Steels_ Metallurgy and Applications, Third Edition

Engineering steels 239

Due to its high density, major precautions have to be taken in the production
of leaded steels in order to avoid segregation effects. Again lead reduces the
frictional effects at the tool-chip interface, where it becomes molten at the
elevated temperatures generated during cutting. Lead is also thought to have
an embrittling effect in the primary shear zone, thereby shortening the chips and
improving surface finish. The lead particles are often present as tails to the MnS
inclusions.
Lead has little effect on the mechanical properties of steel at ambient
temperature since it is generally present as a globular constituent.
The toxic effects of lead are well known and care has to be taken to contain
lead fumes during the production of leaded steels. Similar measures are also
taken during the drop forging of leaded steels but the author is not aware that
hygiene problems arise during the machining of these steels.

Tellurium

Tellurium is an efficient but relatively expensive addition in free cutting steels and
is generally restricted to a maximum level of 0.1%. In larger amounts, it leads
to cracking during hot working. Tellurium is generally present as manganese
telluride, which is a low melting point compound and should therefore act in
a similar way to lead. The high surface activity of tellurium is also considered
important in its action as a free machining additive. Small additions of tellurium,
e.g. 0.01%, are also added to engineering steels in order to produce more globular
MnS inclusions and promote better transverse properties.
Like lead, tellurium presents hygiene problems during steelmaking (garlic
breath) and requires the operation of fume extraction systems.

Selenium

Selenium additions of 0.05-0.1% are made to low-alloy steels, but in free
machining stainless steels many specifications call for a minimum of 0.15% Se.
Selenium is present as a mixed sulphide-selenide and, like tellurium, small addi-
tions of this element are also effective in promoting more globular sulphide
inclusions.

Bismuth

Bismuth is closely related chemically to lead and its free cutting properties have
been known for many years. However, commercial interest in bismuth has only
been significant since about 1980, as greater anxieties have been voiced about
the use of lead. Levels of up to 0.1% Bi are typical in free cutting grades and
bismuth is present as tails to the MnS inclusions. Its action in promoting improved
machining characteristics appears to be similar to that of lead.

Calcium

As indicated earlier, oxide inclusions are hard and abrasive and detract from
the machinability of steels. This is particularly the case with alumina inclusions