This paper describes a newly developed image processing program for grain boundary extraction with some experimental results. It is available on an MS-DOS personal computer without any special equipment. The program deals with image data directly, and extracts a thinned image of grain boundaries from an input original gray image. Its robustness comes from refinement process inserted between binarization and thinning. The refinement process is carefully designed so as to be suitable for the property of a grain images, so that the program is applicable to many kinds of grain images. Since the operators provided in the program are of general purpose inherent, it is also applicable to any kinds of digital images by combining the operators as you like.
A computer program for automated repair of defects in thinned grain boundary images has been developed. The basic data which is necessary to reproduce the geometrical shape of line images are the coordinates of characteristic points and the location of neighboring points connected by lines. Recognition of geometrical structures containing defects is done using the lists of characteristic points, from which a set of data of the distances and angles in the line image surrounding defects is prepared. Repair of grain boundary images consists of erasing undesirable lines and drawing lines between characteristic points. The repair operation involves rewriting of the contents of the data lists. The image reproduced from the repaired data lists is the network of triple points. The rules and method used in the repair operations are similar to those used during manual repair using visual observation. We found that line images containing more than 100 end points as defects could be repaired. With no abnormal structure in the input grain boundary image, the repaired image of the triple point network could be used for recognition of the shape of individual grains. The repair program could be used in a fully automated grain size analyzer.
Grain size numbers of micrographs of ferritic steels measured by commercial automated image analyzers as well as by a newly developed program for personal computer were examined as a function of number of grains counted in a processing field. The dispersion of results due to differences in the commercial equipments has been found to be minimized when about 100 grains are taken into the field of 400×400 pixels. Comparison of values obtained by the new program with those measured manually has lead to the conclusion that the number of grains in the field of 400×400 pixels should be limited to not less than 100 but not more than 300 in order to find a good agreement between the two procedures.
The outline of the software for the grain size measurement developed by the Grain Size Measurement Group in the Committee on Materials Evaluation by Image Analysis Basic Research Association on Specific Subjects sponsored by the Iron and Steel Institute of Japan is described. It consists of three parts being binarization of the input image and grain boundary thinning, repair of the grain boundary, and the measurement and its statistical treatment of grain size. The emphasis is put on the first two processes and the effect of changing the default parameters for binarization and thinning processes on the result of the following repair process is discussed. Then the effect of sample preparation condition on the results of these processes is examined using a common specimen but having different states of polishing and etching. It is shown that for such specimens as polished to 3 μm or finer diamond paste the repair process can be successfully done to give a grain network which well represents the original image. It is found that the variation in etching conditions hereby examined less affects the results. Through these discussions practical usefulness and limitations of the software are visualized.
The aim of this research is to establish a stereological method to characterize three-dimensional (3-D) distribution of grains in a metal from its sectional images with an application of the current image processing techniques. First of all, 3-D models of metals consisting of homogeneous grains and duplex grains were generated by simulating the processes of nucleation and crystallization with various nucleus distributions and growth rates. The frequency distribution of cut-lengths of the ASTM or JSPS standard chart of austenite (No. 7 or 8) obtained by cutting them with equiinterval parallel lines comes into a normal distribution. It was confirmed that there is no significant difference between the standard chart and the homogeneous grain model by statistical tests, and relations among mean cut-length, grain size number, mean diameter and mean volume were derived on the homogeneous grain. For duplex grains, formulae were derived to calculate the degree of duplication and the fraction of volume occupied by different sizes of grains from the distribution of duplex grains. The results can practically be utilized to obtain 3-D information about homogeneous and duplex grain structures with image processing techniques.
The combination of a personal computer and EPMA has been utilized in order to develop a simplified microanalysis equipment system which has nearly the same analyzing power in specifications as CMA or MA in image analysis. Image analyzed data for a duplex stainless steel 329J1 from the present system was compared with that from CMA and found to be in fairly good agreement with color mapping of the concentrations of Cr and Ni. The phase identification for α and γ phases by Cr/Ni partition ratio was also accomplished.
Relation between the intensity of X-ray signal and the quality of the X-ray image with the energy dispersive X-ray Spectrometry (EDS) was analyzed with the application of statistics. One of effective methods to improve the quality of EDS X-ray image is found to be the application of a smoothing treatment of moving average to the digitized EDS X-ray image using a computer image processing.
Microsegregation and precipitation behavior in a nickel-base superalloy (INCONEL 625) were analyzed using a computer-aided X-ray microanalyzer (CMA). During solidification, (Nb, Ti)C and (Ni, Cr)2(Nb, Mo), called Laves phase, precipitated only in the interdendritic regions due to the microsegregation of the solute elements. The formation of microsegregation and the Laves phase, which causes loss of deformability at high temperatures, could be decomposed by slow cooling. Moreover, raising the cooling rate only during solidification was effective in reducing the amount of the Laves phase, because the dendrite structure became finer. The microsegregation of niobium, titanium and molybdenum was discussed, compared with experimental results and mathematical calculations.
X-ray mappings on very large samples are used for the studies of segregations in steel. This has some consequences in terms of electron beam defocusing, calibration and time of acquisition. To minimize this time, it is possible to increase the signal to noise ratio by using optimal linear filters. Traditionally these filtes are used in a univariate case, but as well study many elements simultaneously, the calculation of cross-correlations between elements allows a generalization of these filters to the multivariate case. The model is described with the assumption of a locally stationary phenomenon. It can give the principal components of the spatial structure (i.e., synthetic images at different scales). This facilitates the next step which is to characterize the segregations by classical image analysis of the X-ray mappings. Application to an industrial program of reduction of segregation is presented.
Microfractographical features of corrosion fatigue process of 12 Cr stainless steel turbine blade material are described. The emphasis is focussed upon the initiation and growth of corrosion pits in corrosion fatigue crack initiation process. A quantitative analyzed example of corrosion pit by use of recent developed image analyzing technology is also touched in brief.
Creep damage in pressure parts due to high temperature service is composed of metallurgical damage and mechanical damage. Mechanical damage such as cavitation and microcracks are easily detectable parameter for creep life assessment. On the other hand metallurgical damage resulting from structural degradation and fluctuation of alloy element in matrix and precipitates are difficult to detect quantitatively, in spite of a predominant factor to reduce creep strength and to induce integranular creep cavitation. Through the investigation into the long term used elevated temperature pressure parts removed from a power plant, a new materials-evaluation technique was developed, which is the quantitative analysis of the structural changes such as the alloy element fluctuation in matrix with precipitates in terms of creep damage percentage. Consequently, concentration spectra of alloy element in matrix with coarsening precipitates revealed by computer-aided X-ray microanalyzer (CMA) gave the significant structural parameter to determine the creep damage percentage.
For metallurgical studies or routine control of eutectoid steels, it is necessary to perform automatic measurement of pearlitic interlamellar spacing. Various methods, using a combined system of a scanning electron microscope (SEM) and an image analyzer (IA) have been compared and the optimum procedure has been studied. A conversion coefficient from the apparent spacing measured with SEM+IA system to the true spacing has been determined to be 0.9 with a transmission electron microscope observation on thin specimen.
Computer image processing technology has been applied to cleavage fracture surface analysis and the algorithm for the estimation of 3-dimensional cleavage facet boundaries and 3-dimensional facet areas has been developed. An angle αi, j between relative normal vectors of one mesh Pi, j and adjacent mesh plane Pi+1, j was calculated and the αi, j was compared to the threshold one αth. If αi, j is larger than αth and the αi+1, j calculated between the mesh planes Pi+1, j and Pi+2, j along X axis is smaller than αth, the Pi+1, j mesh plane was decided to be the facet boundary. The same calculation was continued along both X- and Y-axes and facet numbers being contained in the analyzed area were obtained. After the estimation of facet boundaries, 3-dimensional facet areas were calculated dividing by the 2-dimensional facet areas by cos βk which is the angle between average normal vector of facet and vertical vector. By this algorithm, the cleavage fracture surfaces were analyzed of JIS SS41 and HT80 steels and the effect of the threshold angles αth was discussed on the facet numbers estimated, the distribution of the values of α and the 3-dimensional facet areas. The method developed is very useful to analyze the cleavage facet size and to discuss the relationship between the cleavage fracture and microstructures of steels, such as ferritic grain size and prior austenitic grain.
Today there is an established technique to evaluate the thermodynamic properties of individual phases in an alloy system by assessing all the thermochemical information and the information on the phase diagram. This technique has been applied to a large number of binary and some ternary systems. From a practical point of view it would find its most important applications in higher order systems which are found in alloy steels. This lecture will report on an effort to build a thermodynamic database for iron-base alloys, suitable for predicting the formation of various alloyed carbides in steels.