Cast iron program focuses on research.

Reports on an assortment of ongoing research projects that are adding to the already large body of knowledge of cast irons was the focal point of this year's cast iron program. R.W. Heine, University of Wisconsin-Madison, who, for decades, has been a regular contributor to this body of knowledge, this year discussed the high incidence of graphite flotation in ductile iron castings (91-057). This often occurs, he said, because most ductile irons are of hypereutectic composition in the range of %C + 1/ 3%Si [is greater than] 4.3. In these irons, the first phase formed during solidification should be primary graphite beginning near the graphite liquidus temperature and with growth continuing down to the beginning of the eutectic solidification temperature. Upon reaching a critical size, the graphite spheroids may float in the molten iron and produce a flotation layer in the upper regions of a ductile iron casting. The carbon percentage in the iron and the saturation level expressed as carbon equivalent are major contributors to the potential for flotation to occur, Heine explained.

He also discussed several base iron compositions to limit the risk of flotation. Thermal analysis of ductile iron revealed that significant solidification events occur in the temperature region above eutectic solidification. "Flotation is the most obvious result of such proeutectic solidification in hypereutectic irons," Heine said. "Casting properties such as solidification expansion or contraction, shrinkage feeding behavior and nodule count, tensile properties, and microstructures are influenced by this proeutectic stage." He suggested that additional research is needed to better understand the problem of flotation and to learn to control it.

C.R. Loper, Jr. and A. Javaid, University of WisconsinMadison, followed with their study on heavy-section ferritic ductile iron castings (91-106). They found that nodule count or changes in nodularity do not necessarily alter yield strength and only moderately decrease tensile strength. On the other hand, they pointed out that elongation values are reduced. The constant yield strength found in ferritic ductile irons, they said, is noteworthy and assures that static design criteria will be met in heavy-section castings even if a full spheroidal graphite structure is not achieved.

The usage of cored wire has become a popular method of adding reactive, easily oxidized and trim elements to molten metal baths, especially in Europe. J. Rotella, SKW Alloys, and R. Mickelson, Burnham Corp., reported on their work in which a high-magnesium ferrosilicon alloy (in cored wire) was used to produce ductile iron in an operating production facility (91069). The results of numerous trials showed cored wire to be capable of producing good quality ductile iron in a production environment. The technique was used to desulfurize, nodulize and preinoculate molten iron. As a result of successful trials, the calcium carbide, porous plug desulfurizing system used in conjunction with a cupola was eliminated.

The distribution of nodules in ductile cast iron (91056) was the subject of a presentation by T. Skaland, Sintel/Div. of Metallurgy, Trondheim, Norway, and O. Grong, Norwegian Institute of Technology. The researchers stressed the effects of microstructure on cast iron properties, which, they explained, are determined during solidification and the subsequent solid-state (eutectoid) transformation. The number and size of graphite nodules, they said, affect the mechanical and thermal properties of ductile cast iron, so it is important to provide a quantitative description of the nodule distribution in these irons.

K.G. Davis and J-G Magny, PMRL/Canmet, studied the tensile strength of 25-mm-thick gray iron plates cast in permanent iron molds and in sand molds (91-119). They found that part strength was controlled mainly by the iron composition, with variables such as pour temperature and mold type producing very minor effects. An expected correlation between graphite flake size and strength was not realized. The authors explained that despite claims that graphite irons cast in permanent molds are stronger than iron of similar composition cast in sand molds, there was no significant difference between the strength of parts cast in the two different molds.

Interest in austempered ductile iron (ADI) remains high despite slow progress in its applications. B. Kovacs, AFC Technical Center, presented his study of the heat treating of ADI (91-075). His work dealt primarily with the effects of austenitization, quenching and austempering on the mechanical properties and microstructure of ADI castings. What he found was that both temperature and time duration of the various stages in the heat treat cycle has a marked influence on both mechanical properties and microstructure. For example, Kovacssaid, a shorter-than-optimum austenitizing time causes the formation of metastable and/or unstable austenite, which can result in stressing the part during machining. On the other hand, longer-than-optimum austenitizing time causes bainite formation, which reduces the ductility and the impact strength of ADI parts.

While it appears that little attention is given these days to white cast irons, such as NiHard I, IV and high-chrome alloys, G. Laird II, U.S. Bureau of Mines, is studying these irons with the goal of improving their wear properties (91-055). His report systematically explained the effects of composition and heat treatment upon white cast iron microstructures.