Cast Iron Structure and Properties

Abstract

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 bet\~ecn factors of inHuence and target values, it would be necessary to carry out at least 220 experiments (or more than one million) in order to obtain a dear picture. By sorting out some of the influencing factors, one arrives at three complexes which facilitate the approach to putting the questions: 1. The actual alloying elements in cast iron can satisfactorily be comprehended by the "degtee of normality" (or by the carbon equivalent). 2. The influence of the rate of cooling can be controlled by standardizing the test pieces and the molding material. 3. The third group of factors can be represented by the conditions of nuclei formation and of crrstallization, 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 believea. 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 quamity 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 raw materials. The scatter of values obtained in hormaJ prac. tice could be imitated by using iron charges which had been prepared from a variety of pig iron grades and