Laser Surface Melting and Consecutive Point-Mode Forging Hardening of DH36 Marine Steel: Mechanical and Precipitation Behavior

Abstract

This study investigates the effect of the laser surface melting and consecutive point-mode forging process (LSM-CPF) on the mechanical properties and the microstructure of DH36 marine steel. The microstructual revolution during the LSM-CPF process are revealed by metallographic microscope (OM) and scanning electron microscope (SEM) technique, and the strengthening mechanisms for different samples are also elucidated. The results show that the best yield strength (σ0.2) and tensile strength (σb) for the sample treated with 4000 W laser power and 10% reduction ratio are 721.3 and 884.2 MPa, which are 49.55% and 41.54% higher than that of the DH36 matrix, respectively. The hardness of the coatings decreases along the normal direction with the maximum value of 586.4 HV in the CPF zone for the sample treated with 2000 W laser power and 20% reduction ratio. During the low power LSM-CPF treatment, the nanoscale cementite appear as intragranular due to the inhibited carbon diffusion. The coherent boundary of (110)NbC‖(110)Ferrite, [11¯0]NbC‖[001]Ferrite between NbC and ferrite reduces the nucleation barrier to promote the nucleation of acicular ferrite (AF). The strengthening mechanism for samples treated at 2000 W is found to be dislocation strengthening. During high power laser treatment, pearlite transformation is found to occur with a low cooling rate. In this case, the strengthening mechanism is the boundary strengthening of lamellar pearlite and dislocation strengthening.