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测定了各类球墨铸铁在低应力下疲劳裂纹扩展界限应力强度因子△K_(th)值,肯定了球铁是存在ΔK_(th)值的。ΔK_(th)值随着基体中珠光体量、磷含量的增加而降低。铁型复砂铸造工艺、气体软氮化表面强化工艺能在一定程度上提高球墨铸铁的ΔK_(th)值。 当疲劳应力较低时,裂纹前缘的石墨球的钝化作用使球墨铸铁的ΔK_(th)值比一般结构钢高;当应力接近裂纹失稳扩展临界值时,石墨球的应力集中作用降低了基体抵抗裂纹扩展的阻力,从而使球墨铸铁的断裂韧性值低于一般结构钢。 球铁的σ_(-1)值主要描述裂纹萌生所需能量,决定于基体的强度;而ΔK_(th)值则描述裂纹扩展所需能量,依赖于基体的韧性。不同基体组织对二者的影响截然不同,在选材时应考虑二者的综合影响。

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.