Mechanical property and strengthening mechanism on DH36 marine steel after laser surface melting

In this paper, the mechanical properties and microstructure of various laser power surface melting (LSM) on DH36 steel were studied, the temperature gradient (G), cooling rate (η) and solidification (R) were simulated by modified Rosenthal solution and the strengthening mechanism for different laser power was also elucidated. The depth of the coatings exhibited a hike when increasing laser power (P), which is 1.24 mm, 1.41 mm, 1.70 mm and 2.47 mm, respectively. The G and η monotonously increased while the R was independent of laser power. The results showed the LSM process significantly improved the surface mechanical properties. The maximum yield strength and tensile strength were 495.7 MPa and 615.5 MPa. Microhardness of coatings decreased along the z direction with the maximum value is 448 Hv for 1000 W considering the appearance of acicular ferities (AF) in the coating. It indicated mainly adhesive wear for high laser power treatment with a shallow wear depth. Two kinds of strengthening mechanism were observed: second phase strengthening for 1000 W treatment and solid solution strengthening for 2500 W treatment. In the process of low laser power treatment, nanoscale cementies appeared intragranular resulted from the inhabited carbon diffusion. The coherent boundary of 022¯TiC∥022¯Cementite, [233]TiC∥[155]Cementite between TiC and nanoscale cementies reduced the nucleation barrier to promote the nucleation of AF. As for high laser power treatment, micro-alloy elements in DH36 steel traveled through oxidation reaction as large size oxide compounds in grain and on grain boundary.