Effect of matrix grain size on the oxidation and carburization behavior of chromia-forming alloys in high temperature $CO_2$ Environment

Abstract

The effect of matrix grain size on the oxidation and carburization behavior of chromia-forming commercial heat resistant alloys viz. Alloy 600, Alloy 690, and Alloy 800HT has been studied in high temperature $CO_2$ at $650 ^\circ C$ and 0.1 MPa for 500 h. Solution annealing at 1120 and $1180 ^\circ C$ resulted in enlargement of the grain size. Oxidation rate increases on increasing the grain size of the alloys, due to the formation of less protective surface chromia. Similarly, excessive carburization has been observed for solution annealed Alloy 600 and Alloy 800HT specimens. Electrochemical analysis revealed that the chromia formed on higher grain size specimens were excessively porous and defective. It is interpreted that, porous chromia can accommodate increased transportation of C bearing molecules to increase the carbon activity at the oxide-metal interface, therefore proliferating the carburization. Meanwhile, carburization was limited on fine grained Alloy 800HT from cold working and recrystallization, attributed to the formation of protective chromia.