Research On The Hot Working Process Of New Austenitic Stainless Steels

The supercritical water cooled reactor(SCWR)has many advantages such as high thermal efficiency,simple system structure,high safety,good economy,high utilization rate of nuclear fuel,etc.But its special working conditions require high corrosion resistance of the cladding material.310 S austenitic stainless steel is one of the candidate materials for SCWR fuel cladding.But at present,the ordinary 310 S can not fully meet the needs of SCWR.Therefore,in this paper,two new types of 310 S stainless steel which are Zr micro-alloying and Ti-Nb-W micro-alloying has been studied.The microstructure evolution and corrosion resistance of stainless steel in hot working process,such as smelting,forging,hot rolling,solid solution treatment and stabilization treatment,were studied systematically.The appropriate hot working process of these two 310 S was determined finally.The microstructural evolution of two micro-alloying 310 S stainless steels in vacuum induction melting,high temperature forging,high temperature hot rolling and heat treatment was studied in this research.The results show that there is no ferrite in the cast microstructure of the two kinds of stainless steel.After forging and hot rolling,a large number of dispersed carbon particles and some big Zr(C,N)or(Ti,Nb)(C,N)appear in the microstructure.After solid solution treatment,a single-phase austenite structure with uniform grain size appears,and most of the big carbonitrides have been integrated into the austenite matrix.1150 oC is the optimum solution temperature of Zr micro-alloying 310 S,and 1100 oC is the optimum solution temperature of Ti-Nb-W micro-alloying 310 S.The addition of micro-alloying elements increases the sensitization temperature of the two kinds of stainless steels,and after different temperature stabilization treatment,the steels have different precipitates.After the stabilization of 950 oC,a large number of M23C6 or ? phases appear in the microstructure of the stainless steel.After the stabilization of 1000 oC and 1050 oC,there are spherical MC carbides and big,regular Zr(C,N)or(Ti,Nb)(C,N),besides the dispersed carbides.Furthermore,the effect of heat treatment on the corrosion resistance of these two 310 S was studied by means of electrochemical experiments.The electrochemical experiments includes Tafel polarization curve,double loop electrochemical potentiodynamic reactivation(DL-EPR)test and 10% oxalic acid etching test.The results show that the stabilization temperature has little effect on the uniform corrosion resistance of these two kinds of stainless steel.But they can change the intergranular corrosion resistance of stainless steel by affecting the secondary phase.In the range of 950 ~ 1050 oC,with the increase of the stabilization temperature,the resistance to intergranular corrosion is gradually improved.But compared with the solid solution state,the resistance to intergranular corrosion has not been improved or even reduced.M23C6,? phase and Zr(C,N)or(Ti,Nb)(C,N)in the two kinds of stainless steel are very harmful to the intergranular corrosion resistance of the materials.For these two stainless steel,heat treatment of 1150oC×45min or 1100oC×45min can simultaneously play the role of solution treatment and stabilization treatment.These kinds of heat treatment can not only make M23C6 fully dissolve,but also promote the dispersion of ZrC,NbC,TiC and other carbides,so that the material has high resistance to uniform corrosion and intergranular corrosion.