Microstructure Evolution in an Advanced 9 pct Cr Martensitic Steel during Creep at 923 K (650 °C)

An assessment of creep deformation and rupture behaviour of 9Cr–1.8W–0.5Mo–VNb (ASME grade 92) steel

“…2(a). The variations of creep strain with creep exposure exhibited the instantaneous strain on loading, distinct primary creep, narrow region of secondary creep and prolonged tertiary creep regimes at 923 K. Similar creep deformation behaviour has been noticed at 873 K and 973 K. Narrow region of secondary and prolonged tertiary creep regimes have generally been observed in Fe-9%Cr ferritic-martensitic steels [12,13,[20][21][22]. However, presence of clear primary creep regime was not observed under lower stress levels at 973 K ( Fig.…”

“…Stabilised creep rate occurs in the shorter secondary strain regime where the work hardening effect is counter balanced by the recovery process such as dislocation annihilation and rearrangement. Rapid increase in creep rate in the tertiary creep strain is attributed to enhancement in recovery process, growth of precipitates and cavities [12,13,22]. However, the decreasing creep rate behaviour from initial deformation was not clearly observed under lower stress level at 973 K ( Fig.…”

“…Stress exponent (n) values of 12 and13 have been reported in P92 type steels at 923 K [15]. The decrease in n values with increase in test temperatures occurred in the steel due to extensive microstructural modifications at higher test temperatures such as formation (Laves phase) and coarsening of precipitates (Laves phase, M 23 C 6 ), and decreased coherent strain between MX precipitates and matrix results in lower stress dependence of minimum creep rate [12][13][14][15]25]. The stress (s) and temperature (T) dependencies of creep rate of pure metals and single phase alloys are generally described by Bird-Mukherjee-Dorn (BMD) relationship as [26] …”

“…The steel is used in normalised and tempered condition and has tempered martensitic structure. The steel derives its high temperature strength from tempered martensitic lath structure, stabilisation of martensitic lath structure by chromium rich M 23 C 6 type of carbide, martensite phase transformation induced high dislocation density, intra-lath MX type of V and Nb-carbonitrides, and solid solution strengthening from tungsten and molybdenum [12][13][14][15]. Fine dispersion of highly stable MX type of V and Nbcarbonitrides particles are reported to enhance creep rupture strength of the tempered martensite steel not only acting as a barrier to dislocation motion but also by stabilisation of dislocation network.…”

“…Fine dispersion of highly stable MX type of V and Nbcarbonitrides particles are reported to enhance creep rupture strength of the tempered martensite steel not only acting as a barrier to dislocation motion but also by stabilisation of dislocation network. The presence of tungsten in the M 23 C 6 precipitate decreases growth rate of the precipitate during creep exposure, which in turn increases the stability of the martensitic lath structure of the steel on creep exposure [7][8][9][10][11][12][13][14][15][16][17][18][19].…”