DISTRIBUTION OF MX CARBONITRIDES AND ITS EFFECT ON CREEP DEFORMATION IN 9Cr-0.5Mo-1.8W-VNb STEEL

摘要

The effect of the coarsening behavior and precipitation sites of MX carbonitrides on creep behavior was investigated for P92(9Cr-0.5Mo-1.8W-VNb) steel by conventional TEM observations of replica films and by elemental mapping of thin films in energy filtered TEM(EF-TEM), respectively. The MX carbonitrides precipitate not only inside lath grains but also on lath boundaries, and pin lath boundary migration during creep. The stress exponent n of minimum creep rate decreases with increasing temperature. At 750°C, creep rupture strength abruptly decreases due to the decrease of the n value and the quick increase of creep rate in tertiary stage. The coarsening of MX carbonitrides at 650°C to 750°C corresponds to an early stage of Ostwald ripening and is accelerated by creep deformation. The decrease of stress exponent n with increasing temperature is caused by decreasing Orowan stress due to the coarsening of precipitates, M{sub}23C{sub}6, MX and Laves phase. At 750°C, the coherent strain between the MX carbonitrides inside lath grain and matrix significantly reduces at long term, leading to a quick increase of creep rate in tertiary stage.