Internal Defect Detection Technology of Graphite Material

The manufacturing process and defect types of graphite materials are introduced in this paper. A comparative analysis is made on the present nondestructive testing methods, the X-ray detection method is the best for graphite materials. The key parameters of the X-ray detection such as gray scale, contrast, voltage, current, filter slice and integral time are compared and analyzed, then the process parameter setting rule and the high precision detection method of the internal defect inspection of graphite material are confirmed. And a visual simulation analysis of the internal space of graphite material is realized. And method for calculating the rate of porosity of graphite materials based on image processing. Finally the characteristics of graphite material internal defects typical of standard samples are established.
If there are defects inside the graphite material, in the working environment such as high speed, high pressure, vibration shock, high temperature and strong oxidation, the risk of wear, falling off, breaking or even burning will occur during the working process. This paper combines the manufacturing process and defect types of graphite materials to study the process of internal defect detection.
1 Graphite manufacturing process and defect type
The manufacturing process of graphite materials has an important influence on the formation of internal defects. This paper analyzes the types of defects that are easily generated in the production process of graphite materials such as molding, baking, graphitization and raw materials.
(1) The pressing type includes four types of single-face pressing, double-sided pressing, pressing and isostatic pressing, and different defect characteristics can be generated due to different pressure directions. The single-sided pressure is easy to lead to tightness and looseness, and the density is uneven, so that low density and looseness are formed; double-sided pressing is easy to make the upper and lower compact, the middle is loose, and the low density is loose; extruded is prone to cracks, sand holes and low density looseness; The isostatic pressing ensures that the density of the graphite material is uniform, but there are still blisters.
(2) Roasting is easy to produce blisters and cracks, but cracks are small and generally difficult to find. In severe cases, they are layered cracks or cracks.
(3) Graphitization is prone to sand holes and cracks, resulting in uneven density. In severe cases, it can cause obvious cracking and geometrical deformation of the graphite material.
(4) In the production process of graphite material, iron powder is usually selected as catalyst. If the pickling process is not complete and the graphitization temperature is low in the subsequent production process, high-density inclusion defects are easily formed.
2 Non-destructive testing
2.1 Non-destructive testing methods
The internal structure of graphite material has the characteristics of looseness, large pores, coarse grains, uneven density, anisotropy, etc., so that the determination information reflecting its essential characteristics is often absent in strong dynamic noise, and the signal-to-noise ratio of the detection signal is generally It is low, so it is difficult to judge the internal defects. At present, the methods for detecting internal defects of materials are generally ultrasonic testing, acoustic emission testing and X-ray testing. Based on the self-characteristics of graphite materials, X-ray inspection can accurately obtain images of internal conditions of graphite compared with other methods, and intuitively analyze the defect structure inside graphite, but its detection requirements are high, which are closely related to equipment accuracy, parameter setting and subsequent image processing. And the determination of defect characteristics is more complicated.
2.2 X-ray inspection
When a ray is incident on an object, the ray of the ray will interact with the substance atom, causing the ray to be absorbed and scattered, resulting in weakened transmission ray intensity, the degree of ray attenuation and the energy of the ray, the thickness of the object being transmitted, and the density. Correlation, therefore, the main working principle of X-ray inspection is that when the radiation passes through the detected object, the absorbing ability of the defective part and the non-defective part is different, and the difference of the ray intensity after detecting the detected object is judged to be detected. Whether there is a defect inside the material, and the larger the defect size, the more obvious the difference.
3 Influence of process parameters on X-ray detection
Based on the micro-focus X-ray machine, the high-precision detection method of graphite material internal defects is studied. In the X-ray detection process, key process parameters such as voltage, current, filter and integration time directly affect the gray level and contrast of the image, thus affecting The imaging effect of defect features, therefore, in order to more accurately capture the internal micro-defects of graphite materials, the influence of key X-ray process parameters on the detection of internal defects of graphite materials was studied in depth, and the test accuracy test was verified.
(1) Under the same current condition, as the voltage increases, the values ​​of grayscale, contrast and saturation of the image will increase continuously. Under the same voltage condition, the gray level of the image increases with the current. The values ​​of contrast, saturation and saturation will also increase continuously, and the growth rate of each parameter under high voltage and high current conditions is significantly higher than that of low voltage and low current conditions. According to the test data, when the power P=60 W, graphite The internal defect characteristics of the material are most obvious.
(2) In the process of detecting defects in graphite materials, the definition of graphite defect features is inversely proportional to the density of the filter, and the effect of the Al filter with a density slightly higher than that of graphite is optimal.
(3) With the increase of integration time, the image grayscale, contrast and saturation are getting higher and higher. When the integration time is 400 ms, the image uniformity and defect characteristics are optimal.
4 Typical defect feature determination
Based on the above theoretical analysis and key process parameter setting criteria, the typical defects such as crack, sand hole, low density looseness and high density inclusion are accurately captured, and the defect characteristics are analyzed as follows.
(1) Crack
Crack defects are irregular black lines on the product image. They are characterized by irregular orientation, position and size, slender shape, sharp edges at one or both ends, and mostly small widths with varying gaps. Different from other defects, it is a narrow seam defect.
(2) trachoma
The blister defect is a somewhat black area on the product image. It is characterized by the positional size is not fixed, the boundary is smooth, and its shape may be circular, elliptical or rectangular. The blackness is generally large and the features are clear, which is a volume defect.
(3) Low density loose
Low-density loose defects are local light black areas on the product image. They are characterized by irregular boundaries, generally in the form of sheets, and the color is between the material and the background color, which is a volume defect.
(4) High density inclusions
The high-density inclusion defect is a local bright spot area on the product image. They are characterized by bright color, unfixed position and size, irregular boundary, brightness far greater than material and background color and the features are clear, , which is a volume defect.
Author: ZHUANG Suguo, SONG Chun, LI Zhiyu, ZHAO Weigang, WANG Liang