FRACTURE-TOUGHNESS AND CRACK-GROWTH RATE OF FERRITIC AND PEARLITIC COMPACTED GRAPHITE CAST IRONS AT 25-DEGREES-C AND 150-DEGREES-C

This research studied the ambient (25-degrees-C) and intermediate (150-degrees-C) temperatures plane strain fracture toughness (K(Ic)) and crack growth rate da/dN vs stress-intensity variation (DELTA-K) behaviors of compacted graphite (CG) cast irons in an atmospheric environment. As-cast ferritic irons with different percentages of compacted graphite (vermicularity) were produced by using insufficient amounts of spheroidizer. Irons with pearlitic matrix were obtained by heat treating the as-cast structure. The results of fracture toughness testing indicated that (1) for the same matrix, CG irons with higher vermicularity yielded lower K(Ic) values, but their values were still much higher than those of gray (flake graphite) cast iron; (2) for the same vermicularity, CG irons with pearlitic matrix exhibited higher fracture toughness values than those of ferritic matrix; (3) at intermediate temperature (150-degrees-C), the influence of vermicularity and matrix on fracture toughness is the same as at ambient temperature, except that the K(Ic) values were all a bit lower (1 to 8 pct). From crack growth rate vs stress-intensity variation experiments, the Paris equation da/dN = C(DELTA-K)n was derived, where a smaller value of n indicates better crack growth resistance of materials. Compacted graphite cast irons with pearlitic matrix and/or greater vermicularity rendered higher n values and, thus, inferior crack growth resistance. At elevated temperature, the n values were all lower, indicating that the crack growth resistance was improved.