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The role of quenching medium in controlling the deformation of parts during heat treatment Abstract: with the rapid development of China's machinery industry, enterprises have put forward higher requirements for the quality of parts. From the point of view of quenching medium, this paper analyzes the role of professional quenching medium in controlling heat treatment deformation of parts, maintaining brightness and improving internal quality of parts

key words: metal heat treatment, quenching medium, deformation


with the rapid development of China's machinery industry and the increasing improvement of manufacturing level, enterprises have put forward higher requirements for the internal quality of parts - and the simple process, 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

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

based on the rich experience accumulated by Nanjing Kerun company in the long-term R & D, production and sales of quenchants, this paper will introduce the role of special quenchants in improving the heat treatment quality of parts, especially in controlling the heat treatment distortion of parts

I. the root causes of heat treatment deformation of parts

in production practice, the manifestations of heat treatment deformation are diverse, including expansion and contraction deformation of volume and size, as well as bending Distortion, ellipse, warpage, etc. However, as far as the root causes are concerned, 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)

internal stress plastic deformation

uneven heating and cooling and unequal phase transformation during the heat treatment of steel parts will cause the effect of internal stress. With the cooperation of certain plastic conditions of steel parts, internal stress plastic deformation will occur. The internal stress plastic deformation has obvious characteristics: ① the deformation often has obvious directivity; ② the internal stress plastic deformation usually does not change the volume of the part, but only the shape and structure of the part. ③ The most obvious feature of internal stress plastic deformation is that every time the part is subjected to the action of internal stress after heat treatment, it will produce a plastic deformation. The total deformation of the part increases with the increase of the number of internal stress actions, which is also one of the main signs that the internal stress plastic deformation is different from the specific volume deformation. According to the origin of stress and the different characteristics of stress, it can be divided into thermal stress plastic deformation and microstructure stress plastic deformation. Pure thermal stress deformation can be obtained by heating and quenching below AC1 temperature, but pure tissue stress deformation is impossible. The microstructure stress and deformation are closely related to the hardenability of steel, the section size of parts, the height of MS point of steel, quenching medium and cooling method

⑵ specific volume deformation

during heat treatment, the microstructure specific volume of various phase structures is different, and the volume and size changes during phase transformation are specific volume deformation. Under certain conditions of production practice, simple specific volume deformation can be obtained by adopting corresponding heat treatment process. It is characterized by no obvious directionality. If the steel structure 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 of heat treatment times

the specific volume deformation is generally only related to the content of carbon and alloy elements in austenite, the amount of free phase carbides, ferrite, the difference in the specific volume change of microstructure before and after quenching, the amount of residual austenite and the hardenability of steel. The composition of these structures largely depends on the quenching and cooling process. Therefore, the reasonable selection of quenching medium is also an important part of controlling specific volume deformation

II. Quenching and cooling process of steel

⑴ definition and process classification of quenching: quenching is a heat treatment method to heat the steel above the critical temperature and then rapidly cool it at a rate greater than the critical cooling rate to obtain martensite or bainite structure

quenching is one of the most important processes to strengthen parts. It integrates several strengthening mechanisms, such as solid volume strengthening, second phase strengthening, grain boundary strengthening, dislocation strengthening and so on. Before quenching, the overall dimension and geometric accuracy of the workpiece are basically achieved. Even if there is little grinding allowance, 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 quenching depth, the workpiece needs to be cooled violently, but this leads to the occurrence of quenching stress and increases the tendency of deformation and cracking. Therefore, the formulation of quenching process is far more complicated than annealing process. It is necessary to flexibly use the law of martensite and bainite transformation, determine different heat treatment requirements according to the requirements of different parts, and reasonably formulate quenching process specifications and methods. (2) quenching cooling medium: the ideal cooling medium must be in the zone where austenite transformation is most likely to occur and the cooling speed is fast. After avoiding the "nose" zone of C curve, it will be cooled slowly when martensite transformation occurs, so as to minimize the thermal stress and structural stress during martensite transformation. However, this ideal cooling medium has not been found. The ideal quenching diagram of cooling medium is as follows:

point A is the inflection point from vapor film stage to boiling stage, point B is the inflection point from boiling stage to convection stage, the temperature corresponding to point C is the characteristic temperature, and its corresponding cooling rate is the maximum cooling rate

the following diagram shows the integral curve of the cooling speed distribution curve versus time, that is, the cooling characteristic curve (T-T curve), which is combined with the C curve: point a corresponds to point a in the above figure, and point B is similar to point a. It can be seen from the above figure that the greater the cooling rate corresponding to the characteristic temperature point C, the greater the slope of the section AB curve in the following figure, and the easier it is to avoid the "nose" of the C curve, so that non martensitic transformation will not occur, ensuring the hardness after quenching. The higher the temperature corresponding to point a, the shorter the vapor film stage, and the easier it is to ensure the hardness and uniformity of the quenched steel

in the above figure, point B corresponds to the beginning of the convection phase. If the temperature corresponding to point B is too low, its market share will account for 80% of the total amount of glass fiber composites, and the boiling period will be prolonged, resulting in excessive martensitic transformation, large internal stress and large deformation. If the temperature corresponding to point B is high, the boiling period is shortened, the internal stress is reduced, and the deformation is reduced. The temperature at point B is too high. Theoretically, the cooling characteristic curve of some steels will collide with its C curve, resulting in pearlite or bainite structure and insufficient hardness of the steels. However, generally speaking, the temperature at point B is not too high, about 400 ℃, which is generally higher than that at point MS, so that martensitic transformation occurs in the convection stage. In this stage, the cooling rate is slow, so as to reduce the transformation stress and deformation. To sum up, the cooling rate, especially in the boiling period, is very important. It is required to increase the convection starting temperature, which is for small-size parts, thin-walled parts and parts with good hardenability; For large-size thick wall parts or parts with poor hardenability, in order to ensure the hardness after quenching, it is required that the steam film stage is long, quenching can be carried out, the convection temperature starts to be low, or even lower than MS point, so that the workpiece starts martensitic transformation at the steam film stage. Of course, the deformation will increase, but this is a secondary factor

at present, professional quenchant manufacturers have been able to change the cooling characteristic curve through composition adjustment, so that the quenchant is more suitable for the quenching of certain or certain parts, so as to effectively control the heat treatment deformation and improve the heat treatment quality

v. good performance of professional quenchants in actual production

professional quenchants have many advantages, such as high flash point, good thermal oxidation stability and reasonable cooling curve. They are playing an increasingly significant role in actual production and bringing good economic and social benefits to customer factories

because a few domestic professional quenchant manufacturers have mastered the core technology, adopted high-quality raw materials and strict production testing, their products have been able to replace expensive foreign imported similar products, and are better in use effect and service

the following is the quality comparison before and after the use of professional quenching oil in the heat treatment plant:

{a} {b}

the ovality deformation of a certain gear quenched with mechanical oil

(a) and quenched with isothermal grading oil kr498 (b)

the taper deformation of a certain gear quenched with mechanical oil

(a) and quenched with isothermal grading oil kr498 (b)

in bearing production, Deformation and brightness have always been a big problem for heat treatment engineers. In the face of fierce market competition and anti-dumping, major bearing factories have successively updated equipment and quenching oil. At present, due to the use of professional oil, the quality of parts in many bearing factories has been greatly improved, which not only has small deformation, but also has good brightness after quenching. Taking a bearing factory in Shanghai as an example, PLLA can also be made into high-strength surgical suture. After using kr468g special quenching oil, the part deformation data are as follows:

product No.: 6212/01 taper deformation ≤ 3 ≤ 6 ≤ 9 ≤ 12 ≤ 15 ≤ 18 ≤ 21 occupancy rate.5100

VI. conclusion

the extensive attention and use of professional quenching medium in the majority of heat treatment workshops shows its great role in improving the heat treatment quality of parts. We sincerely hope that the professional quenchant products can make greater contributions to the overall leap of China's heat treatment industry! (end)

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