EFFECT OF δ-FERRITE ON THE COMPOSITION AND VOLUME FRACTION OF PRECIPITATES IN P92 STEEL AGED AT 700 AND 750 °C

Using the as-received P92 steel pipe steel samples with/without delta-ferrite as objects and EPMA-EDS+MPST (multiphase separation technique), this work investigated the effects of delta-ferrite on the volume fraction and composition of M23C6 and Laves phases precipitated in the ageing treatment at 700 and 750 degrees C for 1000, 3000 and 5000 h, respectively. The results are as follows. The existence of delta-ferrite promoted aggregation and coarsening of M23C6 and Laves particles precipitated along boundaries with its neighboring lath martensite at the expense of consuming Cr, Mo and W in delta-ferrite during the ageing treatments. Varying rate of the volume fraction of M23C6 with the ageing time for the samples without delta-ferrite would chang at different ageing temperatures, and also the volume fraction of Laves phase would increase with decreasing ageing temperature for the samples with delta-ferrite. Owing to a few amount of Laves formed at higher temperature, the volume fraction of M23C6 increased in the samples with delta-ferrite. And increasing ageing temperature and time stimulated the coarsening of particles precipitated in the pipe samples investigated. It is expected that the related information from this study on the effects of existence of delta-ferrite on the microstructural stability and high temperature properties of P92 can be used as a reference when needed.