Study On Fatigue-Creep Behavior Of The Superheater Pipe Steel Applied In Thermotubes Of Biomass-fired Boiler In High Temperature Alkaline Salt Environment

Plentiful alkali metal,sulfur,chlorine and other inorganic elements will be released when the biomass combust.These will producing alkali metal compounds which will form alkali metal salt deposit on the superheater pipe when blowing into the overheated pip and cause alkali metal salt of hot corrosion damage.Meanwhile,biomass thermotubes is not only subjected to the fatigue effect of cyclic load in starting and stopping operation,but also bearing the creep effect of stable load in stable operation,resulting in thermal corrosion fatigue damage or creep damage in the structure.To study the influence of creep properties and fatigue fracture failure mechanism and of biomass steel pipes under the environment of high temperature and alkali metal salt,this paper uses RDL05 electronic creep fatigue tester,scanning electron microscopy,energy spectrum analysis,metallographic microscope and other auxiliary tools,to carry out the 12 crlmov alloy fatigue creep experiment with different cycle load,alkali metal salt type and creep protected load time.The main contents and results of this research are as follows:(1)Carrying out the high temperature fatigue-creep experiments under the environment of different kinds of alkali metal salt,researching on creep-fatigue performance changes in the relationship of 12 crlmov alloy under different high temperature and alkali metal salt corrosion environment.The high temperature pure chlorine salt is oxidized on the surface of the alloy,which can damage the protective oxide film,form defects on the surface,cause fatigue damage,and reduce the fatigue and creep life of the sample.Adding sulfate has a certain weakening effect of the corrosion of to chlorine salt,which can improve the fatigue-creep property of the alloy.(2)Fatigue-creep tests of 12CrlMoV alloys had been conducted in high temperature alkaline metal salt environment of different cyclic load.The result shows that tensile stress should maintained at 7.2s when put 12CrlMoV alloys in the environment of 600 ℃、KCl-25%K2SO4,the fracture of sample is mainly caused by fatigue.As the stress increases,the damage of thermal corrosion is deepened.Meanwhile,precipitation of coarsening carbides was discontinuously distributed on the grain boundary,weakened fatigue-creep properties.(3)Creep time result in a great influence on the fatigue-creep properties of 12Cr1MoV alloy.As the holding time increases,the fatigue-creep performance changes from fatigue control to creep control.As the holding time increases,the fatigue-creep performance changes from fatigue control to creep control,the origin of destruction is inside the material,inside the material there are many dimples and creep holes,and mostly grow up with inclusions,cementite as core.After the dimples and creep holes grow up,they merge with each other,the fracture shows a creep neck fracture morphology,toughness transcrystalline fracture,fatigue-creep properties is greatly degraded.(4)Researching and analyzing the changes of microstructure under the different time of creep-protection。This shows the precipitation of coarsening carbides was discontinuously distributed on the grain boundary.Spheroidization acceleration of pearlite and cementite expose the ferrite matrix,reduce the binding force between grain boundaries and grains,weaken the creep resistance of alloy materials.