As it happens, the research described in this paper was in the same subject area as some of my own (mostly unpublished) work during the 1950s. Jewett describes his preference for carrying out such research under production conditions and the difficulties of simulating these in the laboratory. I agree with this but had no pilot plant and was able to make all my investigations under full-scale production conditions, which of course had drawbacks as well as advantages. For the last few years this has been one of the areas in which I’ve been trying (so far without success) to persuade the EPMA Hard Materials Group to undertake cooperative research, especially with modern gas analytical apparatus.

To maintain superfine grain sizes during carburising, apart from ammonium paratungstate quality the most important parameters turned out to be the dewpoint, purity and flow rate of the hydrogen atmosphere which (at TECO London) in the ‘50s we made, processed and purified, factors which are barely considered in this paper, which looked only at coarse WC powders as starting materials. In my opinion, the focus should from the start have been on fine, superfine and nano particle sizes.

In the early days of the industry, and especially at Osramwerke in Berlin during the Second World War, there were many attempts to do away with the relatively expensive hydrogen atmosphere for tungsten carburisation. With only point contacts between carbon and tungsten particles, solid-state reaction was virtually ruled out, as shown by experiments in vacuo, where the better the vacuum, the slower the reaction. Much the same applied to inert gases like argon, but not necessarily to nitrogen, which in any case usually contained small amounts of oxygen as impurity. Nevertheless, the necessary reactions were possible with other and cheaper gas atmospheres, especially carbon oxides. For example, many experiments were carried out with so-called direct carburisation of tungsten oxides, where the carrier gas became carbon monoxide and/or dioxide. For alloy carbides involving Ti, Ta and other non-W carbides, even today carburisation may initially involve oxides of the alloying metals and only W as metal powder. Reduction and carburisation by self-generated catalytic carrier gas take place simultaneously, so must be carefully controlled. Here hydrogen atmosphere is only applied in later stages and at higher temperatures, to eliminate oxides and encourage solid solution.

In general, this paper describes a competent piece of research by an acknowledged specialist in the subject and does indeed fill in a few blanks in our knowledge.

This article appeared in the March–April 2019 issue of Metal Powder Report. Log in to your free materialtoday.com profile to access the article.

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