Reduction pretreatment(RP)process,which is implemented after the billet is completely solidified,is an alternative process to further improve billet quality.Finite element method and experiment were used to investigat...Reduction pretreatment(RP)process,which is implemented after the billet is completely solidified,is an alternative process to further improve billet quality.Finite element method and experiment were used to investigate the effect of the RP process on the internal quality of 42CrMo billet.An ultrasonic testing technology was used to study the influence of the RP process on the micro-porosities.Through a three-dimensional finite element model,the mechanism of the RP process on the porosity closure was studied.The experimental results show that the RP process eliminates the macro-porosities of the billet center.The number of micro-porosities in the billets is effectively reduced,and the increase in deformation contributes to the reduction in micro-porosities.Compared with isothermal rolling,the RP process leads to a larger equivalent strain and hydrostatic integration in the billet center because of the temperature gradient,and the effect of the temperature gradient is greater with the increase in deformation.The closure effect of micro-porosity is related not only to the stress state,but also to the distribution of micro-porosity.The increase in three-dimensional compressive stress is beneficial to the porosity closure.If the micro-porosities distributed densely,excessive stress could induce micro-porosity coalescence,which has the risk of inducing cracks.展开更多
Centrifugal casting is a foundry process allowing the production of near net-shaped axially symmetrical components. The present study focuses on the microstructural characterization of centrifugally cast alloys featur...Centrifugal casting is a foundry process allowing the production of near net-shaped axially symmetrical components. The present study focuses on the microstructural characterization of centrifugally cast alloys featuring different chemical compositions for the construction of spheres applied in valves made of alloy IN625 for operation at high pressure. Control of the solidification microstructure is needed to assure the reliability of the castings. Actually, a Ni-base superalloy such as this one should have an outstanding combination of mechanical properties, high temperature stability and corrosion resistance. Alloys such as IN625 are characterised by a large amount of alloying elements and a wide solidification range, so they can be affected by micro-porosity defects, related to the shrinkage difference between the matrix and the secondary reinforcing phases(Nb-rich carbides and Laves phase). In this study, the microstructure characterization was performed as a function of the applied heat treatments and it was coupled with a calorimetric analysis in order to understand the mechanism ruling the formation of micro-porosities that can assure alloy soundness. The obtained results show that the presence of micro-porosities is governed by morphology and by the size of the secondary phases, and the presence of the observed secondary phases is detrimental to corrosion resistance.展开更多
文摘Reduction pretreatment(RP)process,which is implemented after the billet is completely solidified,is an alternative process to further improve billet quality.Finite element method and experiment were used to investigate the effect of the RP process on the internal quality of 42CrMo billet.An ultrasonic testing technology was used to study the influence of the RP process on the micro-porosities.Through a three-dimensional finite element model,the mechanism of the RP process on the porosity closure was studied.The experimental results show that the RP process eliminates the macro-porosities of the billet center.The number of micro-porosities in the billets is effectively reduced,and the increase in deformation contributes to the reduction in micro-porosities.Compared with isothermal rolling,the RP process leads to a larger equivalent strain and hydrostatic integration in the billet center because of the temperature gradient,and the effect of the temperature gradient is greater with the increase in deformation.The closure effect of micro-porosity is related not only to the stress state,but also to the distribution of micro-porosity.The increase in three-dimensional compressive stress is beneficial to the porosity closure.If the micro-porosities distributed densely,excessive stress could induce micro-porosity coalescence,which has the risk of inducing cracks.
文摘Centrifugal casting is a foundry process allowing the production of near net-shaped axially symmetrical components. The present study focuses on the microstructural characterization of centrifugally cast alloys featuring different chemical compositions for the construction of spheres applied in valves made of alloy IN625 for operation at high pressure. Control of the solidification microstructure is needed to assure the reliability of the castings. Actually, a Ni-base superalloy such as this one should have an outstanding combination of mechanical properties, high temperature stability and corrosion resistance. Alloys such as IN625 are characterised by a large amount of alloying elements and a wide solidification range, so they can be affected by micro-porosity defects, related to the shrinkage difference between the matrix and the secondary reinforcing phases(Nb-rich carbides and Laves phase). In this study, the microstructure characterization was performed as a function of the applied heat treatments and it was coupled with a calorimetric analysis in order to understand the mechanism ruling the formation of micro-porosities that can assure alloy soundness. The obtained results show that the presence of micro-porosities is governed by morphology and by the size of the secondary phases, and the presence of the observed secondary phases is detrimental to corrosion resistance.
