Optimizing liquid phase sintering of ferrous powder metallurgical materials

Doctoral thesis, 2008

The properties of powder metallurgical, PM, components are dependent on that the dilemma of full density and suitable microstructure is resolved. Whilst it is often possible to achieve full density, it may or may not coincide with a suitable microstructure. Therefore, this thesis has addressed key aspects in the PM processing chain in the viewpoint of achieving a suitable microstructure after sintering. These key aspects have included Properties of Ferrous Powder, Green Body Consolidation, Sintering and Post-sintering Heat Treatments. To do this two ferrous alloy systems have been considered. A pre-alloyed gas atomized high speed steel, HSS, system that has either been loose powder sintered or sintered after a forming process called starch consolidation. Starch consolidation concerns the manufacture of a porous green body by mixing an aqueous-slurry consisting of spherical powder, starch, dispersant and thickener. After drying, these green bodies have been liquid phase sintered in different atmospheres to manipulate the microstructure locally at the surface or globally. The other considered system has concerned die-pressed grey iron powder mixtures consisting of lubricant coated iron powder and ferrosilicon as well as liquid forming additive in powder form for some mixtures. Common to both systems is that the sintering temperature needs to be accurate in order to achieve a successful microstructure. Furthermore, if the heating rate and the cooling rate are controlled for the grey iron powder mixture system, a microstructure with bainite and nodular graphite leads to yield and ultimate tensile strength of 500 and 600 MPa, respectively. For the HSS system conventional heat treatments were not attempted, but the experimental results and thermodynamic modelling results show that the carbon content in the matrix needs to be around 0.6 wt-% C if the desired microstructure of martensite and carbide is to be obtained. This work has demonstrated through model experiments and X-ray photoelectron analysis that the nitrogen pressure, p, in the atmosphere and the bulk nitrogen content CN is related through CN = constant√p if the surface oxide has been reduced first.