| The super-austenitic stainless steel(SASS)is a kind of high alloy stainless steel with high content of Cr and Mo,its structure is all stable austenite,and it has similar corrosion resistance to nickel-based alloys,therefore it is widely used in plate heat exchanger,flue gas desulfurization equipment,geothermal well pipeline equipment and other very harsh corrosion environments.Due to the high content of Mo and Cr in SASSs,the second phase(such as sigma phase and Laves)precipitates easily in the center of the steel and near the welding heat affected zone during manufacturing or subsequent heat treatment,which directly leads to the material appear stratified,cracking phenomenon.It has a disastrous effect on the production and thermal processing of materials,and decreases the stainless steel pitting and crevice corrosion resistance.In recent years,studies have shown that Rare Earth elements(Re),especially Cerium,can not only significantly affects the microstructure and grain size of steel,but also can improve the segregation of alloy elements.Therefore,the segregation behavior of alloying elements in super austenitic stainless steel can be inhibited by Ce micro-alloying,and thus the corrosion resistance of stainless steel can be improvedIn this study,in view of the processing difficulties and a reduction of corrosion resistance of super austenitic stainless steel S31254 caused by the second phase,the alloy containing 40 ppm cerium(S31254-40Ce)was designed by Ce micro-alloying.The solid solution and aging treatment of S31254-0Ce and S31254-40 Ce were carried out.And the microstructure of the samples were observed by means of metallographic microscope(OM)and scanning electron microscope(SEM).The effects of Ce micro-alloying on the microstructure,precipitation and dissolution mechanism were investigated;In addition,the effects of Ce on grain growth were analyzed by mathematical model fitting.And the pitting resistance and intergranular corrosion resistance of S31254 were analyzed by electrokinetic polarization test,alternating current impedance test and double-loop electrokinetic reactivation method,respectively.The influence of Ce on the corrosion resistance of S31254 was investigated.The conclusions are as follows:(1)Firstly,S31254-0Ce and S31254-40 Ce were treated at 1150?,1200? and 1250?for 0.5h,1h and 2h,respectively.The microstructure shows that the precipitated phase of S31254-0Ce is gradually dissolved with increasing of temperature and time.The precipitated phases of S31254-40 Ce were completely dissolved at 1200? for 2h,while the precipitated phase of S31254-0Ce was still partially dissolved at 1250? for 2h,which indicates that the addition of cerium can accelerate the dissolution rate of the second phase in the solution process.(2)Secondly,it is found that the precipitates of S31254-0Ce and S31254-40 Ce are different after aging at 950? for 10 min,30min,1h and 2h,respectively.As the aging time prolongs,the precipitated phase in S31254-0Ce firstly precipitates at the grain boundary and large rolling deformation zone,then generates at the low-? grain boundary.However,the number and size of precipitates of S31254-40 Ce decreased significantly during the aging process of 10 min ? 30 min.As the aging time extended,the precipitates did not appear as strips in the area with serious rolling deformation,but at the grain boundary of the as-rolled.(3)The potential polarization and impedance spectra of the samples after solid solution heat treatment were tested in 10 wt.% NaCl solution.The results showed that the passive zone of S31254-40 Ce is wider and the self-corrosion potential is higher under the same heat treatment conditions,which means that the addition of cerium can improve the corrosion resistance of the material.Additionally,it is also found that the corrosion resistance of S31254-40 Ce at after solution 1200? for 2h is better than that after solution at 1250? for 2h.which can be ascribed to the production of a large number of annealing twins in S31254-40 Ce at 1200? for 2h,these twins provide more nucleation sites and improves the passive ability of the material. |
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