Materials for engineering, 3rd Edition - (Malestrom)

110
Materials for engineering
features include tool life, production rate, surface finish of component, chip
form and ease of swarf removal. Various elements are added to steel in order
to improve the machinability, of which the most important are sulphur
and lead.
Low-carbon free cutting steels are employed for components whose
mechanical property requirements are minimal and which are mass produced
at high machining rates. Sulphur contents in the range 0.25–0.35% are typical
and sufficient manganese is also added to ensure that all the sulphur is
present as MnS rather than FeS (which causes cracking during hot working).
The MnS inclusions deform plastically and promote chip formation and they
also form a protective deposit on the tool, resulting in reduced cutting forces
and reduction in the tool wear rate. Substantial further improvements in
machinability are obtained by the addition of ~0.25% lead to such steels.
Machinable low-alloy steels are often based on the more costly lead additions
rather than sulphur, since a high level of toughness and ductility may be
required in the transverse direction in (for example) high-strength automotive
transmission components. Re-sulphurized steels tend to exhibit greater
anisotropy of mechanical properties.
Stainless steels
With the addition of about 12% Cr, steels exhibit good resistance to atmospheric
corrosion and these stainless steels owe their passivity to the presence of a
thin protective film of Cr 2 O 3 . They are used because of this corrosion and
oxidation resistance, as well as their pleasing appearance. Because of the
number of elements they contain, their microstructure cannot be simply
represented on the simple phase diagrams we have considered, but they are
normally classified according to their crystal structure as ferritic, martensitic,
austenitic and duplex stainless steels. The alloying elements present can be
classified as ferrite stabilizers, which tend to promote the formation of the
bcc α-phase, or as austenite stabilizers, which tend to promote the facecentred
cubic (fcc) γ-phase. In predicting the room temperature microstructure
of stainless steels, therefore, the balance between the ferrite and austenite
formers has to be considered. An empirical and approximate approach to this
question can be made by means of the Schaeffler diagram, which has been
modified by H. Schneider (Foundry Trade J. 108, 562, 1960) illustrated in
Fig. 3.27. This indicates the structures produced after rapid cooling from
1050°C; the axes are the chromium equivalent and the nickel equivalent. The
former indicates the proportion of elements (expressed as weight percentage)
that behave like chromium in promoting ferrite according to:
Cr equivalent = Cr + 2Si + 1.5Mo + 5V + 5.5Al + 1.75Nb
+ 1.5Ti + 0.75W