Effect Of Al On The Microstructure And Mechanical Properties Of Austenitic Heat Resistant Steel

In order to reduce energy shortage and carbon dioxide emissions,improving energy efficiency and steam parameters of power plant boilers is the most effective way to achieve energy conservation and reduce consumption and reduce environmental pollution.However,the performance of steel for utility boilers has always been the main factor restricting the improvement of utility parameters.At present,the traditional austenitic heat-resistant steel used in power plant boilers mainly uses Cr₂O₃ oxide film formed on the surface in the process of high temperature oxidation to ensure the high temperature oxidation resistance of the material;however,when the service temperature is increased to650?,Cr₂O₃ oxide film on the surface will volatilize and lose its protective effect,and the aggregation of grain boundary carbides will also lead to material failure.The Al₂O₃ and Cr₂O₃ composite oxide films formed on the surface of the new aluminum-containing austenitic heat-resistant steel are relatively stable at high temperature.Meanwhile,the nano-scale NbC strengthening phase precipitated in the crystal can significantly improve the high-temperature creep strength of the material,making the new aluminum-containing austenitic heat-resistant steel a candidate material for the key components of the new generation of ultra-supercritical thermal power generation units.In this paper,three kinds of new aluminum-containing austenitic heat-resistant steels with different Al content were designed.the solid-solution steels were aged at 620?,650?,700?and 750?for different time.The microstructure and precipitated phase of the aged steels were observed by optical microscope?OM?,x-ray diffraction?XRD?,scanning electron microscope?SEM?,energy dispersive spectroscopy?EDS?and electron back-scatter diffraction?EBSD?,and the effects of Al content on the aged precipitated behavior of the aged steels were studied.At the same time,the mechanical properties of aluminum-containing austenitic heat-resistant steel were investigated by tensile test at room temperature after aging and tensile test at high temperature after solid solution.The stress-strain curves under different deformation conditions were obtained by hot compression test of solid-solution austenitic heat-resistant steel containing aluminum at high temperature.The effects of Al content on the rheological stress and microstructure evolution were studied,the main conclusions are as follows:?1?The structure of the aluminum-containing austenitic heat-resistant steel after solution treatment is composed of austenite and a trace amount of fine NbC phase;after aging at 750?for different time,?ferrite and?phase are the main precipitated phases.?phase is precipitated at grain boundary at the beginning of aging,and increases with aging time.When the aging time reaches9h,the?phase of grain boundary is reticulate,and?phase is precipitated in the grain.The?phase of grain boundary gradually changes into?phase and the?phase in grain boundary increases with aging time.The effect of aluminum content and aging temperature on aging precipitation behavior of aluminum-containing austenitic heat-resistant steel is significant.The higher the content of aluminum,the faster the precipitation of?ferrite phase,the faster the phase transition speed,and the more precipitated phases.?2?The effect of aging treatment on the strength and toughness of aluminum-containing austenitic heat-resistant steel is obvious,and the longer the aging time is,the more obvious the effect is.The tensile strength of the samples after aging treatment was significantly higher than that of the samples after solution treatment,but the elongation decreased significantly,especially when the aging time was 72h,which was related to?phase distributed in net shape on grain boundaries.At the same time,the strength of heat-resistant steel increases with the increase of Al content,and the plasticity decreases.The change rule of material hardness is consistent with the change rule of strength.When the temperature is low,grain boundary and in-crystal precipitated phase act together to strengthen the material to some extent,which leads to the increase of material strength and the decrease of plasticity.However,with the increasing of temperature,the softening mechanism plays a major role in the tensile process at high temperature,the strength of the material is significantly reduced and the plasticity is improved.?3?After hot compression,the deformation twins will appear in the process of compression,and the second phase?-ferrite phase will be precipitated at the twin grain boundaries and grain boundaries;among them,the larger the deformation,the more precipitated phase;the more aluminum content,the more precipitated phase on the grain boundary,which hinders the deformation of grain and leads to continuous deformation twins.then the precipitation and deformation of precipitated phase on the grain boundary lead to deformation twins become small grains,and the grain size gradually decreases.Meanwhile,the deformation mechanism is different with different deformation speed.