Analysis Of Oxide Growth And Spalling Behavior On The Inner Wall Of Austenitic Steel Superheater And Reheater Tubes Of Utility Boilers

At present,the austenitic stainless steel pipes used in domestic utility boilers are mainly TP304H,TP347H,TP310hnbn,Super304,etc.The oxidation resistance temperature of austenitic stainless steel is over 700?,which has good processing performance,welding process performance,structural stability,high temperature corrosion resistance and high thermal strength.Under the condition that the outlet temperature of boiler tube steam is constant,the existence of oxide may cause long-term overheating of boiler tube metal,leading to a series of adverse consequences,such as aggravation of oxidation,etc.,making the boiler heating surface austenite The failure of body stainless steel boiler tube is advanced.In order to reduce the accidents caused by boiler tube failure and increase the safe operation time of generator set,it is necessary to control the formation and peeling of oxide skin on the inner wall of austenitic stainless steel tube.This topic is to study the growth and peeling behavior of oxide on the inner wall of tube.A total of 19 sets of austenitic stainless steel pipe samples from 10 power plants were collected in this project.Through the analysis of macro morphology,chemical composition,mechanical properties,metallography,morphology and phase analysis of oxide]ayer,electron microscopy and X-ray energy spectrum,the morphology and chemical composition characteristics of oxide/metal interface of different steel pipes were compared and analyzed,and the 18-8 type materials were analyzed for the oxygen content of boiler pipe steam The results show that the oxide growth mode and the possible influencing factors of the inner wall of the boiler tube,the oxide peeling mode and the possible influencing factors of the inner wall of the 18-8 type austenitic stainless steel boiler tube.According to the analysis of the project,there are four kinds of oxide structures in the inner wall of the austenitic steel pipe of superheater and reheater,namely,two layer structure of primary oxide(inner layer spinel+outer layer ferric oxide),three layer structure of primary oxide(inner layer spinel+ outer layer ferric oxide+outer layer ferric oxide +outermost layer ferric oxide),residual inner layer structure(original outer layer or residual oxide after the outer layer falling off),and successive oxide structure The secondary structure of oxide(inner layer+new outer layer).The growth characteristics of primary oxides are as follows:1)with the increase of operation time,the thickness of primary oxides increases.2)When the thickness of primary oxide is relatively thin,it is easy to form a two-layer oxide structure;as the thickness of primary oxide increases,three-layer oxide structure is gradually formed.3)When the thickness of primary oxide is very thick,the inhibition film of oxide/metal interface will gradually degenerate.4)The resistance of Fe2O3 layer to oxygen diffusion is greater than that of Fe3O4 layer.5)The thickness of inner layer of primary oxide is usually greater than that of outer layer(or the sum of outer layer and outermost layer).The growth characteristics of secondary oxide are as follows:1)after the outer layer of primary oxide peels off,dense new outer oxide containing chromium and nickel is formed on the surface of residual inner layer.2)The growth rate of secondary oxide is slower than that of primary oxide.3)The exfoliation of the primary oxide layer is beneficial to the formation of the inhibition film at the oxide/metal interface of the inner wall.When the thickness of the secondary oxide is thick,the chromium content of the oxide/metal interface inhibition film is low;with the thickness of the secondary oxide becoming thin,the oxide/metal interface of the secondary oxide is easier to form the inhibition film with high chromium content than the oxide/metal interface of the primary oxide.4)The thickness of residual inner layer may tend to be thinner as the outer layer of secondary oxide grows and peels off repeatedly.With the long-term stable operation,the inner oxide thickness of the stable successive oxide gradually thins,the chromium distribution tends to be uniform,and the average chromium content increases after several cycles of outer layer falling off and new outer layer growing,so as to improve the resistance to steam oxidation.The steam oxidation resistance of 18-8 austenitic stainless steel mainly depends on grain size and alloying elements(chromium and carbide forming elements).The finer the grain size,the more stable the carbides formed,and the more chromium dissolved in the solid solution,the easier it is to form a protective inhibition film at the oxide/metal interface and improve the steam oxidation resistance of the steel pipe.Through the comprehensive analysis of the test results,the ability of austenitic steel materials to form inhibition film on oxide/metal interface is enhanced in the following order:TP316H?X8CrNiNb1613?TP304H? 12X18H12T?TP347H.The oxidation structure,primary oxide structure and growth behavior,secondary oxide growth behavior and oxide stripping behavior of 5 kinds of 18-8 type austenitic stainless steels,including 12X18H12T,TP304H,TP347H,X8CrNiNb1613 and TP316H,were preliminarily understood through the experimental research of this project.The research results provide some reference and help for the safe operation of power station units.