Introduction
For many years a great air of mystery surrounded the selection, heat treatment and use of tool steels. Perhaps this was due to the secretiveness of the early steel-makers. However, it may also have been fostered by the jargon and spiel of commercial marketing, and the seemingly endless parade of ‘new’ types and grades of “high speed steel”. To a certain extent this may still be true today.
The advent of new steels for manufacturing and construction together with evolving techniques of mass-production created a growing demand for tougher, stronger and harder tool steels. Clearly these factors facilitated the proliferation of tool steel types and grades. Producers of tool steels reacted quickly to develop new tools that would meet demand as first the new automobile industry, then modern aircraft manufacture and jet engine production and, finally, the exploration of space ushered in new ‘high-tech’ materials.
Cutting and forming these new materials required ever greater levels of precision and accuracy which in turn led to the design and construction of new machine tools. This resulted in steel-makers and metallurgists developing more new tool steels. And so the evolution proceeded.
Over time super HSS, cobalt super-hard HSS, cast tool steels and particle metallurgy HSS cutting tools were introduced. Particle metallurgy HSS could be produced to very tight compositional tolerances and allowed the custom design and manufacture of tools to cut particular materials more effectively and efficiently.
Even with the introduction of tungsten carbide tooling and the subsequent development of many new and exotic carbide grades, the demand for ‘traditional’ tool steels such as HSS has continued as new ‘jet-age’ then ‘space-age’ materials found their way into general manufacturing. Competition, marketing and brand differentiation ultimately boosted consumer demand for new ‘high-tech’ products.
The need and demand for tool steel will no doubt be with us for some ti
me.
Of course HSS is used for many purposes other than cutting tools. For example, the exceptional high temperature wear properties of molybdenum-containing high-speed steels are ideal for new applications such as automobile valve inserts and cam-rings.
In this series of articles the discussion will focus on HSS used in cutting tools.
What follows is not much more than a crude sketch of the history of tool steel and its makers from the latter part of the 1800s to the present. It includes a fledgling survey of the various types and brands of tool steels and HSS. Some of these are still being produced. Others, while now long obsolete, continue to live on in the tool chests and boxes of machinists and other users.
Tool Steel in the 21st Century
Today the common use of the term tool steel means hard steel of a quality used for making tools to be used for cutting and other purposes.
More specifically it refers to varieties of carbon and alloy steels that are particularly well-suited to be made into cutting tools.
For the purposes of this historical survey tool steels have been divided into five basic types or groups:
- water-hardening carbon tool steels;
- oil hardening tool steels;
- shock-resisting tool steels;
- air hardening tool steels; and
- high-speed steels or HSS.
The non-ferrous cutting tool alloys will also be discussed briefly, but not the carbides as these are not steels.
Now although the non-ferrous alloys are also technically not steels, they are a group of tool alloys that share some of the same elemental contents as tool steels and which crop up more often than may be expected. Furthermore, they are often quite useful for certain purposes and operations in the metal industry and the home workshop.
A brief outline of the steel classification system
During the 1970s the American Society for Testing and Materials, the ASTM, now known as ASTM International) introduced a unified numbering system for steel comprising 11 main classes each designated by a letter as follows:
W: Water-Hardening
S: Shock-Resisting
O: Cold-Work (Oil-Hardening)
A: Cold-Work (Medium-Alloy, Air-Hardening)
D: Cold-Work (High-Carbon, High-Chromium)
L: Low-Alloy
F: Carbon-Tungsten
P: P1-P19 Low-Carbon Mould Steels
P20-P39 Other Mould Steels
H: H1-H19: Chromium-Base Hot Work
H20-H29: Tungsten-Base Hot Work
H40-H59: Molybdenum-Base Hot Work
T: High-Speed (Tungsten-Base)
M: High-Speed (Molybdenum-Base)
The current (c.2018) ASTM standard recognises 7 tungsten types and 21 molybdenum types of HSS. In this unified numbering system, the tungsten-type HSS grades (e.g. T1, T15) are assigned numbers in the T120xx series, while molybdenum (e.g. M2, M48) and intermediate types are T113xx.
The current standard (ASTM A600) covers types T1, T2, T4, T5, T6, T8, and T15 and molybdenum-type high-speed steels M1, M2, M3, M4, M6, M7, M10, M30, M33, M34, M36, M41, M42, M43, M44, M46, M47, M48, and M62 in the form of annealed, hot-rolled bars, forgings, plate, sheet, or strip, and annealed, cold-finished bars or forgings used primarily in the fabrication of tools. Two intermediate high speed tool steels designated as M50 and M52 are also included, as is M100A.
Water-Hardening Tool Steels include all the class W tool steels. These steels do not retain hardness well at elevated temperatures, but they do have high resistance to surface wear. Typical applications include blanking dies, files, drills, taps, countersinks, reamers, jewellery dies, and cold-striking dies.
The series of articles continues with Tool Steels: A Brief History — Part 2 Introduction to high speed steel