high speed steel
High speed steel is a tool steel with a high content of alloying elements such as tungsten (W), molybdenum (Mo), chromium (Cr), and vanadium (V). The mass fraction of carbon is 0.7%~1.5%, the mass fraction of chromium is about 4%, the mass fraction of tungsten and molybdenum is 10%~20%, and the mass fraction of vanadium is 1%~5%. Due to the high hardness carbides formed by the combination of alloying elements and carbonization (such as vanadium carbide, with a hardness of up to 2800HV and small and evenly distributed grains), and the strong binding force between alloying elements and carbon atoms, the stability of martensite during heating is improved, allowing the steel to maintain high hardness at 550 ℃~600 ℃, thereby increasing the cutting speed by multiple times compared to carbon tool steel and alloy tool steel, hence the name "high-speed steel". Chromium improves the hardenability of steel, allowing small cutting tools to harden when cooled in air and sharpen their edges. Therefore, high-speed steel is also known as "wind steel" or "sharp steel".
High speed steel has good mechanical properties, can withstand large cutting forces and impacts, and has good processability. It is particularly suitable for manufacturing various small and complex structure and shape cutting tools, such as forming turning tools, various milling cutters, drill bits, broaches, gear cutting tools, and thread cutting tools. At present, there are various types of high-speed steel, which can be divided into ordinary high-speed steel and high-performance high-speed steel according to their cutting performance; According to chemical composition, it can be divided into tungsten based high-speed steel and molybdenum based high-speed steel.
(1) Ordinary high-speed steel
1 Tungsten series high-speed steel
The typical representative of this type of steel is W18Cr4V (abbreviated as W18), which is the most commonly used high-speed steel in China. Due to its low vanadium content and good grinding performance, its Blade is easy to grind sharp and straight, with good overall performance and strong universality. The room temperature hardness can reach 63~66HRC, and the hardness that can be maintained at high temperatures of 600 ℃ is about 48.5HRC. Especially with good heat treatment process, during quenching, there is a small tendency for overheating and strong resistance to plastic deformation. Complex tools that can be used for precision machining, such as thread turning tools, forming turning tools, wide edge precision planing tools, broaches, gear cutting tools, etc. The disadvantage of W18 steel is that the distribution of carbides is often uneven, and the remaining carbide particles are large. If forged unevenly, it will affect the service life of thin edge cutting tools. When manufacturing larger section cutting tools, the strength appears insufficient (the bending strength is only 2-2.3 GPa). Only when manufacturing small section cutting tools can satisfactory strength (3-3.4 GPa) be obtained. In addition, W18 steel has poor thermoplastic properties and is not suitable for use as a hot rolling tool. Due to the aforementioned shortcomings and the increase in tungsten prices in the international market, W18 steel is gradually being replaced by new steel grades.
W14Cr4VMn is another type of tungsten based high-speed steel produced in China with the addition of a small amount of manganese and rare earth element rhenium (Re). The reduction of tungsten content and the addition of rare element rhenium (Re) have improved the distribution of carbides and increased thermoplastic properties. This kind of steel has good forging and rolling processability, slightly higher strength than W18 steel, roughly the same cutting performance as our 8 steel, good grinding performance, wide heat treatment temperature range, and low sensitivity to overheating and decarburization, which is most suitable for making hot rolling tools (such as Fried Dough Twists drills).
2 Tungsten molybdenum high-speed steel
W6Mo3Cr4V2 (abbreviated as M2) is a typical tungsten molybdenum high-speed steel commonly used in China. 1% molybdenum can replace 2% tungsten. The addition of molybdenum reduces the alloying elements in the steel, thereby reducing the uneven distribution of carbides and refining the grains. Compared with W18 steel, M2 steel has an increase of about 17% in bending strength and an increase of over 40% in impact toughness. In addition, large section cutters also have the same strength and toughness, which can be used to manufacture cutters with larger section, or cutters with larger impact force (such as gear shapers) and cutters with weaker structure (such as Fried Dough Twists drills, taps, etc.). M2 steel has good thermoplasticity and good grindability, making it particularly suitable for manufacturing hot formed tools such as rolling or twisting drill bits. It is currently a widely used high-speed steel in various countries. The disadvantage of M2 steel is that its thermal hardness and high-temperature hardness are slightly lower than those of W18 steel, so its high-temperature cutting performance is slightly inferior. In addition, during heat treatment, there is a strong tendency for decarburization and oxidation, resulting in a narrow quenching temperature range.
W9Mo3Cr4V (abbreviated as W9) is a tungsten molybdenum high-speed steel with a high tungsten content and a low molybdenum content. The heterogeneity of the chemical is between W18 and M2, but the bending strength and impact toughness are higher than M2, indicating good hardness and toughness. Its thermoplastic properties are also very good, and its decarburization tendency during heat treatment is smaller than M2. Due to its low vanadium content, its grindability is also better than M2, and it can be used to manufacture various cutting tools (saw blades, drill bits, broaches, milling cutters, gear cutters, etc.). When processing various types of steel, the tool life has been improved to a certain extent compared to W18 and M2, and its cutting performance is equal to or slightly higher than Japan's SKH9 (W6Mo5Cr4V2) steel.
(2) High performance high-speed steel
High performance high-speed steel is derived from ordinary high-speed steel by adjusting its basic chemical composition and adding some alloy elements (such as vanadium, cobalt, lead, silicon, niobium, etc.), with a focus on improving its heat resistance and wear resistance. It is mainly used to process difficult to machine materials such as stainless steel, heat-resistant steel, high-temperature alloys, and ultra-high strength steel.
1 High carbon high-speed steel
The high carbon high-speed steel grades produced in China include 9W18Cr4V (referred to as 9W18) and 9W6Mo5Cr4V2 (referred to as CM2). The carbon mass fraction increases from 0.7% to 0.8% of ordinary high-speed steel to 0.9% to 1.0%, which increases the room temperature hardness to 66~68HRC and the high temperature hardness to 51~52HRC at 600 ℃. It is suitable for reamers, spot facers, taps, and cutting tools that require high wear resistance (220-250HBS), and the service life can generally be increased by 0.5~0.8 times, It can also be used for cutting stainless steel. Austenitic materials and titanium alloys have a wear resistance 2-3 times higher than ordinary high-speed steel, but the increase in carbon content in the steel leads to an increase in residual austenite after quenching, requiring an increase in tempering times and reducing toughness, making it unable to withstand large impacts.
2 Aluminum high-speed steel
Aluminum high-speed steel W6Mo5Cr4V2A1 (referred to as 501) and W10Mo4Cr4V3A1 (referred to as 5F6) are innovative new steel varieties in China. This type of steel has a room temperature hardness of 67~69HRC and a high temperature hardness of 54~55HRC at 600 ℃. Its cutting performance is equivalent to cobalt high-speed steel M42, and the tool life is significantly improved (at least 1-2 times) compared to W18Cr4V, but the price difference is not significant. The gear hob made of this steel allows a cutting speed of ≤ 1.67m/s, but due to its high vanadium content, its grinding workability is poor, and it is highly sensitive to overheating, with a strong tendency for oxidation and decarburization. When using it, it is necessary to strictly master the heat treatment process.
3 Cobalt high-speed steel
Adding cobalt to high-speed steel can improve the thermal stability of the steel. During tempering, the precipitation of carbides increases the dispersion hardening effect, improves the tempering hardness, and thus improves the hardness and oxidation resistance at room and high temperatures. Due to the high thermal conductivity of cobalt, the addition of cobalt can improve the thermal conductivity of high-speed steel and reduce the friction coefficient, thereby increasing cutting speed. For example, M42 (W2Mo9Cr4VCo8) in the M40 series in the United States has a room temperature hardness of 67~70HRC and a high temperature hardness of 54~55HRC at 600 ℃. Its superiority can only be demonstrated during high-temperature cutting, making it suitable for high-speed steel cutting tools for processing high-temperature alloys, titanium alloys, austenitic heat-resistant steel, and other difficult to machine materials. Its cobalt high-speed steel grades include W10Mo4Cr4V3Co10, W12Mo3Cr4V2Co8, W9Cr4V2Co10, etc. M42 has low vanadium content and good grinding performance, The cutting edge can be sharpened, making it particularly suitable for manufacturing precision machining tools. However, the non-uniformity of carbide in cobalt high-speed steel increases, the tendency to decarburize increases during heating, and the strength and toughness decrease. It is not suitable to use thin edged tools or cut under high impact conditions. Due to limited cobalt resources in China, there is currently limited production and use.
4 High vanadium high-speed steel
The mass fraction of high vanadium high-speed steel ranges from 3% to 5%. Due to the formation of a large amount of high hardness and wear-resistant vanadium carbide dispersed in the steel, the wear resistance of high-speed steel is improved, and the grain size can be refined and the overheating sensitivity of the steel can be reduced. This type of high-speed steel is suitable for processing materials with severe tool wear, such as hard rubber and plastic. For low-speed thin chip precision machining tools such as broaches, reamers, taps, etc., high vanadium high-speed steel has a longer lifespan. Its main drawback is poor grinding workability. When using a medium soft praseodymium neodymium corundum grinding wheel with a particle size of 60, the grinding ratio is less than 1, which means that the consumption of the grinding wheel is more than the tool material removed. Commonly used brands include W6Mo5Cr4V3, W12Cr4V4Mo, etc, The high-performance new steel types developed in China include nitrogen containing high-speed steel and silicon niobium aluminum high-speed steel, such as W12Mo3Cr4V3N. Its hardness, strength, and toughness are basically the same as M42, making it a nitrogen containing high-speed steel with good performance. Mainly used for cutting high-strength structural steel (30-52HRC), the grades of silicon niobium aluminum high-speed steel include W6Mo5Cr4V5SiNbAl (referred to as B201), W18Cr4V4SiNbA1 (referred to as B212), etc. The silicon and aluminum in the steel improve the hardness and thermal stability of the steel, while niobium can improve wear resistance and toughness, mainly used for cutting difficult to machine materials. However, this type of high-speed steel has poor grinding workability due to its high vanadium content. In addition, When processing high-strength steel with W12Mo3Cr4V3Co5Si, the effect is good.
(3) Powder metallurgy high-speed steel
Powder metallurgy high-speed steel is a new type of tool material developed in the 1970s. Its process involves atomizing high-speed steel with high-pressure inert gas (argon or nitrogen) or high-pressure water to obtain fine high-speed steel powder, which is then hot pressed to produce tool blanks. Compared with melted high-speed steel, powder metallurgy high-speed steel has the following advantages:
1 Can solve carbide segregation
During the casting of ordinary smelting high-speed steel ingots, coarse carbide eutectic segregation occurs, with carbide grain sizes up to 20-80 μ m. And the carbide grain size of powder metallurgy high-speed steel is 2-5 μ m. And there is no carbide segregation, thereby improving the strength, toughness, and hardness of the steel, with a hardness of up to 69~70HRC. This feature makes powder metallurgy high-speed steel suitable for manufacturing tools that are prone to edge breakage or require sharp edges and high strength and toughness during strong intermittent cutting, such as gear shapers, end mills, etc. Especially suitable for manufacturing large-sized cutting tools, their lifespan can be increased by 2-3 times compared to ordinary high-speed steel, because large-sized ordinary high-speed steel tools are difficult to forge the coarse carbon grains generated by eutectic segregation uniformly.
2 Can ensure isotropy
Due to the process characteristics of powder metallurgy, the isotropy of powder metallurgy high-speed steel is ensured, thereby reducing internal stress and deformation during heat treatment, making it suitable for manufacturing various precision and complex cutting tools.
3 Good grinding performance
The grinding workability of powder metallurgy high-speed steel with a mass fraction of 5% vanadium is equivalent to that of ordinary high-speed steel with a mass fraction of 2% vanadium. The grinding efficiency is 2-3 times higher than that of melted high-speed steel, and the surface roughness value is significantly reduced.
4 Capable of manufacturing superhard high-speed steel
The new process of powder metallurgy high-speed steel provides the possibility of adding high carbides (TiC and NbC) to existing high-speed steel and manufacturing new materials of superhard high-speed steel.
5 Can save steel and labor
When using powder metallurgy to directly press tool blanks, it greatly reduces machining allowance, saves steel and labor. The powder metallurgy high-speed steels produced in China include FT15 (with chemical composition of W12Cr4V5Co5) and FR71 (W10Mo5Cr4V2Co12) produced by the General Institute of Iron and Steel Research, with hardness of 68HRC and 70HRC respectively. Both have advantages such as high temperature hardness and wear resistance, making them suitable for heavy load cutting of difficult to machine materials. The PT1 (equivalent to W18Cr4V) and PVN (W12Mo3Cr4V3N) developed by Shanghai Institute of Materials have a hardness of 67~69HRC, good grinding performance, and better cutting performance than aluminum high-speed steel.
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