Aspects of Applying Hard Chromium Coatings on the Inner Surfaces of Components for Hydraulic Drives with Hydrostatic Guideways

We considered the processes of chromium plating the inner surfaces of the components of cylinders with hydrostatic guideways. We demonstrated the various aspects of the chromium plating process – the limitations placed on the machinery depending on the dimensions of the parts to be plated and the unevenness of the deposition rate of chromium along the length of the part. We developed a diagram of an installation for the application of a hard chromium coating on inner surfaces, which includes a cathode, an anode, a tank, and a pipeline. The distinctive features of the installation are the method of supplying the electrolyte at an angle to the horizontal, which allows us to supply the electrolyte through a turbulent flow swirling along a helical path, and the use of a chromium-plated installation casing. We studied various modes for applying a hard chromium coating on the inner surfaces of a hydraulic cylinder. As a result, we determined the optimal composition of the chromium electrolyte – the ratio of chromic anhydride and sulfuric acid – which is 10:1, respectively, and experimentally selected deposition modes. Chromium coatings obtained through the use of the concentrated electrolyte, which we developed, and the chromium plating method have 5...20% greater hardness and a 10...30-times reduction in porosity with the formation of a shiny, lumpy sludge, which corresponds to corrosion-resistant and wear-resistant coatings needed to manufacture hydraulic drives with highly efficient hydrostatic guideways.

Effect of High-Temperature Quenching on the Phase Composition, Structure, and Wear Resistance of Ni-Hard Chromium-Nickel Cast Iron

Metallographic, micro-X-ray spectral and X-ray structural analyses have been carried out. The quenching temperature increase from 900 to 1150 °C leads to high abrasive wear resistance in Ni-hard chromium-nickel cast iron. The initial hardness decreased before the test from 60 to 35 HRC due to the fact that the phase composition of the metal base before the wear test changes during quenching from medium-carbon martensite with secondary cementite to retained metastable austenite. It contributes to increase the crack resistance of cast iron. Austenite turns into dispersed friction martensite on the working surface in the process of wear testing or operation, as a result of the impact of abrasive particles. This martensite together with a part of cementite provides a high level of hardening and operability of the cast iron secondary microstructure.

Plasma Nitriding of Hard Chromium Electroplated Low Alloy AISI 4340 Steel in Pure Nitrogen Gas Atmosphere

Duplex treatments are surface modification techniques used to enhance the surface properties of steels. In this study, hard chromium electroplated AISI 4340 steel was plasma nitrided in pure nitrogen gas at 600°C for 90 min. The structural and morphological properties were analysed by an X-ray diffractometer and a scanning electron microscope (SEM), respectively. The hardness profile was obtained by a multicycle nanoindentation test and the tribological properties by a ball-on-disk test at room temperature. The results showed that the duplex layer was composed mainly of Cr2N and it had a rough surface morphology with a network of micro-cracks. The highest values of hardness were registered in the nitrogen diffusion layer beneath the outer porous thin layer. The friction coefficient and the specific wear rate after plasma nitriding were reduced by 3 and 5 factors, respectively.