AFS Library

CAST IRON STRUCTURE AND PROPERTIES

3290
Journal: Transactions of the American Foundrymen's Society V 70 P 176-186, 1962 (11 p)
Author: Patterson, W

By comparison with other materials, the range of properties of cast iron is unusually wide. It extends from chilled cast iron to gray iron and spheroidal graphite cast iron. In contrast to many other alloy systems, we are still in the dark regarding the relationship between chemical composition, structure and properties of cast iron. It can be shown that about 20 factors may influence its properties. Assuming linear relations, which cannot always be expected between factors of influence and target values, it would be necessary to carry out at least 22" experiments (or more than one million) in order to obtain a clear picture. By sorting out some of the influencing factors, one arrives at three complexes which facilitate the approach to putting the questions: 1. T h e actual alloying elements in cast iron can satisfactorily be comprehended by the "degree of normality" (or by the carbon equivalent). 2. T h e influence of the rate of cooling can be controlled by standardizing the test pieces and the molding material. 3. T h e third group of factors can be represented by the conditions of nuclei formation and of crystallization, which are governed by the time-temperature curve of the melting process and by the distribution of certain trace elements. This third group exerts a much stronger effect than is commonly believed. A broad field of scatter will be observed when plotting tensile values (obtained in ordinary foundry practice) against carbon equivalent or degree of normality- This scatter cannot be explained as a result of changes in the quantity ratio among the: elements themselves which are accompanying the iron. This influence is obviously small and the scatter can only derive from changes in the melting process and in the selection of rawmaterials. The scatter of values obtained in normal practice could be imitated by using iron charges which had been prepared from a varietv of pig iron grades and melted under conditions of a defined time-temperature inoculation program. At the same time the eutectic cells became coarser and the graphite in the cell branched out much more. This resulted in decreasing quantities of A graphite and increasing quantities of D graphite. When, after superheating, the melt was allowed to stand, the property values began to aller in the opposite direction, and this became more pronounced after inoculating the melt with ferrosilicon. Inoculation with calcium silicide resulted in the highest degree of normality, coupled with the smallest eutectic cells, 100 per cent A graphite and no D graphite. Distinct differences in the level of these values could be observed among the various pig iron grades used, and differences were still well recognizable when synthetic melts, having small additions of pig iron, were melted or when charges with scrap additions were melted in ordinary foundry practice. Evaluation of these results for practical cupola operation will increase, the degree of accuracy aimed at in the production of cast iron.



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