Researchs On Effect Of Nb, Al Element On The High Temperature Performance And Microstructure Analysis Of Heat-Resistant Casting Alloy HP40

Research focused on the problems of high temperature oxidation andcreep rupture etc which produced by using the traditional materials HP40alloyunder temperature oxidation and overheated in the Reformer of the petro-chemical unit and Cracking Furnace. Trying to slove these by microalloyingwhich also research on the oxidation resistance and permanence property ofalloy when adding Nb （Strengthen carbide element） and A1（Increasing anti-oxidation abilities element）. Be designed to improve the mechanical behaviorunder high temperature of the alloy, and the purpose of prolonged thematerial’s lifetime is accomplishedAll mentioned alloy was smelt in vacuum induction furnace and themicrostructure of the HP40alloy with adding different elements which usedthe metallographic microscope and scanning electron microscope （SEM） wasobserved in the as-cast and aged condition. It was found that addition of Nband Al helps improve the morphology and distribution，and play animportant role in strengthening or weakening the grain boundaries. The M₇C₃carbide has transformed into M₂₃C₆, skeleton-like carbides degenerated aftersolution treatment at1180℃and aged for50h at980℃.The carbide of the grain boundary are composed of NbC, M7C3and M₂₃C₆when containing Nb0.8%. Even then these carbide are tiny and distributed in austenitic matrix as askeleton structure, the clumpy carbides will precipitate in matrix withincreasing the contents of Nb, but the precipitation of the grain boundarycarbides didn’t change when added the A13%, A1is mainly solution inmatrix, it didn’t separate out precipitation strengthened phases and change anyform of the carbides.Through the analysis of the oxidation resistant of the HP40alloy withadding different elements under1150℃, all of them are following the rules ofparabola, the surface of the HP40alloy with A1are bright in light brown color,which forming dense oxide layers Al₂O₃through XRD analysis, and improvethe oxidation resistant of the alloy. And the surface of the HP40withoutYttrium was made up of Cr2O3, which is unstable and easily decompose athigh temperature, another, the film of oxide was easy to drop out after a longtime study on the Oxidation, on the view of macroscopic there are tinycorrosions that had led to degradation of oxidation resistant.Then, the rupture strength under900℃&90Mpa were researched, Theresults showed that the alloy with Nb0.8%had the relatively longest rupturelifetime, the HP40alloy（Nb0.8%） with better permanence property got the relatively bigger austenite grain sizes but the smaller area of the grainboundary after observed the endurance test samples by means of opticalmicroscope. And the stress-rupture have some ductile fracture features.To research the carbides precipitation on the grain boundary, which usedthe electron diffraction pattern by transmission election microscopy（TEM） andenergy dispersive X-ray（EDS） to analyse the Structure and Composition of thealloy crystal, it was found that the NbC and Y carbides on the grain boundary,which can separate out as stable carbides during solidification, and offer thenucleation area for the coming precipitation of the Cr carbides. at the sametime, the continuity of the eutectic Cr network carbide were reduced, whichimproved the high temperature Mechanical performance. Another, throughcalculation of the diffraction spot,The zone axis of the NbC and M₂₃C₆are [1-10]&[-110] respectively.