Speaker
Dr
Ida Westermann
(Norwegian University of Science and Technology)
Description
Fracture analysis of a standard construction steel ship 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 ship 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 SEM.
Specimens of the ship deck material and a DOMEX S355 carbon steel were tensile tested in liquid nitrogen. Both steels showed a large 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 effect on the tensile behavior of the specimens.
A small amount of microcracks were found after holding a DOMEX S355 specimen at a constant force below the yield point. In a ship 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.
Author
Ms
Hanne Keseler
(Norwegian University of Science and Technology)
Co-authors
Prof.
Bjørn Holmedal
(Norwegian University of Science and Technology)
Dr
Ida Westermann
(Norwegian University of Science and Technology)
Mr
John Olav Nøkleby
(DNV GL)
Dr
Sastry Yagnanna Kandukuri
(DNV GL)
