Abstract
In the present work, the austenite envelope and the solidification process of nodular cast irons have been investigated. The main results are summarized as follows:
(1) During the solidification of nodular cast irons, nodular graphites are at first formed in melts and grow to some extent in direct contact with melts and then envelopes of austenite are formed through preferential growth of austenite around graphite nodules.
(2) The number of graphite nodules increases rapidly in the initial stage of solidification and it hardly changes in the subsequent solidification. The number of graphite nodules may be specified by the amount of magnesium gas bubbles in melts.
(3) Austenite envelopes are a polycrystalline structure and the regions near grain boundaries remain as liquid state during solidification as a result of segregation of elements with low-melting points.
(4) The existence of grain boundaries in austenite envelopes causes the denodularization of pre-existing graphite nodules through the preferential growth of graphite along boundaries.
(5) The diffusion of iron through channels in austenite envelopes may be the rate determining process of growth of nodular graphite.
