Tool and high-speed steels

Tool steels are used for working, cutting, and forming metal components, moulding plastics, and casting dies for metals with lower melting points than steel. Accordingly, tool steels need high hardness and strength combined with good toughness over a broad temperature range.

The microstructure of all tool steels is based on a martensitic matrix. Molybdenum additions in tool steels increase both their hardness and wear resistance. By reducing the critical cooling rate for martensite transformation, molybdenum promotes the formation of an optimal martensitic matrix, even in massive and intricate moulds that cannot be cooled rapidly without distorting or cracking. Molybdenum also acts in conjunction with elements like chromium to produce substantial volumes of extremely hard and abrasion resistant carbides. Increasing physical demands on tool steels result in an increasing molybdenum content. Depending on their application, tool steels are classified into:

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• Cold-work tool steels (Mo ≤1.8%)
• Hot-work tool steels (Mo ≤3.0%)
• Plastic mould steels (Mo ≤1.3%)
• High-speed tool steels (Mo ≥7%)

AISI-SAE tool steel grades Defining property AISI-SAE grade Significant characteristics Water-quenched W Molybdenum alloying optional Cold-working O Oil-hardening, O6-0.3% molybdenum, cold-work steel used for gauges, cutting tools, woodworking tools and knives A Air-hardening, low distortion during heat treatment, balance of wear resistance and toughness, all molybdenum alloyed - 0.15-1.8% D High carbon, high chromium, 0.9% molybdenum, very high wear resistance but not as tough as lower alloyed steels Hot-working H H1-H19 - chromium base
H20-H39 - tungsten base
H40-H59 - molybdenum base Plastic moulding P Low segregation: reduced alloying of silicon, manganese and chromium
Through hardenability: increased molybdenum and vanadium High-speed T Tungsten base (today mostly replaced by M22) M Molybdenum base Shock resisting S Chromium-tungsten, silicon-molybdenum, silicon-manganese alloying, very high impact toughness and relatively low abrasion resistance Special purpose L Low alloy, high toughness F Carbon-tungsten alloying, substantially more wear resistant than W-type tool steel Typical alloying elements in tool steels and their effects Alloying element Advantages Disadvantages Chrome (Cr) Hardenability, corrosion resistance, wear resistance Lower toughness, poorer weldability Cobalt (Co) Heat resistance, temper embrittlement - Manganese (Mn) Hardenability, strength Thermal expansion Molybdenum (Mo) Hardenability, tempering resistance, temper embrittlement, strength, heat resistance, wear resistance - Nickel (Ni) Yield strength, toughness, thermal expansion - Nitrogen (N) Stress corrosion cracking resistance, work hardening, strength Blue brittleness, aging sensitivity Vanadium (V) Wear resistance, tempering resistance -

Cold-work steels

Cold-work tool steels are tool steels used for forming materials at room temperature or at slightly raised temperatures (~ 200°C). Specifically, tools for blanking metallic and non-metallic materials, including cold-forming tools, are manufactured from these steels.

Fundamentally, cold-work tool steels are high carbon steels (0.5-1.5%). The water-quenched W-grades are essentially high carbon plain carbon-manganese steels. Steel grades of the O series (oil-hardening), the A series (air-hardening), and the D series (high carbon-chromium) contain additional alloying elements that provide high hardenability and wear resistance as well as average toughness and heat softening resistance. 

The four major alloying elements in such tool steels are tungsten, chromium, vanadium, and molybdenum. These alloys increase the steels' hardenability and thus require a less severe quenching process with a lower risk of quench cracking and distortion. All four elements are strong carbide formers, also providing secondary hardening and tempering resistance.

Hot-work steels

Hot-work tool steels are tool steels used for the shaping of metals at elevated temperatures. Their principal areas of application include pressure die casting moulds, extrusion press tools for processing light alloys, and bosses and hammers for forging machines. The stresses encountered here are cyclical, often with abrupt temperature changes and recurring mechanical stresses at high temperatures. Hot-work steels must constantly endure tool temperatures above 200°C during use. To achieve optimum performance, hot-work tool steels require the following properties: 

• Good tempering properties
• Sufficient thermal stability
• High hot toughness
• High resistance to wear at elevated temperatures
• Good thermal fatigue resistance

Cycle times applied in plastic injection moulding, pressure die casting or press hardening (hot stamping) can be reduced considerably by increasing the tool steel’s thermal conductivity, which significantly raises productivity. Heat conductivity is influenced by several material parameters such as microstructure, defects, and alloying elements. 

Armco iron is nearly pure iron with a low defect density and high heat conductivity in the order of 70-80 W/mK. Compared to Armco iron, traditional hot-work steel such as H13 (1.) has much lower heat conductivity  in the range of only 20-30 W/mK. This reduced thermal conductivity is due to high lattice distortion and defect density of the (tempered) martensitic microstructure as well as to a substantial content of alloying elements. All these characteristics interact with phonons, electrons, and magnons as the “vehicles” of heat transport.

Since all hot-work steels have a defect-rich martensitic microstructure, the difference in optimizing heat conductivity lies in the alloying composition. When in solid solution, alloying elements can cause local lattice distortion (size misfit vs. iron), modify the electronic structure, and/or have influence on magnetism. Generally, heat conductivity is reduced as the alloy content increases. Looking at individual elements in a solute state, nickel, chromium, and silicon were found to negatively influence heat conductivity. The effects of vanadium and molybdenum appear less detrimental. After tempering, the amount of solute vanadium, chromium, and molybdenum decrease by carbide precipitation, which diminishes their negative effect on heat conductivity.

Effect of alloying element on properties of hot-work steel Property Si Mn Cr Mo Ni V Wear resistance - - + ++ - ++ Hardenability + + ++ ++ + + Toughness - ± - + + + Thermal stability + ± + ++ + ++ Thermal conductivity -- - -- ± - ±

Plastic mould steels

Tools for processing plastics are mainly stressed by pressure and wear. According to the type of plastic, corrosive conditions can prevail in addition to stresses. The type of plastic and processing method define the key requirements in addition to those generally valid to hot-work steels:

• Economic machinability or cold-hobbing ability
• Smallest possible distortion upon heat treatment
• Good polishing behavior
• High compressive strength
• High wear resistance
• Sufficient corrosion resistance

High-speed steels

When tool steels contain a combination of more than 7% molybdenum, tungsten, and vanadium, and more than 0.60% carbon, they are referred to as high-speed steels. This term describes their ability to cut metals at “high speeds”. Until the s, T-1 with 18% tungsten was the preferred machining steel. The development of controlled atmosphere heat treating furnaces then made it practical and cost effective to substitute part or all the tungsten with molybdenum.

High-speed steel (HSS) has revolutionized the world of machining. At SteelPro Group, we specialize in providing high-quality HSS solutions tailored to your needs. 

What Is High Speed Steel?

High-speed steel is an alloy tool steel renowned for maintaining hardness and wear resistance at elevated temperatures. Classified into tungsten-based (T-series), molybdenum-based (M-series), and cobalt-based (K-series) types, it exhibits exceptional hardness, wear resistance, and heat resistance. With superior red hardness compared to traditional carbon steel tools, it achieves 62-65 HRC after heat treatment. Primarily utilized in high-speed machining operations, it’s ideal for cutting tools like drills and end mills.

Why Is It Called High Speed Steel?

It is called “high-speed steel” because it retains its hardness at high cutting speeds, even when heated to around °F (593°C). This ability, known as red hardness, allows it to perform at speeds much higher than traditional carbon steels.

What Are The Properties of High Speed Steel?

High Alloy Content: High-speed steel contains a mix of elements like tungsten, molybdenum, chromium, vanadium, and sometimes cobalt.

High Hardness: After heat treatment, HSS typically achieves a hardness range of 62-65 Rockwell C (HRC). It retains its hardness at elevated temperatures.

Exceptional Wear Resistance: The carbide content (mainly tungsten, molybdenum, and vanadium carbides) in HSS significantly enhances wear resistance. It maintains cutting edge much longer than other tool steels.

Balanced Toughness: HSS has a good balance between hardness and toughness. While not as tough as cold work tool steels, it is tougher than carbide tools, which are brittle in comparison.

Relatively High Cost: High-speed steel is more expensive than conventional carbon steel due to its alloying elements and specialized heat treatment. However, the cost is offset by its ability to perform at higher speeds.

Overview of High-Speed Steel Types And Grades

To meet the diverse needs of cutting and machining operations, HSS is classified into several categories based on its alloying elements and performance characteristics. Below are the primary classifications:

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Tungsten-Based (T-Series)

Known for its high tungsten content, it provides excellent wear resistance and red hardness, ideal for high-temperature cutting tools like milling cutters.

• M1: A general-purpose HSS suitable for interrupted cutting, offering consistent performance across various applications.
• M7: A N-modified HSS grade with excellent hardness, commonly used for twist drills and other precision tools.

Molybdenum-Based (M-Series)

With a higher molybdenum content, it is more cost-effective than T-Series while delivering comparable performance, widely used for general-purpose tools such as drills and taps.

• M2: The most widely used HSS, balancing durability, cutting efficiency, and cost, ideal for general-purpose tools.
• M2 EUR: A high-C variant of M2, commonly used in Europe, offering slightly higher hardness with a small reduction in toughness.
• M3-1: Designed with increased C and V content, providing better wear resistance and maintaining properties under high-temperature conditions.
• M3-2: Features higher C and V than M3-1, significantly enhancing cutting performance and tool life.
• M4: Known for high wear resistance due to a large volume of hard VC, making it ideal for demanding cutting applications.

Cobalt-Based (K-Series or Subset of M-Serie)

Enhanced with cobalt (e.g., M35, M42), it offers superior red hardness and heat resistance, perfect for cutting hard materials like stainless steel.

• M35: A Co-enhanced version of M2, improving hardness and red hardness, widely used for cutting tools in Europe.
• M42: A super HSS with high Co content, delivering exceptional hardness and wear resistance, relying on hardness rather than large VC amounts.
• CPM Rex76/M48: A premium PM HSS designed for long production runs, heavy-duty machining, and abrasive materials, commonly used for hob tools.

High-speed steel also includes two unique type as follows:

CPM grades like CPM M4 and CPM T15 are made using powder metallurgy, ensuring a uniform microstructure that enhances wear resistance and toughness. Their advanced manufacturing process makes them more expensive but ideal for high-stress precision applications.

• CPM4: A PM variant of M4 that offers superior toughness and reliability, particularly in cold-work applications like punches and dies.
• CPMT15: A PM variant of T15 with greatly improved toughness and grindability, making it a popular choice for broaching tools.
• PM30: A Co-enhanced PM variant of M3-2 that combines high toughness with excellent cutting efficiency, suitable for high-performance applications.

Intermediate high-speed steel has less tungsten and molybdenum, making it more affordable while still strong and tough. It is often used in aerospace and industrial parts where high-speed cutting or extreme heat resistance is not needed, and easier processing is preferred.

• M50: A cost-effective intermediate HSS, suitable for tools like twist drills and woodworking equipment where red hardness is less critical.

Chemical Composition of High Speed Steel Grades

Performance Comparison of High Speed Steel Grades

What Is High Speed Steel Used For?

• Drill Bits
• End Mills
• Taps and Dies
• Reamers
• Saw Blades
• Lathe Tools
• Gear Cutters
• Broaches
• Punches

When Was High Speed Steel Invented?

High-speed steel (HSS) was invented in by Frederick Winslow Taylor and Maunsel White. This marked a breakthrough in cutting tool technology. The following is a brief overview of the key milestones in the history of high-speed steel.

: Robert Forester Mushet developed Mushet steel, the precursor to modern HSS. It contained 2% C, 2.5% Mn, and 7% W, and hardened upon air cooling.

–: Frederick Winslow Taylor and Maunsel White at Bethlehem Steel improved tool steels like Mushet steel through experiments. Their heat treatment process allowed steel to retain hardness at high temperatures, tripling cutting speeds. This became the Taylor-White process, showcased at the Paris Exhibition.

: The first officially classified HSS, AISI T1, was introduced and patented by Crucible Steel Co.

WWII Era: Material shortages led to the development of molybdenum-based HSS, such as AISI M1 and M2, offering cost-effective alternatives to tungsten-based HSS. These grades became widely used.

High-Speed Steel vs. Carbon Steel

If durability and cutting efficiency are your priorities, HSS outperforms carbon steel by a wide margin.

HSS is designed for high-temperature operations, maintaining its hardness and cutting ability even when heated to around °F (593°C). This makes it ideal for machining tools like drills and taps, especially at high speeds. 

In contrast, carbon steel, while more affordable and easier to sharpen, loses its hardness and edge retention under high heat, limiting it to low-speed or hand-tool applications.

High-Speed Steel vs. Carbide

Carbide offers higher performance for specific tasks. HSS remains a versatile, cost-effective option for a broader range of uses.

Carbide tools excel in high-speed, high-precision machining and can cut harder materials with superior wear resistance. However, carbide is brittle, making it less suitable for interrupted cuts or impact-prone operations. 

HSS, on the other hand, is tougher and more forgiving, making it ideal for general-purpose tools and applications requiring durability under stress. 

Partner with SteelPro Group for High-Speed Steel Excellence

SteelPro Group is committed to delivering top-grade HSS products and expert guidance to help you achieve your goals. Whether you’re upgrading your tools or planning new projects, contact us today to discover how our solutions can make a difference. Let’s build your success together!

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