EFFECTS OF PRECIPITATE SHAPE ON HIGH-TEMPERATURE STRENGTH OF MODIFIED 9CR-1MO STEELS

A dispersion of precipitates introduces a threshold stress which opposes deformation. The effect of precipitate shape on dispersion strengthening at elevated temperature has been examined. Three kinds of Mod.9Cr-1Mo steels without Nb(C,N), with spherical Nb(C,N) and with complex Nb(C,N)+V(N,C) ''V-wing'' were designed to have the same microstructures except for the shape of precipitates. The threshold stresses were measured by tensile and creep tests at 600 degrees C. Two sorts of effects have been observed. The first is that complex precipitates restrain dislocation from climbing even in a condition in which dislocation climbs over a spherical precipitate. Mobile dislocations are considerably trapped at the concavity of the V-wings during deformation. The second effect is that the probability of dislocation trapping at V-wing is larger than that at spherical Nb(C,N). These trapping effects play important roles in the strengthening effect of precipitate.