Abstract
The structural changes of P91 steel after different heat treatment and M23C6 lattice expansion are well described by JMA kinetics law; however, the role of molybdenum on the M23C6 lattice expansion is not clearly revealed. The aim of the present work is to investigate the solubility of molybdenum in M23C6 lattice when iron or chromium atoms are replaced by molybdenum and to examine the effect of crystallographic structure changes on the mechanical properties of thermal aged P91 steel. Rietveld analysis of electrochemically extracted residues from the as-received and thermally aged at 600, 650 and 700 °C steel revealed that it is possible to measure and evaluate quantitatively the fraction of 8c crystallographic site occupation by molybdenum atoms of the M23C6 lattice. It was shown that the value of the site occupation factor plotted in natural logarithmic scale increases linearly and obeys Johnson–Mehl–Avrami kinetic law, giving Avrami exponent navg = 0.3356 and activation energy E = 272 kJ/mol. Hardness measurements of the aged samples indicate that the deterioration of properties is closely coherent to the growth of crystallite size.
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