Research On Stainless Steel Impeller By Vacuum Precision Casting With Electromagnetic Stirring

Cast stainless steel impeller has excellent mechanical properties and corrosion resistance, but has relatively poor casting properties. On the one hand, the stainless steel has high melting point, easily oxidized, a wide range of solidification temperature, poor fluidity, a large contraction, on the other hand, the impeller has complex structure and large differences in wall thickness. therefore, produced by common casting process, it has the coarse structure. If it work in poor working conditions, cracks easily occur in high-stress areas, such as blade roots.In order to study the process to improve the quality of the impeller, using the small impeller as study objects, the experiment combines the fine grain of electromagnetic stirring technology and vacuum casting technology, so as to explore and research a new process of stainless steel impeller by vacuum electromagnetic stirring precision casting technology., Based on the precision casting melting and casting are carried our in vacuum, to strictly control the alloy composition of impeller, and exert the electromagnetic stirring during the solidification process, to controlling the solidification structure. The result show that the main factors effecting the impeller forming are pouring temperature, mold shell preheat temperature and the time to start the electromagnetic stirring, for the complete impeller, the optimal conditions are preheating the shell molded at 1000℃incubated 30min, pouring temperature at 1590℃, opening the EMS 3s after pouring liquid metal. EMS can increase the feeding pressure on the impeller, so as to effectively modify the size and position of casting shrinkage and improve the utilization of liquid metal. Two-way EMS further homogenize the temperature distribution in liquid metal, so as to achieve the ingot solidification, eliminating the shrinkage porosity defects. The impeller by new process, with a new clear and complete outline and smooth surface, has no cracks, porosity, shrinkage and other casting defects, providing a technical reference for the impeller manufacturer.Modern material testing and analysis equipment, such as electrochemical workstation, optical microscope, scanning electron microscope, electron microprobe, are used to study and discussion the affection of the new process parameters on the microstructure and properties of the impeller, the results show that, appropriately increasing the stirring time, is benefit to improve the grain refinement. When the one-way stirring time was 8s, the equiaxed grains were evenly refined to the size of 25μm. Ferrite in the casting structure is about 5% and becomes short and scattered, with blunt end. EMS can improve the corrosion resistance of stainless steel, by refining the organizations and inhibiting the element segregation and carbide precipitation. Among them, the EPR reactivation rate of intergranular corrosion in 0.5mol/LH2SO4 solution decrease by 32.8%, from45.8% to 13.0%, the pitting potential in 3.5% NaCl solution increased by0.13517V, from 0.18105V to 0.31622V.