レーザ表面溶融処理により形成されたW系高速度鋼の急冷凝固組織とその焼戻し挙動

概要

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Microstructure and tempering behavior of rapidly solidified layer formed by laser surface melting has been investigated in W-type high speed steels with different Co content. In SKH2 which contains no Co, rapidly solidified layer consists of two regions ; a lower region characterized by forming of δ-dendrites and interdendrite compounds of χ-phase, and an upper region in which dendrites have δ-core surrounded by martensite. The addition of Co suppresses the formation of δ-ferrite and leads to primary formation of meta-stable austenite. This results in formation of super-saturated martensite in lower region of SKH3 containing 4mass% Co and in whole region in SKH4 containing 9mass% Co. The increase of the amount of Co also changes inter-dendrite compounds to M_6C and MC type carbides. This change of microstructure caused by Co addition improves hardness of rapidly solidified layer. Subsequent heat treatment at 823K of rapidly solidified layer of SKH4 produces extremely highly secondary hardening to HV 1000, probably because of the increase of the amount carbides precipitation during tempering.
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