Using the welding parameters to improve the mechanical properties of Liquefied Natural Gas storage tank welded joint

Abstract

The increase demand for LNG as alternative of conventional oil fuel and due to the risk assessment of LNG tank . The increase of focus has been put on the performance of welded LNG tank . Where the improvement of 9%Ni steel plates mechanical properties is still continued by steel making to reduce the cost of fabrication of LNG storage tank and to give more safe structure. These applied by development of heat treatment process that change from quenched till reach to double quenched and tempered . The problem in welded joint is to be more match to mechanical properties of base metal especially with no facility of applying PWHT process . The only way to enhanced the mechanical properties of welded joint by controlling the parameters of using welding process. From the main variables of the arc welding process are the heat input and interpass temperature where the two variables control the thermal cycle of welding process. But there are limitations on the heat input and interpass temperature. The limitations of the welding parameters are greater as the strength of the parent metal increases. Where using the too high heat input as well as high interpass temperature that produce welded joint with low tensile strength, low hardness and low toughness mechanical properties. Although the low heat input or low interpass temperature increase the susceptibility of existing hydrogen crack. The combination of welding parameters, notably interpass temperature, are identified for SMAW process of a 9% Ni steel plates to improve the zones of welded joint strength and toughness properties. Results showed that for thin test specimen , the heat input range from 1.4 to 2 Kj/mm and controlling interpass temperature within 80° C give high tensile strength and improve the toughness properties of welded joint and the ductility improve reduce the probability of brittle fracture happened.