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The Role Of Quenching Medium In Controlling Heat Treatment Deformation Of Parts

With the rapid development of my country’s machinery industry and the increasing level of manufacturing, enterprises have put forward higher requirements for the inherent quality of parts – heat treatment quality. People not only pay attention to the heating of heat treatment, but also pay more and more attention to the cooling of heat treatment.

The two major problems in heat treatment at present are insufficient deformation and uneven hardness, which are closely related to the selection of quenching medium. Therefore, the scientific use of special quenching medium can greatly improve the internal and external quality of heat treatment of parts.

1. The root cause of heat treatment deformation of parts
In production practice, the manifestations of heat treatment deformation are various, including expansion and shrinkage deformation of volume and size, as well as deformed deformation such as bending, twisting, ellipse, and warping. But in terms of its origin, it can be divided into two categories: stress plastic deformation caused by internal stress and volume deformation caused by specific volume change (hereinafter referred to as specific volume deformation).

(1) Internal stress plastic deformation
In the process of heat treatment of steel parts, the non-uniform heating and cooling and the unequal time of phase transformation will cause the effect of internal stress. Under the cooperation of certain plastic conditions of steel parts, plastic deformation of internal stress will occur. Internal stress plastic deformation has obvious characteristics:

① deformation often has obvious directionality

② internal stress plastic deformation usually does not change the volume of the part, but only changes the shape, structure and shape of the part.

③ The most obvious feature of plastic deformation of internal stress is: every time the part undergoes a heat treatment of internal stress, a plastic deformation will occur, and the size of the total deformation of the part increases with the increase of the number of times of internal stress, which is also Internal stress plastic deformation is one of the main signs that distinguish it from specific volume deformation.

According to the source of stress and the different performance characteristics, it can be divided into thermal stress plastic deformation and tissue stress plastic deformation. Deformation by heating and quenching below AC1 temperature can obtain pure thermal stress deformation, but pure tissue stress deformation is impossible. The stress deformation of the structure is closely related to the hardenability of the steel, the section size of the part, the Ms point of the steel, the quenching medium and the cooling method.

⑵ Specific volume deformation
In the process of heat treatment of steel, the specific volume of various phase structures is different, and the volume and size changes that occur during phase transformation are specific volume deformation. Under certain conditions of production practice, a relatively simple specific volume deformation can be obtained by using the corresponding heat treatment process. Its characteristic is that there is no obvious directionality. If the structure of the steel is uniform, the specific volume deformation is the same in all directions, and the specific volume deformation will not continuously change the volume and size of the parts due to the increase in the number of heat treatments.
The specific volume deformation is generally only related to the content of carbon and alloying elements in austenite, the amount of free phase carbides, ferrite, the difference in specific volume change before and after quenching, the amount of retained austenite, and the hardenability of steel. related. These microstructures depend largely on the quenching and cooling process. Therefore, the reasonable selection of quenching medium is also an important part of controlling the specific volume deformation.

2. Quenching and cooling process of steel
(1) Definition and process classification of quenching: Quenching is a heat treatment method in which the steel is heated to a temperature above the critical temperature and then rapidly cooled at a rate greater than the critical cooling rate to obtain a martensite or bainite structure.
Quenching is one of the most important processes to strengthen parts. It combines several strengthening mechanisms such as solid volume strengthening, second phase strengthening, grain boundary strengthening, and dislocation strengthening. Before quenching, the overall dimensions and geometric accuracy of the workpiece are basically achieved, even if there is little grinding allowance, so quenching should not only ensure good structure and performance, but also maintain its dimensional accuracy. In fact, the two are often contradictory. In order to obtain sufficient hardening depth, the workpiece needs to be cooled violently, but this leads to the occurrence of quenching stress, which increases the tendency of deformation and cracking. Therefore, the formulation of the quenching process is far more complicated than the annealing process, and the law of martensite and bainite transformation must be flexibly used, according to the requirements of different parts, determine different heat treatment requirements, and reasonably formulate quenching process specifications and methods.

(2) Quenching cooling medium: The ideal cooling medium must be in the area where the austenite transformation is most likely to occur, and the cooling rate is fast, and after avoiding the “nose” area of the C curve, it should be cooled slowly when the martensite transformation occurs, so that the martensitic transformation occurs. The thermal stress and tissue stress generated during bulk transformation are minimal, but this ideal cooling medium has not yet been found.

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