Effect Of Sulfur On Microstructure And Property Of K4169 Superalloy And The Metallurgical Mechanism Of Its Removal

K4169 alloy has the same composition with Inconel 718,which is the nickel-iron superalloy strengthened by precipitated phase y"(BCT)and ?'(FCC).K4169 alloy has good comprehensive performance under 700?.Therefore,it is widely used in the manufacture of high temperature alloy structural castings,such as aero-engines,gas turbines,turbine disks and blades.In recent years,with the rapid development of aerospace industry,higher requirements have been put forward in order to stabilize the work at higher temperatures.However,the impurity elemental sulfur has a great influence on the microstructure and mechanical properties of K4169 alloy.In order to improve the performance of K4169 alloy,the influence of sulfur on microstructure,properties and the desulfurization mechanism were studied in this paper.The microstructure of K4169 alloy was observed by optical microscope,and the influence of sulfur content on dendrite microstructure was studied.It was found that the microstructure of K4169 alloy was dendritic structure,and with the increase of sulfur content,the dendrite was more developed,the dendrite spacing became larger,and the number of precipitated phases increased gradually.After standard heat treatment of 1095? for 1.5h,air cooling+955? for 1h,air cooling+720? for 8h,furnace cooling to 620?(55?/h)+620? for 8h,the precipitation of the alloy dendrite decreased,the primary dendrite arms became thicker,and the secondary dendrite arms became thicker and shorter.The morphologies,quantity,distribution and composition of the precipitated phase acquired by scanning electron microscope.And the influence of sulfur content on the precipitation phase and composition segregation of K4169 alloy was obtained.The Laves phase presented sieve mesh in the as-cast K4169 alloy,and they basically disappeared after the standard heat treatment.Although the change of sulfur content had little effect on the micro-porosity and carbide in the alloy,it promoted the segregation degree of the alloying elements,especially for the elements of Nb and Ti.The degree of segregation of each element in the as-cast alloy was:Nb>Ti>Mo>Fe>Cr>Al.Compared with the cast alloy,the segregation degree of each element reduced after heat treatment.The influence of sulfur content on mechanical properties was analyzed by means of mechanical property test.At the range of lower than 45 ppm,the tensile strength of K4169 alloy at room temperature changed little with the increase of sulfur content,but the toughness of the alloy decreased remarkably.Compared with the tensile properties at room temperature,the influence of sulfur on the high-temperature durability of the alloy was more obvious.Under the condition of 650?/620MPa,it was found that the durability and elongation of the alloy were decreased significantly with the increase of sulfur content,and the durability of the alloy was fluctuated when the sulfur content exceeded 30 ppm.The carbon carbide and sulfide in the alloy were identified by means of SEM and TEM,and it was found that Nb2SC precipitated in the alloys with the sulfur content of 30 ppm and 45 ppm,and no sulfides were found in the alloys with the sulfur content of 15 ppm and 6 ppm.Sulfur,as the harmful impurity element,promotes the segregation of the alloying elements;sulfur segregates in dendrite and grain boundaries,promotes the formation of harmful M2SC phase,reduces the bonding strength between the alloy crystal interface,and thus reducing the mechanical properties of the alloy,especially the high-temperature durability.Therefore,in order to improve the comprehensive mechanical properties of the alloy,it is necessary to strictly control the content of sulfur in the alloy during actual production.And it is suggested that the sulfur content should be controlled below 15 ppm in the K4169 alloy.Desulfurization of using crucible is a very important means for high-temperature alloys in vacuum induction melting process.The desulfurization behavior of molten Ni-Cr-Fe-Nb in conventional magnesia material crucible and thermodynamics stabilized calcium oxide crucible was analyzed in this paper.The results showed that the calcium oxide crucible had excellent desulfurization ability.And it was mainly because of the following desulfurization reaction in the molten:3(CaO)+2[Al]+3[S]=3(CaS)+Al2O3(s).The desulfurization resultant was found to be CaS by analyzing the inner wall of CaO crucible after melting,and the thickness of the CaS layer on the inner wall of the crucible was approximately 20 ?m.In addition,the results showed that the desulfurization effect of CaO crucible was better than that of MgO crucible,and this was because the CaO crucible reacted with Al2O3 to generate 3CaO·Al2O3 which was in liquid state at the melting temperature,it could be easily transferred away from the interface reaction region which contributed to the desulfurization reaction greatly.However,MgO crucible reacted with Al2O3 to generate MgO·Al2O3 which was in solid state at the melting temperature,so its desulfurization effect was relatively low.The sulfur content in K4169 alloy can be controlled under 5 ppm by vacuum induction melting with CaO crucible,which lays a good foundation for improving the performance of the alloy.