PHASE-TRANSFORMATIONS AND MICROSTRUCTURE CHANGES IN A 12-PERCENT-CR-STEEL DURING TEMPERING AT 1053-K

The evolution of the microstructure and the phase transformations in a 12% Cr-steel during tempering at 1053 K for 30 s, 0.25, 1 and 2 h were studied. The investigations were carried out by transmission electron microscopy. For the identification of the different phases and the determination of their chemical composition electron diffraction and energy dispersive X-ray spectroscopy were used. Four different types of precipitates were found: MX, M3C, M2X and M23C6. They showed the following time dependent precipitation sequence: MX --> MX + M3C --> MX + M2X + M23C6 --> MX + M23C6. Changes in the chemical composition of the various precipitates were determined. The chemical composition of the M23C6 carbide was nearly constant, whereas it varied with time for the M2X and MX carbonitrides. Two different types of MX particles were identified: Ti,V-rich precipitates in the quenched and short-term tempered states and V,Cr-rich particles in the longer-term tempered states. Two types of orientation relationships between the ferrite and the M23C6 were observed: (101BAR)alpha(parallel-to)(1BAR1BAR1)M23C6 and (1BAR11)alpha(parallel-to)(110)M23C6.