Discussion On The Main Controlling Factors Of The Formation Of Carbide Types In Chromium Alloy And Establishment Of Models

Although the chromium-based wear-resistant alloy developed in the 1970s had quickly replaced nickel hard cast iron with its excellent cost performance and widely used in mining,building materials,metallurgy and other industries,its insufficient strength and toughness seriously restrict its service life and application areas,which is the main weakness of chromium-based wear-resistant alloy.The type,shape and size of precipitation carbide during solidification of chromium-based wear-resistant alloys are the main factors affecting the lack of strength and toughness,especially the carbide type.It is well known that carbide types in chromium-based wear-resistant alloys include M₃C,M₇C₃ and M₂₃C₆ carbides.And it is widely accepted that the transformation relationship among the three different carbide types is determined by the Cr/C value,that is,as the Cr/C value increases,the transformation sequence of carbide types is M₃C→M₇C₃→M₂₃C₆.Obviously,the Cr/C value is influenced by both chromium and carbon,and judging the carbide types by this ratio merely is easy to ignore the respective effects of these two elements,which fails to reflect the essential influence of the supply of chromium and carbon atoms and the dynamic conditions of segregation degree on the precipitation type.Based on the limitations of traditional viewpoint that the carbide types in chromium wear-resistant alloys were determined by the Cr/C value,the aim of this study was to reveal the mechanism of recombination-reconstruction between different atoms and the influence of various elements on the formation of carbide types during the mutual transformation of M₃C,M₇C₃ and M₂₃C₆ carbides by analyzing the difference of carbide types in different Fe-Cr-C alloys and the transformation characteristics of carbide types before and after carburization and decarbonization of 3Cr13 stainless steel.（1）The formation conditions and transformation of the two kinds of carbides of M₃C and M₇C₃ are not determined by the ratio of Cr/C ratio.When a certain amount of C segregation is satisfied,M₇C₃ carbide is formed when Cr is sufficient.The insufficient supply of Cr forms M₃C carbides,which has little relationship with the thermodynamic stability of both carbides.In the two sets of samples with the same Cr/C ratio,it was found that when the Cr/C ratio values are basically the same,the types of carbides differ to some extent.（2）The formation conditions and transformation of the two carbides M₇C₃ and M₂₃C₆ are not determined by the Cr/C ratio.Under the threshold of certain chromium content,the formation and transformation are mainly determined by the C content,but not by the thermodynamic stability of both carbides.When C is sufficient,the M₇C₃carbide is formed or transformed.when C is insufficient,the M₂₃C₆ carbide is formed or transformed.（3）The three types of carbides,M₃C,M₇C₃ and M₂₃C₆,have their own lattice structure models and numerical requirements for the threshold value of each element.The M₃Ccarbide is a Fe₃C system lattice structure formed based on Fe and C atoms,and Cr atoms replace some of the Fe atoms as solutes.With the increase of the Cr atoms in the M₃C type carbide lattice structure,the ability of substitution of Fe atom for Cr atom in M₃C carbide system with Fe atom as the main structure will not increase all the time,but reach saturation at a critical value.The critical value measured is about 17.9%.When the degree of segregation of chromium content reaches about 28%,a new M₇C₃carbides appears.This type of carbide has Cr₇C₃ lattice structure formed by Cr-C component as the main atom,and Fe atom replaces the chromium atom as a solute to form the carbide.The M₂₃C₆ carbide is also a Cr₂₃C₆ lattice structure formed by the Cr-C component as the main atom,and is also a carbide formed by replacing the Cr atom with a Fe atom as a solute.However,the requirements for the C atom in these two types of carbides are different.The requirement for C content of M₇C₃ carbide is about8.8%,and the C content of M₂₃C₆ carbide is about 5.5%.The requirement for Cr content of these two types of carbides is basically the same,both of which are about 28%.