Ductile fracture toughness of polycrystalline Armco iron of varying grain size

Abstract

The influence of polycrystalline grain size on the ductile fracture toughness (J_ic) of Armco iron has been studied over the grain size range of 38-1050 μm. Experimental evidence for the various stages of ductile fracture during fracture toughness testing was obtained through scanning electron metallography. J_ic decreases with coarsening of grain size and follows a parabolic relation with d^−½. The critical stretch zone width in fracture mechanics specimens decreases with increasing grain size; at 1050 μm grain size, the stretch zone is not detectable and the material fails by cleavage fracture. The cleavage fracture behaviour at room temperature has been explained in terms of a stress concentration ahead of the crack tip that reaches the level of the cleavage fracture stress. The variation of fracture toughness J_ic with grain size was studied in terms of the plastic zone size. Indirect methods based on K_jc and P_j fail to estimate the plastic zone size correctly. Microhardness measurements were found to be quite suitable for estimation of the plastic zone size. At finer grain sizes, the plastic zone size encompasses a substantial number of grains while at the coarsest grain size (1050 μm it is confined to a single grain, a feature that corresponds with the occurrence of cleavage fracture. The present measurements on Armco iron are shown to be consistent with the J_ic data of Klasen et al. [Mater. Sci. Engng80, 25 (1986)] on microalloyed steels once one has accounted for inclusions and the higher carbon content.