Steels_ Metallurgy and Applications, Third Edition

Engineering steels 237

A

=,., O

0

k-

..J

, , i ,, , ,,, , ,

10g V (cutting speed)

Figure 3.24 Taylor tool life curve

Figure 3.25 Machinability test piece (By permission of British Steel plc)

Sulphur

Sulphur is the cheapest and most widely used free cutting additive in steels.
Whereas most specifications for engineering steels restrict the sulphur content to
0.05% max., levels up to about 0.35% S are incorporated in free cutting steels.
Sufficient manganese is also added to such steels to ensure that all the sulphur
is present as MnS rather than FeS, which causes hot shortness (cracking) during
hot working. The MnS inclusions deform plastically during chip formation into
planes of low strength which facilitate deformation in the primary shear zone.
The MnS inclusions also exude into the tool-chip interface, acting as a lubricant
and also forming a protective deposit on the tool. The net effect is a reduc-
tion in cutting forces and temperatures and a substantial reduction in the tool
wear rate. This is illustrated in Figure 3.26, which relates to low-carbon free
cutting steels and shows the marked decrease in flank wear rate with increase
in the volume fraction of MnS inclusions. Although relatively little improve-
ment in wear rate is achieved at sulphide volume fractions greater than 1.5%,
the chip form and surface finish continue to improve with further additions of
sulphur.