Diffusion phenomena on rubbing surfaces in relation to wear resistance
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When steel parts are working under heavy-duty conditions, there is always a possibility of the formation of carbides on their rubbing surfaces as a result of decomposition of lubricants and subsequent reaction of carbon with the metal. This process is inevitably associated with local seizing and mutual material transfer. As a result, the carbide phase formed during friction betwen two steels of a different composition may differ from the adjacent material in respect to its content of carbide-forming elements. When strong carbide-forming elements are present in a steel with a friction-induced surface carbide layer, heating such a steel to relatively low temperatures leads to localized decarburization which, under certain circumstances, may reduce the wear resistance of steel. In a simple case of a binary alloy, the rate of decarburization depends on the affinity of the alloying element to carbon.
When this affinity in high, the decarburization rate may be slow because of a reduced intensity of the diffusion flux (due to reduced solubility and diffusion coefficients, and because of carbide formation in the contact zone), while elements with a lower affinity to carbon produce a more intense decarburization.
The above considerations do not apply to high-alloy steels in which the extent of carbide-forming elements is so high that practically all the carbon diffusing to the contact interface combines with these elements to form carbides. In this case the reduction in the coefficient of diffusion of carbon in steel does not play any substantial part because its diffusion path is relatively short. It is to be expected that in this case the decarburization rate will increase with increasing tendency of the alloying elements to carbide formation.
It should be pointed out that the dissolution of surface carbide layers takes place also in cases when the carbide layers are produced as a result of friction between similar alloy steels because a large proportion of the carbide-forming elements is dissolved in ferrite (i.e., is not combined in carbides), while the carbide layers contain large quantities of cementite. This means that the decarburization rate is determined in the first place by the chemical composition of the steel and not of the carbide layer.
Facts reported in this article will assist, when necessary, in a rational selection of the friction pair components with a view to reducing or eliminating the decarburization of the surface layers. Needless to say, experimental studies (using the above described methods) will have to be carried out in each specific case. It is also evident that data of this kind are not sufficient to make recommendations about selecting materials for any given friction pair, since all the other factors determining the wear resistance must be taken into account.
KeywordsCarbide Ferrite Wear Resistance Cementite Alloy Steel
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