Abstract
Formerly one of the present authors introduced a new growth theory of cast iron from experiments on some S.G. irons. By his theory not only the peculiar changes of graphite structure and dilatation-curve of the iron but many complicated growth characteristcs of cast iron of the general type were explained very well. In this paper we again deal with the growth of S.G. iron, because as in the previous work the heating atmosphere was air and low vacuum (10-1 mmHg) the growth of iron could not be accurately examined until their final stages. In the present work an S.G. iron was heated up to 300 cycles through 600°→950°→600°C in the vacuum of 3×10-2mmHg. Growth and the metallographic changes of the iron ceased completely after about 200 cycles of heating. Changes of specific volume of the grown iron were also tested. The structure of graphite nodules was observed in detail from the early to final stages by both an optical microscope and electronmicroscope. A considerably difficult problem encountered in the observation of the inner structure of dense graphite nodules in porous iron was solved by using a scanning electronmicroscope developed recently. The results obtained were summarized as follows: 1) The maximum linear growth was 12% after 200 heating cycles, and the rate of growth divided the period of growth into early and later stages. After the dividing point of about 7% of linear growth the rate of growth decreased considerably. 2) On the growth curve the growing period was followed by a small contraction and then by a level line. In the final stages the iron showed thermal expansion and contraction withou tirreversible change by cyclic heating. 3) The early growth up to 7% can be explained by the graphite migration mechanism proposed by one of the present authors. The redistribution of graphite and diffusion porosity due to graphite migration make the iron grow during the heatings. 4) When the growth of the iron reached 7%, its structure changed itself to become completely porous and the cavity of graphite came in contact with the outer atmosphere. This may be confirmed by some calculation on cavity formed by growth. 5) As grown iron was a completely porous structure, it was easily affected by atmosphere and its rate of growth decreased when its length was measured in air and then heated in high vacuum. 6) Contraction of grown iron is similar to that of sintered metal. 7) Metallographic structure of matrix in grown iron changed to a ferritic type by a small amount of carbon in the matrix migrating in the final stages of growth.
