Influence of Cr content in steel 12X1MF on exploitation life of superheater pipes

Abstract

The most widely-used material for tubes in ex soviet power plants is steel 12X1MF. Until today, 12X1MF has proven to be a reliable material for use in plants that operate in conditions of creep (temperature about 540-550 °C and pressure about 14-15 MPa). The microstructure of steel in the initial state depends on the heat treatment that it has undergone. Generally, low alloy CrMo(V) creep-resistant steels applied for construction of energy plants have initial ferritic-bainitic microstructure with fine dispersion of globular carbides precipitated in the ferritic matrix and at grain boundaries. Chromium has a positive effect on the heat resistance (resistance to oxidation) of the steel as well as hardenability and indirectly affects the creep resistance. Chromium should be in a solid solution. After complete heat treatment cycle, chromium carbides in 12Х1MF should not exceed 10-20% of the total Cr+Mo+V carbide content. The effect of chromium in ferritic creep-resistant steels is complex. This paper deals with the case of a superheater pipe failure (fish-mouth rupture) which is partly caused by the fact that the pipe has a lower Cr content than that provided for 12Х1MF steel.