Microstructure formation and precipitation in laser welding of microalloyed C–Mn steel

A 4-kW fiber laser was adopted to weld nano-scale precipitate-strengthened C–Mn steel (NPS steel). Only martensite was found in the fusion zone (FZ), fusion line and coarse-grained heat-affected zone (CGHAZ). The heat-affected zone (HAZ) consisted of both ferrite and martenite/austenite island (M/A island), which were found in both the fine-grained heat-affected zone (FGHAZ) and the mix-grained heat-affected zone (MGHAZ). In the FZ, original niobium titanium carbonitride and niobium titanium carbide precipitates were completely re-dissolved in the re-heating process, and only niobium titanium carbonitride re-precipitated in the subsequent cooling process. The original precipitates coarsened in heat-affected zone (HAZ) like Ostwald ripening phenomenon. Micro-hardness and yield strength of the FZ and the HAZ were higher than those in the base material (BM). Compared with the BM, the strengthening mechanism of the FZ and the CGHAZ were transformation strengthening and grain-refinement strengthening; precipitate strengthening. Transformation strengthening was the primary and important strengthening mechanism in the FGHAZ and the MGHAZ.