High-Performance High-Speed Steel Tool Materials and Heat Treatment Process Revealed

High-Performance High-Speed Steel Tool Materials and Heat Treatment Process Revealed

• Admin
• 2024/10/22

Since its inception, high-speed steel has been widely used as the most basic material for cutting tools in machining, especially the large-scale application of high-performance high-speed steel and powder high-speed steel, so that precision and complex high-speed steel tools still occupy a large share of the tool market. . High-performance high-speed steel can be divided into electric arc furnace smelting high-speed steel, intermediate frequency induction furnace smelting high-speed steel, electroslag remelting smelting high-speed steel, powder metallurgy high-speed steel, etc. according to the final smelting method.

Compared with high-speed steel tools, despite the differences in high-speed steel smelting methods, the durability and service life of high-speed steel cutting tools are more directly related to the chemical composition and heat treatment of high-speed steel. Therefore, it is particularly important to establish the corresponding relationship between “chemical composition-organizational structure-mechanical properties” and select suitable materials and heat treatment processes to ensure the performance of high-speed steel tools.

1. High-performance high-speed steel classification and tool selection

(1) The basic characteristics and classification of the chemical composition of high-speed steel

High-speed steel belongs to high-carbon, high-alloy tool steel for its metallurgical composition. Among them, the content of carbon element generally ranges from 0.7% to 1.6%; the sum of the weight percentages of the precious main alloying elements such as W, Mo, Cr, V, and Co is between 10% and 40%. High-speed steel can be divided into W-series high-speed steel; Mo-series high-speed steel; W-Mo-series high-speed steel and high-performance high-speed steel containing Co, AL, and high V according to the different proportions of the main alloy elements; Another composition characteristic of other types of steel is that the content of Cr element is fixed at wCr=4%.

(2) Main grades and tool selection of general high-speed steel

Among high-speed steel tools, the proportion of tools made of general-purpose high-speed steel is the largest. Since the 1940s, after the composition of W6Mo5Cr4V2 (M2) general-purpose high-speed steel was established, the development and evolution of various high-speed steel grades are mostly based on this brand Unfolding, the material workers passed the assessment of W equivalent, replaced W with Mo, and added Co, AL, V and other elements to improve performance for alloying and smelting, increasing the number of high-speed steel grades by more than 100. M2 steel has fine carbide structure, good carbide distribution and good steel strength, plasticity and toughness. General-purpose high-speed steel can be used in the manufacture of general cutting tools for various machining. Usually the hardness of the workpiece is not more than 280HBW, and the cutting speed is not more than 25-40m/min.

(3) Main grades and applications of high-performance high-speed steel

Unlike general-purpose high-speed steel, which pays more attention to practicability, high-performance high-speed steel is more seeking to improve the comprehensive mechanical properties of tool materials, increase the cutting speed of the tool, and strive for a longer service life, which meets the high performance needs of modern cutting development. High-performance high-speed steel generally refers to steel grades whose heat treatment hardness, temper softening resistance, and wear resistance are significantly higher than those of general-purpose high-speed steel. According to the characteristics of alloying, high-performance high-speed steel can be divided into several types of high-performance high-speed steel such as no cobalt, cobalt-containing, high-vanadium, and cobalt-containing high-vanadium. Table 1 shows several types of high-performance high-speed steels that are widely used.

M42 (W2Mo9Cr4V2Co8) is the first choice of high-performance Co-containing high-speed steel in the tool industry for many years. The steel is based on the general high-speed steel M7 (W2Mo9Cr4V2) by adding 8% Co. This steel grade retains the molybdenum high-speed steel The characteristic of good toughness also significantly improves the cutting performance. M42 steel has high hardness, high wear resistance, high red hardness, and is mainly used for the processing of military products, high-strength quenched and tempered steel, stainless steel, and heat-resistant steel. The disadvantage of M42 steel is that it is more brittle in use, and the tool is prone to premature failure such as tooth chipping, tooth loss, brittle fracture, etc. Recently, it has a tendency to be replaced by M35 with both toughness and strength. The hardness range of M42 and M35 in use is generally controlled by our company: M42 66~68 HRC, M35 65~67HRC. Due to the high price of Co high-speed steel, it is mostly used in the manufacture of precision and complex tools.

(4) Selection of powder metallurgy high speed steel

The production of high-speed steel by powder metallurgy is a relatively advanced new high-speed steel smelting and production process developed in recent decades. Because the molten steel is forced to atomize into ultra-fine droplet particles and quickly cooled into fine powder, the high-speed steel casting structure And carbides have been fully refined, completely changing and improving the overall performance of high-speed steel.

In the 1970s, with the advent of foreign powder atomization technology and the application of large-scale cold and hot isostatic presses, the production of powdered high-speed steel developed rapidly. Powder high-speed steel solves the problem of carbide segregation and greatly improves the strength and toughness of steel. In particular, the difficult forging and rolling of high-alloy and high-performance high-speed steel produced by traditional processes and the high forging waste can be completely controlled. Powder high-speed steel also makes the production of ultra-high alloy content high-speed steel that cannot be achieved by the casting and forging process a reality. At present, the production of powder high-speed steel is mainly concentrated in a few developed countries such as Sweden, the United States, Austria, France, Japan, and the former Soviet Union. China does not yet have a powder high-speed steel manufacturer.

Therefore, the development and production of powdered high-speed steel produced in China has a broad market prospect. The main grades and characteristics of powder high-speed steel are shown in Table 2.

2. Overview of heat treatment process selection for high-performance high-speed steel tools

(1) Quenching heating temperature selection

The content of carbon and main alloying elements such as W, Mo, Cr, and V in high-speed steel determines the temperature range of heating and quenching during the final heat treatment of the tool. The final quenching heating temperature is the most important process parameter in the heat treatment process of the tool. The appropriate quenching heating temperature can ensure that the tool obtains the necessary high hardness (including secondary hardening), high red hardness and good toughness, which will meet the requirements of machine tools and The machining parts require high cutting performance of the tool.

Generally speaking, high-speed steel has a small equilibrium carbon difference and high carbon saturation. It is better to choose a lower heating temperature for quenching. Through research on M2 steel, it is found that for every increase of 0.1% C, the temperature at which the grain boundary melts drops by 11°C; on the contrary, carbon The saturation is low, and quenching and heating can be selected at a temperature above the middle limit, so that the carbide can be more fully dissolved, and higher thermal hardness and wear resistance can be obtained. The morphology and distribution, dissolution and precipitation of alloy carbides present in high-speed steel also affect the selection of the quenching heating temperature. There are many large-sized carbides, severe segregation of carbides, high microstructure levels and large segregation of components. Select the temperature below the middle limit for quenching.

(2) Selection of quenching temperature for different tool types and control of quenching and tempering organization

Under different service conditions, various machining environments have different requirements for tool performance. Under normal circumstances, simple shapes, continuous cutting, single-edge semi-finished products cutting tools, heat treatment can choose higher heating temperature quenching; complex shapes, large thickness differences, large specifications, slender and other high-speed steel tools can choose medium and low temperature limits Quenching.

In addition, for rods, toothed circular saw blades and other products that need to be locally heated in a high-temperature salt bath furnace, the upper limit temperature quenching can be selected; while the whole product is heated in a high-temperature salt bath furnace, for small and medium specifications, The upper limit quenching heating temperature can be selected, and the lower limit quenching temperature can be selected for large specifications. Table 3 takes M42 steel as an example. The quenching heating temperature range, quenching grain size, and overheating qualification level requirements of some tool products are required. The tempering requirements of various high-speed steel tool products must be less than level 2.

(3) Quenching heating time selection and heat treatment of oversize tools

The quenching heating time selection is closely related to the quenching heating temperature. For high-speed steel products, the quenching heating temperature is the first choice. Within the quenching process temperature requirements, the quenching heating temperature is higher, the quenching heating time can be relatively reduced, and the quenching temperature Lower, the quenching heating time can be relatively increased. Generally, the quenching heating time can be calculated according to the effective thickness of the product, and the heating coefficient can be 8-10s/mm. The effective thickness is very small, the upper limit is acceptable, and the minimum heating time for extra-small parts is not less than 2min; the effective thickness is larger, and the lower limit can be removed. The heating time for oversized broaches, hobs, etc. can be selected according to the actual equipment and pressure temperature on site Depending on the situation, it can generally be heated for 8-10 minutes after the indicated temperature of the high-temperature heating furnace returns to the set value through burning. When the product is heated and cooled in the salt bath furnace, a single piece of heating must be maintained.

In addition to the selection of reasonable quenching heating temperature, holding time, and selection of suitable tooling tools, the heat treatment of extra-large-size tools is another important process key to take measures to ensure the heat treatment hardness requirements of the tools and avoid heat treatment cracking or heat treatment of the tools. Severe heat treatment deformation. In the actual heat treatment production process, through the accumulation of years of work experience, the following measures can often achieve better results:

• Improvements must be made to poor design, improper processing, etc. that can cause heat treatment defects, such as reducing sharp changes in section, increasing keyway fillets, and avoiding notched sharp corners to prevent excessive local stress concentration during tool heat treatment.
• Before the final heat treatment, according to the actual situation of the workpiece, select the range of 720～760℃ for high temperature tempering for 2～4h to eliminate the processing stress and prepare the quenching structure.
• Heat treatment, quenching heating and quenching cooling, can adopt multi-temperature preheating and multi-temperature isothermal process; tempering can increase isothermal preheating to prevent tempering cracking.
• After the heat treatment is finished and the finished product is ground, the low temperature aging treatment is carried out.

3. Conclusion

With the wide application of advanced machining equipment such as CNC machine tools and machining centers, as well as the pursuit of high-speed cutting and high-precision cutting targets for difficult-to-machine materials, the development of metal cutting tools must become higher, faster, and stronger. Knives will undoubtedly face huge challenges. Therefore, the production development and selection of high-performance high-speed steel tool materials and heat treatment processes will surely become an important direction for the future development of high-speed steel.

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