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Vacuum heat treatment of high-speed turning tools

1.High-speed turning tool

A turning tool is a tool that is used for turning operations and has a cutting portion. The working part of the turning tool is the part that generates and processes chips, including the cutting edge, the structure that breaks or rolls up the chips, the space for chip removal or storage, and the passage of cutting fluid. The cutting part is subjected to great pressure, frictional impact and high temperature. Therefore, the tool material must have high hardness, high Tang resistance, sufficient strength and toughness, and also need to have high heat resistance, and high-speed steel, which is an alloy tool steel containing more alloying elements such as w, cy, and v. . It can be used as a material for high-speed cutting turning tools.

There are many failure modes of high-speed turning tools: some turning tools are bent under pressure, and some turning tools are subjected to strong vibration, and a piece collapses when impacted (ie, the tool collapses). Some small turning tools are broken as a whole and so on. After all, these situations are relatively rare, and the most common failure form of turning tools is wear. Therefore, high hardness and high wear resistance are one of the most important performance properties of the tool, in addition to the above requirements of red hardness and certain strength and toughness. Therefore, it is very important to design a reasonable vacuum heat treatment scheme to obtain good performance. The turning tool is one of the typical high-speed cutting tools, which requires high wear resistance and hot hardness. The hardness of vacuum heat treatment is above 64HRC, usually high hardness is better.

2.1. Selection of materials

High-speed steel has the characteristics of high red hardness, good wear resistance and high strength. The hardness of high-speed tool steel after vacuum heat treatment can reach 63HRC, and it still maintains a high hardness at a working temperature of about 600°C. When the high-speed steel is at 650°C, the actual hardness is still higher than 50HRC, and its toughness, resistance to Abrasion and heat resistance are good. The cutting speed of high-speed steel tools is 1-3 times higher than that of carbon tool steel and low-alloy tool steel, and the durability is increased by 7-14 times, so high-speed steel is often used to manufacture high-efficiency cutting tools. W2Mo8Cr4V2Co8 (M42) high-speed steel is a high-performance high-speed steel with good performance indexes, and a high-performance high-speed steel with better comprehensive properties.

In order to meet the requirements of high wear resistance and high hardness of turning tools, especially in the case of large cutting amount and cutting difficult-to-machine materials and cutting long workpieces, it is suitable to choose W2Mo8Cr4V2Co8 (M42) high-speed steel.

2.2 Vacuum heat treatment process requirements

The vacuum heat treatment process plays an important role in the production and manufacture of high-speed steel tools. The quality of the vacuum heat treatment process directly affects the performance of the high-speed steel tools. Through the vacuum heat treatment process, the high-speed steel can obtain the required performance and achieve production. Processing requirements. Vacuum heat treatment process is an important way to improve the performance of high-speed steel tools. Through the vacuum heat treatment of high-speed steel tools, the hardness, strength and wear resistance of high-speed steel tools are improved, the service life is prolonged, and the high-speed steel tools can obtain higher performance.

2.3 Processing process

Blanking→forging→preparatory vacuum heat treatment (softening annealing)→cutting→final vacuum heat treatment (quenching, tempering, etc.)→grinding→grinding

2.4 Vacuum heat treatment process

Typical workpiece name: W2Mo9Cr4V2Co8 high-speed tool steel turning tool, the outline is as shown in the figure

Schematic diagram of rectangular section turning tool strip ruler

In the figure h=16mm, b=10mm, L=160mm

2.4.1 Preparatory vacuum heat treatment

(1) Technical requirements

In order to facilitate the subsequent cutting process, it is required to carry out softening vacuum heat treatment to prepare the vacuum heat treatment, so that the hardness of the workpiece after annealing is HBW≤269

(2) Process flow

Heating to 650℃→25～30℃/h Heating to 840～860℃→Holding for 0.3h→10～60℃/h Cooling to 780℃→25～30℃/h Cooling to 650℃→Air cooling or oil cooling

2.4.2 Final vacuum heat treatment

(1) Technical requirements

The final vacuum heat treatment is performed on the workpiece, so that the workpiece can obtain good wear resistance and hot hardness after treatment. The hardness HRC=65～70 is used to obtain the performance that meets the requirements of use.

(2) Process flow

Cleaning→drying→preheating (500～600℃ for the first time, 800～900℃ for the second time)→heating to 1200～1220℃ for heat preservation→cooling (oil quenching)→cleaning→drying→checking→tempering (530 ～550℃ three times tempering)→cleaning→surface treatment inspection

2.5 Analysis of vacuum heat treatment process

2.5.1 Softening Annealing

High-speed steel has a high carbon content and a large amount of alloying elements. After forging, even if it is air-cooled, it will have high hardness. Softening annealing can obtain satisfactory machinability, and prepares the structure for quenching, that is, annealing can reduce Hardness, improve machinability and obtain uniform structure, improve the performance of vacuum heat treatment process. Here W2Mo9Cr4V2Co8 adopts a high temperature annealing process of 840-860 °C. Compared with the ordinary annealing process, the holding time is greatly shortened, and the holding time in the cooling stage is almost cancelled, so the annealing cycle is greatly shortened.

2.5.2 Vacuum quenching

The quenching process transforms the supercooled austenite into martensite or bainite to obtain martensite or lower bainite structure, and then tempering at different temperatures can greatly improve the strength, hardness and wear resistance of tool steel. properties, fatigue strength and toughness, etc., thus endow the tool with the required comprehensive mechanical properties. The high-alloy W2Mo9Cr4V2Co8 high-speed steel has poor thermal conductivity. In order to prevent deformation and cracking of the workpiece during heating and shorten the heating holding time to reduce decarburization, two preheatings are used, 500-600 ℃ primary preheating, 800-900 ℃ secondary preheating. Heat can reduce quenching defects caused by rapid heating in salt bath furnaces.

2.5.3 Vacuum tempering

W2Mo9Cr4V2Co8 high-speed steel turning tools have higher requirements for hot hardness. After quenching, there is still a large amount of retained austenite in the material, and its hardness is low (40-50HRC), and then tempered at 560 °C for 3 times. During fire, the retained austenite decomposes and carbides are dispersed and hardened, and the hardness can be increased to 65-70HRC. Multiple tempering is carried out in order to gradually reduce the amount of retained austenite. The structure after tempering is tempered martensite, fine-grained residual carbide and a small amount of residual austenite.

3.Conclusion

Since high-speed steel contains alloying elements of different compositions, its as-cast structure is a skeletal ledeburite structure with a network distribution. After spheroidizing annealing, vacuum quenching and vacuum tempering, the structure is improved and the performance is improved. . There are sorbite matrix and fine granular carbides, martensite, alloy carbides and a small amount of retained austenite evenly distributed in the microstructure obtained after vacuum heat treatment. Secondary hardening is formed.

However, due to insufficient quenching, insufficient tempering time and decarburization resulting in defective structures. Therefore, in the actual operation in the future, the above reasons should be avoided, the furnace temperature and time should be accurately controlled, and the addition of protective gas quenching will make the high-speed steel have excellent properties such as high hardness, red hardness and wear resistance.

Recommended vacuum heat treatment equipment：

The vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.