High-Speed Steel (HSS), which stands for High-Speed Steel, is a high-alloy tool steel that incorporates a significant amount of alloying elements such as tungsten (W), molybdenum (Mo), chromium (Cr), and vanadium (V). HSS tools exhibit excellent comprehensive properties in terms of strength, toughness, and workability, and they still hold a dominant position in the manufacturing of complex tools, especially for hole machining tools, milling cutters, thread tools, broaches, and gear cutting tools with complex edge shapes. HSS tools are easy to sharpen to a sharp cutting edge.
Depending on their use, HSS can be divided into general-purpose HSS and high-performance HSS.
1. General-purpose HSS tools:
General-purpose HSS can generally be divided into two categories: tungsten steel and tungsten-molybdenum steel. This type of HSS contains carbon (C) at 0.7% to 0.9%. Depending on the tungsten content in the steel, it can be divided into tungsten steel with 12% or 18% W, tungsten-molybdenum steel with 6% or 8% W, and molybdenum steel with 2% W or no W. General-purpose HSS has certain hardness (63-66HRC) and wear resistance, high strength and toughness, good plasticity, and processing workability, making it widely used for manufacturing various complex tools.
- Tungsten steel: The typical grade of general-purpose HSS tungsten steel is W18Cr4V (referred to as W18), which has good comprehensive performance. Its high-temperature hardness at 600°C is 48.5HRC, suitable for manufacturing various complex tools. It has advantages such as good machinability and low decarburization sensitivity, but due to the high content of carbides, uneven distribution, and larger particles, its strength and toughness are not high.
- Tungsten-molybdenum steel: It is a type of HSS obtained by partially replacing some of the tungsten in tungsten steel with molybdenum. The typical grade of tungsten-molybdenum steel is W6Mo5Cr4V2 (referred to as M2). M2 has fine and uniform carbide particles, and its strength, toughness, and high-temperature plasticity are all better than those of W18Cr4V. Another type of tungsten-molybdenum steel is W9Mo3Cr4V (referred to as W9), which has slightly higher thermal stability than M2 steel, and its bending strength and toughness are better than W6Mo5Cr4V2, with good machinability.
2. High-performance HSS tools:
High-performance HSS refers to new steel grades that further increase carbon content, vanadium content, and add elements such as cobalt (Co) and aluminum (Al) to the composition of general-purpose HSS, thereby improving its heat resistance and wear resistance. There are mainly the following categories:
- High-carbon HSS: High-carbon HSS (such as 95W18Cr4V) has high hardness at room temperature and high temperature, suitable for manufacturing tools for processing ordinary steel and cast iron, and tools with high wear resistance requirements such as drills, reamers, taps, and milling cutters, or tools for processing harder materials, not suitable for bearing large impacts.
- High-vanadium HSS: The typical grade, such as W12Cr4V4Mo (referred to as EV4), has vanadium content increased to 3% to 5%, good wear resistance, suitable for cutting materials with great wear on tools, such as fibers, hard rubber, plastics, etc., and can also be used for processing stainless steel, high-strength steel, and high-temperature alloys.
- Cobalt HSS: It belongs to the category of cobalt-containing ultra-hard HSS, with a typical grade such as W2Mo9Cr4VCo8 (referred to as M42), which has a very high hardness, reaching 69-70HRC. It is suitable for processing high-strength heat-resistant steel, high-temperature alloys, titanium alloys, and other difficult-to-process materials. M42 has good machinability, suitable for making precision complex tools, but not suitable for impact cutting conditions.
- Aluminum HSS: It belongs to the category of aluminum-containing ultra-hard HSS, with a typical grade such as W6Mo5Cr4V2Al (referred to as 501), with high-temperature hardness of 54HRC at 600°C, cutting performance equivalent to M42, suitable for manufacturing milling cutters, drills, reamers, gear tools, broaches, etc., for processing alloy steel, stainless steel, high-strength steel, and high-temperature alloys.
- Nitrogen ultra-hard HSS: The typical grade, such as W12Mo3Cr4V3N (referred to as V3N), is a nitrogen-containing ultra-hard HSS, with hardness, strength, and toughness comparable to M42. It can be used as a substitute for cobalt-containing HSS, suitable for low-speed cutting of difficult-to-process materials and low-speed high-precision machining.
3. Melted HSS and Powder Metallurgy HSS:
Depending on the manufacturing process, HSS can be divided into melted HSS and powder metallurgy HSS.
- Melted HSS: Both ordinary HSS and high-performance HSS are made by melting methods. They are made into tools through processes such as smelting, ingot casting, and rolling. A serious problem with melted HSS is the uneven distribution of hard and brittle carbides, which can adversely affect the wear resistance, toughness, and cutting performance of HSS tools due to their large grain size (up to several tens of micrometers).
- Powder Metallurgy HSS (PMHSS): PMHSS is made by atomizing the steel liquid from a high-frequency induction furnace with high-pressure argon or pure nitrogen gas, then rapidly cooling to obtain fine and uniform crystalline structure (HSS powder), and then pressing the obtained powder into a blank at high temperature and high pressure, or first making a steel blank and then forging and rolling it into the shape of a tool. Compared with HSS made by the melting method, PMHSS has the advantages of fine and uniform carbide grains, and its strength, toughness, and wear resistance are significantly improved compared to melted HSS. In the field of complex CNC tools, PMHSS tools will further develop and occupy an important position. Typical grades include F15, FR71, GFI, GF2, GF3, PT1, PVN, etc., which can be used to manufacture large-sized tools, heavy-load-bearing tools, and tools with high impact, as well as precision tools.
