Once the original austenite grain boundaries are revealed, the austenite grain size may be determined by one of the following methods suitable for the purpose. The name of the method used is always mentioned with the grain size specification.
1. ASTM Standard Grain Size Number:
This is the most common method of measuring grain size, where the grain size is reported as ASTM (American Society of Testing Materials) grain size number. The method is used for equaled grains. The grain size is determined in a microscope at a magnification of 100 X by comparing with ASTM standard grain-size charts and the matching number of the grain size is reported.
ASTM standard grain size charts have been shown compared with grain size in specimens in Fig. 2.29 to 2.31 or, the microscope may have special eye-piece, which is provided with a revolving circular plate containing eight glass micrometer disks. On each disk, an accurately ruled hexagonal network is engraved, and the disks are numbered from 1 to 8 depending on the size of hexagons (standards of ASTM).
Different disks are revolved with field of view and when one of the hexagonal networks has matched with the grains of the specimen at 100 X, the grain size number is read directly from the circular plate and then reported. The ASTM grain size number, N, is related to the number of grains per square inch at 100 X magnification, n, by the relationship,
n = 2N-1 …(2.44)
N is the ASTM grain size number, and
n is the number of grains per square inch at 100 X (ASTM E 112 – 63 or IS 4748 – 1968).
Table 2.7 gives ASTM grain size number with corresponding average number of grain at 100 X per square inch, average grain diameter in mm and average number of grains per square millimeter at 1 X. Fig. 2.32 illustrates the same.
ASTM has modified (ASTM standard E 112-1985), the equation to:
G = -2.9542 + 1.4427 In na …(2.45)
G is the ASTM grain size number, and
na is the number of grains per mm2 at 1 X.
Nowadays, the specifications of the steels include the grain size too, as it helps in assessing the quality of steel, or its heat treatment. There is no clear distinction between coarse grained steel, or a fine grained steel as it is relative depending on the application of the steel. However, steel with ASTM grain size number below 3 definitely means coarse-grained steel, while above 6 means reasonably fine-grained steel.
Above ASTM No 8 are ultra-fine grained steels. For example, case hardening steels usually have fine grains of ASTM 5 to 8 according to Mc Quaid-Ehn Test, but some organisations use steels of ASTM number 8. Quenched and tempered tool steels have grain size not coarser than ASTM 7. High speed steels, after hardening from correct austenitising temperature have ASTM 9-10, whereas Mo-based high speed steels have ASTM 10.
Sometimes, steel shows mixed grain sizes then it is commonly expressed as percentages of area occupied by each of the size.
2. Jefferies Planimetric Test:
This method is also suited to equi-axe grained steels. The polished, etched sample is viewed at a certain magnification on the ground-glass screen of the microscope, or on a micrograph to count the number of grains inside an area, commonly 5000 mm2 (that in inside a circle of 79.8 mm diameter) as illustrated in Fig. 2.33.
The magnification is so selected to get at least 50 grains within this area. While counting the number of grains in this area, the grain lying wholly inside the circle is counted as one, but the grain intercepted by the circumference of the drawn circle is counted as half, i.e., the total number of grains in area of 5000 mm2 is given by,
ni, is the number of grain lying completely inside the circle, and
nc is the number of grains intercepted by the circumference.
Then, the number of grains per mm2 at IX, na is given by,
m is the linear magnification used. (There are now available calibrated eye pieces to count the number of grains in a given area at a magnification), then equation (2.45) can be used to get the ASTM grain size number.
3. Heyn’s Intercept Method:
This method is commonly used for high speed steels and when the grains are not equiaxed. The method consists of finding out the number of grains intercepted by a line of known length at a magnification on the ground-glass screen of a microscope, or a micrograph, or by means of graduated eye piece. Normally counting is done of the number of grains intercepted by a line of 0.005 inch in length at a magnification of 1000 X called mean intercept value, I.
The magnification chosen should be able to cover area of at least 0.5 mm2 on the actual sample. Measurements are made at 10 different places and then, the average is taken. The grains touched by the end of the test line are counted as half grains. The length of the test line divided by the average number of grains intercepted by that line gives the mean intercept length, or grain diameter at that magnification. It has been seen that “mean intercept length” grain size is somewhat smaller than the actual grain size. Fig. 2.35 too illustrates this.
The mean intercept value, I, is correlated with ASTM grain size number. If the mean grain size on the intercept scale (I) is expressed as the ASTM grain size number, i.e. as the number of grains per inch2 on the micrograph at 100 X magnification, that is,
Let the number of grains intercepted by a linear length of 0.005″ at 1000 X = I,
then, number of grains per inch at 1000 X = I/0.005
then, number of grains per square inch at 100 X = (2 × I)2
which is equal to n of equation (2.44). Putting this value in that equation,
The Fig. 2.35 illustrates the relationship between Heyn’s mean intercept numbers with ASTM grain size number.
4. Shepherd Fracture Test:
A hardened steel on fracture follows the former austenitic grain boundaries that existed prior to hardening. Thus, a study of fractured surface should give an indication of the austenitic grain size.
In Shepherd test, a scale consisting of a series of standard, hardened and fractured test pieces numbered from 1 to 10, where No 1 is the coarsest and No 10 is the finest fracture, and is indicator of prior austenite grain size, is used for comparing with the actual fractured surface. Fig. 2.36 is used to get the ASTM grain size number.