Assessment of Banding in Steels by Using Advanced Image Analysis

Three methods for quantitative assessment of banded and oriented structures using advanced image analysis are proposed and tested. In method 1, local Fourier transforms are used to extract the locally dominant orientation. The distribution over all possible orientation angles is referred to as the local orientation distribution function. Methods 2 and 3 are based on global Fourier transforms. In method 2, banded structures are quantified by using the difference in projected grey level distribution of the Fourier spectrum in two orthogonal directions. In method 3, the directional image energy as a function of orientation is studied. The image energy, defined by grey level variations, is given by the square of the Fourier spectrum. The concept of energy distribution function is introduced. The methods are successfully applied to two types of materials: pearlite banded structural steels and ferritic-austenitic stainless steels. The proposed methods show good correlation with previously developed methods by Komenda and Sandstrِm for assessing banding based on the Feret slope ratio or the chord slope ratio. It is concluded that all methods give a similar ranking of banded structures.