Study On Subcritical Treatment Technology And Microstructure Properties Of High Chromium Cast Iron Containing Vanadium Molybdenum

High chromium cast iron,as the key wear-resisting material of construction machinery such as cement machine,crusher and tray grate,is widely used in the face of corrosion,wear and other harsh working environment.Its performance determines the service life of components and the production efficiency of the machine.However,the as-cast high chromium cast iron often has some shortcomings such as poor toughness,insufficient corrosion resistance and high temperature performance,and poor wear resistance under some wear conditions,which makes it difficult to adapt to some harsh corrosion wear conditions.In order to further improve the comprehensive performance of high chromium cast iron to meet the requirements of more severe working conditions,it is often through alloying and heat treatment of as-cast.In this paper,four kinds of alloyed high chromium cast iron by adding V and or Mo on the basis of common high chromium cast iron composition were studied.The effects of addition of V and Mo on the as-cast structure and properties of high chromium cast iron were studied.Then the effects of subcritical(thermal)treatment on the structure and properties of as-cast high chromium cast iron were studied.By means of metallographic microscope,scanning electron microscope,XRD,hardness tester,nano indentation and wear tester,the structure and properties of high chromium cast iron were tested and experimentally studied,in order to obtain better performance of alloyed high chromium wear-resistant cast iron through appropriate alloying and optimized subcritical treatment process.The experimental study of cast high chromium cast iron shows that the structure of cast high chromium cast iron with different contents of vanadium and molybdenum is mainly composed of pre-eutectic dendritic solid solution and eutectic structure,and the phases are mainly austenite,carbide and martensite.The addition of Mo element increases the content of residual austenite,and the combined addition of V and Mo elements is beneficial to the refinement of dendritic structure and carbide,and improve the morphology and distribution of the carbide.Mo element is beneficial to improve the corrosion resistance,and the as-cast high chromium cast iron with more Mo has better toughness.The hardness of cast high chromium cast iron varies from 36.9 to 40.6 HRC under different vanadium and molybdenum contents,and the addition of V element is beneficial to improve the hardness.The results of subcritical treatment show that the XRD shows that the phase composition of high chromium cast iron does not change after subcritical treatment,but the content of the constituent phases changes obviously.The martensite transformation of the residual austenite is accompanied by precipitation of alloy carbide,and the microstructure is refined and the distribution of carbide is more uniform.After different heat preservation temperature and holding time of subcritical treatment,four different kinds of V,Mo content of as-cast high chromium cast iron are hardening peak appeared in the insulation at 550℃ temperature,when the temperature reaches 600℃ high chromium cast iron is a drop in the hardness of the phenomenon,high chromium cast iron containing Mo higher subcritical treatment hardening effect is best,maximum increase hardness,At the same time,the holding time needed to reach the peak hardness is also longer,but with the increasing of the holding time,the hardness of high chromium cast iron begins to decline.The subcritical treatment can eliminate the internal stress,refine the grain,refine the carbide,make the high chromium cast iron after the subcritical treatment greatly improve the hardness and not reduce the toughness too much affect the material properties.Wear resistance performance test shows that under the condition of as-cast high chromium cast iron has good anti-wear grain wear and erosion wear resistance performance,after 550℃after different heat preservation time of subcritical treatment,the relative wear resistance of high chromium cast iron presents the downward trend after rising first,wear-resisting performance can be in a peak time of heat preservation,including high chromium cast iron containing Mo more wear-resisting performance obviously.However,the time point of the peak value of the two kinds of wear resistance is not exactly the same as the time point of the peak value of the subcritical treatment hardness,but when the optimal wear resistance is obtained,a certain amount of residual austenite still needs to be retained in the tissue.The comprehensive tests show that the comprehensive properties of high chromium cast iron can be effectively improved by adding appropriate amount of V and Mo elements together with the optimized subcritical treatment.When the content of vanadium and molybdenum is about 0.3% and the subcritical treatment at 550℃ for 6 h,the comprehensive properties of high chromium cast iron are improved to a certain extent.However,when the content of vanadium and molybdenum is 0.3% and 0.7% respectively,the comprehensive performance of high chromium cast iron is significantly improved after subcritical treatment at 550℃ for 8 h.