What is Alloying of High-speed and Tool Steels - Definition

The four major alloying elements in high-speed steels and tool steels are: tungsten, chromium, vanadium and molybdenum. These alloying elements combine with carbon to form very hard and wear-resistant carbide compounds.

Tool Steel

Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools (punches, dies, molds, tools for cutting, blanking, forming, drawing, steering and slitting tools). Their suitability comes from their distinctive hardness, resistance to abrasion and deformation, and their ability to hold a cutting edge at elevated temperatures. With a carbon content between 0.5% and 1.5%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The presence of carbides in their matrix plays the dominant role in the qualities of tool steel.

They are generally grouped into two classes:

Plain carbon steels containing a high percentage of carbon, about 0.80-1.50%
Alloy tool steels, in which other elements (chromium, molybdenum, vanadium, tungsten and cobalt) are added to provide greater strength, toughness, corrosion and heat resistance of steel.

One of subgroups of tool steels is high-speed steels (HSS), which were named primarily for their ability to machine and cut materials at high speeds (high hot hardness). It is often used in power-saw blades and drill bits. This group of tool steels is described in a separate article.

High-speed Steel

High-speed steels, abbreviated as HSS, are a specialized class of tool steels that were named primarily for their ability to machine and cut materials at high speeds (high hot hardness). It is often used in power-saw blades and drill bits. High-speed steel is superior to the older high-carbon steel tools in that it can withstand higher temperatures without losing its temper (hardness). High-speed steels are complex iron-base alloys of carbon, chromium, vanadium, molybdenum, or tungsten, or combinations there of. To achieve good cutting performance from HSS, an appropriate hardening response must be provided in heat treatment.

Central to the performance of high-speed steels is the hardening response achieved during the heat treatment process. Alloying elements are introduced in quantities given by the intended application and by their function in the heat treatment process, whether to increase the solidus temperature or inhibit the growth of secondary hardening precipitates, enabling higher operating temperature.

Alloying Agents in High-speed and Tool Steels

High-speed steel - M8 — High-speed steel – M8

Pure iron is too soft to be used for the purpose of structure, but the addition of small quantities of other elements (carbon, manganese or silicon for instance) greatly increases its mechanical strength. The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties. The four major alloying elements that form carbides in high-speed steels are: tungsten, chromium, vanadium and molybdenum. These alloying elements combine with carbon to form very hard and wear-resistant carbide compounds. The microstructure of high-speed steels consists of a martensitic matrix with a dispersion of two sets of carbides. These carbides are usually known as primary and secondary carbides. Primary carbides are those carbides formed during solidification of the steel. Secondary carbides are those carbides formed during secondary hardening heat-treatment of the steels.

Tungsten. Produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures. Tungsten is used extensively in high-speed tool steels and has been proposed as a substitute for molybdenum in reduced-activation ferritic steels for nuclear applications. The addition of about 10% of tungsten and molybdenum in total maximises efficiently the hardness and toughness of high speed steels and maintains those properties at the high temperatures generated when cutting metals. Tungsten and molybdenum are interchangeable at an atomic level and both promote resistance to tempering which gives improved tool cutting performance at higher temperatures.
Chromium. Chromium increases hardness, strength, and corrosion resistance. The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. Generally speaking, the concentration specified for most grades is approximately 4%. This level appears to result in the best balance between hardness and toughness. Chromium plays an important role in the hardening mechanism and is considered irreplaceable. At higher temperatures, chromium contributes increased strength. It is ordinarily used for applications of this nature in conjunction with molybdenum.
Molybdenum. Molybdenum (about 0.50-8.00%) when added to a tool steel makes it more resistant to high temperature. Molybdenum increases hardenability and strength, particularly at high temperatures due to the high melting point of molybdenum. Molybdenum is unique in the extent to which it increases the high-temperature tensile and creep strengths of steel. It retards the transformation of austenite to pearlite far more than it does the transformation of austenite to bainite; thus, bainite may be produced by continuous cooling of molybdenum-containing steels.
Vanadium. Vanadium is generally added to steel to inhibit grain growth during heat treatment. In controlling grain growth, it improves both the strength and toughness of hardened and tempered steels. The size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength and tends to increase ductility. A larger grain size is preferred for improved high-temperature creep properties. Vanadium is added to promote abrasion resistance and to produce hard and stable carbides which being only partly soluble, release little carbon into the matrix.

References:

Materials Science:

U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.
William D. Callister, David G. Rethwisch. Materials Science and Engineering: An Introduction 9th Edition, Wiley; 9 edition (December 4, 2013), ISBN-13: 978-1118324578.
Eberhart, Mark (2003). Why Things Break: Understanding the World by the Way It Comes Apart. Harmony. ISBN 978-1-4000-4760-4.
Gaskell, David R. (1995). Introduction to the Thermodynamics of Materials (4th ed.). Taylor and Francis Publishing. ISBN 978-1-56032-992-3.
González-Viñas, W. & Mancini, H.L. (2004). An Introduction to Materials Science. Princeton University Press. ISBN 978-0-691-07097-1.
Ashby, Michael; Hugh Shercliff; David Cebon (2007). Materials: engineering, science, processing and design (1st ed.). Butterworth-Heinemann. ISBN 978-0-7506-8391-3.
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

See above:
High-speed Steel

We hope, this article, Alloying of High-speed and Tool Steels, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about materials and their properties.