EFFECTS OF PRECIPITATE SHAPE ON HIGH-TEMPERATURE STRENGTH OF MODIFIED9CR-1MO STEELS

A dispersion of precipitates introduces a threshold stress which oppos es deformation. The effect of precipitate shape on dispersion strength ening at elevated temperature has been examined. Three kinds of Mod.9C r-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 microstructur es except for the shape of precipitates. The threshold stresses were m easured by tensile and creep tests at 600 degrees C. Two sorts of effe cts have been observed. The first is that complex precipitates restrai n dislocation from climbing even in a condition in which dislocation c limbs over a spherical precipitate. Mobile dislocations are considerab ly trapped at the concavity of the V-wings during deformation. The sec ond effect is that the probability of dislocation trapping at V-wing i s larger than that at spherical Nb(C,N). These trapping effects play i mportant roles in the strengthening effect of precipitate.