| It is necessary for alumina-forming austenitic heat resistant steel(AFA steel)to processing excellent high-temperature creep and oxidation resistance to adapt to work in aggressive environment of ultra-supercritical unit.In this study,it has been investigated the effects of different treatments including isothermal aging,annealing and cold rolling on high-temperature creep behavior of AFA steels.The constitutive equation was structured,which revealed the relationship between applied stress and steady-state creep rate of AFA steels at 700℃.The research also illuminated the influences of cold-rolled deformation on high-temperature oxidation behavior in AFA steels.The effects of aging,annealing and cold rolling treatment on high-temperature creep properties of AFA steels were studied.The strengthening mechanisms during creep were analyzed according to the variations of microstructure before and after creep.The results show that the creep resistance is quite outstanding for 1000h-aged,1230℃-annealed and cold-rolled samples,severally corresponding to three treatments,and the steady-state creep rates were 1.54×10-6 s-1,1.61×10-6 s-1 and 4.5×10-10 s-1 respectively.By contrast,the cold-rolled sample has the best creep resistance at high temperature.The dislocation could be effectively pinned by nanosized precipitates formed around dislocations during creep,holding the dislocation density in samples.The reaction between dislocation and precipitates made a decrease of coarsening rate precipitated particles.Orowan mechanism that dislocations bypass the nano-scale precipitates was predominant to promote the creep resistance of AFA steels.The creep behavior of AFA steel under applied stress of 70～160 MPa at 700℃was studied.The constitutive equation of stress and steady-state creep rate was constructed by power law equation.The results show that the relationship between applied stress and steady-state creep rate of AFA steel agree with power law relationship under 70～160 MPa.Under high stresses(130～160 MPa),The error between the predicted values and measured values of the constitutive equation is very small,hence it can scientifically forecast the creep behavior for AFA steel.However,the error is great at lower stresses(70 MPa and 92 MPa),which has a lot to do with creep activation energy.To evaluate the effect of cold rolling on high-temperature oxidation resistance,the high-temperature oxidation behaviors of as-received and cold-rolled samples in dry air at 700℃ were investigated.The results show that the oxides formed on coldrolled samples,which has similar composition comparing with the as-received samples in the initial oxidation stage.With the increasing oxidation time,the gradual spallation of outer scale results in the inner Al2O3 scale to turn into a main force,which is favor in prohibiting inward diffusion of oxygen and expansion of holes.Significantly,the dislocation introduced by cold rolling accelerates the diffusion of Al from the matrix to surface of Al2O3 scale,facilitates the formation of B2-NiAl phases and guarantees the continuous formation of Al2O3 scale. |
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