Sylvain Pelletier

Effect of electrical resistivity on core losses in soft magnetic iron powder materials

Iron powder should be electrically insulated with a dielectric to reduce eddy current losses in components fabricated by powder metallurgy intended for AC soft magnetic applications. However, most of the dielectrics act as distributed air gaps and reduce the apparent permeability of the material. To obtain materials with good magnetic properties, the dielectric amount should then be kept as low as possible while maintaining low eddy current losses. This paper presents the influence of electrical resistivity on core losses in iron powder compacts at low frequency (f < 1 kHz). The results obtained show that a resistivity one order of magnitude higher than the resistivity of iron (~ 0.1 laf~ m) is sufficient to maintain low eddy current losses in powder cores at frequencies below 100 Hz.

Characterization of Nickel Diffusion and its Effect on the Microstructure of Nickel PM Steels

Admixing pure elements to powder mixes can cause the formation of heterogeneous microstructures in sintered parts. For instance, nickel is renowned for forming nickel-rich areas (NRA) in powder metallurgy (PM) nickel steels due to its poor diffusivity in iron matrix (or lattice). The present work is aimed at characterizing the principal diffusion mechanisms of nickel and their influence on microstructures and properties of PM nickel steels. A new wavelength dispersive X-ray spectrometry (WDS) approach linking line scans and X-ray maps to concentration maps is proposed. Grain boundary and volume diffusion coefficients of admixed nickel have been determined in PM nickel steels using Suzuoka’s equation. Results also show that nickel distributes itself in the iron matrix mainly by surface and grain boundary diffusion.

Strain induced martensite formation in PM nickel steels

Abstract Powder metallurgy (PM) nickel steels are often selected because they have high strength, high impact resistance and good abrasion resistance. The microstructure of slowly cooled PM nickel steels typically contains pearlite, bainite, martensite and a fair volume fraction of retained austenite. Since volume diffusion is really low at conventional sintering temperatures [1120–1150°C (≈2050–2100°F)], nickel rich areas are usually found where prior admixed nickel particles were located, i.e. at the surface of iron particles and in sintering necks. Therefore, there is a discrepancy between the rather low mechanical properties of austenite and the high strength of PM nickel steel. Hence, the hypotheses that stress induced or strain induced martensite formation takes place during mechanical testing have been investigated. Results show that martensite forms during mechanical testing, and its final volume fraction is proportional to nickel content.

Effects of additions of nickel nanoparticles on sintering response of PM hybrid low alloy steels

Abstract Addition of pure elements to powder mixes can cause the formation of heterogeneous microstructures in powder metallurgy (PM) parts upon sintering. For instance, it has been shown that additions of nickel particles to an iron powder form nickel rich areas (NRAs), since nickel has low diffusivity in iron at conventional sintering temperature [∼1121°C (2050°F)]. Thus, the present work is aimed at determining if addition of a small quantity of carbon coated nickel nanoparticles to a PM hybrid low alloy steel premix could result in a more homogeneous distribution of nickel in sintered parts. It also characterises the effect of this addition on microstructures and mechanical properties. Grain boundary and volume diffusion coefficients of nickel nanoparticles have been determined using Suzuoka’s equation and wavelength dispersive X-ray spectrometry maps. Results show that addition of nanoparticles initiates lattice diffusion at lower temperature and produces less NRA.

Tensile properties of sinter hardened powder metallurgy components machined in their green state

Abstract With the increasing demand for sinter hardened powder metallurgy components, there is a growing need to solve the poor machining behaviour that characterises them. Approaches based on green machining appear promising to extend tool life and reduce machining costs. The present study deals with the sintered properties of cylindrical tensile specimens green machined. Results show that tensile properties of components machined in their green state are identical to those of components machined after sintering.

CHARACTERIZATION OF WEAR AND PROFILE OF DIAMOND DRILL BIT BY OPTICAL PROFILOMETRY

A new technique to measure wear and profile of impregnated diamond drill bit for mineral exploration is presented. A three‐dimensional surface mapping of worn diamond drill bit surface is obtained by the laser triangulation method. With adequate algorithms all relevant data can be obtained. Consequently, the volume loss between two distances cored can easily be measured with a great precision. Moreover, computerized cross sections of the worn surface give relevant information about the profile resulting from the drilling process.