Environmental Effects on Fatigue Crack Growth in Austempered Ductile Irons

Abstract

Corrosion fatigue crack growth (CFCG) behavior has been investigated for two different grades of austempered ductile irons (ADIs) which were heat treated respectively at two different austempering temperatures, 573 and 633 K. Fatigue crack growth (FCG) tests using compact tension (CT) specimens were conducted in humid air, lubrication oil and three aqueous environments: distilled water (pH = 7.6), sodium chloride solution (pH = 7.3) and sulfuric acid solution (pH = 3). The results showed that fatigue crack growth rates (FCGRs) at 20 Hz in humid air and aqueous environments were comparable and faster than those in lubrication oil. The FCGRs in aqueous environments were found to increase with a reduction in loading frequency or with an increase in solution temperature. The crack growth enhancement by different aqueous media appeared to be equivalent, although the ion species and pH in the bulk solutions were different. The degree of sensitivity to environmental effects for CFCG response in ADI was not essentially influenced by the austempering treatment as the relative difference in FCGR between the two grades of ADIs was not considerably changed in any of the studied.