The influence of the composite casting wall thickness on the arrangement of particulate within the matrix

This paper describes an analysis of a defective microstructure of a mill roll. For this purpose, a piece of a broken roll was collected. That roll was made of mottled cast iron. Its microstructure consisted of modular graphite, transformed ledeburite and a matrix composed of bainite and martensite. Metallographic investigations were performed nearby the fracture in the roll. Observations were conducted on polished sections, first not etched, and then etched, which allowed us to trace carefully the propagation of the fracture. There was found a strict correlation between the microstructure of the roll and the progress occurring in the crack. It was ascertained that the basic reason for the damage to the roll was banded precipitations of ledeburitic cementite. In addition, cementite formed a continuous network. Another microstructure defects of that roll are also precipitations of secondary carbides on the boundaries of former grain of austenite as well as the occurrence of upper bainite in its matrix. The results obtained hereunder allow broadening the data base relative to the genesis of damages to mill rolls, which in future will permit one to design a proper microstructure of cast iron mill rolls. Proper microstructure of cast iron mill roll should be shaped at the stage of designing the chemical composition, conditions of crystallization or heat treatment if any.