Engineering steels 255The chromium content of most grades of high-speed steel is about 4%.
Chromium forms carbides such as M23C6 and M7C3 but these carbides dissolve
fairly readily and are taken into solution at the normal solution treatment
temperatures employed for these steels, i.e. 1200-1300oc. Chromium is therefore
added as a hardenability agent and promotes the formation of martensite. In
addition, chromium is also beneficial in improving the scaling resistance of these
steels at the high temperatures generated during machining operations.
Molybdenum and tungsten are the principal alloying elements in high-speed
steels and can be present in levels up to about 10% and 20% respectively.
From a metallurgical standpoint, the two elements are very similar and in most
types of steel they are interchangeable on the basis of atomic weight %. Both
elements are strong carbide formers and the principal carbide type is M6C.
This carbide is sometimes called eta-carbide and has a low solid solubility in
steel. Therefore molybdenum and tungsten contribute little to the hardenability
of high-speed steels but the small amounts that are dissolved are very effective
in promoting tempering resistance and maintaining a high level of hardness at
the cutting temperature (red hardness). Both elements also induce secondary
hardening reactions in martensite, as illustrated schematically in Figure 3.36.
This figure also indicates that appreciable softening does not take place until
the temperature exceeds about 550~ which therefore represents the effective
maximum operating temperature for these steels.
In the early development of high-speed steels, tungsten was regarded very
much as the preferred element and molybdenum the substitute element to be
tolerated only in special circumstances. However, this prejudice has now largely
disappeared and there appears to be no technical reason for recommending a
tungsten grade in favour of a comparable molybdenum grade.
All high-speed steels contain between 1% and 5% V and as the vanadium
content is increased, the carbon is generally increased by at least 0.1% for
each additional 1% V. Vanadium is a very strong carbide-forming element and
produces extremely hard particles of V4C3 (MC type). This carbide has a very
low solid solubility in steel and again contributes very little to hardenability.
Figure 3.36r
12
-r":_ :: i i i ,,! , i ...... i, , I
0 100 200 300 400 500 600 700
Tempering temperature (~Tempering curve for high-speed steel (schematic) (After Hoyle 2s)