Effect of heat input on microstructural characteristics and fatigue property of heat-affected zone in a FH690 heavy-gauge marine steel

Abstract

We demonstrated here the effect of welding heat input on the microstructure characteristics and fatigue behavior of the multi-pass welded joint of a FH690 grade ultra-heavy steel plate. The welded joint with a heat input of ∼15 kJ/cm exhibited higher ultimate fatigue stress (407 MPa at endurance limit of 10⁷ cycles) than that of ∼50 kJ/cm. Moreover, the welded joint with a heat input of ∼15 kJ/cm fractured within weld metal (WM), while it fractured within the coarse-grained heat affected zone (CGHAZ) with ∼50 kJ/cm. Microstructure characterization revealed that uniform lath bainite was obtained in CGHAZ by ∼15 kJ/cm heat input, whereas granular bainite with martensite-austenite (MA) constituents was obtained in CGHAZ with ∼50 kJ/cm. Transmission electron microscopy (TEM) observations showed that twin martensite was observed in MA constituents. TEM observations suggested that twin martensite played a significant role in fatigue cracking by three aspects: 1) directly crack at interface between MA constituent and matrix; 2) promote the formation of micro-voids within coarse granular bainite; 3) nano grains were observed surrounding the twin martensite near to the fractured surface, suggesting plastic deformation locally occurred in bainite lath, resulting in recrystallization and soften, which promoted crack initiation.