Improvement of the fatigue performance of hydraulic cylinders under high pressure

In industry, structural parts generally operate under variable loads. Under these loads, after a while, cracks that generally start from the surface of the material grow and cause the material to fatigue and break even at stresses far below the yield strength. Fatigue is an important problem for the hydraulic cylinders operating under ultra-high pressures. Since fatigue generally starts at the surface, defects on the surfaces of materials exposed to variable loads may cause the material to fatigue earlier than expected. Therefore, various surface treatments such as burnishing and coating have been developed to minimize surface imperfections. One of the most common of these processes is the burnishing process. Roller burnishing is a process that makes the surface smoother by plastically deforming the roughnesses formed at the end of the pre-treatment and massing them on each other, without removing any chips from the material. With the burnishing of the roughness, residual compressive stresses occur on the surface and this ensures the delay of the crack that will occur on the surface. In this master's thesis, firstly, the effect of surface treatments applied to multi layerhydraulic cylinders operating at high pressures on fatigue life was investigated. Due to the problems occurred during the experiments, improving the surface properties and fatigue life of AISI 4340 material used in the aerospace and hydraulic industry, by means of the roller burnishing was aimed. The effects of the burnishing depth, feed rate and burnishing speed, which are the parameters of the roller polishing process, on the surface roughness, microhardness and fatigue life were investigated. The roller burnishing process parameters have been optimized for the lowest surface roughness and the highest fatigue life. In addition, it has been shown with 3D surface topography images that the tool marks formed in the turning process are reduced by the burnishing operation