Improvement of surface quality of 1.6220 cast steel by calibrating process and effects on material behavior

There is a growing demand for thin walled steel casting structural parts especially in the automotive industry. Therefore an economic casting process is needed, that provides high surface quality. Requirements of spot weldability and corrosion protection need to be met, which is not possible with high surface roughness. Therefore a calibrating step is introduced aimed at reducing the surface roughness of the sand cast steel samples. Besides surface quality, also changes in geometrical dimensions and mechanical properties are investigated. Calibrating was carried out as planar forging on a forging device. This combined process of casting and forging did lead to a significant reduction of the roughness values, but this effect could only be observed in the edge area of the investigated sample plates. Furthermore a deformation of material in thickness direction and flow of material in width direction was noted. A relationship between deformation in thickness direction and straightening was detected. Another one between material flow and reduction of roughness values. Tensile tests and impact tests revealed a more brittle behavior of the material in calibrated state. Also the influence of wall thickness and heat treatment was investigated. Whereas wall thickness (as the local amount of material) mainly determines deformability and in consequence the straightening effect, heat treatment increases the ability of the material to flow, which contributes to the reduction of surface roughness. The inhomogeneity of the surface effect in combination with an extraordinary high required force remain as the main issue in trying to achieve a uniform surface quality improvement by calibrating process.