Study of the Austenitization Process in a P91 Steel

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. (author)