The research of the hardening temperature influence on the structure of the experimental alloy "Iron - Tungsten's carbide"

The purpose of this investigation is the research of structure of experimental alloy Fe-WC produced by method of electroslag refining (ESR) in a cast condition and after the thermal processing.

Present alloy was produced by a ESR method of electrode from steel 40 with addition of powder WC during the process of melting in quantity 22-25% weight.

As modes of the thermal processing were chosen:
1) High-temperature tempering (Тн = 680-700^oC t = 60 min, cooling on the air);
2) Hardening (Тн = 940-960^oC t = 30 min, cooling in the oil);
3) Hardening (Тн = 1180-1200^oC t = 60 min, cooling in the oil).

The offered way of the melt of a material allows to receive fine dendrite structure in a cast state. The structure of an alloy is formed by a fine pearlite matrix with places of the open net of the carbide eutectic. Some undissloved particles of WC is observed. In addition it is necessary to note the presence in structure focused needle precipitations. Presumably this is allocation of the carbide in places of sliding of dislocations. After high-temperature tempering the X-ray structure analysis has shown full absence of the austenite in the structure. After hardening from temperature 940-960^oC presence of austenite in the alloy is marked, at simultaneous reduction of quantity of a phase that did not manage to be identified, and after hardening from temperatures 1180-1200^oC - full absence of unidentified phase and the increase of quantity of residual austenite. It is marked, that the allocation of the specified phase results in growth of the microhardness of a matrix that confirms the assumption of its carbide nature.

Development of a technique of a quantitative estimation of structure of hard alloys by means of program ImageTool

ImageTool is the program of processing and the analysis of images.

For an estimation of a fracture of carbide phases in the alloy such functions as Find object and Count black/white pixels are used. For performance of the specified functions the real half-tone picture should be transformed in binary.

Count black/white pixels is conducted on the binary image. It is received as a result of division into levels of brightness grey (the determination of a threshold) in the initial image. The range of change of brightness of object in the field of view is transformed to digital value in an interval 0 - 255 and displayed as the histogram of integrated distribution of brightness.

Since the subjective factor renders the influences on the result at the determination of a threshold of brightness, for quality estimation of an measurements error a series of experiences with using of various variants of the determination of a division level was realize.

Peaks of the histogram correspond to a carbide phase and a matrix. Therefore, first, it is necessary to conduct an estimation at setting of the bottom threshold of brightness on the minimal value between peaks. The minimum of the histogram for all researched photos was contain in an interval 160-180. For all photos the analysis at an establishment of a threshold 160, 170 and 180 was made.

For all four ways of the determination of the threshold the different ranking by quantity carbide phases was received. The difference in a fracture of a carbide phases in alloys is rather low, so it is possible to explain this difference by a error in the task of a threshold of brightness. Thus, the procedure of comparing of the volumetric contents of phases in various samples may be correctly applied only in that case when the interval of change of the contents of a phase exceeds an error connected to uncertainty of a choice of division level. This error is defined by a consecutive estimation of structure at various values of division level. An alternative way of the task of a threshold is visual definition of accuracy of an outline carbides.

The lack of this method of an estimation carbide phases is absence of an opportunity of exception the smallest objects with the given brightness, but not being carbides on morphology from the calculation. Therefore it was offered to use function Find Object for construction of the binary image which allows to filter objects with the size less threshold.

Objects search is also conducted on brightness levels in gradation grey: the image is scanned until the pixel with the brightness level from the given interval will be found, it is considered, that the required object is found, and then its size is defined by an estimation of an accessory next pixels to the given brightness interval. Then process of scanning proceeds.

Procedure of search in a manual mode begins from the defining of a threshold of brightness grey by means of double scroller.

The histogram is constructed in coordinates the Level of brightness (0-255) - Relative quantity of pixels.

The further analysis of the found objects allows classifying them by the areas (with reception of polychromic images of the found and identified objects). Transferring this image in binary it is possible to count up the percent of carbide phases using function Count black/white pixels.

The main advantage of this method is the opportunity of the defining of a minimum pixels quantity, making required object.