| Laser remelting experiments have been studied on 20 carbon steel and SCrNiMo mould steel after boronization in this thesis. The effects of technological parameters on the depth and microhardness of treated cases are explored, and the alteration and correlative reasons of surface brittleness, structural morphologies and phases composition and the relations of these alteration to properties of anti-thermal impact and wear resistance are investigated by means of optical microscope, scanning electron microscope ( SEM ), X-Ray diffraction (XRD ) and property tests. Moreover, the mechanisms for improving surface brittleness and affecting surface properties are analysed. The results show as follow:(1) the depth of boride layer can be increased considerably by adjusting boronization agent, namely raising the percentage of B4C, to elevate the boron concentration in bolonization agent. The boride layers on the two steels are both up to 0.16mm-0.25mm depth, their hardness increase and homogeneity improves distinctly.(2) the two laser technniques gained tfrom the experiments can melt the boride layers completely into the remelted cases with maximum microhardness decreasing from 1274HV0.2 to 1098 HV0.2, which peaks occur in the subsurfaces duo to melting loss of alloy elements and cooling procedure of melting pool.(3) the surface relative brittleness decreases to 84.4%-81.6% when the boride layers of 20 steel are remelted, and laser remelting can lessen the brittleness of 5CrNiMo steel surfaces to a certain extent. The scheme L2 of laserremelting (laser power 860W, scanning speed 600mm/min, spot offset +7mm, overlapping ratio 18%, shielding gas 0.55Mpa argon) behaves better with the surface relative brittleness decreasing by 71.3% maximum.(4) the laser remelting has improved the anti-thermal impact property of the two steels because of postponing effectively microcracks yielding and prolongating, which provides hot moulds with a new technical access to extend life-span(5 ) the abrasive wear resistance properties on both surfaces of the two steels have been reformed obviously after laser remelting with relative wear resistance increasing by 23%-142%, the effect on 20 steel is more evident. The boride layer possesses abrasive wear with fracture mechanism in the highest flight, while the layer of laser remelting with slide friction wear and fatigue wear in the highest flight.The impact-wear resistance of the two steels has been improved to some extent and that of 20 steels has been improved more. The boride layer and the laser remelting layer possess different impact-wear mechanisms. The boride layer gives priority to intergranular brittleness fracture and transgranular fracture, and exhibits friction wear, bursting apart and surface desquamation, while the laser remelting layer only reveals friction wear.(6) within the area of laser remelting, intrinsic dentation structures of the two metals have been reformed completely and defects such as rarefaction and pore in boride have totally removed. The remelting area can be divided into three areas according to their fine morphologies which all compose martensite containing boron, Fe2B and multi-alloy phases.Laser remelting achieves decreasing surface brittleness effectively and increasing properties of anti-thermal impact and wear resistance to some extent mostly by means of alternating boride morphologies, eliminating FeB and defects within boride layer, improving the interface bonding intensity, fining the microstructures, lessening surface hardness properly, enhancing surface fracture toughness and redistributing boron concentration. The numerous multi-alloy phases within remelting area and strong support to the materials' surfaces by martensites gained during remelting process within transition area are also in favour of improvement of wear resistance properties.
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