Ductile & CG Iron Casting Skin— Evaluation, Effect on Fatigue Strength & Elimination Phase II (09-10#01)
Coordinator: Ohio State University and AFS Cast Iron Division (5)
The elimination of the flake skin is one of the key elements of unlocking the full design potential of compacted graphite iron (CGI). Capitalizing on the results of a previous AFS sponsored effort, 04-05#02, “Study of the Effect of the Casting Skin on the Tensile Properties of Light Weight Ductile Iron Castings,” the Department of Materials Science and Engineering at The Ohio State University (OSU) proposes to conduct research with the goal of understanding the mechanism of formation of casting skin in CGI, evaluating its effect on selected mechanical properties, and developing the methodology for its complete elimination. The results of this research will be of immediate applicability to the industry without major capital investment.
The research strategy is designed to develop the knowledge re- quired to improve and ultimately eliminate the skin quality of CGI castings and to generate data on its impact on the static me- chanical and fatigue properties of CGI, as well as on the efficien- cy of shot blasting in improving these properties. Additionally, the study may help in the definition of the minimum thickness of the layer that must be removed by machining to avoid negative skin quality effects. The research will capitalize on the experience in the characterization of casting skin accumulated in the Virtual Solidification and Casting Laboratory (VisionCast) at OSU.
The success of this investigation will rely on the completion of extensive experimental work that will provide critical data required in the understanding of casting skin formation and elimination. The developed correlations between process variables, casting skin quality and mechanical properties will provide the impetus to fur- ther expand the applications of CGI. During Phase I, the mecha- nism for skin formation and test specimens design was validated. During Phase II, fatigue specimens will be cast and the influence of skin formation on fatigue will be determined. Also, potential actions to reduce or eliminate skin formation will be investigated.
Status Update: Papers resulting from Phase I were presented at the 2011 Metalcasting Congress. Phase II has begun with test specimens for fatigue testing cast at OSU’s new metalcasting lab. Those wishing further information on the project should contact Prof. Doru Stefanescu, The Ohio State University, at
stefanescu.1@osu.edu.
Ductile Iron Structure/Property Optimization & Enhancement—Phase II (10-11#02) Phase I (09-10#02)—Complete
Coordinator: Stork CRS, AFS Cast Iron Division (5), Ductile Iron Society and Consortium
ASTM A536 contains examples of the yield strength, tensile strength and percent elongation relationships expected in
ductile cast iron. Frequently, both castings and test bars will exhibit mechanical properties that are significantly better than the minimums expected as indicated in the ASTM specifica- tion. One example is the information generated by the re- searchers from testing a pearlitic ductile iron expected to have mechanical strength properties that would be consistent with the pearlitic 80-55-06 grade but exhibiting high ductility of almost 15% elongation.
To our knowledge, research has not been conducted to de- termine what is required to consistently achieve such proper- ties with this extent of ferrite. It is known that strengthening mechanisms in metal include solid solution strengthening with both substitutional and interstitial elements. Work hardening is another means to strengthen metals. Phase transformation, pre- cipitation hardening and grain refinement are still other mecha- nisms utilized to strengthen metals.
This study will characterize the graphite and other fac- tors that control the matrix mechanical properties that are independent of nodule count, size and morphology. The characterization will include tensile and compression test- ing of sections from example casting and test bars, as well as impact testing. The impact test temperature will be at room temperature and -30C (-20F), as required in GGG 40.3, for example. The ductile to brittle transitions temper- ature determination could also be made for select samples. The microstructure of the test bars will be evaluated us- ing quantitative metallography. Optical microscopy will be used to determine ferrite grain size. Chemical analysis will also be required. It will be essential to procure castings in this first task that exhibit those properties which both meet as well as exceed the ASTM A536 minimum expectations. The second task will characterize all of the data generated from Task 1 using multiple regression analysis to deter- mine how the different factors interact with the mechani- cal properties with the intent to isolate those factors that contribute to the high tensile strengths and yield strengths (and other properties) when the structure is predominantly ferrite. From these results a determination will be made to go to Phase II, which includes DOE tests to reproduce these results.
Status Update: Phase I of the project is complete. Historic data and 53 samples received from four metalcasting facilities have been analyzed and tested for tensile properties, hardness, chemical analysis and quantitative metallography. Michigan Technological University is conducting x-ray diffraction and transmission electron microscopy of selected samples. Phase II will focus on DOE heats and validation of improved prop- erties and all of the 9 initial heats have been poured. The project results continue to be reviewed during updates given to consortium members and the steering committee. Infor- mation on the project is limited to the consortium members and will be released at a later date to AFS corporate members. Those wishing to participate should contact John Tartaglia, Stork CRS, at
John.Tartaglia@us.stork.com.
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International Journal of Metalcasting/Fall 2011
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