Study On Failure Behavior Of Super304H Under The Synergism Of Simulated Fireside Corrosion And Loading

Super304H has been widely used in super supercritical thermal power boiler over/reheater because of its high permissible stress,high stress-rupture strength,great high-temperature oxidation resistance,high-temperature corrosion resistance and moderate price.At present,studies on Super304H were limited to two aspects about independent oxidation/corrosion research and stress-rupture life in static air.The study was rarely reported about the early failure characteristics and mechanism of corrosion failure and corrosion product layer growth in real service environment,especially under the synergism of fireside corrosion and loading.Bursting accident caused by corrosion and fracture of alloy under the synergy of fireside corrosion and loading is an important common risk for safe operation of thermal power units.Therefore,studying its failure behavior and mechanism has remarkable theoretical values and engineering significance.In this paper,X-ray diffractometer(XRD),scanning electron microscope(SEM),X-ray energy dispersive spectrometer(EDS)and electron backscatter diffraction(EBSD)were used to study the high temperature corrosion behavior of Super304H in the environment of simulated fireside corrosion at 650?.The results showed that the average thickness of corrosion layer of Super304H was respectively 2.5,5.2,11.5 and 27.8?m after corrosion of 72,151,321 and 700h;and the corrosion layer is composed of loose Fe2O3 outer layer,relatively dense Cr2O3 inner layer and interfacial porous product.The sulfur product accumulates and expands at the grain boundary.The diffusion and dissolution of the oxide film lead to the formation of the "bludge like" Fe2O3 protruding corrosion products and the protective Cr2O3 film gradually exfoliates.With the prolongation of the corrosion time and growth of corrosion product,break-up and exfoliation of the corrosion products led to the further reduction of the effective cross section of the steel,and the corrosion environment shortened the service life of the Super304H under different loading,and the stress-rupture strength in simulated fireside corrosion environment is 80MPa,which approximately differs by 43%compared with that in static air environment(140MPa),and the fireside corrosion environment is the main factor leading to the loose pores of the corrosion products at the alloy surface and the appearance of a large number of "bludge like" protruding products;the loading increases the hole nucleation rate of the corrosion layer/metal interface and thus reduces the stress rupture property of Super304H.The thickness of corrosion layer and corrosion affected zone is significantly increased by loading,220Mpa/546h was 2?m and 10?m respectively,OMpa/546h was 0.6?m and 1.8?m.In-situ test of FIB combined with TEM showed that spinel phase was CuCr2O4,a large number of dislocation plugs,dislocation entanglement and fine crushed grain were formed in the corrosion affected zone under loading.