Discontinuous precipitation during isothermal transformation in a 9-12% chromium ferritic steel

Abstract

A study of discontinuous reaction in a 9-12% chromium ferritic steel was performed by means of many characterization methods, such as physicochemical phase analysis, electron backscatter diffraction, electron probe microanalysis. Isothermal aging treatments at 700C after solution annealing were carried out for the period over 216 h. Cr-rich M2N were formed as the main precipitates, together with a small amount of Cr-rich M23C6 and Nb-rich MN. With the increase of isothermal aging time, precipitates were observed to be preferentially nucleated at the prior austenite grain boundaries and grew toward the interior of grains with the morphology of pearlite, and eventually formed a lamellae structure. The characterization of carbide and nitride was presented in detail. Besides, the growth of the discontinuous reaction was also documented. It was found that the isothermal decomposition reaction of over-cooled austenite could be summarized as:-+Cr-rich carbonitrides. The growth of discontinuous precipitation involved the diffusion of chromium (short range) and nitrogen (long range).