Numerical modelling and experimental investigation of thermal and microstructural evolution of E36 marine steel during high heat input welding

In recent years, the size of steel structures has increased rapidly in the marine and shipbuilding industries, and high heat input welding has been developed to improve the efficiency of manufacturing process. However, during high heat input welding, the impact toughness of the heat-affected zone (HAZ) deteriorates seriously. In this study, the effect of thermal cycle and oxide metallurgy on microstructure evolution and mechanical properties of heat affected zone in high heat input weldments is investigated using a combined numerical modelling and experimental method.

A Finite Element Analysis (FEA) model is developed to investigate the effect of three different heat source paths, i.e., linear, sinusoidal, and oscillating-stop, on the resulting thermal evolution in thick steel plate welding. The developed FEA model is implemented for electro-gas welding of E36 marine steel plates with thicknesses ranging from 20 to 40 mm and heat input of 157 kJ/cm. The modelling results are validated against experimental observations, and then recommendations for choosing an appropriate heat source path in high heat input welding are proposed for different thicknesses of steel plates.

Based on the thermal modelling, the phase evolution of E36 and E36Nb marine steel using welding with a heat input of 100 kJ/cm and 250 kJ/cm is investigated, and the parameters in the Leblond-Devaux (L-D) equation and Koistinen-Marburger (K-M) equation for phase evolution of E36 and E36Nb steel are determined. A multiple-physics thermal-phase evolution FEA model is then developed and verified using experimental data. The thermal-phase model with the parameterized L-D and K-M equations can predict the amount of ferrite, pearlite, bainite and martensite in HAZ during high heat input welding.

Finally, the effect of oxide particles on microstructure evolution and mechanical properties of the heat affected zone of E36 marine steel is investigated. Strong de-oxidizing elements Ti, Mg, and Ca are added to E36 steel during steelmaking to obtain fine, dispersed and uniformly distributed oxide particles in the steel. It is found that oxide particles reduce the coarsening of parent austenite grains and increase the nucleation of ferrite during welding. Therefore, the mechanical properties of HAZ can be maintained during high heat input welding.