Short fatigue cracks in austempered ductile cast iron (ADI) T. J. MARROW and H. C ¸ ETINEL Manchester Materials Science Centre, UMIST and University of Manchester, Grosvenor St., Manchester M1 7HS, UK Received in final form 28 January 2000 ABSTRACT The growth of short fatigue cracks was investigated in an austempered ductile cast iron (wt% 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu), austenitized at 870 °C and then austempered at 375 °C for 2 h. At stress amplitudes close to the fatigue limit endurance limit of 10 7 cycles, subcritical crack nuclei initiated at graphite nodules. The crack nucleus deceler- ated and arrested after propagating a short distance. The position of an arrested crack tip was characterized using an electron backscatter diffraction technique, demonstrating that short fatigue cracks in austempered ductile cast iron (ADI) can be arrested by boundaries such as those between ausferrite sheaves or packets and prior austenite grains. Refinement of the prior austenite grain size decreased the size of subcritical crack nuclei. It is proposed that the arrest and retardation of short crack nuclei are controlled by the austenite grain size and graphite nodule size. This determines the fatigue endurance limit. NOMENCLATURE a 0 =crack nucleus length a =crack length D =graphite nodule diameter r p =reverse plastic zone size R-ratio =ratio of minimum to maximum stress in the fatigue cycle Y =geometric compliance factor 870/375 =ductile iron microstructure austenitized at 870 °C and austempered at 375 °C for 2 h 1050/870/375 =ductile iron microstructure austenitized at 1050 °C and cooled, then re-austenitized at 870 °C and austempered at 375 °C for 2 h DK =stress intensity factor range DK th =fatigue crack propagation threshold stress intensity factor K t =stress concentration factor s 0 =fatigue limit stress amplitude s y =yield stress shapes and the as-cast microstructure has good machin- INTRODUCTION ability. With the additional benefit of low density and low raw material and heat treatment costs, they find Ductile cast irons applications in crankshafts and large gears in transport and lifting applications, 1,2 in which resistance to fatigue Ductile cast irons (also known as nodular or spheroidal irons) have superior mechanical properties to conven- failure is an important requirement. The fatigue resistance in ductile irons, expressed as tional cast irons. This can be attributed largely to the spheroidal shape of the graphite nodules. Austempered the plain specimen fatigue limit, generally increases with tensile strength and matrix hardness as the matrix micro- ductile irons (ADI) have an ausferrite matrix of ferrite and retained austenite. They possess a useful combi- structure varies from ferrite through ferrite/pearlite and ausferrite to quenched and tempered martensite. 1,3–7 nation of high tensile strength, toughness and ductility, with good resistance to fatigue and wear. Good casting However, there is a maximum in the fatigue limit in ADI microstructures at intermediate strength. 4,5 This is behaviour allows near net shape forming of complex © 2000 Blackwell Science Ltd. Fatigue Fract Engng Mater Struct 23, 425–434 425