Grain Size Analysis

Grain size analysis of metal and alloy samples such as aluminum or steel is a very important step in the whole quality control process of metallographic laboratory. The standard grain size is divided into 12 grades, 1-4 grade is coarse grain, 5-8 grade is fine grain, and 9-12 grade is ultra-fine grain size. It is a well-known fact that smaller grain size usually improves the mechanical properties of the sample, so it is very important to control the alloy composition and process to achieve the desired grain size. In addition, grain size analysis is also used to investigate the causes of possible material failures. According to ASTM E112-13 (Standard Test Methods for Determining Average Grain Size), T,C&A Lab can perform grain size analysis on a wide range of metallic materials and nonmetallic materials whose structures having appearances similar to those of the metallic structures. Welcome to contact our experts for consultation.

The grain size test methods T,C&A Lab provides include:

• Comparison method

The comparison method does not need to calculate any grains, intercepts, or intersections. There is generally a certain deviation (±0.5 grain size units) when evaluating the grain size by the comparison method. The repeatability and reproducibility of the evaluation value is usually ±1 grain size number.

• Planimetric method

The planimetric method is to calculate the number of grains in a known area, and use the number of grains per unit area to determine the grain size level. The measurement result of the planimetric method is unbiased, and the accuracy of ±0.25 grain size units can be achieved through reasonable counting. The repeatability and reproducibility of the evaluation value are less than ±0.5 grain size units. The key to the grain size of the planimetric method is the count of grains that are clearly divided by the grain interface.

• Intercept method

The intercept method is to calculate the number of intercepts at the intersection of a test line segment (or network) of known length with the grain interface, and use the intercept per unit length to determine the grain size level. The accuracy of the intercept method is a function of the calculated intercept or intersection. The accuracy of the effective statistical results can reach ±0.25 grain size units. The measurement result of the intercept method is unbiased, and the repeatability and reproducibility are less than ±0.5 grain size units.

Case study

After obtaining the specific microscopic photos, the average grain diameter can be calculated according to the following process.

• Determine picture magnification

First measure the size of the microphotograph, choose one of length or width, and then measure the actual length or width of the sample.

Magnification=picture distance/actual distance

• Find out the number of grains per unit

After calculating the magnification, the grain size level can be determined. First, calculate the number of grains in the sample.

The number of grains = the number of complete grains + 0.5 times the partial grains. The grain boundaries of the complete grains are all observable.

Then calculate the actual area, actual area = picture length/magnification rate x width/magnification rate.

According to the calculation formula in ASTM standard: N=2(n-1). Where N refers to the number of grains per square inch under 100 times magnification, and n refers to the grain level. The N can be obtained after unit conversion. Finally, the grain grade number n can be calculated.

• Calculate the average grain diameter

Average grain diameter = actual length of the sample / number of grains in the intercepted part

Actual length = cut length / magnification

Instruments and data

Note: this service is for Research Use Only and Not intended for clinical use.

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