Nb-rich MX Research Articles

Precipitates change of P92 steel under 3.5 MeV Fe13+ ion irradiation at 400 °C

9–12% Cr ferritic/martensitic steels are promising candidate materials for applications in fourth generation nuclear reactors. 9%Cr ferritic/martensitic steel P92 was normalized at 1050 °C for 0.5 h and subsequently tempered at 765 °C for 1 h. The precipitates of the normalized-and-tempered steel after 3.5 MeV Fe13+ ion irradiation at 400 °C to 0.66 dpa were studied using transmission electron microscopes. After irradiation, the number of precipitates increased, especially for the number of large-sized precipitates, and there was an increase in the overall size of precipitates. Pre-existing Cr-rich M23C6, Nb-rich MX and V-rich MX precipitates in the normalized-and-tempered steel still existed in the irradiated steel, and the M23C6 precipitates remained almost unchanged in their composition. Irradiation-induced Nb-rich M6C5 carbide with a base-centered monoclinic structure, V-rich M2X (V2C type) carbonitride with a simple orthorhombic crystal structure, and Fe71Cr26(Mo,W)3 (σ-FeCr type) intermetallic compound with a simple teragonal crystal structure were identified in the irradiated steel. The formation of these irradiation-induced phases is also discussed.

Mechanisms for improving tensile properties at elevated temperature in fire-resistant steel with Mo and Nb

Fire resistance of structural steels used in buildings is typically evaluated based on the ratio of the yield strength at 600 °C to that at room temperature. Fire-resistant steel shows a yield strength ratio of more than 2:3 between 600 °C and room temperature and achieves an excellent high-temperature strength. Alloying elements such as Mo and Nb are used to improve the strength at elevated temperatures. In this study, the effects of the addition of Mo and Nb on the fire resistance of steel were carefully investigated using SEM, TEM, in-situ TEM, EPMA, 3D-APT, positron annihilation lifetime spectroscopy, and first principles calculations. The fire resistance in Mo and Nb added steel (Mo + Nb) was shown to be drastically improved compared to plain carbon steel (CMn). The improvement in the high-temperature strength in Mo + Nb was attributed to the precipitation of fine Nb-rich MX particles and a solid solution of Mo and Nb hindering the dislocation movement, and thereby minimizing the dislocation annihilation at elevated temperature. The solid solution of Mo and Nb lowers the vacancy formation energy and allows a vacancy to form more easily, leading to a large lattice distortion which results in a slower dislocation mobility in Mo + Nb.

Influence of weld thermal cycle and post weld heat treatment on the microstructure of MarBN steel

Martensitic steels strengthened by Boron and Nitrogen additions (MarBN) were developed for high temperature/high stress service in power plant for periods of many years and are being considered as a promising candidate for the replacement of the more conventional Grade 91/92 steels. In the present study, extensive microstructural observation of physically simulated Heat Affected Zone (HAZ) MarBN material has been carried out after dilatometry simulations to link the variation in microstructure with weld thermal cycles. The microstructure in the MarBN HAZ has been observed to vary from a refined equiaxed morphology to a duplex microstructure consisting of refined grains distributed on the pre-existing Prior Austenite Grain Boundaries (PAGBs) as the peak temperature of the weld thermal cycle decreases. The temperature range corresponding to the formation of the duplex grain structure coincides with the temperature regime for the dissolution of the pre-existing M23C6 carbides. An even distribution of the M23C6 carbides within the martensitic substructure was also observed after Post Weld Heat Treatment (PWHT), which is beneficial for the creep performance of the weld HAZ. The MX precipitates are more resistant to thermal exposure and are not completely dissolved until the peak temperature reaches 1573 K (1300 °C). The Nb-rich MX precipitates are the predominant type observed both after weld simulations and PWHT. The hardness between the materials experienced with the thermal cycles with different peak temperature does not significantly vary after PWHT conducted in an appropriate condition, which is likely to mitigate an unfavoured stress condition in the localised area within the HAZ.

Precipitate Phases in Normalized Ferritic/Martensitic Steel P92

Precipitate phases in the P92 F/M steel after normalization at 1323 K (1050 °C) for 30 min followed by air cooling to room temperature have been investigated by transmission electron microscopy (TEM). Three types of phases consisting of M3C, MX and M2C have been identified. Needle-like Fe-rich M3C carbide, (Fe0.87Cr0.13)3C, has a simple orthorhombic lattice, and a size of 7-18 nm in short axis and 73-190 nm in long axis. Sphere-shaped Nb-rich MC carbide with a f.c.c. crystal structure has a chemical formula of about (Nb0.69V0.15Fe0.1Mo0.06)C and a size of 12 to 88 nm in diameter. Large-sized Fe-rich M2C phase exhibits a block-like shape, and has a hexagonal crystal structure. TEM examination indicated that the number density of both the Fe-rich M3C carbide and Nb-rich MX phase is higher than Fe-rich M2C carbide phase.

Creep properties, creep deformation behavior, and microstructural evolution of 9Cr-3W-3Co-1CuVNbB martensite ferritic steel

Creep deformation behavior and microstructure evolution of G115 steel were systematically investigated for temperatures of 625–675°C under uniaxial tensile stress of 120–220MPa. The relationship between minimum creep rate and applied stress followed the Bird–Mukherjee–Dorn (BMD) equation. The modified BMD equation was proposed using threshold stress to elucidate the actual creep deformation mechanism. The values of the threshold stress were determined to be 177.8, 91.4 and 87.6MPa at 625, 650, and 675°C, respectively. The true creep activation energy and the true stress exponent were 275.76kJ/mol and 6, respectively. Thus, the dominant creep deformation mechanism was identified as dislocation climb. Three types of precipitates can be revealed after creep deformation: W-rich Laves, Nb-rich MX, and Cu-rich phases. The creep damage of G115 steel after creep deformation was characterized by martensite cracks and martensite fractures owing to the hardness and brittleness of the lath martensite structure. Further, a dense array of deep and equiaxed dimples appeared in the central region of fracture surfaces under the tested creep conditions. Ductile fracturing was the main fracture mechanism during creep deformation.

Microstructure evolution and fracture mechanism of a novel 9Cr tempered martensite ferritic steel during short-term creep

In this work, the microstructure evolution and fracture mechanism of a novel 9% chromium tempered martensite ferritic steel G115 were investigated over the temperature range of 625–675°C using uniaxial creep tests. The creep curves consist of a primary transient stage followed by an apparent secondary stage, and an accelerated tertiary creep regime. The relationship between the minimum creep rate and the applied stress followed Norton's power law. Based on the EBSD analysis, there were no obvious textural features formed after creep deformation, and with the increase in creep time, the number of subgrains slightly increased, and then sharply increased, indicating dynamic recrystallization (DRX) occurs after creep deformation. In addition, three types of precipitates can be observed after creep deformation: W-rich Laves phase, Nb-rich MX, and Cu-rich precipitates. The Nb-rich MX with a square shape and Cu-rich precipitates with an ellipsoidal shape remain very stable. However, the W-rich Laves phases distributed mainly on the grain boundaries have rod-like, chain-like, and bulky shape, which are coarsened significantly. Representative fractographs of the G115 steel after creep deformation exhibit significant necking with an elliptical shape. A dense array of deep and equiaxed dimples appear in the central region under the tested creep conditions. Ductile fracturing is the dominant fracture mechanism during short-term creep deformation.

Effects of two different types of MX carbonitrides on austenite growth behavior of Nb-V-Ti microalloyed ultra-high strength steel

The size evolution of two different types of MX carbonitrides and its influence on austenite grain growth behavior of Nb-V-Ti microalloyed ultra-high strength steel has been investigated. Based on microstructure examination, the spherical Nb-rich MX is much finer than the cuboidal or rectangle shape Ti-rich MX. The average size of austenite grain and two types of MX carbonitrides sluggishly increased with temperature in the low temperature region. However, a rapid growth has been observed at relative high austenitizing temperatures due to the dissolution and coarsening of precipitates. A grain growth model was developed in view of the pinning effect of different types of carbonitrides. The pinning contribution of two types of MX carbonitrides was calculated using pinning parameter equation, and the pinning effect of Nb-rich MX is much larger than that of Ti-rich MX. This suggests that fine Nb-rich carbonitrides play a more predominant role in retarding austenite grain growth.

Microstructural aspects of superior creep resistance of a 10%Cr martensitic steel

The microstructural evolution and the dispersion of secondary phases were studied in a low-nitrogen 10%Cr martensitic steel with 3% Co and 0.008% B additives at 650°C under an applied stress of 140MPa. It was demonstrated that the superior creep strength of this steel can be attributed to the high resistance of M23C6-type carbides and Nb-rich MX carbonitrides against coarsening, resulting in a stable of the tempered martensite lath structure (TMLS) under short-term creep conditions. The TMLS remains slightly changed under creep: lath coarsening occurs with a two-fold decrease in the lattice dislocation density. M23C6-type carbides were found to give the main contribution in hindering the transformation of interlath boundaries to subgrain boundaries, impeding the migration of low-angle boundaries by exerting a large pinning pressure. A high Zener drag pressure is maintained up to rupture. The precipitation of a Laves phase under creep conditions results in a minor contribution to the overall pinning pressure. V-rich MX carbonitrides tend to dissolve with increasing time. No formation of a Z-phase was detected. M23C6 carbides retain their orientation relationship with ferritic matrix up to rupture. No significant strain-induced coarsening of M23C6 carbides, Laves phase, or MX carbonitrides was observed.

Effect of W on tempering behaviour of a 3 %Co modified P92 steel

The tempering behaviour of two 9Cr–3Co–0.5Mo–0.2 V–0.05Nb–0.005B steels containing 2 and 3 wt% W was examined. The decomposition of retained austenite induces the formation of semi-continuous films of W-rich M23C6 carbides along boundaries at 525 °C. The increase in W content from 2 to 3 wt% modifies the normal carbide precipitation sequence in the vicinity of these boundaries. The formation of W segregation at prior austenite grain/interlath boundaries leads to formation of the thermodynamically metastable W-rich M6C carbides. Finally, these M6C carbides transform to a stable Laves phase. Two-phase separation of MX carbonitrides into Nb-rich and V-rich dispersoids appears in both steels after tempering at 750 °C. This separation is thermodynamically metastable and attributed to the precipitation of Nb-rich MX carbonitrides with a round shape on dislocations at T ≤ 450 °C and V-rich MX carbonitrides within ferritic matrix at 750 °C. Moreover, in the steel with 2 %W, the additional separation of V-rich MX carbonitrides into W-rich particles located in the vicinity of the prior austenite grain/lath boundaries and W-free ones located within the interiors of laths was observed.

Precipitate Phases in an 11% Cr Ferritic/Martensitic Steel with Tempering and Creep Conditions

Precipitates in an 11% Cr ferritic/martensitic steel containing Nd with tempering and creep conditions were investigated using transmission electron microscope with energy-dispersive X-ray spectroscopy. The precipitates in the steel with a tempering condition were identified to be Cr-rich M 23C6 carbide, Nb-rich/V-rich/Ta–Nb-rich MX carbides, Nb-rich MX carbonitride, and Fe-rich M 5C2 carbide. Nd-rich carbonitride, which is not known to have been reported previously in steels, was also detected in the steel after tempering. Most of the Nb-rich MX precipitates were dissolved, whereas the amount of Ta-rich MX precipitates was increased significantly in the steel after a creep test at 600 °C at an applied stress of 180 MPa for 1,100 h. No Fe2W Laves phase has been detected in the steel after tempering. (Fe, Cr)2W Laves phase with a relatively large size was observed in the steel after the creep test.

Precipitation of Nanosized MX at Coherent Cu-Rich Phases in Super304H Austenitic Steel

The present investigation of transmission electron microscopy reports the precipitation of nanosized and cubical-shaped incoherent Nb-rich MX at the coherent Cu-rich phases in the austenitic matrix of the Super304H steel. In addition, the nanosized Nb-rich MX phases were often observed to precipitate on dislocations during creep. It is concluded that the dense incoherent Nb-rich MX and coherent Cu-rich precipitates with a nanosized diameter contribute excellent creep resistance in the steel.

Characterization of precipitates in X12CrMoWVNbN10-1-1 steel during heat treatment

The characterization of precipitates in X12CrMoWVNbN10-1-1 steel during the heat treatment was carried out for revealing the evolution of the precipitates. In addition to other microstructural parameters (such as dislocation and subgrains), the precipitate also plays an important role for microstructural stability which is a prerequisite for long term creep strength. In this paper, the precipitates during the heat treatment for this steel were characterized using physicochemical phase analyses and transmission electron microscopy. It was found that the Fe-rich M3C carbides and Nb-rich MX particles were detected in the samples cooled in furnace from austenitization at 1080°C for 16h. However, after water cooling, only Nb-rich MX particles existed. During tempering at 570°C for 18h, the formation of Cr-rich M7C3 was detected but was replaced partially by Cr-rich M23C6. Additional Cr-rich M2N nitride was also found. After two successive tempering (570°C+690°C) for 24h, Cr-rich M7C3 was completely replaced. The microchemical analyses of the extracted residues during heat treatment were also discussed. The results gave rise to an indication that the precipitation of precipitates nearly completed in first tempering and the transformation from Cr-rich M7C3 to Cr-rich M23C6 mainly occurred in the second tempering.

9Cr-3W 강의 크리프 특성에 미치는 Mo 첨가의 영향

The effect of the Mo addition on the high temperature creep properties of the 9Cr-3W steel was also evaluated. Two experimental steels, (9Cr-3W and 9Cr-3W-0.5Mo), were prepared using a vacuum induction melting process, followed by hot rolling and heat treatment processes. Three types of precipitates, (<TEX>$M_{23}C_6$</TEX>, Nb-rich MX and V-rich MX) were observed in a typical tempered martensitic matrix. Significant effects of the Mo addition on the tensile properties were not observed. However, the creep properties at <TEX>$650^{\circ}C$</TEX> under applied stresses of 140 and 150 MPa were considerably enhanced by the Mo addition. The microstructural observation after the creep test indicated that the addition of Mo could function to retain the recovery of the martensitic matrix, thus resulting in the enhanced creep properties of the 9Cr-3W-0.5Mo steel. Furthermore coarsening of the <TEX>$M_{23}C_6$</TEX> carbides and formation of Laves phases were observed in both samples after the creep tests.

The precipitation strengthening behavior of Cu-rich phase in Nb contained advanced Fe–Cr–Ni type austenitic heat resistant steel for USC power plant application

Copper has been used as a strengthening element in newly developed Fe–Cr–Ni type austenitic heat resistant steel for inducing Cu-rich phase precipitation to meet high temperature strength requirement for 600°C Ultra Super-Critical (USC) coal fired power plants for many years. However, the precipitation behavior and strengthening mechanism of Cu-rich phase in these advanced austenitic heat resistant steels is still unclear. In order to understand the precipitation strengthening behavior of Cu-rich phase and to promote high strength austenitic heat resistant steel development, 18 Cr9 NiCuNb steel which is a Cu-added Nb contained advanced Fe–Cr–Ni type austenitic heat resistant steel has been selected for this study to be aged at 650°C till to 10,000h. Micro-hardness and room temperature tensile test were conducted after long-time aging. SEM, TEM, HRTEM and three dimensional atom probe (3DAP) technology accompanying with thermodynamic calculation have been used to investigate the Cu-rich phase precipitation behavior during 650°C aging. The experimental results showed that Cu atoms can quickly concentrate in clusters at very early precipitation stage to form the fine nano-size Cu-rich “segregation areas” within less than 1h at 650°C. With increasing aging time at 650°C Cu atoms continuously concentrate to Cu-rich segregation areas (clusters) and simultaneously other kinds of atoms such as Fe, Cr and Ni diffuse away from Cu-rich segregation areas to austenitic matrix, and finally to complete the transformation from Cu-rich segregation areas to Cu-rich phase. However, there is only Cu atoms concentration but not crystallographic transformation from early stage of Cu-rich clusters forming to the final Cu-rich phase formation. Even the Cu atom becomes the main composed element after 500h aging at 650°C the Cu-rich phase still keeps coherent relationship with austenitic matrix. According the experimental results in this study, Cu-rich phase precipitation sequence which starts from the Cu atom segregation followed by the Cu diffusing from matrix to segregation areas and Fe, Cr and Ni atoms diffuse out from Cu-rich areas to matrix without crystallographic transformation is proposed. The Cu-rich phase is the most dispersed phase and contributes the most important strengthening effect among all precipitated phases (M23C6, MX and Cu-rich phase). It has been found that Cu-rich phase is very stable and still keeps in nano-size even for 10,000h aging at 650°C. The unique precipitation strengthening of Cu-rich phase in combination with nano-size Nb-rich MX phase and grain-boundary M23C6 carbide contributes excellent strengthening effect to 18 Cr9 NiCuNb austenitic heat resistant steel.

Electron Microscopic Characterization of the Microstructural Evolution of 9-12% Cr Steels upon Aging

In this study, the microstructural behavior of two kinds of 9-12% Cr steels (specimen A and B) used for power plants with and without W, Co, B which were subjected to aging during various time and temperature were investigated by electron microscope and related analytical techniques. The results indicate that there are four kinds of precipitates in these materials, i.e., M23C6, Nb-rich and V-rich MX, W-rich and Mo-rich Laves phase, and Z-phase. Upon aging, the area fraction of M23C6 increased whereas that of Laves phases decreased despite an increase in size. The area fraction of W-rich Laves phase was much higher than that of Mo-rich Laves phase, indicating that W addition compared to that of Mo addition is more powerful in the formation of Laves phase precipitation (specimen A). Specimen A aged at 700°C exhibited the formation of Z-phase. The formation of Z-phase is known to have a tendency of dissolving MX precipitates in similar alloys. The high activity of Cr at 700°C is believed to be the cause of Z-phase formation.

Effects of the fabrication process parameters on the precipitates and mechanical properties of a 9Cr–2W–V–Nb steel

The effects of the fabrication process parameters such as a tempering temperature, cold rolling and annealing condition on the precipitates and mechanical properties of a normalized 9Cr–2W–V–Nb steel were evaluated. Nb-rich MX precipitates were found in the specimen tempered at 550 °C while M 23C 6, Nb- and V-rich MX ones were observed in the specimen tempered at 750 °C. A cold rolling and an annealing at 750 °C of the specimen tempered at 550 °C induced the formation of large inhomogeneous M 23C 6 carbides, causing a reduced tensile strength. However, the cold rolling of the specimen tempered at 750 °C provided fine precipitates due to a fragmentation of some of the M 23C 6 carbides, and an annealing at 700 °C for 30 min was found to be suitable to recover the degraded mechanical properties from a cold working.

Effect of cobalt on the microstructure of tempered martensitic 9Cr steel for ultra-supercritical power plants

The effect of Co on the microstructure of advanced 9Cr–3W–0/5Co–0.2V–0.05Nb–0.08C–0.05N steel, designed for application at highly efficient ultra-supercritical power plants, has been systematically studied, using specimens containing 0%, 1%, 3% and 5% Co. Calculation by Thermo-Calc suggested and observation of microstructure after heat treatment confirmed the presence of about 6% and 0.4% of δ-ferrite in Co-free and 1% Co steel, respectively. Coarse primary Nb-rich MX particles preferentially form inside δ-ferrite, as according to Thermo-Calc Nb solubility in the body-centered cubic phase is approximately double of that in the face-centered cubic phase. Detailed examination of precipitates was conducted on samples after heat treatment. The total amount of precipitation increases with increasing Co content, which is especially significant between 1% and 3% Co steels. Precipitate density around prior austenitic grain boundaries is highest for 3% Co steel, and lowest for 1% Co steel. The mean diameter of precipitates does not differ significantly.

Effect of Ausageing on the Precipitation Behavior of MX Carbonitride in Modified 9Cr-1Mo Steel

The precipitation behavior of MX carbonitride during a normalizing heat treatment with and without ausageing was investigated in a modified 9Cr-1Mo steel. The normalizing heat treatment was performed at 1150 oC for 1800 s. Ausageing was conducted at 765 and 500 oC for 1800 to 86400 s during the cooling from the heat treatment. The matrix of the steel was austenite single phase during normalizing and ausageing, except for that ausaged at 765 oC for 86400 s. The initial austenite grain size and hardness were not influenced by ausageing, except for the sample ausaged at 765 oC for 86400 s. Although Nb-rich MX (NbX) and cementite were observed, V-rich MX (VX) was not observed under any of the conditions investigated. The amount of NbX in the steel ausaged at 500 oC was at least twice as large as that under the other conditions, and the amount in the steel ausaged at 760 oC was slightly larger than that in the steel that did not undergo ausageing. The precipitation of NbX took place during ausageing in the austenite matrix. On the other hand, it is well known that VX precipitates during tempering. An equilibrium mole fraction of VX in the austenite matrix calculated by Thermo-Calc. was larger than that of NbX at the ausageing temperatures. It is proposed that VX is an equilibrium phase at the ausageing temperature; however, VX nucleation takes much longer in the austenite matrix. It is postulated that the precipitation of VX is more strongly influenced by the interfacial energy rather than supersaturation. It is concluded that the precipitation of MX carbonitride, especially NbX, can be controlled by ausageing during cooling after a normalizing heat treatment.

Creep deformation in a modified 9Cr-1 Mo steel

The short term creep properties and microstructural changes associated with creep were studied for constant stress specimens of a modified 9Cr-1Mo steel tested at 923, 873, and 823 K. The variation in the minimum creep rate with stress could be well described by the Norton equation (ε = Aσnexp(-Q/RT)) and many of the variations of this equation described in the literature. The values for the activation energy and stress exponent were comparable to values reported previously. The Monkman-Grant equation was found to describe well the relationship between the minimum creep rate and the time to rupture. The microstructural changes have been considered in terms of precipitates previously identified in these steels. A coarse structure (size ≤200 nm), comprising M23C6, M2X and a Nb rich MX type precipitate, was observed using SEM. A finer scale structure (size < 200 nm) was observed, using TEM, to be predominantly comprised of M23C6, M2X and a V rich MX type precipitate, with smaller amounts of M6X, V4C3, etc. After creep the coarse M23C6 and M2X precipitates were found to coarsen and the fine V rich MX type precipitates were found to increase in number. It is considered that precipitation of the V rich MX precipitates compensates for the coarsening of the M23C6 and M2X, thus offsetting the reduction in creep strength that would have been produced by the coarsening.

Two-phase Separation of Primary MX Carbonitride during Tempering in Creep Resistant 9Cr1MoVNb Steel

Precipitation behaviour during heat treatment has been investigated on 9Cr1MoVNb steel. Coarse primary and fine secondary MX carbonitrides are observed in the as normalized condition. On the other hand, coarse primary MX is not detected after tempering at 1 038 K, and Nb-rich and V-rich fine secondary MX carbonitrides and M23C6 carbide are observed in the as tempered condition. Variations of metallic elements concentrations of Nb-rich MX and V-rich MX in the as tempered condition are within the relatively small ranges. Average diameter of 34 nm for V-rich MX is slightly larger than that of 24 nm for Nb-rich MX in the as tempered condition. Both size and metallic elements concentrations of the fine secondary MX observed in the as normalized condition are almost the same as those of Nb-rich MX observed in the as tempered condition. A composition of metallic elements of the coarse primary MX varies along the line between those of Nb-rich MX and V-rich MX in a Cr-Nb-V ternary phase diagram. It has been considered that, at normalizing temperature of 1 323 K, Nb-rich MX and V-rich MX forms solid solution and composition of the primary MX varies along the tie line between those two phases. Two-phase separation of primary MX into Nb-rich MX and V-rich MX may be caused by the miscibility gap in primary MX and another two phases at the tempering temperature of 1 038 K.