces pressure on the surface of the hardened layer, it is not easy to produce cracks. In fact, it is found that thermal stress cracks will occur on the surface of the hardened layer when the cooling process is violent due to the change of the cooling characteristics of the quenching medium. This crack occurred continuously for more than 10 times in a crankshaft produced by us. The analysis shows that the reason is still the deterioration of cooling characteristics due to the aging of quenching medium, and the cause of this accident is that Tvmax is too low. This makes us realize that for induction quenching, the cooling rate in the whole low temperature zone is more significant than the absolute value of V300. Therefore, for the cooling medium used for induction quenching, we believe that Tvmax should be strictly controlled as for V300. In addition, we believe that it is still of practical significance to use a handheld Sugar Meter for on-site detection. However, it should be improved that it is not necessary to convert the concentration of the medium, but only to maintain the reading within a reasonable value range during the periodic detection time

III. use and maintenance of medium

in recent ten years, through the use of water-soluble quenching medium, we also found that the use and maintenance of medium is also very important. Because the water-soluble quenching medium will accelerate the aging speed when it is used frequently or not used for a long time, the cleaning cycle should also be set up in the daily management. When it is not used for a certain period of time, it should be properly stirred to slow down the aging process

IV. conclusion

from the above analysis, we can conclude that the following measures should be taken to control the water-soluble quenching medium

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