Selecting a method of surface hardening for thin-walled repair bushings

One of the effective ways to restore the serviceability of the “shaft-seal” joint is the use of thin-walled repair bushings. When installed on shafts, they restore the joint tightness without disassembling the unit in shortest time and field conditions. The team of authors have produced thin-walled repair bushings with a wall thickness of 0.25 mm. They have made research to choose a surface hardening method for thin-walled bushing to achieve a microhardness of above 21,000 MPa. This value exceeds that of quartz abrasive and aluminum oxide, which are the main components of dust. Having analyzed the methods of surface hardening, the authors approved the applicability of electrolytic chromium plating and physical vapor deposition of coatings (the PVD method) for thin-walled repair bushings. Thin-walled bushings were surface hardened by electrolytic chromium plating (Хтв 21 (Cr hard)) at a temperature of 45 to 55°C, titanium nitride (TiN) coating was obtained by the PVD method at a temperature of 200 to 450°C. The coating thickness was measured with a Konstanta K5 electromagnetic thickness gauge used for protective coatings. The microhardness of the hardened surface was determined with a PMT-3M device. The roughness of hardened working surfaces of thin-walled repair bushings was measured with a MarSurf M400 profilometer (a surface finish gauge). After surface hardening of thin-walled bushings, the thickness of chrome coating was 21to 30 microns, and that of titanium nitride - 4.2 to 5.2 microns. Electrolytic chromium plating resulted in a microhardness of bushing surface of 11,000 to 12,000 MPa, while the PVD method provided a microhardness of 18,900 to 21,700 MPa. The profile diagram of the hardened working surface of a bushing showed that the roughness value decreased by one degree of fineness. The research results have proved the applicability of the chosen methods of hardening thin-walled repair bushings with a thickness from 0.25 mm.