Evolution of Crystallographic Structure of M 23 C 6 Carbide Under Thermal Aging of P91 Steel
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Evolution of Crystallographic Structure of M23C6 Carbide Under Thermal Aging of P91 Steel Aru¯nas Baltusˇnikas, Albertas Grybe˙nas, Rita Kriu¯kiene˙, Irena Lukosˇiu¯te˙, and Vidas Makarevicˇius (Submitted May 24, 2018; in revised form August 31, 2018) 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. Keywords
M23C6, P91 steel, Rietveld refinement, site occupancy, thermal aging, XRD
1. Introduction P91 steel was successfully implemented in power industry for boiler components as tubes and pipes due to its excellent long-term creep behavior at higher operating temperatures and pressures (Ref 1-3). The microstructure of P91 steel after proper normalizing and tempering consists of tempered martensite lath, block and packet, containing a high density of free dislocations and fine carbonitride precipitation inside the matrix and extensive carbide precipitation along the grain boundaries (Ref 2, 3). The main alloying element in P91 steel is chromium which is most important for solid solution strengthening and for attaining sufficient oxidation and corrosion resistance of steel. Molybdenum also contributes to the high-temperature strength of solid solution, but the content of Mo should not exceed 1% because it accelerates the growth of M23C6 carbides and it tends to form the Laves phase (Fe2Mo) which removes the element (Mo) from solid solution causing a loss of the steel strengthening. The properties of P91 steel are extremely sensitive to the microstructure, and the thermal treatment or creep exposure would produce the softening of solid solution (Ref 4, 5). The M23C6 carbides play an important role in increasing the creep strength of martensitic/ferritic 9-12% Cr steels by retarding the grain and subgrain growth. The M23C6 carbides containing Cr, Fe, Mo and W in the tempered P91 and P92
Aru¯nas Baltusˇnikas, Albertas Grybe˙nas, Rita Kriu¯kiene˙, Irena Lukosˇiu¯te˙, and Vidas Makarevicˇius, Laboratory
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