Development of a method for computer processing of fractographic images to assess the cohesion of inclusions to the matrix in the weld metal after its operational degradation and hydrogenation

Abstract

In this paper, the method for automated processing of digital fractographic SEM images based on the modified local Otsu approach was developed. The effectiveness of the developed method was demonstrated. The method is used to identify inclusions or their traces at the bottom of dimples in weld metal fractures with subsequent assessment of their quantity. It is assumed that a decrease in the cohesion of inclusions to the matrix is one of the signs of damage to the weld metal at the microstructural level. Consequently, the more inclusions were found at the bottom of ductile fracture dimples, the stronger their connection with the matrix and, accordingly, the less damage in the weld metal. The effect of operational degradation of the weld metal on a circumferential weld on thick pipes of thermal power plant steam pipelines is analyzed. For this purpose, the features of cohesion of inclusions with the matrix in the weld metal in the initial state and after operation on the main steam pipeline for 23 years were compared. The influence of hydrogen (absorbed by the weld metal during long-term operation, as well as during its additional electrolytic charging) on the cohesion of inclusions in its structure with the surrounding matrix is also considered. A decrease in cohesion between these two components of the weld metal structure under the influence of all analized factors is shown. Thus, a method for quantitative assessment of fractographic features caused by the decohesion of inclusions from the matrix developed on the basis of analysis and computer processing of scanning electron microscope images, can be used to assess the operational damage of the weld metal at the microscopic level.