Abstract
Fracture analysis of a standard construction steel platform deck, which had been exposed to a liquid nitrogen spill, showed that the brittle fracture started at a flaw in the weld as a consequence of low-temperature embrittlement and thermal stresses experienced by the material. In the present study, the permanent effect of a cryogenic spill on the fracture properties of carbon steels has been investigated. Charpy V-notch impact testing was carried out at 0 °C using specimens, from the platform deck material. The average impact energy appeared to be below requirements only for transverse specimens. No pre-existing damage was found when examining the fracture surfaces and cross sections in the scanning electron microscope. Specimens of the platform deck material and a DOMEX S355 MCD carbon steel were tensile tested immersed in liquid nitrogen. Both steels showed a considerable increase in yield- and fracture strength and a large increase in the Lüders strain compared to the room temperature behavior. A cryogenic spill was simulated by applying a constant tensile force to the specimens for 10 min, at -196 C. Subsequent tensile tests at room temperature showed no significant influence on the stress-strain curve of the specimens. A small amount of microcracks were found after holding a DOMEX S355 MCD specimen at a constant force below the yield point. In a platform deck material tensile tested to fracture in liquid nitrogen, cracks associated with elongated MnS inclusions were found through the whole test region. These cracks probably formed as a result of the inclusions having a higher thermal contraction rate than the steel, causing decohesion at the inclusion-matrix interface on cooling. Simultaneous deformation may have caused formation of cracks. Both the microcracks and sulphide related damage may give permanently reduced impact energy after a cryogenic exposure.
Highlights
Liquid nitrogen is a widely used cryogenic liquid, which has a boiling point of approximately -196 °C
Cryogenic liquids need to be kept at very low temperatures, their storage and transportation set specific demands for the materials used in the storage tanks
No particular difference is observed between the stress-strain curves, except that the upper yield point gradually disappeared for the DOMEX S355 MCD material when the tensile force applied under cryogenic conditions was increased
Summary
Liquid nitrogen is a widely used cryogenic liquid, which has a boiling point of approximately -196 °C. Another study concerning the low-temperature tensile properties of carbon steel was made by Goodenow and Bucher [5] in 1969 They confirmed the results seen by Owen et al, that tensile fracture of carbon steel at temperatures down to -196 °C occurred after the yield point had been reached. They defined a plastic instability transition temperature, TPI, as the temperature below which inhomogeneous plastic deformation would continue after the specimen had yielded. The combination of low-temperature embrittlement and thermal stresses experienced by the material during the cryogenic exposure had resulted in failure of the platform deck. The material was subsequently examined to see if any permanent damage had resulted from this treatment
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