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Instructions For Controlling The Quenching Distortion Of Forged Gears

TIME : 2021-08-10 HITS : 8


Quenching of forged gears is the main link that causes distortion. The basic method to control quenching distortion is to cool all parts of the gear as uniformly as possible. In addition, there are some noteworthy issues:

1. The influence of the hardenability of the steel itself on distortion.

The higher the hardenability of the steel, the larger the volume involved in the transformation of the structure. When the workpiece is fully hardened and the whole is martensite, the volume difference between before and after quenching reaches the maximum, and the volume change of the steel with 1% carbon content is about 1%; if only half of the hardened, that is, half of the volume is quenched into martensite , The volume difference before and after quenching will be twice as small as the former. Therefore, the smaller the hardenability, the smaller the quenching distortion. On the contrary, the quenching distortion of the forged gear is greater.

In many forged gears, in order to solve the distortion problem, the method of reducing the hardness of the core is often adopted. However, considering the strength of the gear, the hardness of the core cannot be too low, because an important reason for the fatigue failure of many gears is the deviation of the core hardness. Therefore, this has become a major contradiction in gear production. In order to solve the contradiction between gear strength and heat treatment distortion on the hardness requirements of the gear core, the hardenability of steel must be reasonably limited.

Experiments show that as long as the hardenability (or core hardness) of the steel is similar, the distortion is also similar, which provides favorable conditions for controlling the distortion. For gear quenching distortion, the level of hardenability of steel is important, but more important is the bandwidth of steel hardenability, that is, the degree of fluctuation of hardenability. It is precisely because the hardenability of steel is of great significance to the quenching distortion of gears, all countries have included hardenability into steel standards. In recent years, the width of hardenable bands has been further narrowed. For example, the German "Technical Conditions for Delivery of Carburized and Hardened Steels" newly stipulated narrow hardenable steels, and the bandwidth has been reduced from 8 HRC of ordinary hardenable steels to 5 HRC. The new standard issued by our country in 2004 also reduced the width of the hardenable band compared with the original standard.

2. Forced pressure quenching.

Many domestic forged gear manufacturers advocate free quenching in concept, in order to simplify the process, facilitate operation, and reduce costs. The development of pressure forced quenching technology and equipment has also been greatly affected. Unfortunately, it is difficult to control the distortion of gears with special structures like bevel gears by free quenching. For decades, the distortion of bevel gears has plagued my country's forging gear industry. In fact, in heat treatment production, for the bending distortion of workpieces such as small twist drills and slender rods, everyone recognizes the use of pressure straightening to achieve stable mass production; and for the thin-walled and large-walled gear manufacturing in forged gears. For parts such as disc bevel gears and auto synchronizer gear sleeves, the use of press quenching can also eliminate or reduce the influence of various potential distortion factors existing in the production process and heat treatment process under strong pressure. The price paid for free quenching is much lower. Therefore, press forced quenching should be an important and indispensable process.

Recently, a moulded induction hardening process has been developed abroad for the hardening of special gears, especially bevel gears and synchronizer gear sleeves, and has achieved significant results. This new process combines the advantages of induction hardening and compression hardening, which can greatly reduce distortion, and can reduce or eliminate subsequent processes. Moreover, since induction hardening is quenched with water-based coolant, the gear after heat treatment does not need to be cleaned. At the same time, the built-in inductor can not only be used for heating and quenching, but also for heating and tempering parts without additional equipment, so the production cost is greatly reduced.

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