Thermal ageing process at laboratory scale to evaluate the lifetime of Liquefied Natural Gas storage and loading/unloading materials

Cryogenic materials used for the manufacture of the tanks and the piping systems for Liquefied Natural Gas (LNG) need to remain tough, ductile and crack resistant with a high level of safety. To evaluate the degradations of the mechanical properties of the 9% Ni steel, the 304L stainless steel and the weld metal of the 9% Ni/9% Ni and 304L/304L weld joints, submitted to thermal cycles (cryogenic temperature/room temperature), tensile tests and small punch tests were carried out. Then, the fracture surfaces were observed by scanning electron microscope to identify the fracture modes. X-ray diffraction analysis was employed to reveal phase changes which could promote the degradation in the mechanical properties of these materials. From all the results obtained with mechanical tests after 3000 thermal cycles, no degradation or decrease of mechanical properties has been revealed. The Scanning Electron Microscope observations are in agreement with a ductile fracture behaviour. X-ray analysis results confirm the absence of phase change in these materials. Therefore, it seems that the influence of accelerated thermal ageing at very low temperature on mechanical properties of these materials is negligible.