W. M. Garrison, Carnegie Mellon University, Pittsburgh, PA
Summary: It would appear that the fracture toughness of ultra-high strength steels decreases as the inclusion volume fraction is increased. However, I am not sure we are able to explain why or if we understand how fracture toughness depends on inclusion volume fraction. In this discussion we will focus on three heats of AF1410 steel. In the first head the inclusion volume fraction is 0.00034, the sulfides are CrS and the inclusion spacing is 2.2 µm. In the second heat the inclusion volume fraction is 0.00014, the sulfides are MnS and the inclusion spacing is 2.0 µm. In the third heat the inclusion volume fraction is 0.000065, the sulfides are CrS and the inclusion spacing is 1.51 µm. In all cases the inclusion spacing, XΩ, is taken to be the average nearest neighbor center-to-center spacing between inclusions and is given by XΩ = 0.554 (Nv)-1/3 where Nv is the number of inclusions per unit volume. To assess the effect of inclusion volume fraction it is best to compare the effects of inclusion volume fraction at constant inclusion spacing because of the effect of inclusion spacing on toughness. This means that Nv stays constant as the inclusion volume fraction is increased and we increase the inclusion volume fraction by making the particles larger. The crack tip opening displacement at fracture (dIC ). after tempering at 510°C scales linearly with the inverse of the inclusion radius. This suggests that dIC should scale linearly with f-1/3. Indeed this seems to be the case with the straight line going through the origin. Similar behavior is exhibited after tempering at 425°C but the slope of the line is much less. Void generation experiments indicate that the toughness results are not influenced by resistance of the inclusions to void nucleation. Therefore, it is suggested the observed results can be explained by the effect of inclusion size on the rate of void growth as suggested by the Rice and Tracey equations for void growth. .
