Control and application of cooling characteristics of water-soluble quenching media (2)
3. Effect of aging on cooling characteristics As time passes, the water-soluble quenching medium gradually ages, although its concentration does not change, its cooling characteristics are deteriorating. This is a common feature of water-soluble quenching media, because the organic polymer breaks the polymer during long-term placement or continuous heating and cooling. Oil quenching media also have the same problem, because their own cooling rate is slow, and the relatively long aging cycle does not cause quality problems, so it is not taken seriously. The aging of the water-soluble quenching medium is shown in Figure 3. As can be seen from Figure 3, the aging characteristics of AQ251 are a decrease in Tv max and an increase in v300. This change is very unfavorable for heat treatment. The combination of changes in Tvmax and v300 is reflected in the sharp increase in the cooling rate of the medium in the low temperature stage, and the tendency of product quenching is more obvious. 4. Control method Before we realize the aging phenomenon of water-soluble quenching medium, we generally control the influence of the concentration and temperature on the cooling characteristics. The specific method is to select the appropriate concentration range according to the product material, structural characteristics and heat treatment requirements, and according to the reverse solubility of the water-soluble quenching medium, it is determined that the temperature before quenching is lower than the reverse melting point of 20 to 30 °C. The concentration is generally obtained by multiplying the refractive reading by the hand-held sugar meter by the refractive index. With the understanding of the aging phenomenon of the medium, we believe that the refractive index is affected by the kinematic viscosity, resulting in inaccurate concentration detection data, and the refractive index must be obtained periodically by viscosity detection. This approach does make concentration detection more accurate, but ignores the effects of aging on cooling characteristics. As the loss caused by the aging phenomenon gradually increases, many companies also realize that the concentration detection can not meet the control requirements of the cooling characteristics, and the method for detecting the periodic cooling characteristics of the quenching medium appears. The aging of water-soluble quenching medium is a gradual process, and the change of cooling characteristics will not be reflected quickly in one cycle. In addition, many products have a wide range of adaptation to cooling characteristics, so the cycle detection results are not judged by a qualified standard, not to mention Said to adjust the cooling characteristics. However, the control of cooling characteristics should be accepted as a consensus at this time and accepted by users. However, it is necessary to further explore the form of control. As the heat treatment product becomes more and more narrow in terms of the change in the cooling characteristics of the quenching medium, the quality problems caused by the aging of the medium are fully exposed. One case is due to the fact that depending on the material and mechanical properties of the product, the cooling characteristic value close to the quenching edge is required; the other case is that the quenching medium ages so that the cooling characteristic reaches the edge where the quenching tendency occurs. At this time, more strict control measures should be taken to prevent the cooling characteristics from developing to the edge of aging. In fact, we have found the boundary conditions for the control of the cooling characteristics. The following is an example of the AQ251 used for crankshaft quenching and tempering in our factory to introduce the control method of cooling characteristics. When the factory first used the AQ251 for crankshaft quenching and tempering, the main material used for processing was the 42CrMoA railway locomotive crankshaft. The mechanical performance requirements of the crankshaft are at a medium level. Since 1996, there has not been a locomotive crankshaft quenching and cracking accident. The cooling characteristic value V300 of 13% AQ251 has increased from 49 °C/s to 64 °C/s. In 2003, our factory entered the marine crankshaft market and began to produce 42CrMo4 marine crankshaft. The material is basically the same as 42CrMoA, and the mechanical performance requirements are greatly improved, especially for the impact work. According to the conventional heat treatment, the technical requirements cannot be met. When quenching medium is selected, it is considered to use 6% to 8% of AQ251. After the process test, small batch production, the mechanical properties meet the technical requirements of the product. However, in June 2005, there were cases of quenching cracks in seven consecutive quenched and tempered crankshafts. After analysis, it is mainly due to the aging of AQ251, but according to the analysis of the cycle characteristics of the cooling characteristics, the amplitude of the characteristic value is not as large as that of the 13% AQ251 used in the field, but the absolute value has reached the boundary of the quenching. Subsequently, we developed control methods based on various tests, combined with the use of other water-soluble media. We believe that for quenching and tempering products, the lower limit of v300 should be selected according to the technical requirements of the product and its materials, and then the upper limit of v300 should be determined according to the on-site production experience (such as the detection of cooling characteristics when crankshaft cracks). Select a cooling characteristic detection period (for example, 1 month) to record the usage during this period, and also perform cooling characteristics detection when the number of uses exceeds a certain limit. In view of the aging characteristics of the water-soluble quenching medium, the qualification criteria for the two test results should be different. For the detection of the period v300, it can be judged whether it is qualified according to the upper and lower limits. For the period detection, v300 should be based on a certain amount less than the upper limit as the criterion for judging whether it is qualified. For induction hardening, we not only control v300, but also control Tvmax. In general, since induction hardening produces only pressure on the surface of the hardened layer, cracking is less likely to occur. In fact, we have found that when the cooling process is severe due to changes in the cooling characteristics of the quenching medium, thermal stress cracks will occur on the surface of the hardened layer. The crack once appeared in more than 10 consecutive types of crankshafts we produced. The reason for the analysis is still that the quenching medium aging deteriorates the cooling characteristics, and the accident is caused by the Tvmax being too low. This makes us realize that for induction hardening, the cooling rate of the entire low temperature zone is more significant than the absolute value of v300. Therefore, for the cooling medium used for induction hardening, we believe that Tvmax should be strictly controlled like v300. In addition, we believe that the use of handheld sugar meters for on-site testing is still practical, but it should be improved that it is not necessary to convert the concentration of the medium, but only within a reasonable range of values ​​during the period of detection. Third, the use of media maintenance In the past ten years, through the use of water-soluble quenching media, we have also found that the maintenance and use of media is also very important. Because the water-soluble quenching medium will accelerate the aging speed when it is used frequently or not for a long time, a cleaning cycle should be set up in daily management, and when it is not used for a certain period of time, it should be properly stirred to slow down its aging. process. Fourth, the conclusion From the above analysis, we can conclude that the following points should be done to control the water-soluble quenching medium: Previous page 
(1) According to the product materials and technical requirements, combined with the influence of concentration and temperature on the cooling characteristics, select the appropriate concentration and temperature range.
(2) Select a reasonable cooling characteristic detection period and temporary inspection regulations, and determine the corresponding qualification standards to ensure that the cooling medium is in a qualified state.
(3) Formulate daily use and maintenance specifications and clean them regularly.