Precipitation Characteristics,Thermodynamics And Kinetics Of Carbides In Solidified Structure Of Chromium Alloys

The wear resistance of chromium-based alloys depends to a large extent on the morphology,size,and type of carbides in the microstructure.The type and characteristics of carbides mainly depend on the thermodynamic and kinetic conditions of precipitation.Therefore,this dissertation discusses the mechanism of the solidification and precipitation of chromium-based alloy carbides from the perspective of thermodynamic and kinetic conditions change and reveals the carbides by changing the compositional and cooling rates of Cr and C in chromium alloys.Form the characteristics of the type,size,shape distribution and evolution rules,in order to lay the technical foundation for further obtain high-performance chromium wear-resistant alloy structure.Research indicates:?1?The precipitation type of carbides in the solidification structure of chromium-based alloys is converted from M₃C type to M₇C₃ type or M₂₃C₆ type mainly controlled by Cr amount;M₂₃C₆ type and M₇C₃ type two types of carbides can be converted to each other and converted.The condition is controlled by the change in the content of C but the effect of Cr content is not significant.This point of view is completely different from the past that is always determined by the ratio of Cr to C ratio,Cr/C ratio.In the experiment,XRD analysis of low-carbon ferrochromium containing1.69%Cr and 58%Cr indicated that the carbide in the structure was M₂₃C₆;the medium carbon ferrochromium containing 3.5%C and 31%Cr was solidified.The type of carbide in the structure is M₇C₃ type.The energy spectrum analysis shows that the content of Cr in carbide is about 58%.The content of high-carbon ferrochrome containing 8.0%of C and 58%of Cr shows that the carbide type is mainly M₇C₃ type.The strong binding of Cr and C atoms during the formation of carbides in the solidification structure of chromium-based alloys was found by the energy spectrum analysis.The formation of M₂₃C₆ type carbides and M₇C₃ type carbides are all Fe-rich row of Fe.The biggest difference is the difference between them.The amount of C is different.?2?The type of carbide changes depends not only on the degree of thermodynamic stability,but also on the condition of the constituents to a greater extent.When the C content is sufficient,the M₂₃C₆ carbide will be converted into a more stable M₇C₃ when the conditions change.When the content is insufficient,the M₇C₃ type carbide will be thermodynamically relatively unstable and become M₂₃C₆ type.Therefore,the C atom concentration of the component conditions is the decisive factor that determines the mutual transformation of the M₇C₃ type carbides and the M₂₃C₆ type carbides.?3?The difference of cooling rate in the solidification process of chromium alloys changes the thermodynamic and kinetic conditions of carbide precipitation,and the degree of influence is closely related to the system components.Eventually,the precipitation morphology,size and distribution of carbides are affected to varying degrees.It was found in the experiment that for the chromium alloy No.4 sample with Cr content of 9%,the type of carbides in the solidified structure under the cooling of the sand mold is M₃C type and M₇C₃ type,and the carbides are in the shape of blocks,and the size is coarse;under the metal cooling,the morphology of the carbides does not change much,and the size is smaller than that of the sand mold;under water cooling conditions,the continuity of the carbides in the structure becomes weaker,and the size becomes smaller and the carbides are arranged in a strip shape.When the Cr content is17%,only M₇C₃ type carbides are formed in the solidified structure under sand cooling,and the carbide size is reduced and the shape is rod-like;under the condition of metal cooling,the change degree of carbide morphology is smaller than that of the sand type to the metal type when the amount of Cr is 9%,but the distance between the carbide is narrower and the length size is obviously smaller and the distribution is more uniform under the rapid water cooling.?4?The change of carbide morphology and size with cooling conditions is based on the change of thermodynamic conditions when nucleation grows.The faster the cooling rate is,the lower the precipitation temperature is,the higher the thermodynamic driving force in the nucleation is,the higher the nucleation rate is.And carbide is not only small in size but also more granular in morphology.In terms of dynamics,as the cooling rate increases,the degree of supercooling increases,the solidification temperature decreases,and the decrease in temperature causes the diffusion displacement speeds of different atoms in the system to become smaller to a certain extent,especially the C atom can not diffuse in time,which leads to stop growth of carbide because of carbon poor at the frontiers of solid-liquid interface.Therefore,the carbide size will decrease and be interrupted or granular.