Grain size is the average size of the particles in a metal. All metals are made up of tiny crystals fused together. When viewed under a microscope, the boundaries between these crystals can be counted, allowing the grain size to be calculated. Metals with smaller grains tend to be stronger, though large grains can sometimes be useful as well. Testing grain size is thus useful for quality control in the development and production
Grain Size Measurements
Metals, except in a few instances, are crystalline in nature and, except for single crystals, they contain internal boundaries known as grain boundaries. When a new grain is nucleated during processing (as in solidification or annealing after cold working), the atoms within each growing grain are lined up in a specific pattern that depends upon the crystal structure of the metal or alloy. With growth, each grain will eventually impinge on others and form an interface where the atomic orientations are different.
As early as the year 1900, it was well known that most mechanical properties were improved as the size of the grains decreased. A few notable exceptions exist where a coarse grain structure is desired. Alloy composition and processing must be controlled to achieve the desired grain size. Metallographers examine polished cross sections of specimens from appropriate locations to determine the grain size.
Measure Grain Size
The Grain Size Module in IQmaterials utilizes the General Intercept Procedures described in both ASTM and ISO grain size norms. This module offers an intuitive interface and convenient walk-through Wizards that improve the accuracy and throughput of your testing.
Grain Size Measurement Features:
- Measure the grain size using ASTM or ISO norms
- Choose from intercept overlays, including:
- Diagonal lines
- Horizontal lines
- Vertical lines
- Set the boundary detection level with the interactive histogram tool
Circular test line pattern is applied for grain boundary counting.
The grain size of a metal or single phase alloy is an estimate of the average grain diameter, usually expressed in millimeters. The metallurgical techniques used to determine grain size are not necessary for this discussion, the major point to remember is that grain size is an important material characteristic. As the average grain size decreases, the metal becomes stronger (more resistant to plastic flow) and as the grain size increases, the opposite effect on strength occurs. In general, for a given alloy and thickness, ductility increases with grain size and strength decreases. This occurs because the smaller the grains, the shorter the distance dislocations can move. Therefore it is desirable to use metal of the smallest average grain size which can be economically fabricated into the desired part.
In addition to strength, grain size will also effect formability, directionality, texture and surface appearance. Table 1 shows the effect of change in grain size on tensile strength, yield strength and elongation.
Table 1. Annealed Tempers Mechanical Properties, Brass Alloy C26000 Flat Products, Thickness 0.04 inches
|Temper Grain Size
||Tensile Strength ksi
(0.5% Ext.) ksi
|Elongation in 2.0 inches, %
Table 2 describes some common grain size ranges and their recommended applications for manufacturing parts.
Table 2. Available Grain Size Ranges & Recommended Applications
|Average Grain Size, mm.
||Typical Operations and Surface Characteristics
|0.005 - 0.015
||Stampings and shallow forming. Parts will exhibit good strength and a very smooth surface.
|0.010 - 0.025
||Stampings and shallow drawn parts. Parts will exhibit high strenght and smooth surface. Generally used for metal less than 0.010" thick.
|0.015 - 0.030
||Stampings, shallow drawn parts and deep drawn parts requiring buffable surfaces. Generally used for metals less than 0.12" thick.
|0.020 - 0.035
||Used for many drawn parts. This grain size range includes the largest average grain size which will produce parts essentially free of orange peel. Generally used for metal thickness up to 0.032".
|0.025 - 0.040
||Deep drawing especially for material 0.015" to 0.020" thick. Brass with grain size of 0.040 mm may exhibit some roughening of surface when severely stretched.
|0.030 - 0.050
||Stampings that do not require buffing or polishing and drawn brass parts with relatively good surface finish. Generally used for metal 0.015" to 0.025" thick.
|0.040 - 0.060
||General deep and shallow drawing of brass. Moderate orange peel may develop on surfaces. Normal size range of 0.020" to 0.040".
|0.015 - 0.030
0.060 - 0.090
|Deep drawing of difficult shapes and deep drawing of metal and thicker. Parts will have rough surfaces with orange peel if they are not smoothed by ironing.
ASTM Committee E-4 has been a world leader in the standardization of grain size measurement methods. Initially, Methods E 2 recommended the ]effries planimetric method as the preferred measurement method. This method is more difficult to apply on a production basis than the intercept method due to the need to mark off the grains as you count them to minimize counting errors. This is unnecessary with the intercept method.
With the 1974 revision of Test Methods E 112, the intercept method, as modified by Halle Abrams, became the preferred analysis technique. The three-circle intercept method, as described in Test Methods E112 since 1974, provides a more precise estimate of the grain size in much less time than required by the planimetric method. In manual methods, it is essential to recommend the most efficient method for any measurement.
Test Methods E 112 is designed for rating the grain size of equiaxed grain structures with a normal size distribution; the standard is presently being revised to provide better instructions for rating the grain size of deformed grains. Other standards have been introduced by E-4 to handle the measurement of occasional, very large grains present in an otherwise uniform, fine grain size dispersion (E 930, Methods of Estimating the Largest Grain Observed in a Metallographic Section (ALA Grain Size)) or for rating the grain size when the size distribution is non-normal, for example, bi-modal or "duplex" (E 1181, Methods of Characterizing Duplex Grain Sizes). Committee E-4 has recently developed a grain size standard for ratings made using semiautomatic or automatic image analyzers (E 1382, Test Methods for Determining the Average Grain Size Using Semi-Automatic and Automatic lmage Analysis). No other standards writing organization has developed standards similar to Methods E 930, Methods E 1181 or Test Methods E1382.