254 Steels: Metallurgy and Applications:3^6
8
< oo 410
sl
2 I |l.~--D-Type Inclusions
Sept. Apr. Sept.
1984 1985 1986
Time(each point is the average of 10 casts)Figure 3.35 SAM counts of steel delivered from major steel supplier (After Hampshire
and King 26)High-speed steels
High-speed steels are so called because of their ability to maintain a high level of
hardness during high-speed machining operations. They are characterized by high
carbon contents, sometimes up to 1.5%, and major additions of strong carbide-
forming elements such as chromium, molybdenum, tungsten and vanadium. Up
to 12% Co is also included in some of the more complex grades. The constitu-
tion, manufacture and properties of high-speed steels have been reviewed very
thoroughly in a book by Hoyle. 28
Role of alloying elements
The microstructure of high-speed steels consists of primary and secondary
carbides in a matrix of tempered martensite. The primary carbides are coarse
particles which are not dissolved during solution treatment for the hardening
operation and the secondary carbides are fine particles that are precipitated in
martensite during tempering treatments. The role of the major alloying elements
is outlined below.
A high carbon content is required in order to produce a hard martensitic matrix
and also to form primary carbides. Both constituents are effective in providing
abrasion resistance during metal cutting. However, the amount of carbon that
can be accommodated in high-speed steels is limited on two counts. Firstly,
carbon lowers the solidus temperature of steels substantially and therefore reduces
the maximum temperature that can be employed for solution treatment prior to
hardening. Secondly, carbon has a powerful effect in depressing the Ms-Me
temperature range and a very high level of carbon in solution will lead to
excessive amounts of retained austenite.