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## Determination of the Average Grain Size of Metals in Heat Treatment

1. The Classification of Cut-off Point Method and The Matters Needing Attention In Its Application1. 1 Matters Needing Attention in the Straight-line Intercept Method

One or several straight lines, appropriate measurement grid length and magnification, ensure that at least 50 intercept points can be intercepted.

Calculation of intercept point.

Choose 3-5 fields of view for measurement.

Measurement of non-equaxial grains.

1. 2 Matters needing attention in single circle intercept point method

It is suitable for materials with obvious difference in grain size at different positions of the sample.

The measuring grid circle used can be any circumference, usually 100mm, 200mm, 250mm are used.

Choose the appropriate magnification to ensure that each circle produces about 35 intercept points.

The required several circles are arbitrarily distributed on the inspection surface as large as possible, and the number of fields of view is increased until the calculation accuracy is obtained.

1.3 Precautions for the three-circle intercept method

Tests have shown that when the cut-off point of each sample is counted up to 500, reliable accuracy is often obtained.

The measurement grid consists of 3 concentric and equidistant circles with a total circumference of 500mm. The grid is used to measure arbitrarily selected fields of view, and the number of intercept points is recorded each time.

Choose an appropriate magnification to make the three-circle sample grid produce 50-100 intercept points in each field of view, so that 400-500 total intercept points can be obtained through the selected 5 fields of view.

When the measurement grid passes through the confluence point of three crystal grains, the intercept point is counted as 2.

2. Determination of the Grain Size of Non-equaxial Crystal Samples2.1 For rectangular bars or plates, the grains should be measured in the longitudinal, transverse, and normal cross-sections, and round bars should be measured in the longitudinal and transverse directions. If the isometric deviation is not large (3:1 aspect ratio), a circular measurement grid can be used for analysis on the longitudinal specimen. If a linear orientation measurement grid is used for measurement, any two of the three main sections can be used to measure three orientations.

2.2 Area method

n

_{-flat}=（n_{-horizontal}n_{-vertical}n_{-Normal plane}）^{1/3}^{ }n

_{-flat}: the average number of grains per mm².G=3.322lgn -2.9542

2.3 Cut-off point method

The circular measurement grid can be randomly placed on the three main detection surfaces. Or use a straight line segment to count the intercept points on 2 or 3 main detection surfaces in 3 or 6 main directions. For the measurement of the grain size of the equiaxed shape deviation is not too serious (≤3:1 shape ratio), the circular measurement grid is used to measure the longitudinal plane.

In grain size measurement, 6 possible orientations of the measurement line

Calculation of grain size by intercept method

G=6.6439lgP

_{1}-3.288P

_{1}= (P_{1l}·P_{1t }·P_{1n}) 1/3p

_{1l}, p_{1t }, and p_{1n}are the number of intercept points per unit length on the basic plane of the vertical, horizontal, and normal phases, respectively.Through the number of intercept points, the average intercept is calculated, and the grain elongation and anisotropy coefficient can be determined.

3. The Expression Method of Grain Size Value3.1 It is usually expressed in terms of average grain diameter, average intercept, average cross-sectional area of grains, and grain size grade number G.

3.2 Because the various numerical expressions of grain size are calculated from the logarithm or reciprocal of the initial measured value na (the average number of grains per mm2) or p1 (the number of intercept points per mm). Therefore, the grain size of a group of samples cannot be simply averaged.

3.3 If uneven crystal grains are found in the sample, after comprehensive observation, if it is an accidental or individual phenomenon, it can not be calculated. If it is more common, the area percentages of different grades of grains in the field of view should be calculated. If the area percentage of the crystal grains is not less than 90% of the field of view area, only the number of grades of the next type of crystal grains will be recorded. Otherwise, use different grade numbers to represent the grain size of the sample, where the first grade number represents the dominant grade number.

3.4 If you need to evaluate the mixed grain size, you can evaluate the average grain size of each part of the mixed crystal grains according to the unimodal distribution, and determine the percentage of each part of the crystal grains. Such as 70% for level 8 and 30% for level 3.

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