Sintering behaviour of compacted water-atomised iron powder: effect of initial state and processing conditions

Abstract

Conventional ferrous powder metallurgy relies on uniaxial die-pressing of powder before sintering. The operation causes particle rearrangement and deformation and the initial state of the compact ultimately determines the sintering behaviour. In this study, sintering was investigated for water-atomised iron powder compacted at three different pressure levels, with and without admixed graphite. Electron back-scatter diffraction of the powder and compacts showed a large increase in low-angle grain boundaries after compaction. The sintering of the compacts was investigated by means of dilatometry in hydrogen. The initial compaction strongly influences sintering shrinkage, particularly in the low-temperature, high-diffusivity ferrite region which can account for up to 80% of the recorded shrinkage. Only a small fraction of the total shrinkage originates from the austenite phase, even at high sintering temperatures. However, the ratio of sintering shrinkage in ferrite and austenite changes with compaction pressure, carbon content and heating rate.

Influence of the heating rate on sintering shrinkage of plain iron

“…The same result was found by Ziu et al [9] on 316L green specimens shaped by MIM and by Wendel et al [4] on prior cold compacted Fe and Fe-0.4%C.…”

“…1. less consumption of the driving force at the lower temperatures that increases shrinkage, as suggested by Wendel et al [4]; 2. less shrinkage at each temperature. These two counteracting effects balance each other.…”

“…Both the anisothermal and the isothermal shrinkage depend on the green density [1], as a result of the combination of the thermodynamic driving force and of the deformation of the powder particles promoted by cold compaction [2]. Deformation introduces a large density of structural defects in the interparticle regions, as it has been demonstrated by Torresani et al [3] and confirmed recently by Wendel et al [4]. Structural defects enhance bulk diffusivity through the dislocation pipe diffusion mechanism [5,6] (structural activity).…”

Sintering anisotropy of binder jetted 316L stainless steel: part I – sintering anisotropy

“…spark plasma sintering, hot pressing, hot isostatic pressing, etc.) use the advantage of adding pressure during sintering to improve densification [2,3]. Conventional press and sinter processes typically reach higher green density by the pressure-assisted green part shaping process.…”

Power Molding Inductors Prepared Using Amorphous FeSiCrB Alloy Powder, Carbonyl Iron Powder, and Silicone Resin

“…Except for the 3K7C sample, the initial permeabilities of the heat-treated samples after compaction were significantly higher than those of the heat-treated samples before compaction. This is because of the high internal strain caused by the severe plastic deformation of carbonyl iron powders during uniaxial molding [ 21 ]. According to previous research [ 22 ], the high permeability of nanocrystalline alloy has been linked to two causes: (1) the magnetostriction of the alloy, which is proportional to the Si content, decreases dramatically after crystallization; (2) the effective magnetic anisotropy decreases dramatically for the very small grain size of α-Fe crystals due to ferromagnetic exchange interactions.…”