A New Method For Making Rapid And Accurate Estimates Of Grain Size

THE grain size of a metal or alloy is one of the most important factors determining its properties. In steels, for example, grain size affects hardenability, toughness and machinability; in brasses, grain size is related to hardness and deep-drawing quality. In the field of heat-resistant alloys, grain size has a marked effect on ductility, creep and endurance strength. In order to determine these effects, as well as to control the properties of materials, many accurate and rapid measurements of grain size must be made. This paper presents a new and improved method for the estimation of grain size. USUAL METHODS of EVALUATING GRAIN SIZE A frequently used method of estimating grain size is that of comparing the image of a polished and etched specimen on the ground-glass screen of a metallographic microscope with standard grain-size charts. Several types of standards are available for use with different classes of microstructures. Geometric Grids.-The Metals Handbook1 has a series of hexagonal grids ranging in size from ASTM No. I to 8. Idealized Grain Networks.-Idealized grain-boundary networks for ASTM grain sizes I to 8 are included in the American Society for Testing Materials Tentative Standard E19-39T. Standard Micrographs.- The Metals Handbook and ASTM Tentative Standard EI9-39T contain micrographs of hypoeutectoid and hypereutectoid steels ranging in grain size from No. I to 8. The grain size of annealed brass (diam¬eter of the average grain in millimeters) is obtained by comparison with 10 micrographs of varying grain size in ASTM Tentative Standard Es-39T. A handicap to accurate evaluation of grain size by this method is the fact that it is difficult to compare photographs in a dimly lighted room with the image on the ground-glass screen, or, if the general illumination is increased so that the printed standards may be clearly seen, the contrast of the image of the unknown is decreased. It is easier to estimate grain size by comparing a micrograph of the unknown with the grain-size standards. However, photographing every specimen on which the grain size is desired is too costly in time and materials to be practicable. For hardened steels, the Shepherd or Jernkontoret fracture grain-size standards are very accurate and useful, but this method obviously cannot be applied to ductile alloys such as brass, stainless steels and austenitic heat-resistant alloys. Jeffries' method for grain-size measurements consists of counting the number