Role of in-service conditions in operational degradation of mechanical properties of portal cranes steel

Abstract

Long-term service of steel structures causes a deterioration of the initial mechanical properties of metal and, therefore, decreases their workability. Sea portal cranes are operated in the regime of intensive cyclic loading that leads to exhaustion of steels plasticity. Therefore, these structures undergo essential degradation which reveals itself firstly in a decrease of brittle fracture resistance. Some units of 33 year operated crane are investigated from the point of a possible loss by rolled steel its initial mechanical and corrosion properties. A level of operational stresses in different units of the real crane measured by the strain gauge method varied somewhere by a factor of three. A strong dependence between the assessed level of in-service stresses and a drop of impact strength determined by Charpy testing is established. This indicates a crucial role of the factor of operational stresses in a decrease in brittle fracture resistance of the operated metal. Comparison of the test results for the specimens cut in mutually perpendicular directions confirmed the known regularity: impact strength of the transverse specimens relative to the rolling direction is lower than the longitudinal ones. This difference is revealed to become more pronounced with the build-up of metal degradation and, respectively, with an increase of operational stresses. It is shown that the level of in-service degradation of the metal along rolling direction rises with a reduction of the rolling plate thickness. It is explained by the aggressive action of marine atmosphere which caused not only corrosion of marine structures but also serves as a factor of steel hydrogenation and a development of microdamaging in metal that facilitates delamination in fibrous structure steel. Polarization resistance is defined for the tested units as the electrochemical parameter most sensitive to materials in-service degradation. A clear correlation between the mechanical and electrochemical metal characteristics is shown: the metal with a lower brittle fracture resistance has a lower polarization resistance. This relationship is suggested to use for non-destructive examination of metal state of portal cranes in a course of their long term service.