Research And Development On The Compound Electromagnetic Continuous Casting For Ni-based Corrosion Resistant Alloy

Ni-based corrosion resistant alloy is widely used in the fields of aerospace,oil industry and nuclear energy due to its features of high temperature resistant and corrosion resistant.However,the high energy consuming,low production efficient and quality defects of billet are the main difficulties in terms of the conventional product routine.The continuous casting technique to produce Ni-based corrosion resistant alloy has become the front technique,which increases the production rate and reduces the product cost.Japan has first succeed to use the continuous casting method to produce Incoloy800H and Incoloy825 alloy.In the present,few steel plant has the continuous caster setup for this technique in China.However,Incoloy800H alloy billets obtained from the caster setup has many surface quality problems such as depth oscillation marks and surface cracks,and many internal quality problems such as large grain size,the severse segregation of element,centerline shrinkage and internal cracks.For Incoloy825 alloy,there is no report on the success of continuous casting in China.Therefore,in the present work,the continuous casting technique for Ni-based corrosion resistant alloy was investigated in detailed based on the problems raised in the industry.In this thesis,the solidification behavior of Incoloy825 alloy was investigated.A designed single side electromagnetic stirring(EMS)equipment was adopted to study the alloy solidification process.The results showed that the application of EMS can increase the exquiaxed grain ratio and refine the grain:the average equiaxed grain ratio increased from 16.7%to 77.5%and the averaged grain size decreased from 5.8 mm to 0.21 mm compared that without EMS,respectively.The dendritic etching solution for dendrite structure was adopted to study the dendrite evolution processes,which can explain the mechanism of columnar-to-equiaxed transition(CET)and grain refinement.Furthermore,based on the Campanella’ s dendrite drifting theory,a new dendritic broken-drift criterion for Incoloy825 alloy with EMS was setup by us.The results showed that EMS can decrease the dendritic segregation ratio of element and reduce the number of large TiN inclusion.Therefore,the internal quality and the mechanical feature of cast state Incoloy825 alloy were enhanced.In this thesis,the electromagnetic continuous casting(EMCC)for Incoloy800H alloy was successfully conducted using the vertical EMCC caster.The rectangular billets have a cross-section dimension 225×100 mm.The effect of EMS on the internal quality was discussed.The results showed that the equiaxed grain region was enlarged with EMS for the obtained rectangular billet.The average equiaxed grain ratio was increased from 2%(without EMS)to 34.61%(with EMS),and the average grain size is decreased from 8.8 mm(without EMS)to 1.64 mm(with EMS).However,due to the poor high temperature plasticity,the relatively wide brittle range for Incoloy800H alloy,and the high cooling rate in the continuous casting condition,the obtained billets have the deep oscillation marks and surface cracks.In this thesis,the compound electromagnetic continuous casting method for the Ni-based corrosion resistant alloy was first proposed.The high frequency electromagnetic field was applied in the mould slit region and the low frequency electromagnetic field(EMS)was applied in the vicinity of the second cooling region.The high frequency electromagnetic field is used to improve the surface quality and the EMS is used to improve the internal quality.The Incoloy800H alloy billet(100×100 mm)was successfully obtained.In this thesis,based on the results obtained from the compound EMCC technique,the mechanism for electromagnetic field to improve the surface and the internal quality was discussed.The results showed that the magnetic force was generated by high frequency electromagnetic field,pointing to the mould centre due to the applied alternating current.This results in the radius of meniscus is increased therefore the channel is widen for the flux and the lubrication condition between the initial solidification shell and the mould in increased.Furthermore,the Joule heat will increase the temperature of the liquid alloy and the cooling rate for the initial solidification shell is decreased and uniformed,which further lead the initial solidification point moved towards the casting direction and stabilized the solidification process.With the effect of both the electromagnetic field and the Joule heat,not only the oscillation marks,but also the depression and the cracks on the billet surface were disappeared,that the surface quality was significantly improved.The oscillation mark depth decreased from 0.75 mm(without high frequency electromagnetic field)to 0.18 mm(with high frequency electromagnetic field)while the applied power is 66 kW with a frequency of 20 kHz.When the electric power is increased to 100 kW,the wavy-shape oscillation mark appeared.This is due to the electromagnetic force difference between the slit center and the segment center,which caused the variation of the initial solidification point.On this condition,the depth of the oscillation mark was increased from 0.18 mm to 0.32 mm.In this thesis,the effect of EMS on the internal quality in the compound EMCC technique was discussed.EMS can increase the equiaxed grain ratio of square billet(increase to 41.45%)and decrease the grain size from 10.83 mm to 1.28 mm.Furthermore,the dendritic segregation of element was reduced.The centerline shrinkage and the internal crack disappeared as well.The EMS can also inhibit the formation of TiN inclusions in the interdendritic region and decrease the number of the large TiN inclusion:the number is decreased from 371×10-4/μm2 to 1.59×104/μm2.In this thesis,for Incoloy800H alloy,three types of TiN inclusions were analyzed:large TiN,TiN cluster and the TiN strings,respectively.The results showed that the interface between the three type TiN inclusion and matrix often act as stress concentration sites to form internal cracks.Furthermore,some TiN clusters accumulate in the interdendritic region which resulted in the appearance of the shrinkage and the cracks by blocking the compensating channel of molten alloy.With EMS,the results clear showed that the number of the above mentioned three type of TiN inclusions were decreased and therefore,the rate of the crack appearance is decreased.Furthermore,the applied EMS can help to uniform the temperature field in the molten alloy,refine the grain size and reduce the degree of element segregation.These effects can help to avoid the appearance of internal crack for Incoloy800H alloy billet.