Two noteworthy problems in controlling gear quenching distortion

Gear quenching is the main link causing distortion. The basic method to control quenching distortion is to make all parts of the gear as evenly cooled as possible. In addition, there are some noteworthy issues:



1 Effect of hardenability of steel on distortion



The higher the hardenability of the steel, the larger the volume involved in the structural transformation. When the workpiece is completely quenched and the whole is martensite, the volume difference before and after quenching reaches the maximum, and the volume change of steel with 1% carbon content is about 1%; If only 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 gears will be greater.



In many carburized gears, in order to solve the distortion problem, the method of reducing the center hardness is often adopted. However, considering the gear strength, the center hardness cannot be too low, because one of the important reasons for many gear fatigue failures is the low center hardness, so 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 gear center, the hardenability of steel must be reasonably limited.



The experiment shows that as long as the hardenability (or core hardness) of steel is similar, its distortion is also similar, which provides favorable conditions for controlling distortion. For gear quenching distortion, the hardenability of steel is important, but more important is the hardenability bandwidth of steel, that is, the fluctuation degree of hardenability. It is precisely because the hardenability of steel is of great significance to the gear quenching distortion that all countries have incorporated the hardenability into the steel standard. In recent years, the width of the hardenability band has been further narrowed. For example, the German Technical Conditions for the Delivery of Carburized and Quenched Steel has newly stipulated the narrow hardenability steel, and its bandwidth has been reduced from 8 HRC to 5 HRC of ordinary hardenability steel. The new standard issued by China in 2004 also reduces the width of hardenability band compared with the original standard.



2 Pressure forced quenching



Many domestic gear manufacturers advocate free quenching in order to simplify the process, facilitate operation and reduce costs. The development of pressure forced quenching process and equipment has also been greatly affected. Unfortunately, it is difficult to control the distortion of gears with special structure such as bevel gears by free quenching. For decades, the distortion of bevel gears has been troubling the gear industry in China. In fact, for the bending distortion of workpieces such as small fried dough twist drills and slender rods in heat treatment production, everyone agrees to use pressure straightening, so as to achieve stable batch production; For parts such as thin-walled large circular bevel gears and automotive synchronizer gear sleeves in gear manufacturing, press quenching can also eliminate or reduce the influence of various potential distortion factors in the production process and heat treatment process under strong pressure, which is much lower than the cost of free quenching. Therefore, press forced quenching should be an important and indispensable process method.



Recently, a mold pressing induction hardening process has been developed abroad for quenching special gears, especially bevel gears and synchronizer gear sleeves, and has achieved obvious results. This new process combines the advantages of induction quenching and die pressing quenching, which can greatly reduce the distortion and reduce or cancel the subsequent processes. Moreover, because induction quenching uses 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.