Steels_ Metallurgy and Applications, Third Edition

258 Steels: Metallurgy and Applications

that the Ms-Mf temperature range is raised and the retained austenite transforms

to martensite on cooling to room temperature. Therefore at the end of the first

tempering treatment, the microstructure will consist of:

1. Primary (undissolved) carbides;


2. Tempered martensite containing secondary carbides;


3. Untempered martensite;

and a second tempering treatment is required in order to temper the newly formed

martensite. The second tempering treatment is again carded out at 530-570~

and for most grades this achieves the required microstructure, i.e. primary

carbides in a matrix of tempered martensite. However, for optimum performance

and maximum dimensional stability, it may be necessary to carry out a third

tempering treatment, particularly in high-cobalt grades.

Standard specifications and uses

In the UK, high-speed steels are specified in BS 4659:1989 Tool and die steels,
which also includes details of hot-work tool steels, cold-work tool steels and
plastic moulding grades. The specified ranges for the main alloying elements
in high-speed steels, together with the hardness requirements in the annealed
and hardened conditions, are summarized in Table 3.13. The grade designations
adopted for the various types of tool steel in this specification follow the system
laid down by the American Iron and Steel Institute (AISI) except that in all cases
they are preceded by the letter 'B'. Thus grade T1 in the American standard
is designated BT1 in the UK standard. In both cases, the letter 'T' refers to
grades in which tungsten is the principal alloying element and the letter 'M' is
used for molybdenum or tungsten-molybdenum grades. In the German system,
the designations have the prefix 'S' (Schnellstahl) and the numbers that follow
represent the levels of tungsten, molybdenum, vanadium and cobalt in that order.
The chromium content is not specified but is most likely to be of the order of
4%. Thus M2/BM2 (6% W, 5% Mo, 4% Cr, 2% V) in the American and British
standards have the same alloy design as $6-5-2 in the German standard.
Although 13 grades of high-speed steels are listed in BS 4659: 1989, the three
popular grades of high-speed steel are listed in Table 3.14.
In the United States, M10 (8% Mo, 4% Cr, 2% V) is also a common grade and
with the three steels listed above makes up nearly 90% of the general purpose
high-speed tools. However, an M10-type steel is not listed in the British standard.
Hoyle 2s has prepared a basic guide to tool selection and this is shown in
Table 3.15. This selection reflects the benefit of cobalt additions in maintaining
a high level of red hardness at high cutting speeds and the use of higher carbon
and vanadium contents in order to increase the resistance to abrasive wear when
cutting very hard materials.
Although high-speed steels constitute the major type of cutting tool, sintered
carbides now feature very prominently in this market and ceramic materials,
such as alumina and cubic boron nitride, are also being used for fast machining
operations or for very hard workpiece materials.