Cooling medium and quenching deformation

Those who are initially involved in the heat treatment process are prone to the recognition that the faster the quenching medium cools, the greater the quenching deformation of the workpiece. In fact, the choice of quenching media is not so simple. The quenching deformation is a problem that must be solved under the premise that the workpiece is not quenched and the quenching hardness and the depth of the hardened layer can meet the requirements. Therefore, a workpiece that is quenched under any particular conditions has a cooling rate range that is most suitable for its quenching medium. Excessive cooling rates can cause quench cracking and excessive quenching deformation. The too slow cooling rate can not only harden the workpiece, but the quenching deformation problem is often more serious. Generally speaking, the cooling rate of the oily medium is slow, while the cooling rate of the aqueous medium may be very fast. In addition to paying attention to the cooling speed of the medium, the influence of the use conditions and the use method on the cooling rate is a problem that must be paid attention to. Compared with oily media, the change of water temperature has a great influence on the cooling characteristics of aqueous media. Therefore, the aqueous medium is particularly suitable for use in single-piece quenching and in the case of mesh belt furnaces where the workpiece can be sprinkled and quenched. The oily medium is suitable for both single-piece applications and for simultaneous quenching of multiple pieces.

A large number of production practices have shown that under the conditions that can meet the requirements of hardening, the quenching deformation of the workpiece after quenching in oil is usually smaller than that in the aqueous medium; the quenching deformation in the hot oil is smaller. Compared to oily media, workpieces that are autoclaved or quenched in a low temperature salt bath are less deformed. The quenching deformation in the oil is large, mainly manifested by the large degree of dispersion of the deformation. The reason is that we believe that when the workpiece is quenched in aqueous and oily media, it always undergoes a transition from the vapor film stage to the boiling cooling stage. This shortcoming is also referred to as the "characteristic temperature problem" of aqueous and oily media. This problem does not exist with high pressure gas quenching or quenching in a low temperature salt bath. (For details, please refer to the "Cooling speed belt method for analyzing and controlling quenching deformation problems" serialized in the first to sixth issues of 2006.)

Although aqueous and oily media have long been widely used, we don't know much about their cooling mechanism. Recent studies have found that in quenching and cooling in aqueous and oily media, a small area on the surface of the equivalent thickness of the workpiece (referred to as "surface point"), the end of the vapor film stage and the temperature value of the surface point There is no uniquely determined correspondence between them. The experimental observation is that in a certain temperature range, among the many surface points having the same equivalent thickness on the same workpiece, which point changes from the vapor film envelope state to the boiling cooling state at what temperature, but it is said that Quasi, that is, there is considerable randomness. Since the current general theory of three-stage partitioning of quenching and cooling in liquid media cannot explain such phenomena, we propose a "four-stage theory of quenching cooling in liquid media". Compared with the current three-stage division, an "intermediate stage" is added to the four-stage theory. It is because of the existence of the intermediate stage that the so-called characteristic temperature problem is caused. Some technical methods have been developed using the four-stage theory. These techniques not only control the temperature range that occurs in the intermediate stage, but also allow any particular part of the workpiece to complete the intermediate stage of the process within a set time frame. According to the four-stage theoretical knowledge, the quenching and cooling process of the workpiece is recognized, and the quenching cooling technology of the intermediate stage of the workpiece cooling is controlled by the related technical method, which is called “fine quenching cooling technology”. We hope that through the multi-faceted participation of the heat treatment industry and the promotion and application from easy to difficult, the fine quenching and cooling technology can be gradually improved. In the future, the application of fine quenching and cooling technology will overcome the characteristic temperature problems of aqueous and oily quenching media. In the near future, the degree of quenching deformation of workpieces quenched with oily or even aqueous media is likely to be reduced to the level of high pressure gas quenching and low temperature salt bath quenching. (For details on the four-stage theory and fine quenching and cooling technology, please refer to: "Heat Treatment Technology and Equipment", Issue 6, 2006, and 10 articles are being published continuously).

At present, forging blank isothermal normalizing has been promoted and applied in the domestic heat treatment industry. Compared with ordinary air-cooled normalizing, isothermal normalizing can obtain a more excellent and uniform preparation. Therefore, the quenching deformation generated in the subsequent quenching or carburizing quenching is smaller. The key process of isothermal normalizing production is the rapid cooling of the forging billet after normalizing. The requirement for rapid cooling in the early stage is to lower the temperature of the forging blank to the set isothermal temperature before the pearlite transformation occurs in the supercooled austenite. Only in this way can we ensure a uniform isothermal transformation product of the forging blank. Due to the different steel grades and sizes of different workpieces, there should be a variety of cooling media available for quick cooling in the early stage of isothermal normalizing. For workpieces made of steel of smaller size and better hardenability, a cooling medium with a slower cooling rate may be used. For workpieces made of steel of larger size and poor hardenability, the cooling rate must be selected. Cooling medium. The cooling media currently available for quick cooling in the early stage of isothermal normalizing are arranged in the order of their cooling rate from slow to fast as follows:

Normal air cooling - fast air cooling - constant speed coolant - oil - aqueous medium - water.
In the case where the size is large and oil is required, in order to avoid the occurrence of pyrotechnics, an aqueous medium may be replaced.

In addition to the isothermal normalizing of the forging billet, the secondary heating and quenching of the workpiece after carburizing, the cooling speed after carburizing and the quality and uniformity of the obtained microstructure, the intrinsic quality, deformation degree and grinding of the workpiece after reheating and quenching Cracking sensitivity and so on have a significant impact. This method can also be solved by using a method equivalent to isothermal normalizing. The key to doing this work is also to choose the appropriate pre-cooling medium.