Cemented carbide hardmetals have been produced since 1910
and have gradually taken over from tool steels as the major
material for high speed metal cutting and forming.
One such forming operation is that of cold heading used
in the production of nail and screw fasteners. This operation
subjects a cemented carbide die to cyclic compressive and
tensile stresses and may be considered as a fatigue process.
Cold heading manufacturers have always been aware that some dies
will produce millions of components and yet others will fail
after a much smaller number and, in view of this, have reasoned
that the surface finish of the die may be limiting factor in
This research has been carried out to investigate how the
surface finish .affects die life and to find if there is any
numerical correlation between surface finish and life.
Commercially manufactured cemented carbides were subject
to fatigue trials using standard miniature Wohler specimens.
The grades of materials used were B, N and TT with a cobalt
content of 6, 6 and 25% respectively. Grades N and TT were
commercial grades with a grain size of 1.5-3µm, whereas B grade
was an experimental grade with a grain size of 3.0 – 5.0µm.
The fatigue tests were carried out on Wöhler rotating
bending fatigue machines with stress levels on the specimens of
between 700 and 1400 MNm².
Initial testing was carried out with specimens in the as
received ground condition. In order to demonstrate the effect
of surface treatment on the fatigue performance of the test
grades, the test specimens were subject to various mechanical
and thermal treatments. [...continued]
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.