Effect of tempering time on the oxidation behavior of 21/4Cr-1Mo steel

Abstract

21/4Cr-1Mo ferritic steel is generally used in the normalized and tempered condition for steam-generator tubes in fast breeder reactors. The microstructure of the alloy changes on tempering, depending on the type of carbides formed, as a function of tempering time. At the very early stage of tempering (up to ∼30 min), most of the cementite (Fe₃C) dissolves, causing trapping of Cr and Mo to form their carbides. With an increase in the tempering time more Cr and Mo are used up for carbide formation. This leads to a decrease in the available Cr content, which is generally responsible for the formation of a protective oxide scale. It is thus expected that a decrease in the oxidation resistance will occur with an increase in tempering time.

Oxidation tests were carried out on specimens prepared after five tempering treatments; viz. 10 min, 30 min, 60min, 120min, and 180min at 998 K. Oxidation was carried out in air both for long duration (1000 hr at 773 K) and for short duration (6hr at 873, 973, and 1073 K). Results indicate a decrease in the oxidation resistance with increasing tempering time. At 1073 K, however, no significant change in oxidation was noticed. This could be perhaps due to the vanishing of the tempering effect by the dissolution of carbides at this temperature.