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The function of vacuum heat treatment for Cutting Tools

1. Cutting tools

1.1 High-speed steel: It is an alloy tool steel containing more alloy elements such as W, Cr, and V. For example, w18cr4v is the most commonly used tool material and is widely used to manufacture various more complex tools. Such as drills, milling cutters, reamers, broaches and other forming tools.

1.2 When the tool is working, due to friction, the temperature of the tool edge will inevitably increase. When the cutting speed reaches a certain level, the edge temperature can reach 500-600°C. As the cutting speed increases, the working temperature of the tool edge may also increase. This requires that the tool material not only has the necessary hardness, strength, wear resistance and certain toughness of general tool materials, but also requires the tool to have high hardness, strength and wear resistance at higher temperatures (commonly known as red hardness or thermal hardness). rigid). Carbon tool steel and low alloy tool steel can maintain their working performance below 200°C. When the tool is heated above 250°C, the hardness drops significantly and the cutting efficiency is lost. After heat treatment, high-speed steel has good thermal hardness, so it is widely used in production practice.

2. Vacuum heat treatment of cutting tools

Drill bit, material: W18CV4V, diameter 18mm, requirement HRC63-65, deformation requirement: less than 0.5mm.
The production process of drill bits: raw materials – forging – vacuum heat treatment (vacuum annealing) – mechanical processing – vacuum heat treatment (vacuum quenching), vacuum tempering – finishing (grinding).

3. Vacuum annealing of high speed steel (drill bit)

3.1 The purpose of vacuum annealing of high-speed steel is to reduce the hardness, facilitate cutting, eliminate forging stress (internal and external stress) and prepare the structure for subsequent machining and vacuum quenching.

3.2 Vacuum annealing process: pad the workpiece to 870-880℃, keep it for 4 hours, quickly cool to 740-750℃ (cool to the bend of the C curve) and keep it for 6 hours, so that the austenite is isothermally decomposed, and then cool to 500-550 ℃ air cooling out of the oven.
The hardness after vacuum annealing was tested using a Brinell hardness tester and was HB 207-255. Its microstructure has fine carbides evenly distributed on the sorbite matrix.

4. Vacuum quenching and vacuum tempering of high-speed steel

4.1 The required hardness, strength, thermal hardness and wear resistance of high-speed steel cutting tools are obtained through correct vacuum quenching and vacuum tempering. Therefore, the vacuum quenching and tempering process determines the performance and life of the cutting tools and is the key to vacuum heat treatment. .
Its process:

4.2 Vacuum quenching process operation:
The vacuum quenching process of high-speed steel drill bits includes three parts: preheating (temperature, time), heating and insulation (temperature, time), quenching and cooling.
Quenching preheating: The purpose of preheating high-speed steel is mainly to reduce temperature differences and stress caused by temperature differences, reduce deformation, and prevent cracking. Secondly, it is to prevent decarburization and improve production efficiency.
High-speed steel contains a large amount of alloying elements, has poor thermal conductivity and low plasticity. If the workpiece is directly heated to room temperature above 1200°C, great stress will be generated, which may easily cause deformation and cracking during heating. The stress caused by overheating during cooling also increases the tendency of deformation and cracking. In order to reduce thermal stress, graded preheating is used in practice, such as 450-500°C preheating in the process, and then rising to 800-850°C preheating after heat preservation. Preheating shortens the high-temperature heating time and helps prevent the workpiece from being damaged. Oxidative decarburization.

4.3 Vacuum quenching, heating and heat preservation
After the workpiece is preheated at medium temperature at 800-850°C, it is transferred to the vacuum quenching furnace for heating at 1280°C, with a heating coefficient of 8-15 seconds/mm. According to the effective thickness and diameter of a straight shank drill bit of ￠18mm, the holding time in the vacuum quenching furnace is 10 minutes. At this time, the internal structure of the drill bit allows carbon and alloy elements to be dissolved into austenite to the maximum extent. At the same time, the austenite grains are not allowed to grow too much, creating convenient conditions for meeting technical requirements after quenching and cooling.

4.4 Quenching and cooling
The purpose of quenching and cooling is to obtain the necessary structure and properties, but deformation and cracking also often occur during cooling. Practice has proven that it is crucial to choose the correct quenching method according to the shape, size and technical requirements of the workpiece.

5 Vacuum tempering of high-speed steel drill bits

5.1 After quenching, high-speed steel can produce obvious secondary hardening during vacuum tempering, which has a direct impact on the hardness and hot hardness of the steel.
Vacuum tempering process: The tempering process of drill bits is operated in a vacuum tempering furnace. After three times of high-temperature tempering, not only the stress is eliminated, the strength and plasticity are improved, but the hardness is also increased, resulting in secondary hardening. The hot hardness of high-speed steel is also obtained during the tempering process. These two points are the salient features of high-speed steel tempering.

5.2 Effect of microstructure transformation performance during vacuum tempering
During vacuum tempering, martensite, retained austenite and carbides in the steel will all change. First, the quenched martensite transforms into tempered martensite; second, the retained austenite transforms into Quenched martensite.
During the tempering process, alloy carbides of vanadium and tungsten precipitate, reducing the content of vanadium, tungsten, and chromium and precipitating, and fine particle sizes are dispersed and distributed on the martensite matrix, causing the hardness to increase and causing secondary hardening.
Secondary hardening is also related to the transformation of retained austenite into secondary martensite during the cooling process after tempering. The transformation of low-hardness retained austenite into high-hardness secondary martensite is also the reason for the increase in hardness. .
The high hardness obtained during vacuum tempering of high-speed steel will remain high in the subsequent cutting process even if the cutting part rises to about 600°C because: 1) Vanadium and tungsten carbides, mainly vc type, both Dispersion precipitation causes secondary hardening, and it has good stability and is difficult to aggregate. 2) After the carbides have been precipitated, the martensite still contains a relatively high amount of tungsten, making it difficult for the martensite to continue to decompose. Due to these two reasons, the hot hardness of high-speed steel is guaranteed.

5.3 Analysis of vacuum tempering process
In the vacuum tempering process, the tempering temperature is 560±10℃, and the tempering times are three times for 1 hour. Why temper three times? This is because most of the high-speed steel transforms into martensite after quenching, and the amount of retained austenite is 20-25% or even higher. After the first tempering, about 15% of the remaining austenite transforms into martensite. There are still about 10% retained austenite and about 15% newly transformed untempered martensite, which will also generate new stress and have a certain impact on performance. For this reason, secondary tempering is required. At this time, 5-6% of the retained austenite is transformed into martensite. For the same reason, in order to transform the remaining retained austenite and the quenched martensite into tempered To fire martensite and relieve stress, a third tempering is required. After three temperings, about 1-3% of the remaining austenite remains.
After tempering, the W18Cr4V steel drill bit obtains: tempered martensite (alloy martensite containing tungsten vanadium), fine carbide particles and a small amount of retained austenite, with a hardness of HRC65. The microstructure is dark black martensite distributed with bright white carbide particles. After testing, it meets the technical requirements.