Various methods have been introduced to predict postweld hardness of the heat affected zone (HAZ) for 9% Ni steel which is a primary steel adopted in the construction of liquefied natural gas (LNG) storage facilities. Two models were derived for the evaluation of the HAZ hardness, and then validated. The formulae developed in this investigation are sufficient to predict the hardness of the HAZ for 9% Ni steel . For the model using a rule of mixture, it is suggested that the morphology of martensite should be taken into consideration. Since the prediction of hardness depends on the calculation of the critical cooling time and hardness of microstructural constituents, a formula to estimate the hardness of martensite in HAZ was given. For empirical equation relating welding parameters, calculation results were found to give a fairly good description of the postweld HAZ hardness.

Effects of welding parameters on the weld bead appearance and mechanical properties in twin-wire tandem submerged arc welding of a high-strength low-alloy steel were investigated. Steel plates were welded using different combinations of welding current, voltage, inter-wire spacing and welding speed. The weld joints were characterized in detail by metallography. Hardness and tensile properties of the specimens were measured. The experimental analysis reveals that good weld appearance can be obtained for a range of inter-wire spacing from 55mm to 80mm. Leading wire current, trailing wire voltage and welding speed are the major factors affecting weld bead profile and microstructures in weld metal and fusion zone. Greater welding speed tends to produce higher cooling rate and favors the formation of strengthening phases like acicular ferrite and bainitic structures resulting in better mechanical properties.

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Based on welding heat transfer theory, heat elastoplasticity theory and phase transformation theory, welding residual stress in multi-layer welded joint of 9% Ni steel for liquefied natural gas (LNG) storage tanks was simulated using SYSWELD finite element analysis software. Taking into account nonlinear relationships between temperature and mechanical properties, together with influence of transformation latent heat on temperature field, double ellipsoid heat source model was applied in the simulation of the three fields (temperature field, microstructure field and stress-strain field). Distribution pattern of welding residual stress in 9%Ni steel welded joint was obtained through numerical simulation. Results showed that heat affected zone (HAZ) showing high-level residual stress was the main stress concentrated part of the welded joint. Longitudinal residual stress appeared to be tensile in the weld and HAZ, and compressive in regions remote from the weld. A match of low heat input and high interpass temperature should be employed to lower residual stress level in HAZ. Simulation results were well consistent with theoretical analysis.

The welding temperature field of twin-wire submerged arc welding (SAW) in an X80 pipeline steel welded joint was analyzed using a three-dimensional (3D) finite element (FE) model. Taking into account nonlinear relationships between temperature and mechanical properties, a coupled thermo-mechanical FE solution was used to obtain the temperature distribution for varying set of welding conditions. Effect of welding speed, inter wire spacing on welding temperature field were studied and presented. It is found that welding speed and inter wire spacing play a significant role in deciding the temperature distribution of twin-wire submerged arc welding. Simulation results were well consistent with theoretical analysis.