The evolution of precipitates of 22Cr–25Ni–Mo–Nb–N heat-resistant austenitic steel in long-term creep (original) (raw)
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A crept modified 2(1/4)Cr–Mo steel (T24) was investigated in order to understand differences in creep behavior which are related to different cooling rates after austenitizing (air-cooling and water quenching). Two 823 K creep stress levels were compared, 150 MPa (rupture times of the order of: 50,000 h) and 200 MPa (rupture times of the order of: 500 h). The air cooled specimens showed lower rupture times at 200 MPa (short term creep conditions) while the differences in rupture times were not pronounced at 150 MPa (long term creep tests). The minimum creep rates were always faster for the air cooled material state. Microscopic techniques, documenting length scales from the mm (optical microscopy) to the nm range (transmission electron microscopy) were used to study microstructures. Water quenching results in the formation of martensite while air cooling produces bainitic microstructures. After creep, the water cooled specimens still show a finer microstructure (tempered martensite which is equivalent to a fine bainitic structure) than the air cooled specimens (coarse bainite). This mesoscopic microstructural difference can be documented using optical microscopy and scanning electron microscopy, and accounts for the differences in minimum creep rates and rupture times. No significant differences were found when using transmission electron microscopy (TEM) to study the two microstructures (fine and coarse bainite) after creep. M 23 C 6 (on internal interfaces), MX precipitates (on internal interfaces and in the matrix) and M 6 C carbides were detected in both microstructures. TEM observations suggest that interactions between small and stable MX precipitates and dislocations provide the high creep strength. Long term creep conditions result in the formation of molybdenum-rich M 6 C carbides which were not observed after short term creep exposure. These M 6 C carbides form close to M 23 C 6 particles. Moreover, it was found that the Mo-content of the MX precipitates increases during long term creep. Crown
Creep behavior of a new precipitation-strengthened 15Cr–15Ni austenitic stainless steel with optimized content of Nb, C, N and Mo, subjected to a special multicycled aging-quenching heat treatment process, was investigated at a temperature of 750 1C and stress range of 78–200 MPa for up to 10,000 h. The steel exhibited excellent creep rupture strength which exceeds even that of the commercial NF709 alloy. The value of the creep exponent n at the applied testing temperature/stress conditions was found to be of around 5.6, demonstrating that dislocation creep is the dominant deformation mechanism during the performed creep tests. The crept microstructures showed the presence of a high number of copper precipitates, and of fine dispersion of densely distributed intragranular nano-sized (Nb,Cr)N nitride precipitates which transformed to more coarsening-resistant Z-phase precipitates with the prolonging creep exposure time. The nitrides, occurring in dominant quantities in the microstructures, showed superior coarsening-resistance during creep exposure while copper precipitates exhibited a relatively high coarsening rate. A comparison between creep rupture strength of the studied steel and of its commercial equivalent grade SUS 304 JI HTB revealed that the nano-sized (Nb,Cr)N nitrides and/or Z-phase precipitates essentially played the key role in the observed improved resistance to dislocation creep of the material. Considerations over the observed creep behavior and aspects of high-temperature microstructural stability such as coarsening of strengthening precipitates, issues related to phase transformation processes suggested that the steel in the present study exhibits capability for applications in USC fossil power plants with steam parameters of around 700 1C/35–40 MPa.
Journal of Materials Engineering and Performance, 2019
A modified 9Cr-1Mo steel has been exposed to three separate austenitization temperatures, i.e., at 950, 1025, and 1100°C for normalization. After subsequent tempering at 750°C, the normalized and tempered samples were creep-tested at temperatures of 550, 600, and 650°C. The creep strength of the investigated samples was evaluated in terms of minimum creep strain rate and time to rupture. The effects of microstructure, precipitate and boundary misorientation on the creep behavior of the samples have been studied with TEM and EBSD analyses. Further, the evolution of crystallographic texture after creep tests has also been studied. The presence of an intermediate size of martensitic microstructural units (i.e., prioraustenite grain, martensitic packets, etc.) and combination of fine coherent and incoherent Nb(C,N) precipitates has provided superior creep strength for the samples normalized at 1025°C, when these are subsequently subjected to low-temperature (i.e., 550°C) and high-temperature (i.e., 650°C) creep tests, as compared to other conditions of normalizing heat treatment.
Materials Characterization, 2015
In this paper, the high-temperature creep resistance and effects on the austenite reversion and the dynamic evolution of precipitates of maraging 300 steel were investigated. The main strengthening mechanism in a solution treated and aged material is the fine needle shaped Ni 3 (Ti,Mo) precipitates densely dispersed in a single martensitic phase. The specimens were submitted to creep tests at temperatures of 550, 600 and 650°C and stress conditions of 200, 300 and 500 MPa. Stress exponent (n) varied from 6.0 to 7.2 and activation energy for creep (Q c) from 364 to 448 kJ/mol, associated to the tangled and cells arrangements of the dislocations, show that the dominant creep mechanism is controlled by dislocations climb and slip. The experimentally determined threshold stresses are about 25 MPa at 550°C and close to 4 MPa at 600 and 650°C. Due to high-temperature creep exposure, part of martensite was reverted to austenite in a range of 17.2% to 48.5%, depending upon the time, temperature and applied stress. At the same time, the Ni 3 (Ti,Mo) precipitates were coarsened and Fe 2 Mo precipitated, leading to undesirable alloy's strength reduction. Volume fraction of reverted austenite showed strong negative correlation with hardness. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45°shear lip.
Creep strength breakdown and microstructure in a 9%Cr steel with high B and low N contents
Materials Science and Engineering: A, 2020
A 9%Cr-3%Co steel with high B and low N contents exhibits creep strength breakdown at a temperature of 923 K after 3000 h. Specific feature of this steel is a distinct difference between short-term and long-term creep regimes for transient, steady state and tertiary creep stages. This behavior is unusual for high Cr steels and attributed to low density of M 23 C 6 carbides precipitated on lath boundaries during tempering. Precipitation of Laves phase along these boundaries during transient creep followed by its coarsening affects significantly the creep mechanisms and results in the well-defined difference in mechanical behavior between the short-and long-term creep regimes. In contrast, the strain-induced formation of a small amount of Z-phase particles scarcely changes the creep behavior.
Precipitation Evolution in the Austenitic Heat-Resistant Steel HR3C upon Creep at 700 °C and 750 °C
Materials
HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be σ105700= 57.1 MPa and σ105750=37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), M23C6, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. M23C6 preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular M2...
Design and characterization of microstructure evolution during creep of 12% Cr heat resistant steels
Materials Science and Engineering: A, 2010
12% Cr heat resistant steels with a fine dispersion of nano precipitates were designed supported by thermodynamic modeling. A detailed characterization of the microstructure evolution at different creep times (100MPa/650°C/8000h) was carried out by scanning transmission electron microscopy (STEM). The results of the microstructure analysis are correlated with the mechanical properties in order to investigate the influence of different precipitates
Metals
Five Co-modified P92-type steels with different contents of Cr, W, Mo, B, N, and Re have been examined to evaluate the effect of the chemical composition on the evolution of Laves phase during creep at 650 °C. The creep tests have been carried out at 650 °C under various applied initial stresses ranging from 80 to 200 MPa until rupture. An increase in the B and Cr contents leads to a decrease in the size and volume fraction of M23C6 carbides precipitated during tempering and an increase in their number particle density along the boundaries. In turns, this affects the amount of the nucleation sites for Laves phase during creep. The (W+Mo) content determines the diffusion growth and coarsening of Laves phase during creep. Susceptibility of Laves phase to coarsening with a high rate is caused by the large difference in Gibbs energy between fine and large particles located at the low-angle and high-angle boundaries, respectively, and can cause the creep strength breakdown. The addition ...
Materials Science and Engineering: A, 2011
Ferritic-martensitic steels of the 9%Cr1%Mo type have been extensively used in power plant components, heat exchangers, piping and tubing, etc., due to an excellent combination of properties such as creep resistance, toughness and resistance to oxidation at high temperatures. In these steels the stabilizing role of MX carbonitrides (M = Nb, V; X = C, N) is one of the main factors responsible for the resistance under creep conditions. The control of precipitation and coarsening of MX phases during prolonged, high temperature tempering or post-weld heat treatment is then a key point to obtain the desirable microstructure and hence, to achieve high temperature resistance under service conditions. In the present contribution we report the evolution of the precipitated phases during heat treatment at 780 • C for increasing times in the range 40 min to 7 h for an ASTM A213 T91 steel. The Nb and V contents in solid solution were determined as a function of the time of treatment and maxima were observed for 5 and 5.66 h, respectively. Creep tests to rupture were also conducted at 600 • C-190 MPa for as-treated specimens. A maximum creep rate was observed to occur in coincidence with the maximum values of Nb and V contents in solid solution. We suggest possible relationships between the observed second phase evolution and the creep resistance behavior.
Microstructure Evolution During Creep of Cold Worked Austenitic Stainless Steel
IOP Conference Series: Materials Science and Engineering, 2018
The 14Cr-15Ni austenitic stainless steel (SS) with additions of Ti, Si, and P has been developed for their superior creep strength and better resistance to void swelling during service as nuclear fuel clad and wrapper material. Cold working induces defects such as dislocations that interact with point defects generated by neutron irradiation and facilitates recombination to make the material more resistant to void swelling. In present investigation, creep properties of the SS in mill annealed condition (CW0) and 40 % cold worked (CW4) condition were studied. D9I stainless steel was solution treated at 1333 K for 30 minutes followed by cold rolling. Uniaxial creep tests were performed at 973 K for various stress levels ranging from 175-225 MPa. CW4 samples exhibited better creep resistance as compared to CW0 samples. During creep exposure, cold worked material exhibited phenomena of recovery and recrystallization wherein new strain free grains were observed with lesser dislocation network. In contrast CW0 samples showed no signs of recovery and recrystallization after creep exposure. Partial recrystallization on creep exposure led to higher drop in hardness in cold worked sample as compared to that in mill annealed sample. Accelerated precipitation of carbides at the grain boundaries was observed during creep exposure and this phenomenon was more pronounced in cold worked sample.