The determination of traces of O, N and C in the refractory metals Mo and W—an international effort

Abstract

The influence of traces of O, N and C on the physical and especially the mechanical properties of the refractory metals Mo and W is discussed. The technological and economic importance of determination of O, N and C in Mo and W is elucidated. The Commission of the European Communities launched a relevant multidisciplinary Community Programme as early as 1969. The present state, within this programme, of the determination of O, N and C in Mo and W is outlined. Additional studies by the refractory metals group of the chemistry section of the Gesellschaft Deutscher Metallhütten and Bergleute (GDMB) are also reported on. Oxygen. Two round-robin tests were conducted by the “non-metals in refractory metals” group of the Community Bureau of Reference (BCR) for the determination of oxygen in molybdenum. Reducing fusion, 14-MeV neutron-, photon- and charged-particle activation analysis yielded comparable results of about 15 ppm O. Homogeneity studies were conclusive and the reference material was certified and is available from BCR. Oxygen concentrations in tungsten turned out to be even lower, certainly below 5 ppm. Only activation analytical methods will be adequate to determine the true oxygen content and work in this direction is being undertaken. Nitrogen. Relevant BCR round-robin tests for traces of N in Mo and W were not conclusive. Discrepancies were also found in a first round-robin test by the GDMB. It was possible, however, to reveal systematic errors frequently encountered in the classical Kjeldahl method, which turned out not to be applicable to the determination of nitrogen below 1O ppm. Only a newly devised micro-Kjeldahl method is capable of determining nitrogen down to 1 ppm and results for Mo are in good agreement with those of fusion methods. Nitrogen contents in W are presumably in the 100 ppM range and only determinable by ultrahigh-vacuum diffusion extraction and activation methods. Carbon. Carbon contents in Mo and W are also often presumably in the range of 1–10 ppm and thus not determinable by classical combustion methods. Additionally, discrepancies occur between results of combustion in resistance-heated furnaces with temperatures up to 1300° and in high-frequency induction furnaces with temperatures up to 2000°. The GDMB-group is investigating this phenomenon.