Gradient boride layers formed by diffusion carburizing and laser boriding

Laser boriding, instead of diffusion bonding, was proposed to formation of gradient borocarburized layers. The microstructure and properties of these layers were compared to those-obtained after typical diffusion borocarburizing. First method of treatment consists in diffusion carburizing and laser bonding only. In microstructure three zones are present: laser bonded zone, hardened carburized zone and carburized layer without heat treatment. However, the violent decrease in the microhardness was observed below the laser bonded zone. Additionally, these layers were characterized by a changeable value of mass wear intensity factor thus by a changeable abrasive wear resistance. Although at the beginning of friction the very low values of mass wear intensity factor I-mw were obtained, these values increased during the next stages of friction. It can be caused by the fluctuations in the microhardness of the hardened carburized zone (HAZ). The use of through hardening after carburizing and laser bonding eliminated these fluctuations. Two zones characterized the microstructure of this layer: laser bonded zone and hardened carburized zone. Mass wear intensity factor obtained a constant value for this layer and was comparable to that-obtained in case of diffusion borocarburizing and through hardening. Therefore, the diffusion bonding could be replaced by the laser bonding, when the high abrasive wear resistance is required. However, the possibilities of application of laser bonding instead of diffusion process were limited. In case of elements, which needed high fatigue strength, the substitution of diffusion bonding by laser bonding was not advisable. The surface cracks formed during laser re-melting were the reason for relatively quickly first fatigue crack. The preheating of the laser treated surface before laser beam action would prevent the surface cracks and cause the improved fatigue strength. Although the cohesion of laser bonded carburized layer was sufficient, the diffusion borocarburized layer showed a better cohesion. (C) 2014 Elsevier Ltd. All rights reserved.