ON THE THRESHOLD VALUE ΔK_(th) OF RARE EARTH-MAGNESIUM DUCTILE IRON
Xu Chi (Shanghai Institute of Metallurgy; Academia Sinica)
Cite this article:
Xu Chi (Shanghai Institute of Metallurgy; Academia Sinica). ON THE THRESHOLD VALUE ΔK_(th) OF RARE EARTH-MAGNESIUM DUCTILE IRON. Acta Metall Sin, 1981, 17(6): 661-666.
Abstract Estimation of threshold value ΔKth in fatigue crack growth of different REMg ductile irons under lower stress revealed that the existence of ΔKth in the irons is affirmed. The ΔKth decreases in value with the increase of either pearlite occurred in matrix or phosphorus content. The processing such as the metal-backed shell mold casting and gas soft-nitriding may increase the ΔKth in certain extent. Under lower fatigue stress, the blunt effect of graphite nodules at the tip of crack causes the ΔKth value to be greater than that of common structural steel. While the stress approaching the critical value of unstable crack propagation, the stressconcentration effect of graphite nodules decreases the resistance to crack propagation, thus causing its fracture toughness lower than that of the steel. The σ_(-1) value which represents the energy required for crack initiation is dependent upon the strength of ductile iron matrix, and the ΔKth value which represents the energy required for crack propagation is dependent upon the toughness of matrix. The effects of various matrix microstructure on these two values are contrary. A combined consideration of both values would be taken on selection of materials used.
