Grain size
Thread Content
Grain size is a scale that indicates the size of the grains. Methods for grain size measurement: 1. Comparison method. Principle: The grain size grade is determined by comparing it with a standard grading chart. The standard grading chart is a series of images with different average grain sizes, representing various grain size levels. The sample to be tested is observed under a microscope and compared with the grain morphology in the standard grading chart to determine the grain size grade that is closest. Step: Prepare the specimen: The sample is processed through steps such as cutting, mounting, grinding, and polishing to create a specimen suitable for observation under a microscope. Selecting a microscope: A metallographic microscope is usually used, with the magnification adjusted to an appropriate level, typically around 100 times. Observing the sample: Place the prepared sample under a microscope and adjust the focus and lighting to make the grains clearly visible. Comparison rating: The observed grain morphology is compared with standard rating charts to determine the grain size grade. Multiple comparisons can be made in different fields of view to improve accuracy. II. Area method – Principle: The grain size grade is calculated by measuring the area of the grains. Assuming that the grains are equiaxed, the grain size is determined based on the relationship between the average grain area and the grain size grade. Steps: The steps for preparing the sample and selecting the microscope are the same as those in the comparative method. Measuring grain area: Under a microscope, a certain number of grains are randomly selected, and the area of each grain is measured using image analysis software or manual measurement tools. Calculate the average area: Add up the areas of all the grains that have been measured, then divide by the number of grains to obtain the average grain area. Determine the grain size grade: Determine the grain size grade based on the table or calculation formula relating the average grain area to the grain size grade. III. Intercept Method – Principle: A test line of known length is used under a microscope to intersect the grain boundaries, and the grain size grade is determined by counting the number of intersections. Steps: The steps for preparing the sample and selecting the microscope are the same as those in the comparative method. Placement of the test line: Under a microscope, place a test line of known length (which can be straight or circular) on the specimen image such that it intersects with the grain boundaries. Count the number of intersections: Count the number of intersections with the grain boundaries along the test line. Multiple measurements can be taken in different fields of view to improve accuracy. Calculate the grain size grade: Determine the grain size grade using the formula that relates the number of intersection points, the length of the test line, and the grain size grade. Note: The quality of sample preparation has a significant impact on the test results; it is necessary to ensure that the sample surface is smooth, free of scratches and deformation, with clearly visible grains. Select an appropriate magnification and detection method based on the characteristics of the material and the detection requirements. In the comparative method, attention should be paid to the scope of application and limitations of standard rating charts; different standards may yield different rating results. In the area method and intercept method, using image analysis software can improve the accuracy and efficiency of measurements, but attention should be paid to the accuracy and reliability of the software. Multiple measurements and comparisons are carried out to improve the accuracy and reliability of the detection results.
The scale that indicates the size of grains is called grain size, and it is commonly expressed as the number of grains per unit volume (or per unit area), or the average linear length (or diameter) of the grains. In industrial production, grain size grades are used to indicate the size of the grains. The standard grain size is divided into 12 grades: grades 1 to 4 represent coarse grains, grades 5 to 8 represent fine grains, and grades 9 to 12 represent ultra-fine grains. When metals crystallize, each grain forms as a result of the growth of a crystal nucleus; therefore, the size of the grains depends on the number of nuclei and the relative rate of grain growth. The number of crystal nuclei is represented by the nucleation rate. The higher the nucleation rate, the greater the number of nuclei per unit volume, and the finer the grains. The smaller the growth rate, the more batches of nuclei are formed during growth, and the greater the number of nuclei, resulting in finer grains. Conversely, the smaller the nucleation rate and the greater the growth rate, the coarser the grains become. Therefore, the grain size depends on the ratio of the nucleation rate N to the growth rate G; the larger this ratio N/G, the finer the grains. The number of grains per unit volume, ZV, is given by ZV=0.9 (N/G)3/4, while the number of grains per unit area, ZS, is given by ZS=1.1 (N/G)1/2. It can be seen that grain size can be reduced by promoting nucleation and suppressing growth; conversely, grain size can be increased by suppressing nucleation and promoting growth.
GBT 6394-2017 Method for determining the average grain size of metals