Statistical analysis of EPMA results for numerical modelling of micro-segregation in steel

Abstract

Measurement of concentration by electron beam probe micro analysis (EPMA) gives data about the distribution of alloyed chemical elements of the solidified steel. Those experimental data are essential for mathematical prediction of the structure. Particularly, distribution coefficients which give the concentration ratio at the solid-liquid-interface are parameters for micro-segregation simulations. Depending on the structure like columnar or equiaxed dendritic the local extent of element concentrations causes a characteristic pattern. Statistical methods are used to find out characteristic values because an EPMA gives back the concentration scans of a measured line or of an area (mapping) which can consist of e.g. 6·5 × 104 values for each analysed element at about 0·25 mm2. The length scale ratios of sample size and measurement grid have to be optimized according to the structure in the area of interest. A comparison of statistical concentration distributions was carried out from both aspects, measurement of real steel samples and mathematical approach based on micro-segregation formula. Columnar and equiaxed dendritic structures have been investigated in terms of statistical element distribution, and the resulting data are applied to modelling of solidification. Determined distribution coefficients k of [Mn] depend on the as-cast structure, the chemical steel composition, and the kind of definition of the min- and max-values of EPMA-readings.