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The effects of high-temperature austenitizing treatments (prior austenite grain size) and internal hydrogen on the fracture mode during fatigue crack growth studies in AISI 4340 steel have been investigated. The results show no correlation between the prior austenite grain size and reverse plastic zone at the maximum incidence of intergranular fracture in specimens without hydrogen. Fatigue striations were more often observed in the coarse-grained specimens austenitized at 1200°C. The presence of internal hydrogen resulted in predominantly intergranular fracture in the fine-grained specimens at low stress intensity ranges. However, the amount of intergranular fracture was inversely related to the prior austenite grain size. The fracture mode was predominantly intergranular at intermediate stress intensity ranges for all grain sizes. The results were discussed in terms of variations in plane strain fracture toughness with austenitizing temperature and hydrogen diffusion to the prior austenite grain boundaries.
austenitizing temperature, grain size, plane strain fracture toughness, internal hydrogen, stress intensity range, crack growth, intergranular-transgranular fracture, fatigue striations
Advanced Engineer, Materials Engineering, Westinghouse Electric Corporation, Alafaya Trail, Orlando, Fla.