Study of the Austenitization Process in a P91 Steel

Abstract

It is well known that two main families of precipitated particles are present in 9Cr1Mo grade 91 steels after a standard manufacturing process that includes, as final stages, normalizing and tempering, i.e., M23C6 carbides (M = Cr, Fe, Mo) and MX carbonitrides (M = V, Nb, Cr). In previous works it was reported that samples of a T91 steel austenized at 1050oC for times between 0 and 40 minutes following heating at a rate of 50 oC/s exhibit a heterogeneous austenitic grain size distribution after austenite holding for 20 to 30 minutes from the austenite plateau start. Moreover, M23C6 dissolution and a change in the chemical identity of the MX precipitates (from major V-rich to major Nb-rich) occur during the first 5 minutes of austenite holding at 1050 oC. In this contribution the detailed evolution of second phase precipitates during the first minutes of holding in the austenite phase field for an ASTM A335 P91 steel, obtained by means of a high speed, high resolution dilatometer is studied and followed by scanning and transmission electron microscopy. M23C6 precipitates were not observed from the first minute of austenitization. MX precipitates change progressively their character from V-rich to Nb-rich. The observed diminution of the Ms, temperature values would be related to the dissolution of M23C6 carbides and V-rich MX carbonitrides.