Research On Stainless Steel For Impeller Used In Phosphoric Acid Slurry Pump

The composition of phosphoric acid slurry is very complicated. During the process of conveying slurry, the impeller of slurry bump suffers from the corrosion and the erosion wear brought by slurry in liquid-solid double states. After quite a long time of verification, the stainless steel with high content of carbon and chromium named Cr30stands out from so many kinds of alloys those are resistant of corrosion, because of its higher toughness, certain ability of corrosion resistance and expensive metal nickel saving from the perspective of economy. However, the Cr30has some disadvantages itself:in order to improve its ability of wear resistance, the content of carbon is very high, which leads to excessive consumption of chromium near the grain boundary and intergranular corrosion; the matrix of Cr30is ferrite with high brittleness, poor plasticity and low rate of finished products. Therefore, it makes important significance to change the distribution of carbide, improve the toughness, corrosion resistance and forming ability.This topic is implemented under the laboratory conditions. First, to get familiar with the organization and performance of Cr30,we find that the carbide becomes diffused with the appearance of austenite,then take measures of combining micro alloying, modification, austenitizing(with nitrogen and nickel) and heat-treatment to improve its properties and change the morphology of carbide, and test with the help of equipments such as video microscope, hardness tester, impact tester and SEM, XRD, etc. Electrochemical workstation LK2005A is also used to test the polarization curve.The results turn out to be:The K/Na modifier cannot change the distribution of carbide, neither can heat-treatment because of no phase changes during different temperatures, but modifiers can somehow refine the grain; when nitrogen is brought in, the grains get refined and carbide gets diffused, heat-treatment works and makes the carbide more diffused. There exits austenite in the matrix when nickel is used, and the carbide mesh begins to break. After nitrogen is brought in after nickel, the carbide is diffusive, and modifier works, making the matrix uniformed. At the same time, the tip of carbide becomes obtuse with the treatment of heat.Results analysis:Nitrogen can combine with chromium generating nitride which can dissolve with carbide. Besides, nitrogen can also work as the surface active element to gather near the grain boundary as the nuclei for the carbide, at the same time, nitrogen can low the diffusion coefficient of chromium, blocking the growth of carbide, leading to the growing numbers and diffusion of carbide without any obvious morphology changes. There exists austenite in the matrix after nickel is brought in, the morphology of carbide begins to change, and when nitrogen is used after, the austenite part becomes more and the morphology of carbide changes vastly. The modifiers can uniform the matrix, and heat treatment can makes the carbide more diffusive leading to high hardness.Performance test:the stainless steel with high content of carbon and chromium Cr30in the original state has a stiffness of22.7HRC, a toughness of2.1J/cm2, the critical potential of polarization curve is-0.544V, and the corrosion speed is2.1288mg/cm2·h, while the Cr30after improvement has a stiffness of42.9HRC, a toughness of22.6J/cm2, the critical potential of polarization curve is-0.352V,and the corrosion speed is0.11629mg/cm2·h.