Evolutions Of As Cast Structures And Mechanisms Of High Temperature Phase Transformations In Nitrogen Bearing Duplex Stainless Steel

Duplex stainless steel(DSS)is a high performance stainless steel which comprising nearly equal amounts of ferrite and austenite,and nitrogen as an economic element is more important to DSS,and has also attracted more and more attention.But at present,regarding to the effects of nitrogen on DSS,these existing studies are not deep enough,and such reports about evolutions of as cast structures and mechanisms of ferrite/austenite high temperature phase transformations in nitrogen bearing DSS are also not enough,so it limits the further use of nitrogen in DSS and the appearence of the new economic DSS.Characterisics of as-cast macrostructures,microstructures and ferrite/austenite high temperature phase transformations in nitrogen bearing DSS were studied by means of the laboratory small ingot experiments.ProCAST simulation software and Thermo-Calc ther-modynamic calculation software,through metallographic microscope,X ray diffraction and electron backscattered diffraction,electron microprobe and the ultra high temperature confocal laser scanning microscopy.Based on the industrial tests,the characteristics and heredity of as-cast microstructures and rolling mircostruc-tures were also discussed.The results are as follows,In DSS,characteristics of the columnar crystals and equiaxed crystals are obvious,and boundaries of both the distribution areas are distinct.With the increase of nitrogen content,the region of equiaxed crystals expands and that of columnar crystals decreases,and both of them become fine and compact.The simulation results are basically consistent with the experimental results,which better proves the influence of the nitrogen content on the macrostructures.The metallographic structures show that DSS is consisting of two phases with distinct characteristics,and the presence of characteristic diffraction peaks(111)? and(110)? exactly confirms the existence of austenite and ferrite in DSS,meanwhile,the increase of nitrogen content promotes increasing the intensity of characteristic diffraction peak(111)?.The volume fraction of austenite increased with increasing the nitrogen content,and that of ferrite is just opposite.When the nitrogen content is low,the austenite grains preferentially precipitate along the ?/? grain boundaries,and are mainly needlelike in shape,while the other grains which formed in the ?-ferrite matrix are mainly granular in shape and small in size.When the nitrogen content is high,the size of austenite grains increase,the needlelike austenite which form along the ?/? grain boundary has a certain orientation,and the austenite in the ?-ferrite matrix is mainly needlelike and wheatlike in shape.With the nitrogen content increasing,the number of austenite grains increased firstly and then declined,which indicated that there existed a competitive relationship between the austenitic nucleation rate with its growth rate,and the nitrogen content has a significant effect on the relative size of them.The thermodynamic calculation results also show that high nitrogen content can not only increase the initial precipitation temperature of austenite and promote the diffusion of elements,but also broaden the precipitation temperature range of austenite.The results of in-situ observation and thermodynamic calculations show that high nitrogen content can promote the precipitation and growth of y during the??? transformation.High nitrogen content leads to the higher initial precipitation temperature of ?,and ?-cells preferentially precipitate along the 8/8 grain boundaries and are flaky in morphology,some intragranular y-cells form subsequently in the ?-ferrite matrix and are irregular and needlelike in shape.When the nitrogen content is low,it results in the lower starting precipitation temperature of ?,and a triangular ?-cell prefers to form along the ?/? grain boundary,then some needlelike y-cells form subsequently in the ?-ferrite matrix,meanwhile,there are also some needlelike ?-cells precipitating along the ?/? grain boundaries.Secondly,the in-situ observation results combining with the diffusion controlled growth theory reveal the influence mechanism of the cooling rate on the precipitation morphology of y.When the cooling rate is slow,it leads to the higher initial precipitation temperature of ?,and ?-cells preferentially precipitate along the ?/?grain boundaries and are flaky in morphology,some intragranular y-cells form subsequently in the ?-ferrite matrix and are needlelike in shape.When the cooling rate is fast,it results in the lower starting precipitation temperature of y,and y-cells prefer to form along the ?/? grain boundaries,but some needlelike y-cells form subsequently in the ?-ferrite matrix and are needlelike in shape.The in-situ observation results show that the nitrogen content and heating rates have an important influence on the decomposition of y during the ???transformation.The starting and finishing phase transformation temperature increase with the nitrogen content increasing in the ??? transformation,the Thermo-Calc thermodynamic calculation results also confirm this,but it has no effect on the phase transformation mechanism,the main form of the phase transformation is still the migration of the interphase boundary.Secondly,the finishing phase transformation temperature increase with the heating rate increasing,when the heating rate is slow,the ??? transformation occurs by means of the migration of the interphase boundaries in the outside and inner of austenite,but when the heating rate is fast,the transformation only happens in the outside of austenite by the same way.Industrial tests show that the differences between the casting microstructure and the rolling microstructure are obvious,the shapes of austenite in as-cast microstructures are needlelike or wheatlike,the morphologies of austenite in rolling microstructures are rodlike along the rolling direction and islandlike along the transverse direction.The rolling process has no obvious effect on the volume fraction of austenite and ferrite,but has remarkable influence on the grain size.The rolling process changes the absolute size of grains,but does not change the relative size.The ?/? interphase boundaries in as-cast microstructures are easy to distinguish,but it is opposite in the rolling microstructures.