PREPARATION OF NBS WHITE CAST IRON SPECTROCHEMICAL STANDARDS Robert E. Michaelis and LeRoy L. Wyman Institute for Materials Research Several methods have been investigated for the Cast iron may be described generally as an alloy of iron and carbon where the carbon content is between 1.7 and 4.5 percent. This is more carbon than can be retained in austenite at the eutectic temperature; hence on slow cooling the casting normally will exhibit a matrix of pearlite with many graphite flakes dispersed throughout. Such material is known as gray iron and was so named because of the characteristic "gray" fracture imparted by the graphite flakes. The tonnage production of gray iron exceeds that of all other cast metals combined. Also included in the general term "cast iron" are pig irons, white irons, chilled irons, and malleable irons. The rapid methods of spectrochemical analysis have been employed widely in the cast iron industry for the determination of most metallic constituents. In recent years, however, developments in spectrochemical instrumentation, particularly the vacuum emission spectrometer, made possible the complete analysis of cast irons including determinations for carbon, phosphorus, and sulfur. This markedly extended the use of spectrochemical analysis in the cast iron field. Also of significant interest has been the increasing application of x-ray spectrochemical methods for the analysis of materials such as cast iron. The major limitation to quantitative spectrochemical analysis is that the methods are not absolute, but are dependent on the technique of comparison with samples of known composition, referred to as standard samples. Thus the problem of calibration is of paramount importance to the accuracy of spectrochemical analysis. Standard samples must not only be of high homogeneity and have accurate values assigned to them, but also extreme caution must be exercised in matching the standards and the unknown samples with respect to all variables such as size, shape, composition, and metallurgical condition. The acute need by both industry and government for reliable and accurate cast iron spectrochemical standards was made clear to the National Bureau of Standards in the early 1950's by surveys and communications; and in 1955 an active program for the preparation of suitable standards was initiated. The main purposes of this paper are to describe the development of a satisfactory method of preparation for cast iron standards and to evaluate the application of this method to a set of eight prepared NBS spectrochemical standards. This work was supported in part by the American Iron and Steel Institute. 2. CONSIDERATION FOR PREPARATION The preparation of spectrochemical standard samples to be provided in the "as cast" condition presents several difficulties not encountered in those which can be provided in the wrought condition. For wrought standards, nonhomogeneous portions can be determined and discarded prior to working to the final size. Equally significant is the fact that the final homogeneity in the wrought standard can be improved markedly over that of the selected material by appropriate working and heat-treating operations [1]. For "as cast" standards, the required chemical homogeneity and desired metallurgical structure are obtained solely through metal pouring and casting procedures. Additional difficulties in the preparation of cast iron standards over most other cast materials are encountered because of the variety of complex metallurgical structures which may be formed on the solidification of the cast iron from the molten state, and which may markedly influence the spectrochemical results. The final metallurgical structure depends primarily on the composition, pouring temperature, and rate of cooling. 3. TYPE OF CAST IRON FOR STANDARDS Since gray iron is by far the most widely used type of cast iron, it commanded initial consideration. On investigation at NBS and elsewhere, however, it was shown that the graphite flakes, in terms of amount, size, shape, and distribution, had a pronounced effect on the volatility rates in the determination of some elements by optical emission analysis. In white cast iron virtually all of the carbon exists in the combined form and there is essentially no free graphite. Although limited in engineering applications, the white cast iron was considered to be the most advantageous material for the spectrochemical standards for the following reasons. (1) It would provide a uniform metallurgical structure for a series of standards to cover wide concentration ranges for the elements of interest. (2) It would offer a casting material that could be prepared to provide the requisite homogeneity of chemical composition. (3) Investi gation had shown that test samples with an induced white structure could be prepared in the foundry for most types of iron; these could thus be analyzed directly relative to the white iron standards. The white structure can be induced in at least a portion of the test samples either by using a rapid chill-casting procedure or by making certain additions known to promote whiteness, such as tellurium or bismuth, or by a combination of both, to the test ladle or mold. (4) Finally, the white cast iron provides the opportunity to make the important determination of total carbon by vacuum optical emission analysis, which would not be possible if much of the carbon was present in an uncombined form. 4. CHEMICAL COMPOSITIONS In addition to those elements which may be considered the main components of an alloy, modern metallurgy is most intimately involved with the effects of minute amounts of other elements. These may be impurities inherited from the raw materials or they may be picked up from furnace linings, crucibles, etc. In any event, most contaminants are deleterious to the final alloy. However, trace quantities of some elements, not normally impurities, may be added deliberately to enhance the alloy performance. Of further concern to the chemical composition requirements with respect to trace elements is the fact that the material sources and the processing methods may vary appreciably among the producers for a particular alloy. As a consequence, each may have his particular problems with respect to control of these elements. Thus the compilation |