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Vacuum Heat Treatment Furnace

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Pay Attention to Vacuum Tempering to Improve the Quality of Quenched Parts

Vacuum tempering of quenched parts is the last process of heat treatment of workpieces. The purpose of vacuum tempering is to prolong the service life of the workpiece. Its function is: stabilize the structure, transform the metastable quenched structure into a more stable tempered structure; stabilize the size, eliminate the quenching stress, reduce brittleness, and avoid deformation and cracking of the workpiece; improve performance, so that the workpiece can obtain the required strength, toughness and hardness. It can be seen that vacuum tempering is an important process of vacuum heat treatment, but it is often neglected and causes a lot of losses.

Relationship between Microstructure and Properties of Vacuum Tempering and Technology

The performance of the steel parts after vacuum tempering is mainly determined by the vacuum tempering temperature. In actual production, the vacuum tempering temperature is selected according to the hardness requirements of the steel parts. Due to the deviation of the amount of elements in the steel parts, as well as different quenching parameters and size, etc., the hardness after quenching will be affected, so the vacuum tempering temperature is finally selected by the actual quenching hardness. When choosing, pay attention to avoiding the brittle zone of vacuum tempering, or take measures.

The holding time of vacuum tempering also has some influence on the hardness of steel parts. For the same hardness requirement of a steel piece, it can be obtained by vacuum tempering at a lower temperature for a longer period of time, or at a higher temperature for a shorter period of time. Therefore, the determination of the vacuum tempering time is considered together with the vacuum tempering temperature. At the same time, the effective thickness of the steel piece, steel type, heating medium, and furnace loading are also considered. In terms of structure transformation and reduction of hardness, the half hour of vacuum tempering changes the most. For high temperature vacuum tempering, the hardness will be significantly reduced in one minute. The relief of stress is also the most obvious in the early stage of vacuum tempering. Therefore, it is generally sufficient to use vacuum tempering for 1 to 3 hours. Of course, it is better to confirm after summarizing and verifying in production practice.

Application of Vacuum Tempering

The service performance and service life of quenched parts are related to factors such as material smelting quality, forging process, vacuum quenching process, steel type selection, vacuum heat treatment, etc. These factors are like the links in the chain. Any problem in any link will lead to early failure of the workpiece. Among them, vacuum tempering treatment is equally important, so in actual operation, it is necessary to carefully select the temperature and holding time, complete the vacuum tempering treatment correctly, and give full play to the role of vacuum tempering to achieve good results.

Improvement of Vacuum Tempering Process

With the in-depth development of material science and the continuous updating of metal materials, the requirements for workpieces and processing technologies vary greatly, and the vacuum heat treatment process is also changing with the development. At the same time, based on theory and practice, the vacuum tempering process is constantly being optimized and improved.

For small cutting tools such as high-speed steel drill bits, try the vacuum tempering method twice, that is, the first vacuum tempering at 585-590°C, and the second vacuum tempering at 560°C. The result is that the cutting life is not lower than the conventional three-time vacuum tempering at 560°C;

Hierarchical low-temperature vacuum tempering method, that is, the vacuum tempering process of high-speed steel tools after vacuum quenching is: 320 ~ 380°C x 1h once + 560°C x 1h twice, resulting in a service life that is more than 40% higher than the traditional three-time vacuum tempering method. There is also a process of low quenching and low return, that is, w18Cr4V steel is vacuum quenched at 1180-1200°C and vacuum tempered at 500°C, and the life of the processed punch is increased by 7 to 8 times;

3Cr2W8V steel hot stamping die, the vacuum quenching temperature is increased to 1150 ~ 1200 ℃, and the vacuum tempering temperature is increased to 680 ℃ (conventional 560 ~ 620 ℃), its service life is three times that of the conventional treatment method, and the failure is not cracking, but deformation and fatigue failure. A factory’s 3C2w8V steel vise mold was originally heated at 1150°C, then air-cooled, and then vacuum tempered at 580°C for three times. It was often cracked during use, but instead of tempering, the service life was improved, and it was invalid due to wear, and it also saved energy consumption.

Due to the high Ms point of low carbon steel (such as 16Mn is 410 ℃), tempering will occur spontaneously during the rapid cooling process of forming martensite by vacuum quenching, and the separate vacuum tempering operation can be avoided. Self-tempering can be used for induction heating and quenching of large-sized parts, and separate vacuum tempering is also saved.

Some hot oil quenched parts after carburizing can no longer be tempered, especially for workpieces working under bending fatigue load, which can make full use of higher residual compressive stress, obtain higher fatigue strength and multi-impact resistance, and simplify the process and save energy consumption, but attention should be paid to problems such as stable size and grinding cracking tendency, and measures should be taken.