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Technical Paper

Notch Sensitivity and Fatigue in Austempered Ductile Iron

The properties of ADI are suitable for automotive crankshafts, camshafts, and gears; but, like many high strength materials, ADI is notch-sensitive. In this study of notch sensitivity and fatigue, baseline properties were provided by smooth-wall, and ground and polished specimens. These specimens had a rotating-bending fatigue strength of 393 MPa (57.0 ksi) for a life of 107 cycles. The fatigue strength of notched test bars was evaluated after fillet rolling and shot-peening. Notched specimens were tested with stress concentration factors ranging from 1.0 to 2.4. As the severity of the fillet radius increased from 3.05 mm. (0.120 in.) to 1.02 mm. (0.040 in.), the fatigue strength decreased from 335 MPa (48.6 ksi) to 230 MPa (36.7 ksi). The notch sensitivity factor, q, increased from 0.41 to 0.55. Shot-peening was performed with steel shot, with coverage ranging from 100-400%.
Technical Paper

Monotonic and Cyclic Property Design Data for Ductile Iron Castings

Data on the low and high cycle fatigue response as well as monotonic (non-cyclic) and cyclic flow properties are required by design engineers for the proper design of many components. Currently, there are no readily available sources for such information on ductile/nodular cast iron. The purpose of this study was to determine property design data similar to those in SAE standard J1099 for SAE Grade D5506 ductile iron. In this study, the monotonic and cyclic strin hardening properties, and the variation of elastic modulus after repetitive cyclic testing were determined for eight ductile iron castings selected to represent the range of properties possible in Grade D5506 ductile iron. Strain-life testing was conducted on four of the eight materials to calculate the strain and stress versus fatigue life (reversal) constants.
Technical Paper

Improving the Properties of Austempered Ductile Iron

The influence of austenite content on the properties of austempered ductile iron (ADI) was investigated. The “reacted” austenite content in the ADI structure was controlled by varying austenitizing temperature and, thus, the carbon content of the austenite during austenitizing. Phase transformation studies revealed how to vary the carbon content during austenitizing. The investigation included “step-austenitizing”, where the material was first heated above the critical temperature and subsequently cooled and held below the upper critical temperature. Intercritical heat treatments were also performed to produce a mixed ferrite + ausferrite microstructure containing nominally 50% proeutectoid ferrite + 50% ausferrite. Cast plates containing 3.4C-2.7Si-0.3Mn-1.1Ni-0.16Mo were austempered at 725F (385C). The evaluated properties included tensile properties, machinability (in drilling) and thermal expansion coefficient.