Instructions for Controlling Gear Quenching Distortion
Gear quenching is the main link that causes distortion. The basic method to control quenching distortion is to make all parts of the gear cool evenly as far as possible. In addition, there are some noteworthy issues:
1、 The influence of hardenability of steel itself on distortion. The higher the hardenability of 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 smaller than the former. Therefore, the smaller the hardenability is, the smaller the quenching distortion will be. On the contrary, the quenching distortion of gear is greater.
In many carburized gears, in order to solve the distortion problem, the method of reducing the core hardness is often used. However, considering the gear strength, the core hardness cannot be too low, because an important reason for many gear fatigue failures is the low core 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 requirements of gear center hardness, 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 gear quenching distortion, and all countries have incorporated the hardenability into the steel standards. In recent years, the width of the hardenability band has further narrowed. For example, the German Technical Conditions for Delivery of Carburizing and Quenching Steels has newly stipulated narrow hardenability steels, and its bandwidth has been reduced from 8 HRC to 5 HRC for ordinary hardenability steels. The new standard issued in 2004 also narrowed the width of the hardenability band compared with the original standard.
2、 Pressure forced quenching. Many domestic gear manufacturers advocate free quenching in concept in order to simplify the process, facilitate operation and reduce costs. The development of pressure forced quenching process and equipment is also greatly affected. Unfortunately, free quenching is difficult to control the distortion of gears with special structure, such as bevel gears. For decades, the distortion of bevel gears has plagued 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 automobile 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 cheaper than free quenching in general. 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. This new process combines the advantages of induction quenching and die pressing quenching, which can greatly reduce distortion and reduce or cancel subsequent processes. Moreover, since induction quenching uses water-based coolant quenching, the gear after heat treatment does not need to be cleaned. At the same time, the built-in inductor can not only heat and quench, but also be used to heat and temper parts without additional equipment, so the production cost is greatly reduced.
