Microstructure And Properties Of New High Chromium Cast Iron With Wear And Corrosive Resistance Used In Desulfurization Slurry Pump

Desulfurization slurry pump is an important component of the desulfurizationsystem in thermal power plants, work under weak acid environment with corrosion of alow concentration chloride ions and abrasion of soft, hard particles. Existingdesulfurization slurry pump-used high chromium cast irons primarily present followingshortcomings: a mismatch between wear and corrosion properties, brittleness and poorcastability, make these materials cannot meet actual demand of producing, therefore, itis necessary to develop a new high chromium cast iron with good wear and corrosionresistance to overcome these shortcomings and reduce costs. Based on existing highchromium cast irons, by adding nitrogen, titanium and other alloy elements, a new highchromium cast iron with superior performance has been developed.First, by studying the effect of carbon and chromium on the hardness and corrosionresistance of high chromium cast irons as well as the impact of nitrogen on thecorrosion resistance of high chromium cast irons, determined the appropriatecomposition range of carbon, chromium and nitrogen, and thus developed a new wearand corrosion resistant high chromium cast iron used in desulfurization pump. Then,through the study of effect of nitrogen on as-cast microstructure and mechanicalproperties of the new high chromium cast iron, established links between microstructureand mechanical properties and ascertained effective tissue morphology which couldimprove the hardness and toughness of high chromium cast iron. Finally, by study theeffect of different heat treatment on hardness of the new high-chromium cast iron, seeka appropriate heat treatment technology to improve its hardness and evaluated itscasting performance, preparing for the applications of the new high chromium cast iron.According to the study,the as-cast hardness of high chromium cast iron mainlydepend on carbon and chromium, the relationship between them as follows:HRC=61.9-1.32Cr/C; the corrosion resistance of high chromium cast iron is closelyrelated to carbon and chromium, the use of low-carbon high-chromium component isconducive to improve the corrosion resistance of high chromium cast iron; addingnitrogen can significantly improve the corrosion resistance of high chromium cast iron,because nitrogen can increase the base electrode potential and enhance self-healingcapabilities of passivation film; finally developed a new kind of desulfurization slurrypump-used high chromium cast iron, the composition range of some main elements are:carbon content of1.2~2.3wt%, chromium content of23~27wt%, a nitrogen contentof0.1~0.6wt%.The microstructure and mechanical properties of of the new high chromium cast iron with different nitrogen content was analyzed and found that its as-castmicrostructure mainly consists of austenite, M7C3carbides and Ti2N titanium nitride;with increasing nitrogen content, the microstructure is significantly refined which mayhave something to do with titanium nitride can be acted as heterogeneous nucleation ofM7C3carbide; nitrogen can significantly improve the hardness and impact toughness ofhigh chromium cast irons because of nitrogen significantly improving theirmicrostructures, research shows that fine and well-distributed microstructure makes thenew high chromium cast iron have good hardness and toughness.Finally, by evaluating the heat treatment technologies and castability of thedesulfurization slurry pump-used high chromium cast iron,the results indicated thatafter air quenching at the temperature range of900~1000℃, the hardness is improved,increased from38HRC to46HRC; while after isothermal annealing, the hardnessincreased dramatically, up to about50HRC. What kind of heat treatment technology isselected to improved hardness of new high-chromium cast iron need to weigh theirimpact on the wear and corrosion resistance. After treated with air quenching andisothermal annealing, there were some tiny secondarys carbides dispersedlyprecipitating in the matrix and improve the hardness of high chromium cast irons. Thenew high chromium cast irons have good castability which meet demands of actualproduction. Although there is a certain hot tearing tendency, hot cracks can be avoidedthrough the use of appropriate cast processing measures.