オーステナイト系耐熱鋼25Cr-20Ni-1.3Nb焼結体の微細組織とクリープ特性に及ぼすW添加の影響

抄録

The effect of W addition on microstructure and creep properties of sintered 25Cr-20Ni-1.3Nb austenitic heat resistant steel is investigated. The 25Cr-20Ni-1.3Nb sintered compact had a structure in which fine spherical NbC was uniformly dispersed in the γ-Fe matrix grains and Cr₂₃C₆ was discontinuously precipitated in a thin film at the γ grain boundaries. In addition, the creep life of those compacts was lower than that of the cast material of the same composition. The addition of W resulted in higher creep properties than the cast material. The improvement in creep properties is due to the reduction in void ratio and grain growth. Fine W powders fill the pore between coarse powders of 25Cr-20Ni-1.3Nb and this leads to an increase in packing density. The increase in W addition caused a change in NbC precipitation from fine particles at intragranular to a film-like shape at grain boundaries. Therefore, the disappearance of fine NbC in the matrix grains, which had suppressed grain growth due to the pinning effect, leads to grain growth and improves the creep property.