Microstructural evolution mechanisms and microhardness of high-strength steel fabricated by underwater local dry laser cladding

In this work, the effects of overlap ratio and laser power on the formation quality of underwater local dry laser cladding were investigated, and an optimized processing window was established. Based on the optimized parameters, the effect of thermal cycles on microstructural evolution and microhardness was analyzed, while the underlying mechanisms were clarified through temperature field simulation. The results showed that surface flatness first improved and then deteriorated with increasing overlap ratio, reaching an optimum at 40%-45%. When laser power increased from 2.75 kW to 3.0 kW, surface flatness was further enhanced, and lack-of-fusion defects between adjacent layers were eliminated. Temperature field analysis revealed that the number and intensity of subsequent thermal cycles gradually decreased with increasing deposition passes. Consequently, the decomposition of lath martensite (LM) and the formation of polygonal ferrite (PF) were progressively suppressed. Microstructural characterization showed that the deposition zone, fusion zone, and overlap zone that experienced subsequent thermal cycles mainly consisted of lath martensite (LM), tempered martensite (TM), lath bainite (LB), and polygonal ferrite (PF), while granular bainite (GB) was observed only in the deposition zone. In contrast, regions that did not experience subsequent thermal cycles mainly consisted of LM and proeutectoid PF owing to the absence of tempering and recovery processes. As a result of the increased retention of hard martensitic structures, the microhardness gradually increased from the first pass to the sixth pass.