| A W-surface-alloyed layer was obtained on alloy tool steel by the irradiation of CO₂ laser in order to improve wear-resistance and heat-resistance. Beneath the W-alloyed layer a solid-state transformed layer was formed and then was followed by matrix. W-concentration was homogeneous throughout the alloyed layer by well-interdiffusion of components. Behind the interface of liquid/solid, however, the solid-state diffusion of W was neglected. As the laser scanning speed was decreased normal grains were obtained near the laser-irradiated surface, and meanwhile dendritic structures were formed around the alloyed zone by rapid cooling. Many fine carbides were precipitated in grains, but they were not found in dendritic grains. But by increasing scanning speed fine dendritic structures were produced throughout the alloyed zone due to rapid cooling. As long as W-particles became larger, they were not completely dissolved and at the same time the size of retained W-particles became larger. As the laser energy density was increased and scanning speed was decreased, the width and depth of the surface-alloyed layer became wider and deeper. However, as the size of W-particles was increased, the width became wider but the depth was decreased. When the size of W-particles was small, the battiness of alloyed layer was almost constant due to the thorough dissolution of W-particles and the intermixing of components. As the size of W-particles became larger, the hardness was increased and resulted irregular in alloyed layer due to retained W-particles. As the W-particle size was increased and laser scanning speed was decreased, the hardness of solid-state transformed martensitic region increased. However, as laser scanning speed was decreased further below a certain threshold level, the hardness was decreased by tempering effect of martensitic structure due to the passage of heat in the liquid region. |
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