Magnetic susceptibility of an atomized 304L stainless steel powder: Particle size effect
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I.
INTRODUCTION
THE number of major and minor components in stainless steels makes it difficult to infer general relations between nominal composition and practical properties. When atomized powders of such materials are concerned, further complications stem from (1) pure surface effects, such as surface segregation and oxidation, and (2) true size effects which may be the consequence of the size-dependent cooling rate of the particles in the course of the atomization process. Reports can be found in the literature on the surface oxidation state of powders of various atomized steels 1~,2,3] and nickel base alloys. [4.5,6]Composite surface oxide layers are observed with a typical thickness of a few nanometers, but the state of knowledge is not such as to allow the prediction of the surface phase composition and related local magnetic properties in other systems. As for the present material, the surface oxide layer has been thoroughly investigated and characterized by a combination of surface and bulk analytical techniques. [71 In particular, it was shown to be 8.3-nm thick, from the dependence of the total oxygen content of the powder on particle diameter, and to be composed essentially of manganese, iron, and chromium oxides, based on X-ray photoelectron spectroscopy and ion microprobe analysis and on the selective reduction of the surface iron oxide by hydrogen. Its thickness and composition (54 at. pct Mn 2+, 30 at. pct Fe 3ยง and 16 at. pct Cr 3+) are essentially independent of particle size. The magnetic properties of an Fe-Cr-Ni alloy are dependent upon whether it is austenitic or ferritic. Schematically, austenitic steels are paramagnetic materials at room temperature and above, whereas ferritic steels are ferromagnetic. In its final form, as powder metallurgy extruded pipes or rods, the 304L steel investigated here is austenitic, and it is the observation of a ferromagnetic contribution to the susceptibility of the raw atomized powder and its unexpected dependence on particle size that are the subjects of the present article. G. GASC, Graduate Student, and P. BRACCONI, Research Assistant, are with the Research Laboratory for the Reactivity of Solids, University of Burgundy, Dijon 21004, France. Manuscript submitted October 15, 1991. METALLURGICAL TRANSACTIONS A
II.
MATERIAL AND TECHNIQUES
The powder is a product from Anval-Valinox, Montbard, France. Its elemental chemical composition is listed in Table I. The corresponding Cr and Ni equivalents calculated from Reference 8 amount to 19.6 and 12.2 wt pct, respectively. In the following, they are used in association with the cooling rate to predict the solidification mode. From De Long et a l . ' s diagram [9] (a modification of Sch~effler's diagram), a weld of the present material would be likely to contain from 2 to 4 wt pct ferrite at room temperature. The ternary Fe-Ni-Cr phase diagram l~~ shows that for equilibrium temperatures ranging from ~-600 ~ to ~1350 ~ the investigated composition lies in the austenite phase field (3/loop). The particles are ess
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