Consistent Casting of High Strength Ductile Iron: Effect of Late Stream Inoculation
The objective of this project was to determine if the ferroalloys introduced by in-stream inoculation reduce fatigue strength, impact toughness, and machinability.
Modified 7/8-ince (22mm) Y-block castings were poured in pearlitic ductile iron at three commercial foundries. Each foundry poured casting from the high end of their carbon equivalent range, i.e. greater than 4.55% CE. Five groups of test castings were evaluated. Mechanical testing was conducted on all five groups, and four were selected for fatigue testing. The chemical compositions were determined. Microstructure was characterized by percent nodularity, nodule count and ferrite content. Several conclusions were made from the results of this study:
1. Fatigue life failures initiated at large graphite nodules, nodule clusters, non-metallic inclusions, and micro-shrinkage porosity.
2. High carbon equivalent led to wide ranging nodule size distributions and nodule clustering.
3. Sometimes, late-stream inoculation increased nodule count without reducing nodule size. Castings with nodule counts over 260/mm2 contained nodules over 120 um diameter.
4. All heats had comparable tensile properties, yet the fatigue limits varied by as much as 40% with late inoculation practice.
5. There is a need to better understand and optimize the effects of late-stream inoculation and how to maximize the properties of late-stream inoculated ductile iron has been reinforced.
Recommendations: Both fine and coarse nodule sizes in ductile irons increase the scatter in fatigue properties. To improve the fatigue properties of ductile iron, it is important to reduce a wide-range nodule size distribution. Increases in the design properties of ductile iron castings are achieved by reducing scatter in these properties.