Recovery and Precipitate Analysis of 9 Pct Cr-1 Pct MoVNb Steel during Creep.

The effect of tempering temperature and creep exposure on the microstructure of a modified 9Cr steel was investigated. Creep-interrupted specimens, including the grip portion, were investigated precisely using mainly X-ray and inductively coupled plasma (ICP) spectroscopy. After saturation of precipitation due to creep exposure, the amount of extracted residue decreased once and then increased within a short period (dip). Chemical analysis showed that during the dip, the precipitates temporarily dissolved into the matrix and precipitated again. The size of the Cr₂₃C₆ increased gradually during creep, but the growth rate was relatively small, as compared to the Ostwald ripening. The size of the VN particles in the specimens tempered at 800 °C in the early stage of creep was very fine, approximately 20 nm, and tended to decrease further with the progress of creep. The size variations of the precipitates and the dip were explained from the annihilation or migration of precipitation sites, i.e., dislocations and boundaries, during creep. Transient creep for the specimens tempered at 500 °C was controlled by a reduction of the mobile dislocation density. On the other hand, transient creep for 800 °C was due to precipitation hardening of fine VN particles with the progress of creep, which was supported by the increase in both the lattice strain and the activation energy with creep. [ABSTRACT FROM AUTHOR]