Application of microscope image analyzer in metallographic analysis
Measuring grain size is a frequently performed inspection item in metallographic inspection work. The traditional method is to refer to the standard pictures in the relevant standards (GB6394-2002) and evaluate the grain size level by comparing with the standard pictures. This method is simple and fast, but the subjective error is also relatively large. If you use the other two methods specified in GB6394, namely the area method and the intercept point method (arbitration method), although accurate measurement results can be obtained, these two methods are very inconvenient to use and their tediousness is daunting. If an image analyzer is used to measure the grain size using the intercept point method, the grain size level can be determined directly and quickly.
The intercept point method determines the grain size by counting the grain boundary intercepts on a measuring grid of a given length. The calculation formula of the grain size grade index G is:
G＝-3.2877＋6.6439lg(M×N／L)
In the formula: L - length of measuring grid used (mm)
M - magnification for observation
N - the number of intercept points on the measurement grid L
L and M are known numbers. As long as N is measured, the image analyzer can obtain the grain size level. During the actual measurement work, due to various precipitates that may exist inside the grains and grain boundary fractures caused by improper corrosion control, it brings certain difficulties to accurate measurement. It is necessary to use the corrosion and expansion functions in the image analyzer to remove the grains. The precipitates within and the grain boundaries are reconstructed to obtain a complete grain image.
Quantitatively measuring parameters such as the percentage of microstructure in metal materials and studying their impact on mechanical properties is one of the main uses of image analyzers in metallographic analysis. For example: Determine the percentage of ferrite and pearlite in gray cast iron, ductile iron, cast steel and low carbon steel; the percentage of martensite and ferrite in dual-phase steel; carburizing and quenching hardened layer and Oberite ball The residual austenite content in iron; the phosphorus eutectic content in high-phosphorus brake shoes; the eutectic silicon content in cast aluminum alloys, the beta phase content in bearing bush white alloy, etc. These tasks are easily accomplished using the basic functions of the image analyzer. If quantitative metallographic analysis is performed on different matrix structures of a certain material and compared with its mechanical properties, the quantitative correspondence between the microstructure and mechanical properties can be deeply studied.
Due to the surface roughness of the base material under the coating or the influence of the electroplating process, the coating has uneven thickness. In order to solve the measurement error caused by uneven thickness, when the image analyzer measures the coating, it first displays the cross-sectional shape of the coating. Draw many straight lines parallel to each other, perpendicular to the coating surface, and across the coating on the screen, so that each straight line can measure a coating thickness data, and then process these data to obtain the average thickness and maximum thickness of the coating. , minimum thickness and other parameters. If the object to be measured is a very small metal wire with coating on its circumference, then take its cross-sectional image and draw many straight lines along the radial direction at different angles from its center, and it can also be measured.
Determine the depth of decarburized layer and carburized layer
First, measure the ferrite content of the matrix structure, and then draw a movable straight line parallel to the surface on the screen. Calculate the ferrite content passing through the straight line. As the straight line moves toward the center, when the ferrite content in the matrix structure is found, When the solid content matches the area, the distance between the straight line and the surface is the depth of the decarburized layer or carburized layer.
