Steels_ Metallurgy and Applications, Third Edition

240 Steels: Metallurgy and Applications

which are formed in engineering steels due to the practice of adding aluminium as

a deoxidant or grain-refining element. However, the adverse effects of alumina

can be reduced with the addition of calcium, which results in the formation

of calcium aluminate. These inclusions soften during high-speed machining and

form protective layers on the surface of carbide tools. Calcium is also effective in

reducing the projected length of MnS inclusions, thereby improving the transverse

properties of resulphufized steels.

Low.carbon free cutting steels

For some engineering components, the mechanical property requirements are
minimal and by far the most important requirement is a high and consistent level
of machinability. Hose couplings and automotive spark plug bodies are examples
of such components which are mass produced at high machining rates. Sulphur
contents in the range 0.25-0.35% are typical but, as illustrated in Figure 3.27,
substantial improvements in machinability are obtained by the addition of lead
to resulphurized steels. Where extremely high rates of machining are required,
a low-carbon free cutting steel might be treated with sulphur, lead and bismuth,
e.g. 0.25%, S, 0.25% Pb, 0.08% Bi.

50-

20

.,..,. E:

E

| 10

80

.'I=:

+0.09%

Pb

+0.20%

Pb

I I I I t J

100 120 140 160 180 200

Cu~ing speed (m/min)

Figure 3.27 Effect of lead on machinability (After Wannel et al. 16)