Abstract
The authors have investigated the retarding effect of nitrogen on the graphitization of white cast iron.
In agreement with some other investigators, the results showed that acid-soluble nitrogen (NS) retarded the first stage graphitization more severely than acid-insoluble nitrogen (NI) in white cast iron. Moreover, it was deduced that of the acid-soluble nitrogen, the solid soluted one in cementite of white cast iron would retard the decomposition of cementite as a result of strengthening the bonding of cementite.
In this paper, the retarding mechanism of nitrogen on graphitization was investigated thermodynamic alloy in connection with the distribution of the nitrogen during the solidification process of molten iron and also during the malleablized annealing process of white cast iron.
The main results obtained are summarized as follows:
(1) In the case of supersaturation of nitrogen over the equilibrium solubility in molten cast iron, the nitrogen tends to be condensed in eutectic cementite.
(2) The first stage graphitization is retarded in proportion to the concentration of the nitrogen in cementite.
(3) In the course of the first stage graphitization, a part of nitrogen diffuses through the specimen into argon gas atmosphere, and the acid-soluble nitrogen in austenite decreases.
(4) Most of the remaining nitrogen is dissolved in austenite. The more nitrogen is dissolved in austinite, the more it becomes to involve in the eutectoid cementite.
(5) The second stage graphitization is retarded in proportion to the concentration of the solute nitrogen in the eutectoid cementite.
(6) After the second stage graphitization, most of nitrogen is changed to stable silicon nitride, and the NS content in ferrite is less than 16 ppm.
