Dependency of fracture toughness on the inhomogeneity of coarse TiN particle distribution in a low alloy steel

An investigation of the effect of the coarse TiN particle distribution on the fracture toughness of a steel, as determined by crack-tip opening displacement (CTOD), was carried out using a range of samples from a Ti-treated steel that had been thermally cycled to simulate a coarse grained heat-affected zone (CG HAZ) microstructure. Experimental results from tests carried out at room temperature showed that the inhomogeneous spatial distribution of the coarse TiN particles in the microstructure ahead of the fatigue precrack caused the samples to fail with significantly different CTOD values. Detailed fractographic investigation showed that, with an increased number of overall fracture initiation sites (FISs) and number density of local cleavage initiation sites (CISs) caused by coarse TiN particles, the fracture toughness CTOD values generally decreased. The increase in FIS number and CIS number density has been related to the inhomogeneous coarse TiN distribution ahead of the fatigue precrack and so the sampling of microstructural areas with a high number density of coarse TiN particles. The mechanism by which the coarse TiN particles cause cleavage fracture initiation is discussed.