| ABSTRACT: | QUALITY INSPECTION OF SINTERED TUNGSTEN-COBALT POWDER PRODUCTS BY
MAGNETIC TECHNIQUE
E. Gorkunov, A. Ulyanov, and A. Chulkina
Institute of Engineering Science, Russian Academy of Sciences (Urals Branch); Yekaterinburg;
Russia.
The physical structure of sintered W-Co powder alloys comprises hard non-magnetic particles of
tungsten carbide cemented by plastic interlayers of cobalt, which is ferromagnetic. Strength
characteristics and magnetic parameters are governed mainly by the composition and structural state
of the cobalt binder, which provides a tool for magnetic inspection of the quality of sintering.
Coercivity HC is the main structure-dependent parameter of sintered powder alloys. The physical
nature of HC in sintered alloys however cannot be discussed in terms of Kersten’s “theory of
inclusions” because of the high volume of carbide grains (as much as 80 to 95%). To reveal the nature
of HC, we have made an assumption that the cobalt interlayer between tungsten carbide grains behaves
as a ferromagnetic film of variable thickness, with the spacing being equal to the average carbide grain
size. The magnetic hysteresis of the film depends upon two factors – the film thickness gradient (Hch)
and the interaction of the domain walls with various defects in the crystal lattice of the cobalt film
(Hcd), and it is given by
A/cm,
where (Hcd) = 32A/cm – experimentally measured coercivity of domain wall movement in the cobalt
interlayer; is the average carbide grain size; C is cobalt content in the alloy; ?WC and ?Co are the
density of tungsten carbide and cobalt respectively. Experimentally measured values of HC provide
fairly good agreement with the analytical ones for W-Co sintered alloys of various carbide grain size
and carbon content, provided the average thickness of cobalt interlayers exceeds 0.4 – 0.6 ?m. If
is less than 0.4 ?m, the cobalt interlayers are likely to get gaps leading to strength loss and
changing the nature of magnetic hysteresis. Thus measurements of HC make it possible to evaluate the
average grain size in sintered powder products after sintering. The content of cobalt in a particular
sintered alloy can be estimated by measuring its saturation magnetization. Magnetic inspection allows
one to predict cutting properties of hard-carbide cutting tools.
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