Observation and Quantification of Crack Initiation from TiN Particle in a Ferrite-pearlite Steel

Abstract

This article shows a microscopic observation of the cracking of TiN particle and its quantification in ferrite-pearlite steels as a model case of the crack initiation in inclusions. Three model steels containing different amount of TiN particles were produced by vacuum melting, hot rolling, and heat treatment. The microstructures of model steels were evaluated by optical microscopy, image analysis, and EBSD analysis. From the results of Charpy impact test and fractography, it was confirmed that TiN particles behave as initiation sites of cleavage fracture in two of the three model steels. Tensile tests using circumferential notched round bar specimens were interrupted at a certain stroke condition to obtain deformed specimens with distributed strain and stress. By measuring TiN particles’ diameter and cracking under various strain and stress conditions on the deformed specimens, we made a dataset in which individual TiN particles’ cracking were associated with various information, i.e., TiN particles’ size, strain, and stress condition. Through the statistical analysis of the dataset, the probability of TiN particles’ crack initiation was formulated as a function of TiN particles’ diameter, macroscopic strain and stress.