Fine Steel Structure after Microarc Molybdenum Steel Saturation

Introduction. In modern production, it is important to increase reliability and durability of steel products. One way to achieve this is by creating high-hardness, wear-resistant coatings on their surface. These coatings can be formed using the method of diffusion saturation, which involves the introduction of carbide-forming elements into the metal. Traditional methods for creating these coatings are time-consuming, taking up to 8 hours or more. To accelerate this process, researchers have proposed using high-energy methods such as laser and plasma treatments. However, these methods require specialized equipment that can be expensive. In this paper, we consider a method for creating a high-hardness molybdenum-based coating by microarc alloying. This method involves exposing a processed steel product immersed in coal powder to multiplicative microarc discharges that occur between the metal surface and the surrounding powder medium. The discharges are generated when an electric current is passed through them. This method allows for a significant increase in the process of diffusive surface saturation. It is characterized by simplicity and low energy consumption. The properties of the resulting coatings are primarily determined by their fine structure. Therefore, studying this structure is a crucial task. The aim of this research was to investigate the features of the fine structure of the steel surface layer after microarc molybdenum plating.

Materials and Methods. A coating containing finely dispersed ammonium molybdate powder and an electrically conductive gel as a binder in a volume ratio of 1:1 was used as a source of molybdenum for diffusion saturation. The coating was applied to the surface of cylindrical samples made of 20 steel with a diameter of 12 mm and a length of 35 mm. Then they were immersed in a metal container with a carbon powder with a particle size 0.4–0.6 mm. An electric current was passed through this powder for 6 minutes, with a surface current density of 0.53 A/cm². A Neophot-21 microscope, an ARL X'TRA-435 diffractometer, a ZEISS CrossBeam 340 scanning electron microscope with an X-ray microanalyzer, and a NanoEducator scanning probe microscope were used to study the fine structure of steel.

Results. After microarc molybdenum saturation of steel samples, a coating with a multilayer structure and a complex phase composition was formed. On the surface of the material, there was a slightly etched layer with a thickness of 50–55 µm, under which there was a carbonized layer with eutectoid structure and a thickness of approximately 200 µm, and the original ferrite-pearlite structure was preserved lower. The base of the slightly etched layer was a dispersed ferrite-carbide mixture containing about 47% wt. % of Mo and having a microhardness of 8–9 GPa. This layer contained carbide inclusions up to 5 µm in size, containing 94 wt. % of Mo and having microhardness up to 21 GPa. The surface relief was characterized by the presence of carbide inclusions of 3–5 µm in size, as well as multiple nanoscale inclusions protruding above the surface to a height of 10 to 150–200 nm.

Discussion. The results of the study, obtained using metallographic analysis, scanning electron microscopy, X-ray phase analysis and atomic force microscopy, showed that during microarc molybdenum steel saturation, a diffusion layer was formed containing nanoscale particles of the carbide phase. These particles reached a volume fraction of up to 70% and were located at the base of the layer. This layer was a ferrite-carbide eutectoid mixture. A quantitative assessment of the strengthening effect of these particles confirmed that the presence of such particles, characterized by high microhardness, determines the high hardness of the resulting coating.

Conclusion. Microarc molybdenum steel saturation is an effective method for creating coatings with exceptional performance characteristics. These coatings are characterized not only by their high hardness, due to the presence of nanoscale carbide particles located in a ferrite-carbide base, but also by their improved mechanical properties. This makes them promising for use in various industries where high wear resistance and durability of products are required. The research findings indicate that microarc molybdenum steel saturation significantly reduces processing time and avoids the use of expensive equipment, which makes it more affordable for industrial implementation.

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