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 t he fracture toughness of a steel, as determined by crack-tip opening displa cement (CTOD), was carried out using a range of samples from a Ti-treated s teel that had been thermally cycled to simulate a coarse grained heat-affec ted zone (CG HAZ) microstructure. Experimental results from tests carried o ut at room temperature showed that the inhomogeneous spatial distribution o f the coarse TIN particles in the microstructure ahead of the fatigue precr ack caused the samples to fail with significantly different CTOD values. De tailed fractographic investigation showed that, with an increased number of overall fracture initiation sites (FISs) and number density of local cleav age initiation sites (CISs) caused by coarse TiN particles, the fracture to ughness CTOD values generally decreased. The increase in FIS number and CIS number density has been related to the inhomogeneous coarse TiN distributi on ahead of the fatigue precrack and so the sampling of microstructural are as with a high number density of coarse TiN particles. The mechanism by whi ch the coarse TiN particles cause cleavage fracture initiation is discussed .