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What is High Speed Steel

High-speed tool steel (HSS) is a complex iron-based alloy specifically designed for machining materials at high cutting speeds. When used for high-speed cutting of metals, high temperatures are generated, causing ordinary steel to lose its hardness and cutting ability. However, high-speed tool steel does not lose its hardness, even when heated to near red-hot temperatures, and maintains good cutting hardness.

Bileşim ve Alaşım Elementleri

High-speed tool steels are primarily composed of carbon, chromium, vanadium, molybdenum, or tungsten, or combinations thereof, and sometimes substantial amounts of cobalt. The total alloy content in high-speed steels generally ranges from 20% to 40%. All high-speed steels, whether molybdenum or tungsten types, contain about 4% chromium. The various alloying elements in high-speed steel are the core factors that determine its hardenability, high wear resistance, resistance to thermal softening, and excellent toughness, making it ideal for industrial cutting operations.

Key alloying elements and their effects include：

• Tungsten (W): The T series of HSS contains 12% to 20% tungsten, along with chromium, vanadium, and cobalt as other major alloying elements. Tungsten is a very strong carbide-forming element, and its substantial addition produces large volumes of high-temperature stable alloy carbides, significantly increasing wear resistance and “red hardness”. An early popular grade, 18-4-1 (T1), contains about 18% tungsten.
• Molibden (Mo): The M series contains approximately 3.5% to 10% molybdenum, with chromium, vanadium, tungsten, and cobalt as other alloying elements. Molybdenum, like tungsten, is a strong carbide former and contributes significantly to tempering resistance and maintaining high hardness at cutting temperatures (“red hardness”). Molybdenum types are generally less expensive and have higher abrasion resistance and less distortion in heat treatment than T-series steels.
• Krom (Cr): All high-speed steels contain about 4% chromium. Chromium forms carbides (e.g., M23C6 and M7C3) that dissolve readily and are taken into solution at typical heat treatment temperatures, promoting hardenability. It also improves scaling resistance at high machining temperatures.
• Vanadyum (V): The vanadium content in HSS varies, and typically, when vanadium content is increased, carbon content is also increased. Vanadium is a very strong carbide former and forms very hard, abrasion-resistant MC carbides. Increasing vanadium offers greater wear resistance and hot hardness. Vanadium carbide also acts as a grain-refining agent.
• Kobalt (Co): Cobalt is added to some HSS grades to enhance cutting ability and is a major alloying element in certain T and M series steels. Its primary effect is to increase hot hardness, thereby improving cutting efficiency when high tool temperatures are reached during cutting. Cobalt elevates the melting point and can increase heat-treating temperatures. While it increases red hardness, cobalt additions slightly increase the brittleness of high-speed tool steels. Cobalt does not form carbides but enhances the precipitation hardening effect of other alloying elements.
• Karbon (C): High carbon content is crucial for producing a hard martensitic matrix and forming primary carbides, both of which provide abrasion resistance. Carbon content typically varies from 0.70% to 1.5%. Low carbon grades are tougher, while high carbon grades offer higher hardness and wear resistance.
• Nitrogen (N): Nitrogen is usually present in air-melted HSS in amounts from 0.02% to 0.03%. Deliberately increasing nitrogen to 0.04-0.05% with higher silicon can slightly increase maximum attainable tempered hardness and change carbide morphology.

Isıl İşlem

1. Austenitleştirme. When high-speed steel is heated to approximately 840°C, ferrite transforms into austenite, and some alloy carbides may dissolve. When heated to 1120°C or greater, all M23C6 carbides dissolve, and up to 50% of M6C and MC carbides may also dissolve. This dissolves carbon into the austenite matrix, providing the necessary alloy and carbon content for hardenability, hot hardness, and resistance to tempering.
2. Söndürme. After austenitizing, HSS can be cooled in still air to near maximum hardness, but is often quenched in warm oil for faster heat removal and higher hardness levels. Quenching transforms most of the high-carbon austenite to martensite, but some austenite may be retained.
3. Temperleme. The final microstructure after tempering primarily consists of tempered martensite and well-distributed hard carbides. High-speed steel is a secondary hardening steel. Multiple tempering can convert residual austenite into martensite.

Özellikler

1. Sertlik. Hardness is resistance to penetration by a diamond-hard indenter, measured at room temperature. HSS typically contains sufficient carbon to permit hardening to 64 HRC. General-purpose HSS, like M1 and M7, are usually heat treated to 64-66 HRC, while cobalt-containing HSS generally achieve 65-67 HRC. Super high-speed steels, particularly the M40 series, can be heat-treated to 70 HRC.
2. Sıcak Sertlik (Kırmızı Sertlik). This is the ability of the steel to retain high hardness at elevated temperatures. It is primarily increased by cobalt, vanadium, and molybdenum. HSS tools can retain a keen cutting edge up to temperatures around 650°C (1200°F).
3. Aşınma Direnci. This is resistance to abrasion. It is strongly influenced by the matrix hardness and composition, precipitated secondary carbides (M2C and MC), and the volume and nature of excess alloy carbides. Higher hardness generally leads to higher wear resistance, especially under abrasive cutting conditions.
4. Sertlik. HSS generally possesses good toughness for effective use in industrial cutting operations. They are notably tougher than carbide materials, especially in interrupted-cut applications. However, extremely high hardness and high carbide content can reduce toughness compared to other tool steels. P/M HSS offers improved toughness due to uniform and fine carbide distribution.
5. Sertleştirilebilirlik. HSS hardens so deeply that almost any commercially encountered section will have uniform hardness from center to surface.

Classifications

Here, we use the American Iron and Steel Institute(AISI) classification.

1. T-Series (Tungsten High-Speed Tool Steels): These steels contain 12% to 20% tungsten and are designated by the letter ‘T’. T1, also known as 18-4-1 (18% W, 4% Cr, 1% V), is a classic example. The T1 type does not contain molybdenum or cobalt. Cobalt-base tungsten types range from T4 through T15 and contain various amounts of cobalt.
2. M-Series (Molybdenum High-Speed Tool Steels): These steels contain approximately 3.5% to 10% molybdenum and are designated by the letter ‘M’. M-series steels are generally less expensive and have higher abrasion resistance and less distortion in heat treatment than T-series steels. Common general-purpose molybdenum steels include M1, M2, and M7.

Coatings

High-speed tool steels can be coated with titanium nitride, titanium carbide, and other coatings by physical vapor deposition (PVD) techniques for improved performance and increased tool life. PVD is preferred over the older chemical vapor deposition (CVD) process for HSS because it operates at lower temperatures and eliminates the need for subsequent heat treatment.

Uygulamalar

High-speed tool steels are widely used for most common types of cutting tools. These include:

• Single-point lathe tools (tool bits, cutoff tools, inserts).
• Drills, reamers, taps, milling cutters, end mills, hobs, saws, and broaches.
• Tools for heavy cuts or high-speed machining.
• Form tools.
• Hot forming dies, fine blanking, and other hot- and cold-forming applications.
• Bearing applications (e.g., aerospace bearings).
• High-load, high-temperature structural components.
• Special tools requiring specific properties, such as broaching involute splines in truck transmission gear blanks.

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