Steels_ Metallurgy and Applications, Third Edition

242 Steels: Metallurgy and Applications

Whereas silicate inclusions impair the machinability of low-carbon free

cutting steels, they are not particularly damaging in medium-carbon steels when

machined with carbide tools. This is due to the fact that higher temperatures

are generated in the cutting of these steels, leading to the softening of the

silicate inclusions. However, alumina particles are again particularly detrimental,

reducing the machining performance of both plain and resulphurized medium-

carbon steels.

Machinable low-alloy steels

In high-strength automotive transmission components, a high level of toughness
and ductility may be required in the transverse direction. For this reason, free
machining versions of low-alloy engineering steels were based traditionally on the
more cosily lead additions rather than sulphur. However, as indicated earlier, the
addition of inclusion-modifying agents can reduce the anisotropy in mechanical
properties and provide improved service performance in resulphurized steels. This
is illustrated in Figure 3.29, which shows the improvement in transverse impact
properties obtained in SAE 4140 steel 17 with calcium treatment. Bearing in mind
that high-strength engineering steels are generally fine grained with aluminium,
calcium treatment will also lead to an improvement in machinability with carbide
cutting tools due to the formation of calcium aluminate rather than alumina.

Machinable stainless steels

Although out of context in terms of chapter heading, it was thought that benefit
might be gained in continuity of technology by dealing with machinable stainless
steels at this stage.
Austenitic stainless steels have high rates of work hardening and this results in
poor machinability. However, this problem has been exacerbated to some extent

30-

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SAE 4140 Steels

Ib ~kk_ Q and T to 950 N/mm 2 9 Standard

9 ~' 9 Calcium treated

I , I l l ' I

0 0.02 0.04 0.06 0.08 0.10 0.12

Sulphur (wt %)

Figure 3.29 Effect of calcium treatment on transverse upper shelf energy of SAE 4140
steels (After Pickett et al. 17)