TPT January 2012

A rticle Improving standard methods of grain size determination in high-alloy steel and alloy products D.Yu. Klyuev, Ye.Ya. Lezinskaya, V.V. Perchanik (National Metallurgical Academy of Ukraine) N.A. Koryaka (ITA Representative in CIS, Ukraine)

Austenite or ferrite grain size is the major parameter of steel and alloy single-phase structure. Determination of this parameter is complicated by two reasons: • Opacity of the subject being measured • Imperfection of the methods used for grain size determination Many methods for determination of metal grain structure parameters appeared in the 20 th century. They have made it possible to lay down basic principles of a new trend in metal science: stereological metallography. Stereometric Metallography by S.A. Saltykov, Stereology in Physical Metallurgy by K.S. Cherniavsky, Quantitative Microscopy by De Goff and Rines and Quantitative Stereology by Underwood are fundamental works generalising the experience gained in this direction during the last century. Nearly a century-old history of existence of standard methods of determination of grain size in steels and alloys underwent several stages of its development. At first, methods of austenite and ferrite grain identification were developed. American Society for Testing and Materials (ASTM) pioneered creation of such standards. In the early 1930s, ASTM Committee E4 developed a number of standards including various methods of grain size determination which formed the basis for analogous standards adopted in many industrially developed countries, eg SIS 11 11 01 (Sweden), UNI 3245 (Italia), GOST 5639 (Russia), NF A04-102 (France), and SEP 1510 (Germany) which was transformed later into DIN 50601 and ISO 643. Methods in the above standards determine average dimensions (diameter, area) or the number of the grains visible in a microsections plane. These methods are based on the development of reference photographs with which structures seen in the microsections plane are compared and the grain size is determined by the number of the grains seen in one square inch at magnification ×100. Grain size numbers have appeared much more later on, in standard E112. It is significant that evaluation by numbers was called forth by the needs of metal product producers who had to control grain size in

their finished products. Introduction of conventional units (numbers) has made much easier the grain size control operations but it significantly worsened accuracy as compared with the quantitative methods. Nevertheless, all present-day standard methods of determination of grain size in metals and alloys use exactly these conventional units. All methods come to either visual comparison with photographic references and expression in conventional units (numbers) amounting from 18 (in GOST 5639) to 30 (in ASTM Е 112), or counting the number of grains in a unit of length (chord method), area or volume. Chord method is of a lowest accuracy because of imperfect means of determination of spatial dimensions and shapes of individual crystallites in opaque materials of metal products. It does not ensure a realistic view of such important parameters of a spatial structure as grain anisotropy, shape, etc. The existing methods developed for the reconstruction of spatial structures by its flat layout in a metallographic section are rather laborious and, again, coming to the determination of average dimensions which does not allow establishment of the cause of material property irregularities undoubtedly connected with structural irregularities, ie grain anisotropy. High-alloy steels and alloys used in high-duty structures (power equipment, electronics, chemical equipment, space technology, etc) are the materials the most prone to grain anisotropy. It should be pointed out that the present-day methods of manufacture of products form such materials, eg tubes for fuel element cans, allow formation of a rather homogeneous structure in these products [1-3] . At the same time, the available standard methods for evaluation of such structures are rather imperfect. What concerns methods for evaluation of grain anisotropy, they are practically absent. Domain of above standards, except GOST 5639, spreads just to the structures in which grain size distribution approaches the normal one. And at the same time, if the image in the microsections plane is closest to those in photographic references given in the standards, the most trustworthy assessment results can be obtained. The higher grain anisotropy of the structure, the higher is the error of its assessment.

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J anuary 2012

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