Steels_ Metallurgy and Applications, Third Edition

238 Steels: Metallurgy and Applications

200

150

A

E

50

tD

L.-^100

r

e--

Ii

I I I

1.0 2.0 3.0

Area MnS (%)

Figure 3.26 Effect of sulphide content on machinability (After Wannel et al. 16)

For many years, it has been postulated that the morphology of the MnS
inclusions plays a major role in the machining performance of free curing steels,
large globular inclusions being far more effective than thin elongated inclusions.
However, sulphide morphology is influenced markedly by the state of deoxidation
of the steel, a heavily killed steel promoting the formation of MnS inclusions that
are easily deformed into elongated inclusions during hot working. In addition,
killed steels tend to contain hard, abrasive oxide inclusions which have an adverse
effect on machinability. Therefore it is extremely difficult to differentiate between
the effects that might be due to sulphide morphology and those that are clearly
due to the presence of hard, abrasive oxide inclusions.
One of the disadvantages of high sulphur contents is that they impair the
transverse ductility of steels, particularly when MnS is present as long elongated
inclusions. As indicated later, inclusion-modifying agents can be added to high-
sulphur steels in order to promote a more favourable sulphide morphology.

Lead

After sulphur, lead is the next most common additive and additions of
0.15-0.35% Pb are incorporated in free cutting steels. Such levels are soluble in
molten steel but are precipitated as discrete particles of lead during solidification.