Significance and Use
Refractory metal powders, such as tungsten and molybdenum, are usually produced by hydrogen reduction at high temperatures. Thus, they usually contain numerous large, strongly-sintered agglomerates. Many of the manufacturing processes using these powders involve a milling step or some similar treatment or depend on the individual particulate size, not on the agglomerate size. Thus, a knowledge of the individual particulate size distribution, not the agglomerate size distribution, is usually desired from a particle size analysis of these powders. This practice provides a procedure for breaking down agglomerates into their constituent particles (de-agglomeration), without excessive fracture of the individual particles. The procedure is often referred to as laboratory milling or rod milling.
The laboratory milling conditions specified in this guide have been in use since 1965 as part of Test Method B 430. This guide was first published as a separate, stand-alone standard in 1995 because of its applicability in preparing powder samples for analysis by other methods (for example, Test Methods B 761 and B 822), in addition to Test Method B 430. Information on the development and establishment of the milling conditions here specified can be found in the footnoted reference.
The milling procedure described in this practice does not necessarily break down only agglomerates without fracturing individual particles; some particle fracture may occur in certain powders. However, use of this practice does provide consistent particle size analysis results that have been found to relate well to powder behavior in numerous manufacturing processes.
This practice shall be used for the de-agglomeration of the refractory metal powders and compounds listed in 1.1, when an evaluation of the individual particulate size distribution is required from the subsequent particle size analysis. It shall not be used when the agglomerate (as-is or as-supplied) size distribution is desired.
This practice may be used in preparing samples for Test Methods B 330, B 430, B 761, B 822, and other particle size analysis methods, prior to the dispersion procedure of Guide B 821, if used.
1.1 This practice covers the de-agglomeration of refractory metal powders and their compounds in preparation for particle size analysis.
1.2 Experience has shown that this practice is satisfactory for the de-agglomeration of elemental tungsten, molybdenum, rhenium, and tantalum metal powders, and tungsten carbide. Other metal powders (for example, elemental metals, carbides, and nitrides) may be prepared for particle size analysis using this practice with caution as to effectiveness until actual satisfactory experience is developed.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific precautionary statements, see Note 2.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
B243 Terminology of Powder Metallurgy
B330 Test Method for Fisher Number of Metal Powders and Related Compounds
B430 Test Method for Particle Size Distribution of Refractory Metal Powders and Related Compounds by Turbidimetry
B761 Test Method for Particle Size Distribution of Metal Powders and Related Compounds by X-Ray Monitoring of Gravity Sedimentation
B821 Guide for Liquid Dispersion of Metal Powders and Related Compounds for Particle Size Analysis
B822 Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering
de-agglomeration; laboratory-milled; laboratory milling; molybdenum; particle size analysis; powders; refractory metals; rhenium; rod-milled; rod milling; tantalum; tungsten; tungsten carbide; De-agglomeration; Laboratory-milled powders; Molybdenum; Particle analysis--metallic materials; Powders; Refractory metal powders; Rhenium powder; Rod-milled powders; Tantalum (Ta) powder; Tungsten; Tungsten carbide
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