Grain size overview grain size represents the size of the grain size scale. The grain size of metal has a great influence on many properties of metal. The effect of grain size is the effect of grain boundary area. The smaller the grain size, the larger the grain boundary area and the greater the influence on the properties. For the Mechanical Properties of metals at room temperature, generally, the smaller the grain size, the higher the strength and hardness, and the better the plasticity and toughness.

2. The basic methods of determining average grain size there are three basic methods in general: Comparison method, area method and cut-off point method. The comparison method does not need to calculate the grain size and cut-off moment. Compared with the standard series grading chart, there is a certain deviation (± 0.5 grade) in evaluating the grain size by comparison method. The reproducibility and reproducibility of the assessed values are usually ± 1. 2. Area method: The area method is to calculate the number of grains in a given area and to determine the number of grain size grades by the number of grains per unit area. The accuracy of ± 0.25 grade can be achieved by counting the grain size. The result of area method was unbiased and the reproducibility was less than ± 0.5% . Level 5. The key to the grain size of area method is the counting of grains with distinct grain boundaries.

3. cut-point method: The cut-point number is the cut-point number of the intersection part of the test line (or grid) with known length, and the cut-point number of unit length is used to determine the grade of grain size. The accuracy of cut-off point method is a function of the number of cut-off points or the intercept. The accuracy of ± 0.25 grade can be achieved by valid statistical results. The results of cut-off point method are unbiased, and the reproducibility and reproducibility are less than ± 0.5 grade. For the same accuracy level, the cut-off point method is faster than the area method because it does not need the cut-off point or the number of cut-off points.

3. METALLOGRAPHIC case study the above is only a general measurement method is too general, if we really get a specific micro-photo, how do we do? Now let’s look at the specific operation and calculation method. Determine the magnification of a photograph take the lead in measuring the size, length, or width of a microphotograph, then measure the actual length or width magnification = picture distance/actual distance 2, find out the number of grain size grades calculate magnification can determine the number of grain size grades. First, the number of grains in the sample must be calculated. Grain number = complete grain number + 0.5 x partial grain. The grain boundaries of intact grains are observable. Then calculate the actual area = picture length/magnification x width/magnification according to the ASTM STANDARD FORMULA: N = 2(n-1) where n is the number of grains per square inch magnified by 100 times and N is the number of grain grades. The value of n can be obtained by unit conversion. Finally, the number of grain grades N can be calculated. 3. The average grain diameter the average grain diameter = the actual length of the sample/the actual length of the cut part = the length of the cut line/the magnification

 Source: Web Collation

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