A general scheme relating the chemical and instrumental methods available for isolation and identification of microconstituents in steel is presented. Several techniques are discussed in more detail. These include electrolytic solution, selective centrifuging, and use of the petrological microscope. High-silica values were obtained when thiocyanate and citrate were used as electrolytes in electrolyses of the Klinger and Koch type. The anomalous values were traced to oxidation of silica in the steel to silicic acid. Washing the residue with sodium carbonate eliminated this problem. The authors describe use of a potentiostat for selective solution of complex austenitic 15Cr-25Ni steels containing additions of other metals. Use of the threshold centrifuge to separate residues by weight and size for X-ray diffraction analysis is discussed. An example of identification of spinels in a globular silicate inclusion by the petrological microscope is also presented. Throughout the paper the complementary nature of the various methods and the use of methods in combination are stressed.
The identification of phases in solid steel specimens is subject to well known limitations. With suitable techniques, however, phases present in exceedingly small amounts can be isolated and identified. The work to be described is represented by three main stages.1.The techniques of isolation. The aim has been simplicity with the maximum efficiency of separation. Simple open and closed type cells are used and a few selected electrolytes for different conditions.2.The second stage is the identification of the separated particles. The X-ray photographs do, however, provide a considerable amount of other data which are often as important as the identity of the constituents. Thus, information can be obtained about the shape and size of particles and lattice spacing changes in solid solution phases. A supplementary determination of the solid solution content of the matrix is often useful.3.The third stage involves the interpretation of the results in relation to the strength and other properties of the steel or alloy. Here such questions arise as the effects of particular carbides or intermetallic phases, solid solution hardening and the times involved in various reactions and their relation to creep, etc.A number of examples are given and some limitations noted.
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