Cast working rolls are commonly in use in the finishing stands of modern warm strip mills. These rolls consist of a wear resistant shell and a core made of cast iron. The shells are produced by centrifugal casting. In the last years the wear resistance of working rolls, with shells consistin of so called “Indefinite Chill”- alloys was strongly improved by adding niobium or vanadium in order to maintain the hot strip mills managements demands.
Unfortunately this increase of wear resistance goes hand in hand with a decrease of process controllability with respect to graphite formation. For that reason new methods of process control will have to be implemented, so that the amounts of vanadium and niobium can be increased.
In this study graphite formation in indefinite chill alloys was investigated by applying Differential Thermal Analysis (DTA). Special attention was paid to eutectic reaction. No qualitative differences in the DTA plots with respect to graphite formation could be monitored. The reason is the small amount of stable eutectic in comparison with stable eutectic.
In semi quantitative analyse there are some indications that there is delayed melting of samples with a greater amount of graphite.
Furthermore the application of oxygen sensors was tested. The sensor type CE 1096 of the manufacturer Heraeus Electro-Nite which is also applied to specify steel melts was tested. The sensor type Celox Foundry of the same manufacturer was also tested. This sensor was especially developed for foundry industry.
The study aimed at the usability of the sensors and the investigation of the relation between oxygen activity of the melt and the amount of graphite of the cast roll’s shells. Both sensors showed a good performance at oxygen activities of about 1,5 ppm. For melts with oxygen activities below 1 ppm many of the results were uncertain or even useless.
The success of deoxydation and inoculation could be pointed out. A relationship between graphite formation tendency and oxygen activity could not be found.
There is no significant relationship between oxygen activity and solidification type within the investigated range of alloy compositions.