Influence of Dissolved Oxygen on the Reaction between Molten Iron and Reaction-Bonded Si3N4

Fundamental investigations of the reaction between Si3N4 and molten iron have shown that Si3N4 material is desirable in steelmaking process. This paper describes the influence of dissolved oxygen in iron on the corrosion of reaction-bonded Si3N4. The results are as follows:Si3N4 specimens dipped into iron containing more-than-300-ppm dissolved oxygen for 30 minutes at 1630°C and rotated at 300rpm were not attacked. However, molten iron of less-than-300-ppm oxygen attacked Si3N4 as oxygen contents decreased. Optical microscope and EPMA observations showed the formation of 50-150-μm thick SiO2 layer on the Si3N4 surface which did not corrode. Otherwise, there was none of the thick layer on specimens dipped in below-300-ppm-oxygen iron. This reaction mechanism was discussed thermodynamically. Iron with a high dissolved oxygen level forms a large amount of SiO2 at the beginning of the reaction, which acts as a protection layer on the Si3N4 surface. On the other hand, a low dissolved oxygen level iron causes the Si of Si3N4 to dissolve into iron without SiO2 formation.