THE MORPHOLOGY AND AGING BEHAVIOR OF DELTA-FERRITE IN A MODIFIED 9CR-1MO STEEL

Dual phase (martensite + delta-ferrite) microstructures were developed in a modified 9Cr-1Mo steel, by austenitising at 1523-1623 K, followed by water-quenching. These duplex structures were thermally aged at 973 K for ageing periods varying from 30 min to 21 h. Morphological aspects of delta-ferrite phase and its response to age-hardening were studied by optical, scanning electron and transmission electron microscopy, X-ray diffraction, electron probe microanalysis and microhardness testing. It was observed that austenitizing at 1523 K produced fine, acicular delta-ferrite while the delta-ferrite formed by austenitising at higher temperatures (1573-623 K) were massive, irregular-shaped and banded. Moreover the presence of delta-ferrite caused an abnormally strong (110) reflection, observed in X-ray diffraction patterns of martensite plus delta-ferrite structures. This behaviour is thought to be due to development of (110) texture in delta-ferrite phase. Thermal ageing at 973 K caused age-hardening of delta-ferrite with a peak hardness attained after 3.6 ks of ageing. Electron microscopic results suggest that the observed hardening was caused by the formation of Fe2Mo Laves phase.