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.

Keywords: Leblond–Devaux equation; heat-affected zone; high heat input welding; marine steel; numerical simulation; phase evolution.

Figure 1

(a) Thermal-Mechanical Physical Simulation System; (b) schematic diagram of the sample during an experiment; (c) specimens’ geometry for the HAZ coarse grain zone experiment and SH-CCT experiment; and (d) heating and cooling profiles of SH-CCT experiments.

Figure 2

Optical microscope images of E36 microstructures with 11 different cooling rates as indi-cated during the thermal cycle of SH-CCT experiments. B stands for bainite, M for martensite, P for pearlite, F for ferrite, AF for acicular ferrite, and PF for proeutectoid ferrite.

Figure 3

Optical microscope images of E36Nb microstructures with 11 different cooling rates as indicated during the thermal cycle of SH-CCT experiments. B stands for bainite, GB stands for granular bainite, M for martensite, P for pearlite, F for ferrite, AF for acicular ferrite, and PF for proeutectoid ferrite.

Figure 4

Continuous cooling transformation curves of E36 and E36Nb, where F, P, M, and B represent the phase-formed zones for ferrite, pearlite, martensite, and bainite at a given cooling rate. (a,b) Experimental SH-CCT diagrams of E36 and E36Nb using a Thermal-Mechanical Physical Simulation System with the corresponding hardness measurements. (c,d) Simulated SH-CCT diagrams of E36 and E36Nb, respectively, using COMSOL software.

Figure 5

Predicted phase evolution for E36Nb with a heat input of 250 kJ/cm at different welding times as indicated. (a–c) Fraction of austenite; (d–f) fraction of ferrite; (g–i) fraction of pearlite; and (j–l) fraction of bainite.

Figure 6

Predicted phase evolution of selected points during thermal welding cycle by COMSOL software; (a,b) E36 with 100 kJ/cm and 250 kJ/cm heat inputs; (c,d) E36Nb with 100 kJ/cm and 250 kJ/cm heat inputs.

Figure 7

Phases in the coarse-grain zone in HAZ for E36 and E36Nb welded with different heat inputs of 100 and 250 kJ/cm. B stands for bainite, P for pearlite, F for ferrite, GB for granular bainite, AF for acicular ferrite, PF for proeutectoid ferrite.