Fractographic Substantiation of the Loss of Resistance to Brittle Fracture of Steel after Operation in the Marine Gantry Crane Elements
O. O. Nemchuk, H. V. Krechkovska
G. V. Karpenko Physico-Mechanical Institute, NAS of Ukraine, 5 Naukova Str., UA-79060 Lviv, Ukraine
Received: 05.11.2018; final version - 25.03.2019. Download: PDF
Portal cranes are operated for a long time in the intensive cyclic loading, which leads to the loss of the initial mechanical properties of steels, especially of their resistance to brittle fracture. The different stresses arise in different structural elements during the crane operation, and therefore it can be expected that the degradation degree of metal in these elements will also be different. The impact toughness tests using Charpy specimens (KCV), cutting from the St3sp sheet steel, after 33 years of operation in the responsible elements of the ‘Sokol’ type marine gantry crane, showed a significant decrease in the brittle fracture resistance of metal due to exposure of the higher operating stresses. The effect of steel texture on its operational degradation degree is investigated and a more intensive decrease in the impact toughness of transverse specimens (relative to the rolling direction) is shown. During the fractographic studies, the informative signs of the fracture surfaces of specimens with the minimum and maximum KCV levels, which correlated with the minimum and maximum (from the analysed variants) stresses in various elements of the crane during their operation, are compared. The areas on fracture surfaces directly behind the stress concentrators, corresponding to the stage of ductile nucleation of failure from the concentrator’s bottom, and the central parts on them, corresponding to subsequenting fracture propagation, are allocated. With use macro- and microfractography analysis, it is substantiated that the main feature of the operational degradation of steel is associated with its tendency to low-energy delamination along interfaces between non-metallic inclusions and ferritic matrix, which elongated along the rolling direction. This effect is maximally revealed, on the one hand, for steel used at higher operating stresses, and, on the other hand, for transverse specimens, when the main direction of the fracture propagation coincides with the rolling direction.
Key words: portal crane, operational degradation of steel, resistance to brittle fracture, delamination, hydrogen influence, fractography analysis.
PACS: 46.50.+a, 62.20.M-, 62.20.me, 81.40.Np, 81.40.Lm, 81.70.Bt
Citation: O. O. Nemchuk and H. V. Krechkovska, Fractographic Substantiation of the Loss of Resistance to Brittle Fracture of Steel after Operation in the Marine Gantry Crane Elements, Metallofiz. Noveishie Tekhnol., 41, No. 6: 825—836 (2019) (in Ukrainian)