Effects of heat input on microstructure evolution and corrosion resistance of underwater laser cladding high-strength low-alloy steel coating

The effects of heat input on the microstructure evolution and corrosion resistance of the high-strength low-alloy (HSLA) steel coating obtained by wire-feed underwater laser cladding were studied. Optical digital microscope, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscope (TEM), electron back-scattered diffraction (EBSD), electrochemical tests, laser microscope and other characterization methods were used. As the heat input increased, the smoothness of the underwater cladding coating improved, with little changes in elemental distribution and phase composition. However, the content of acicular ferrite increased, while the amounts of lath bainite, lath martensite, and granular bainite decreased. As the heat input increased from 0.4 to 0.55 kJ/mm, the grain size increased from 25.3 to 55.9 mu m, the proportion of low-angle grain boundaries rose from 61.3 % to 69.1 %, and the texture intensity of the (110) crystal plane increased from 16.22 to 23.83. Meanwhile, the dislocation density decreased from 4.9 x 1014 to 4.1 x 1014 m- 2. These changes enhanced the corrosion resistance of underwater coating. As the heat input increased from 0.4 to 0.55 kJ/mm, the corrosion current density decreased from 2.13 x 10-5 to 3.35 x 10-6 A/cm2, and the corrosion potential increased from -0.823 to -0.529 V. Additionally, the depth-towidth ratio of the corrosion pits decreased from 0.255 to 0.053. This study laid the foundation for high-quality in-situ underwater repairs of ships and submarines made of high-strength steel.