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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