Effects of Fine Vanadium Nitride Precipitates on Internal Stress during High-Temperature Creep of 12%Cr-15%Mn Steels

The objective of the present study is to investigate the effects of fine precipitates of vanadium nitride on the creep deformation behaviors of 12%Cr-15%Mn austenitic steels through the measurement of internal stress and effective stress. These stresses were obtained by strain dip tests. The internal stress of VN free materials was determined as a unique function of applied stress. On the other hand, that of VN containing materials was not obtained uniquely as a function of applied stress. This indicates that fine precipitates of VN with high number density are strong barriers for the dislocation motion in spite of wide range variation of stress applied on the dislocations. At lower applied stress, the change of deformation rate during the strain dip test of the VN containing materials was very small and could not be detected with the increase of deloaded stress. Consequently this means that the forward or backward motion of almost all dislocations were inhibited by the precipitates. At higher applied stress, the VN containing materials showed the decrease of deformation rate with the slight deloading during strain dip test. This means that there exist movable dislocations which can glide without being blocked by the precipitates. The difference of stress dependence of minimum creep rate between at lower applied stress and at higher one was considered to be caused by such interaction of the fine precipitates with dislocation motion.