Secondary treatment of pm

Machining:
 Sintered parts do have some porosity and
it is, therefore, necessary to differentiate their machining behaviour from
those of fully dense wrought products. The major differences are, as follows:
(i) when a porous metal is machined, the depth of work hardening is more
important than for the wrought metal, as in case of former, pores create stress
concentration. (ii) The temperature at the tool end causes oxidation of the
pore surface. (iii) The surface porosity enhances tool vibration, which is
submitted to fatigue. 1 Pleney and Eudier 
have elegantly summarized various stages of tool wear. The stages are: –
Adhesion wear of tool – Influence of the oxidation of parts – Tool oxidation –
Tool wear by fatigue. Although P/M parts are often specified because machining
operations can be eliminated, in some cases it is more economical to leave
certain part details desired for machining, rather than to incorporate them in
the pressed configuration.  Parts made by
P/M techniques generally require machining methods that differ from those used
for wrought or cast parts of similar composition because of the inherent
porosity of P/M parts.  The following are
some of the causes of machining problems in P/M parts: (i) Pores may be closed
by smearing of the metal surface. ii) Cutting fluids may cause difficulties by
entering the parts. iii) Parts may become charged with abrasives when ground;
honed or lapped. Variations in machinability may be caused by differences in sintering
conditions and by different alloy compositions. 
In steels the degree of surface carburization or decarburization would
have significant effect on machinability. Machinability will also be affected
if the parts are either over or under sintered. As parts increase in density,
they begin to approach the same machinability as wrought and cast parts.  This is due to the fact that porosity
simulates a series of interrupted cuts.