the powder with softer pure ironpowder, and Fig.1 shows the twopowders that were eventually used.Mixtures containing 25. 50 and 75%ofsteel were tried and the most prom­ising of these proved to be the 50:50mixture. If such a mixture is pressedand sintered at temperature suffici­ently high to give a reasonably denseproduct. diffusion of carbon from thesteel into the iron took place and thelooked for duplex structure was lost.

The powder consoli­dation process whichproved most successfulwas rotary forging at amodest temperature of700C. In this process thepowder mixture wasplaced in cylindricalmetal conta~ners 30 mmin diameter, degassed at320C and sealed. Thecontainers were thenheated for 30 minutes at700C and forged on arotary forging machine.Fig.2 shows a section ofa forging die and Fig.3shows the general arran­gement. With intermedi­ate re-heating for 15 min.at 700C a final defor-

b mation =Qm (Doz/DZ) =2.52 was achieved, and

this resulted in full densification.Microscopical examination showedthat the two starting materials hadretained their separate identities andthat both had similarly elongated toproduce a fibrous structure as can beseen in Fig.4. A transverse sectionshown in Fig.5 illustrates the discon­tinuous phase formed by the steel fib­res in the matrix of iron. It reveals alsoa rotational effect indicated by thearrow.Specimens broken in tension

objective in this study was to find auseful outlet for fines resulting fromthe production, by comminution, ofsteel grit for grit blasting.The steel particulate has the fol-lowing composition: 0.95Si-O.75Mn-0.4Cr-0.85C-O.2 0 2, andbecause of its high hardness (1250HVj is not possible to densify andshape this material by the normal PMprocess. In order to overcome thisobstacle it was thought that usefulresults might be obtained by mixing

pure iron and a hypereutectoid steelpowder of a wrought material havinga duplex structure in which the steelacts as a reinforcing agent. The

marteau

matrice

a

lin, France, recently described at theParis PM Colliquium on PMMicrostructures results from a studyof the production from a mixture of

Rotary Forged PM IronReinforced by Steel Fibres

~~pnnapal

cage

galet

FIG.2 Geometry of the rotal}' forging die used to consolidate theiron/steel powder mixture.

FIG.l (a) Iron powder (b) steel powder

H. Taghizadeh and his co-workers atthe Ecole Nationale Superieure desMines in St. Etienne, France, and atWheelaborator-Allevard in Goncel-

FIG.3 General arrangement of the rotal}' forging machine. FIG.4 Longitudinal section of the product with 50% steel.

MPR June 1990 405

sites of inclusions. X-ray micro­analysis idontified the oxides of alu­minium. silicon. and titanium. Fig.6shows one such inclusion and thedo-cohesion that it has caused duringthe tensile test. This is attributed tothe fact that tho inclusions. which aremuch harder than the matrix. do notdeform.In comparison with powder forgedalloy steels tho duplex material has\wy good tonsile proporties and it issuggestod that such materials couldhavo somo interesting applications.AGD

FIG.5 Trans~'erse section.

showed an elastic limit of 520 MPa,and a breaking strength of 725 MPawith an elongation of almost 3%.However. when the time of interme­diate reheating during the forgingoperation was increased to 40 min.the strength values obtained weresome 20% lower.Examination of tho fracture surfacosshowed a typical ductilo lcup andcono) fracture with no sign of cleav­age, and it is suggested that the poorductility may be consequence of theslight residual porosity at the iron!steel interfaces which are also the

Novamet Specialty Products CorporationAn INCO Company

INOVAMETI

CONDUCTIVEPIGIENTS

Also:

Conductive Silver-Coated Nickel

Nickel Coated Alumina

INca spp

For more information write to:

INCO SPECIALTY POWDER PRODUCTS1-3 Grosvenor Place, London SW1X 7EA, England

FIG.6 De-cohesion in the region ofan inclusion.

406 MPR June 1990