Technological Basis Of Near Net Shape Electroslag Casting TBM Cutter Ring In Composite Super-gravity Field

TBM(Tunnel Boring Machine)cutter ring is used as the "sharpest tool" of the shield machine and is widely used in hard rock tunnel's projects such as railway,pipeline and national defense.In the tunnel construction process,the TBM cutter ring is used very much,and the consumption cost of the cutter ring accounts for 10%to 20%of the total construction cost.At present,domestic TBM cutter rings do not have competitive advantages in terms of quality and price,which makes foreign TBM cutter rings occupy most of the market share,and the production process of foreign cutter rings is strictly confidential.The traditional production process of TBM cutter ring is smelting-ingot-casting-forging-ring rolling-rough machining-heat treatment-fine finishing-finished product,which is a long production process and a low material utilization.Therefore,under the premise of ensuring the wear resistance of the TBM cutter ring,the development of a low-cost TBM cutter ring production process has become an urgent problem for Chinese companies.The research idea of this subject is to use super-gravity technology to directly cast molten steel into a cutter ring,which shortens the production process of TBM cutter ring and improves the utilization rate of materials,thereby reducing the construction cost of tunnel engineering.Therefore,if the molten steel can be directly cast into a cutter ring,the production process of the TBM cutter ring can be shortened,and the utilization rate of materials can be improved,leading that the construction cost of the tunnel project will be reduced.Based on this,the production process of electroslag crucible smelting and near net shape electroslag casting cutter ring in super-gravity field is proposed in this paper.The compositions of the low-oxygen smelting slag in electroslag crucible is studied by ion-molecular coexistence theory and slag-steel interface reaction experiment.With the aid of electroslag casting experiments in super-gravity field,the effect of super-gravity parameters on the solidification structure,oxides and carbides of the cutter ring was studied.Combining the experimental data of abrasive wear and impact wear,the optimal parameters of the near net shape electroslag casting TBM cutter ring in super-gravity field are determined.The ultra-low oxygen content in the steel is one of the key factors that determine the quality of the cutter ring.Therefore,the influence of the compositions of the slag and the FeO content in the smelting process on the oxygen content in the steel of cutter ring was studied.The results show that the basicity(%CaO/%SiO2)and FeO content in the slag are important factors on the oxygen content in the cutter ring steel,and other components in the slag have little effect on the oxygen content in the steel.Increasing the basicity of the slag is beneficial to reduce the oxygen content in the steel,but when the basicity is greater than 2.0,it has little effect on the oxygen content in the steel.Under different conditions of the basicity,the influence of FeO content on oxygen content in steel is analyzed.The results show that when the basicity is constant,the oxygen content in the steel increases with the increase of the FeO content;but when the basicity and FeO content change simultaneously,the oxygen content in the steel is jointly affected by the basicity and FeO content.A constant supergravity field with a rotational speed of 500 r·min-1,a composite supergravity fields with three rotational speeds of 500 r·min-1+600 r·min-1+750 r·min-1,and a composite supergravity fields with five rotational speeds of 500 r·min-1+600 r·min-1+750 r·min-1+850 r·min-1+950 r·min-1 are designed to study the solidified structure of the cutter ring under different supergravity fields.It has proved the influence of the high gravity field alignment on the secondary dendrite spacing and the size of the original austenite.The results show that the increase of the rotational speed of the super-gravity field contributes to the reduction of the secondary dendritic average spacings and the original austenite grain sizes.Compared with cutter ring casting in the constant rotational speed super-gravity field,the solidification structure of the cutter ring casting in the composite super-gravity field with the stepwise increasing rotational speed is smaller.Among them,the composite supergravity field with five rotational speeds has the best effect on the refinement of the solidification structure,and the secondary dendrite average spacings and original austenite grain sizes can be reduced by 30%?40%.The distribution law of oxides and liquid carbides electroslag casting cutter ring in the super-gravity field is studied,and the influence of the rotational speed in the super-gravity field on the size distribution of oxides and primary carbides is analyzed.The results show that the removal of oxides is affected by both the movement time of oxides and the solidification time of molten steel.In the range of rotational speed studied in this paper,the removal effect of oxides in the composite super-gravity field with three rotational speeds is the best.According to the analysis of the experimental statistical results,it can be seen that the increasing rotational speed in the super-gravity field is beneficial to reduce the particle size of the primary carbides.Based on the thermodynamic model of the formation of primary carbides and the growth kinetic model,the influence mechanism of rotational speed is explored.Increasing the rotational speed in the super-gravity field is beneficial to reduce the segregation ratio of interdendritic elements,increase the solid phase rate of liquid precipitated carbides,and reduce the growth particle size of primary carbides.Combined with the test results of abrasive wear and impact wear,the optimal speed parameters of the TBM cutter ring near net shape casted by electroslag in super-gravity field are determined.The results show that the best wear resistance is the cutter ring casted in the three rotational-speeds composite super-gravity field,and the cutter ring produced with the process parameter has been used in underground construction projects in Guangxi,Zhejiang,and Xinjiang.Its wear resistance is no less than the cutter ring produced by traditional technology.The engineering application results show that the process of electroslag casting TBM cutter ring in composite super-gravity field with three rotational-speeds proposed in this paper is feasible,and the research results have successfully passed the evaluation of the scientific and technological achievements in the Chinese Institute of Metals.