DIFFUSIONAL S URFACE A LLOYING O F C ARBON S TEEL

Russian Physics Journal. Vol. 39, No. 3, 1996 DIFFUSIONAL SURFACE ALLOYING OF CARBON STEEL O. V. Sizova, L. B. Zuev, and N. Ya. Kudryavtseva UDC 621.785 Research on the physicomechanical properties and structure of the strengthened layer obtained by diffusional alloying of carbon steel reveals the laws governing the surface-layer destruction of strengthened and unstrengthened steel in an abrasive and corrosional medium. Carbide layers obtained by multicomponent saturation are shown to have greater corrosion and wear resistance. One of the basic methods of protecting steel surfaces against wear in rigorous conditions - high pressure and aggressive media - is the application of wear-resistant coatings combining hardness, plasticity, and corrosional resistance. The most common method of producing such coatings is to saturate the steel with carbide-forming elements such-as chromium, titanium, and vanadium [1-3]. A range of surface-alloying methods corresponds to different working conditions of steel parts - for example, slipping with and without lubrication, contact with fixed and loose abrasive, or friction in an aggressive medium. In practice, this means that a strengthened layer that has good wear resistance in some conditions will be actively degraded in others, often simply as a result of variation in one or two of the operational parameters. Therefore, it is very important to understand the mechanical aspects of particular types of wear, the specifics of the structural-phase state of the surface, and its deformation as a result of contact interaction. The lack of systematic research on the structural features and physicomechanical properties of the coatings prevents the wide use of surface-strengthening methods. In the present work, the structure and properties of the strengthened layer obtained from carbon steel by diffusional alloying are investigated, along with its failure mechanism under the action of abrasive and an aggressive medium -- water containing chlorides, nitrides, and alkalis. Tests are conducted in the range of normal mechanochemical wear, i.e., wear controlled by the dynamic equilibrium of secondary-structure formation and destruction. The abrasive medium consists of silicon-dioxide particles of size 0.8-30 /~m combined into conglomerates. Wear occurs as the samples rotate in a tank of abrasive at a rate of 200-220 rpm. The corrosion resistance is determined, like the wear resistance, from the mass loss of the samples in definite time intervals. Comparative data on the corrosion resistance of strengthened layers are necessary since most steel parts operate in a wet medium, as a rule. The oxide films formed at the surface are flocculent and easily break down, exposing the underlying metal. Analysis of the data obtained permits the prediction of the operating capabilities of such surface- strengthened steel. Diffusional alloying is based on a powder mixture containing titanium, chromium, and vanadium (1:1:1). The micro- structure of the samples is studied on a NEOPHOT-21 metaliographic microscope and a TESLA BS-500 electron microscope. A DRON-5 diffractometer is used for x-ray structural analysis and determination of the composition of the strengthening phases. The microstructure of the samples after diffusional alloying is shown in Fig. 1. The diffusional surface zone is light and free from etching; this is characteristic of surface layers obtained in both one-component and multicomponent coatings [1]. The thickness of the light layer is -40#m and its microhardness is 28,000 MPa. X-ray structural analysis of the hard layer shows that the coating consists of a complex carbide based on iron and chromium and corresponds to the formula