抄録
The purpose of this study was to investigate the effects of humidity on the characteristics of fatigue crack propagation in spheroidal graphite cast iron with two phases which is ferrite and bainitic-ferrite. First, using spheroidal graphite cast iron (FCD400) as a specimen, two kinds of heat treatments were conducted; one was austempering (ADI, FADI82 and FADI55), and the other was ferrite haploidization processing (FDI). Fatigue crack propagation test was then carried out using these resultant four materials. The experiment conformed to ASTM. Stress ratio R was 0.1, and the specimen used was of the 1CT type with a thickness of 12.5mm. The test was carried out at room temperature and under three kinds of humidity: 0, 40, and 80%. The relationship between the characteristics of fatigue crack propagation and the crack closure generated on the fracture surface was investigated.
Threshold stress intensity factor range, ΔKth of all materials increased with increasing humidity. In high ΔK regions of FADI and ADI, the fatigue crack propagation speed was accelerated because this seems to be effect of a phenomenon similar to the water embrittlement of bainitic-ferrite included in the matrix. In low ΔK regions of ADI, FADI and FDI, the fatigue crack propagation speed was lower because of the oxide-induced crack closure of the ferrite included in the matrix. Consequently, although the phenomenon similar to water embrittlement occurred also in the low ΔK regions of FADI82 and FADI55, the oxide-induced crack closure was also found to occur markedly. This confirmed the superior characteristics of fatigue crack propagation of specimens containing ferrite and bainitic ferrite.
