Steels_ Metallurgy and Applications, Third Edition

Engineering steels 241

160

140

~^120

IO0

90 1 O0 110 120 130 140

HV30

Figure 3.28 Effect of hardness on machinability (After Wannel et al. 16)

The hardness of low-carbon free cutting steels is important and the effect
appears to be associated with the achievement of an optimum embrittling effect
for chip formation. This is illustrated in Figure 3.28, which indicates an optimum
hardness of 105-110 HV in the normalized condition. Therefore the composition
of these steels, including residual elements such as chromium, nickel and copper,
needs to be controlled in order to achieve the optimum hardness. Additions of
phosphorus and nitrogen may also be employed in achieving the desired embrit-
tlement effects. Above the critical hardness level, embrittlement and ease of chip
formation give way to increased abrasion and reduced tool life.
As indicated earlier, oxide inclusions are particularly damaging to
machinability and the deoxidation practices adopted for low-carbon free cutting
steels have to be controlled very carefully in order to minimize the level of
these inclusions. With the move from ingot production to continuous casting, this
problem has become more acute as the steels need to be more heavily deoxidized
in order to minimize gas evolution during solidification.

Medium.carbon free cutting steels

Medium-carbon steels, containing 0.35-0.5% C and up to 1.5% Mn, are often

used in the normalized condition for engineering components requiring a tensile

strength of up to about 1000 N/mm 2. The free cutting versions of these steels

are generally based on sulphur contents of 0.2-0.3% but, because of their higher

strength, they are significantly harder to machine than low-carbon free cutting

steels. Therefore benefit can be gained in selecting a grade of steel with the

lowest level of carbon consistent with achieving the required strength in the end

product.