A Parameterized Leblond–Devaux Equation for Predicting Phase Evolution during Welding E36 and E36Nb Marine Steels

Abstract

High heat input welding can improve welding efficiency, but the impact toughness of the heat-affected zone (HAZ) deteriorates significantly. Thermal evolution in HAZ during welding is the key factor affecting welded joints’ microstructures and mechanical properties. In this study, the Leblond–Devaux equation for predicting phase evolution during the welding of marine steels was parameterized. In experiments, E36 and E36Nb samples were cooled down at different rates from 0.5 to 75 °C/s; the obtained thermal and phase evolution data were used to construct continuous cooling transformation diagrams, which were used to derive the temperature-dependent parameters in the Leblond–Devaux equation. The equation was then used to predict phase evolution during the welding of E36 and E36Nb; the quantitative experimental phase fractions of the coarse grain zone were compared with simulated results to verify the prediction results, which are in good agreement. When heat input is 100 kJ/cm, phases in the HAZ of E36Nb are primarily granular bainite, whereas for E36, the phases are mainly bainite with acicular ferrite. When heat input increases to 250 kJ/cm, ferrite and pearlite form in both steels. The predictions agree with experimental observations.

Numerical Simulation of Temperature Evolution, Solid Phase Transformation, and Residual Stress Distribution during Multi-Pass Welding Process of EH36 Marine Steel

“…With the development of heat source models and simulation methods, numerical simulation enables accurately characterizing the temperature history, microstructure evolution, and residual stress distribution throughout the welding process. For example, Fu et al [24] implemented a parameterized Leblond-Devaux equation to predict the phase evolution during welding E36 and E36Nb marine steels, which agrees well with the experimental observations. However, the focus of numerical simulation research is limited to single-pass welding, which is insufficient for the welding requirements of thicker plates.…”

“…Then, the followed cooling process leads to the occurrence of diffusional transformations (pearlite, bainite, and ferrite) and diffusionless transformation (martensite) with the increase in cooling rate. The L-D model has been widely used to predict both diffusional and displacive phase transformations, demonstrating superior predictive accuracy and broad adaptability compared to the Zener-Hillert equation, Kirkaldy's rate equation, and the Johnson-Mehl-Avrami-Kolmogorov equation [24]. The Leblond-Devaux (L-D) equation is premised that the rate of the transformation is directly proportional to the extent of the deviation from the equilibrium, suggesting that the equilibrium fraction is asymptotically attained as time progresses.…”

“…The Leblond-Devaux (L-D) equation is premised that the rate of the transformation is directly proportional to the extent of the deviation from the equilibrium, suggesting that the equilibrium fraction is asymptotically attained as time progresses. The phase transformation model utilizes the L-D diffusion phase transition formula, expressed as follows [24]:…”

“…For the diffusionless transformation formulation, the phase transformation model employs the Koistinen-Marburger equation. This was selected as the coefficient formula [24]:…”

“…To estimate the accuracy of the current model that was established on the phase transformation parameters obtained from the SH-CCT curves, we first compare the numerical simulation results and experimental results from Fu et al [24]. The simulated microstructure evolution under varying temperature histories is shown in Figure 4.…”