Sources of trace elements in primary raw materials used in production of superalloys

Abstract

The stringent demands of producers of Ni-based superalloys set lower permissible levels of deleterious trace elements than those that are normally considered significant by the geologist or the extractive metallurgist. The classification of metallic elements as siderophile, chalcophile, and/or lithophile is found to be a good general guide, as the most harmful superalloy trace elements have strongly chalcophile characteristics, i.e. they are concentrated in sulphides. Ore associations of the major superalloy metals Ni, Co, Cr, Mo, Al, and Ti are reviewed, and it is suggested that only in the case of lateritic Ni and Co ores is any form of ore source selection likely to alleviate problems associated with deleterious trace elements. In all other cases, the problems are passed on to the extractive metallurgist, whose product purity is reviewed from the point of view of the trace elements Sb, As, Bi, Cd, Ga, In, Pb, Mg, Se, Ag, Te, Tl, Sn, and Zn. The purity of carbonyl and some electrolytic Ni seems to be adequate for superalloy specifications. The lower levels of Co and the other alloying elements used make it possible to be less stringent with respect to impurity levels than with Ni, but in every case some impurity levels quoted seem high. This places a considerable burden on the superalloy manufacturer who, it is suggested, should know his source of primary metal, look carefully at the contribution of deoxidants in alloy manufacture, and examine the volatility of trace elements. The ‘gaseous’ impurities (H₂, O₂, N₂) can appear in primary refined metals as a result of extraction and refining practice or by contamination during handling.