T91 Steel Research Articles

LBE corrosion fatigue life prediction of T91 steel and 316 SS using machine learning method assisted by symbol regression

This study employs machine learning models assisted by symbol regression to achieve satisfactory corrosion fatigue life prediction for T91 steel and 316L stainless steel (SS) used in 4th generation advanced nuclear power plants. Symbol regression features improve the performance of all considered machine learning models. The artificial neural network (ANN) model shows the most significant improvement of 22 % decrease in RMSE regarding to the model without symbol regression features. Additionally, the well-trained ANN model is transferred to predict 316L SS with a 50 % reduction in training samples, highlighting its potential for more efficient model training and deployment.

Experimental and Numerical Analysis of Residual Stresses in Similar and Dissimilar Welds of T91 and Super304H Steel Tubes

Experimental and Numerical Analysis of Residual Stresses in Similar and Dissimilar Welds of T91 and Super304H Steel Tubes

Correlation between impact angle and corrosion-erosion damage behavior of ferritic/martensitic steel exposed to flowing oxygen-saturated lead-bismuth eutectic

Impact angle is considered to be one of the main factors that influence the damage behavior of materials exposed to flowing lead-bismuth eutectic (LBE) conditions in Gen IV LBE-cooled fast reactors, especially in the areas where the flow changes its direction sharply, such as sudden expansions or elbows. In the current study, the corrosion-erosion damage behaviors of T91 ferritic/martensitic steel under different impact angles were investigated in flowing (∼3 m/s) oxygen-saturated LBE at 500 °C. The results show that the damage behaviors of T91 steel are closely related to the impact angles. When the flowing direction is parallel to the specimen surface, i.e., 0° impact angle, the oxide scales show partially damaged features in the form of local buckling and exfoliation after exposure for 300 h. With continued exposure for 600 h and 1000 h, the specimen surface seems to be covered completely by the undamaged oxide scales again. In sharp contrast, when the flowing direction is perpendicular to the specimen surface, i.e., 90° impact angle, the oxide scales are almost totally delaminated after exposure for 600 h and 1000 h. As a consequence, the liquid LBE permeates into the T91 steel substrate along the boundaries of prior austenite to induce intergranular corrosion. Correspondingly, the possible reasons behind such phenomena are discussed. Meanwhile, the damage process models of oxide scales associated with 0° and 90° impact angles are established, respectively.

Influence of boron segregation on the hot ductility of T23 steel CGHAZ

Boron segregates in a non-equilibrium segregation mode at lath, packet, block boundaries as well as prior austenite grain boundaries (PAGBs) in 2.25Cr1.6WVNb steel coarse grain heat-affected zone (CGHAZ) during the thermal cycle, and tends to segregate only at PAGBs in the same mode during the high-temperature tensile test. Boron segregation along PAGB increases GB plasticity but not inter-granular strength. This is because the boron segregation at PAGBs occupies the vacancies and other defect positions, which inhibits the nucleation and growth of micro-voids, and reduces plasticity consumption. As a result, the hot ductility of CGHAZ is enhanced.

TEM comparative study on oxide films of 316L and T91 steel exposed to 350–500 °C steam

The oxide films of 316L and T91 exposed to 350–500 °C steam were investigated using transmission electron microscopy (TEM). Independent of the exposure temperature, a triplex oxide structure with outer magnetite, inner Cr-rich (Fe1.4Cr1.6O4), and Ni-rich layer formed on 316L, while only a duplex layer with outer magnetite and inner Cr-rich (Fe2.2Cr0.8O4) layer formed on T91. As the fast channels for oxidant and the obstacles for solid-state diffusion, nanopores are distributed evenly in the Cr-rich inner layer and are more abundant in 316L than in T91. The oxidation behavior of the materials was understood based on the microscopic characteristics of the oxide films.

Improvement of low-cycle fatigue behavior of modified 9Cr-1Mo steels at 450 °C in liquid LBE environment by the addition of Si element

This study examined the low-cycle fatigue performance of two modified 9Cr-1Mo steels containing Si at levels of 0.38% and 1.2%, respectively. The investigation was conducted under conditions of 450 °C oxygen-saturated and oxygen-poor lead–bismuth. The elevated oxidative corrosion behavior at this temperature aligns more closely with operational conditions. Comparing the steels with differing Si contents offers a clearer insight into the impact of Si element incorporation on the material's mechanical properties. The results indicated that the high-Si steels had a higher fatigue life, and their fatigue fractures exhibited quasi-dissociation. The high-Si steel displayed elevated oxidation resistance, as indicated by the slower growth of the oxide layer. Owing to significant oxidative corrosion, the fatigue life of the low-Si steel was shorter in oxygen- saturated LBE compared to oxygen-poor LBE. The LME-induced cracking of modified T91 steel in LBE initiated at the grain boundaries. During the expansion phase, a blend of ductile and brittle behaviors was observed, occurring through and alongside grain expansion patterns. Due to the elevated Si elemental content, high-Si steel exhibited weakened susceptibility to LME in oxygen-saturated LBE, owing to the creation of SiO2 through the reaction between Si and dissolved oxygen in the LBE, which shielded the steel matrix.

EFFECT OF BURNING BIO-MASS FUEL ON THE CORROSION PERFORMANCE OF STEELS IN THE 34.5MW BOILER

The usage of fossil fuels such as coal is now being reduced and shifted to bio-fuel in the boilers. It is because fossil fuels are exhaustible and generate CO2 emission during burning. Whereas, bio-fuels are abundantly available at low cost and also produce fewer greenhouse gases. However, burning of such fuels create a lot of corrosive species which can damage the components used for construction of boilers. Ferritic, austenitic, and martensitic steels are widely used to construct various parts of the boiler. Hence, in this study, four different grades of steels known as T91, SS304, SS316, and SS410 have been placed in the actual boiler environment for 493h. The boiler utilizes wood chips, rice husk, sawdust, bamboo base and leaf cuttings as burning fuel and operates at 850±50∘C. The results indicate that T91 steel showed a maximum rate of corrosion followed by SS304, SS316, and SS410. Martensitic steel 410 showed the minimum loss in weight among other steels. No intergranular corrosion was noticed in 410 steel, but the oxide so formed on the surface was fragile and porous although the chromium content is less in SS410 as compared to SS304 and SS316.

Study on Modification Effect and Mechanism of Pulse Detonation-Plasma Technology Treatment on T8 Steel

Study on Modification Effect and Mechanism of Pulse Detonation-Plasma Technology Treatment on T8 Steel

Influence of surface condition on the high-temperature corrosion of 12Cr1MoVG, T91, and TP347H steels in waste-to-energy boilers

High-temperature corrosion of superheater and reheater is a major obstacle for improving steam parameter of waste-fired boiler. In the present study, three commercial alloys (12Cr1MoVG, T91 and TP347H) were investigated under a simulated atmosphere. The anti-corrosion performance of Inconel 625 overlay weld coating on 12Cr1MoVG was also investigated in a real waste-to-energy (WtE) plant. The experiments were carried out by a two-zone lab-scale tube furnace where the gas temperature was 600℃ and the sample temperature was 380℃. Different sample surface conditions of the corrosion test specimens were set up includes clean surface, NaCl/KCl deposition, and ash deposition. Corrosion kinetics were plotted on the basis of specimens weight gain. The results showed that chloride salts are the main corrosive species in ash deposit. The corrosion rate of different deposits and clean surface was in the following order: KCl/NaCl > ash deposit > clean surface. The corrosion products were Fe2O3, Cr2O3, Fe3O4, NiO, (Fe, Ni)Cr2O4, FeCl2, and FeCl3. The presence of chromium was beneficial for high-temperature corrosion resistance. The anti-corrosion performance of the three alloys in descending order was TP347H > T91 > 12Cr1MoVG. This is consistent with the chromium and nickel content in the alloys. The engineering investigation showed that Inconel 625 overlay weld coating can effectively protect base material from the attack of corrosion species includes ash deposit and MSW incineration atmosphere, for long-term in real WtE plant.

Influence of temperature on the microstructural evolution of SIMP and T91 steels in liquid lead-bismuth eutectic: Experiments and molecular dynamics simulations

The microstructural evolution of ferritic/martensitic steels SIMP and T91 in oxygen-saturated liquid LBE at different temperatures (400, 470, 550 and 620 °C) has been studied by experiments and molecular dynamics simulations. It is shown that the SIMP steel offers better oxidation resistance at 400–550 °C due to the formation of protective SiO2 particles and bands. At 620 °C, both the SIMP and T91 steels undergo dissolution corrosion with similar dissolved layer thicknesses. Molecular dynamics simulations reveal that the number of Fe-O pairs decreases with increasing temperatures and the enhanced dissolution of Fe makes the surfaces of the steel difficult to oxidize at 620 °C.

A comparative study on characteristics of composite (Cr3C2-NiCr) clad developed through diode laser and microwave energy

A typical ferrite/martensitic heat-resistant steel (T91) is widely used in reheaters, superheaters and power stations. Cr3C2-NiCr-based composite coatings are known for wear-resistant coatings at elevated temperature applications. The current work compares the microstructural studies of 75 wt% Cr3C2- 25 wt% NiCr-based composite clads developed through laser and microwave energy on a T91 steel substrate. The developed clads of both processes were characterized through a field emission scanning electron microscope (FE-SEM) attached with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and assessment of Vickers microhardness. The Cr3C2-NiCr based clads of both processes revealed better metallurgical bonding with the chosen substrate. The microstructure of the developed laser clad shows a distinctive dense solidified structure, with a rich Ni phase occupying interdendritic spaces. In the case of microwave clad, the hard chromium carbide particles consistently dispersed within the soft nickel matrix. EDS study evidenced that the cell boundaries are lined with chromium where Fe and Ni were found inside the cells. The X-ray phase analysis of both the processes evidenced the common presence of phases like chromium carbides (Cr7C3, Cr3C2, Cr23C6), Iron Nickel (FeNi3) and chromium-nickel (Cr3Ni2, CrNi), despite these phases iron carbides (Fe7C3) are observed in the developed microwave clads. The homogeneous distributions of such carbides in the developed clad structure of both processes indicated higher hardness. The typical microhardness of the laser-clad (1142 ± 65HV) was about 22% higher than the microwave clad (940 ± 42 HV). Using a ball-on-plate test, the study analyzed microwave and laser-clad samples' wear behavior. Laser-cladding samples showed superior wear resistance due to hard carbide elements. At the same time, microwave-clad samples experienced more surface damage and material loss due to micro-cutting, loosening, and fatigue-induced fracture.

Endurance Strength and Residual Life Assessment of T23 Steel after 96000 Hours of High Temperature and High Pressure Service

The macroscopic morphology, oxide layer characteristics, rupture strength and residual life of T23 steel after 96000 hours of service at high temperature and high pressure were analyzed. The results show that T23 steel pipe has good macroscopic appearance, without obvious abnormal features such as cracks, folds, rolling breaks, double skin, scabs, delaminations and sharp scratches. The chemical composition of different pipe sections conforms to the standard, and the structure is mainly bainite, with uniform distribution and no obvious coarse phase. There is an obvious interface between the metal substrate and the internal oxide layer, and there is an obvious wavy line at the interface. The oxide layer is relatively dense and continuous. The tensile properties at room temperature and high temperature are good. At room temperature, the yield strength exceeds 420MPa, the tensile strength exceeds 540MPa, and the elongation exceeds 19%. After 96000 hours of use, the remaining creep life of T23 split platen superheater submitted for inspection is 84000 hours, and the remaining creep life of T23 rear platen superheater submitted for inspection is 40000 hours.

Steam oxidation behavior of T92 steel tube for high temperature reheater after long-term service in ultra-supercritical boilers

The steamside oxidation behavior of T92 steel tube for high temperature reheater after long-term service in ultra-supercritical boilers was investigated. The structure evolution process, phase composition and element distribution of the oxide film were studied. The oxide film consisted of outermost layer with hematite, subouter layer with magnetite and inner layer with Fe-Cr spinel. The structure of the transition zone of the oxide film and related oxidation mechanism was discussed in detail. A steam oxidation kinetic model of the T92 reheater based on the evolution of the inner layer thickness of the oxide film was established.

Effect of additive (Zr) and dopants (Pd & Sb) on the depth of migration of lighter lanthanides (Nd & Ce) from U-bearing metal alloys into T91 steel

The metal alloy fuel, which is considered as the advanced nuclear fuel, has its own limitations; namely, lower liquidus temperature, higher swelling, and is highly prone to breach the cladding due to fuel-cladding chemical interaction (FCCI). The mobile and insoluble fission products (FPs), especially lanthanide FPs are mainly responsible for FCCI. In the present study, the influence of additive (Zr) and dopants (Pd and Sb) on the depth of migration of two lanthanide FPs, namely Nd and Ce, from different uranium-bearing metal alloys into T91 (ferritic-martensitic steel) cladding material is investigated. Seven metal alloys, namely, U-Nd, U-Nd-Pd, U-Ce, U-Ce-Pd, U-Zr-Nd, U-Zr-Nd-Pd, and U-Zr-Nd-Sb, were prepared using arc melting technique. The amount of lanthanide FPs taken in the above alloys corresponds to about 10 at.% burn-up of a typical fast breeder reactor (FBR) fuel. The pulsed laser deposition (PLD) method was used to obtain alloys in the form of thin film on T91 cladding, with simultaneous in-situ annealing. Subsequently, these films were subjected to elemental depth profiling using secondary ion mass spectrometry (SIMS). The depth of migration for Nd and Ce from the seven metal alloy systems into T91 cladding was determined based on SIMS data. The extent of the individual and simultaneous effect of additive and dopant on the depth of migration was determined. Both dopants (Pd and Sb) are effective individually in binding Nd and Ce within the fuel matrix (in the U matrix) by forming stable inter-metallic compounds. The effect of Zr on the migration of lanthanide FPs is negligible in the presence of dopants. Stable inter-metallic phases responsible for binding lanthanides FPs are identified. A comparative study on the efficiency of two dopants (Pd and Sb) is also discussed. For the first time, the depth of migration of Nd and Ce from U metal alloys into T91 cladding was measured in the presence of dopants (Sb/Pd) by using a novel technique of PLD followed by SIMS. These studies are very useful for understanding FCCI.

Effect of steam on the creep behavior of T92 steel at 650°C

One self-designed testing system was utilized to investigate the creep behavior of the T92 steel in steam environment. The creep tests were conducted at 650 °C with a certain stress range from 100 MPa to 160 MPa in the steam environment and air respectively. The specimens tested in steam environment shows typical multi-layered oxide scale consists of an outer scale of magnetite and an inner scale of Cr-rich spinel. Compared to the specimens in air, the steam specimens possessed larger minimum creep rate and reduced plasticity. The stress dependence of minimum creep rates exhibited a power law with similar exponents of 12.0 and 12.6 in steam environment and air respectively, indicating similar operating mechanism of creep. However, the activation energy in steam environment was smaller than that in air. Microstructural observation demonstrated that the degradation of tempered martensite lath structure and coarsening of the precipitations were mainly responsible for creep damage both in steam environment and air. The results concentrating on the possible interaction between the oxidation scale growth process and creep were discussed on the basis of oxidation process, a potential effect of hydrogen on creep and potential hydrogen embrittlement.

Corrosion Performance of Commercial Alloys and Refractory Metals in Conditions for Electrorefining of Spent Nuclear Fuels

Molten LiCl-KCl salt and liquid cadmium are proposed as the electrolyte and the reactive cathode for the electrorefining of spent nuclear fuels, but they can be corrosive to the structural alloys. The down-selection of existing materials through corrosion testing is necessary to ensure the longevity of the electrorefiner vessel and electrode assemblies. Haynes C276, Inconel 600, AISI 316L stainless steel, and 42CrMo low-alloy steel were exposed to a LiCl-KCl melt at 500 °C for 500 h in an argon atmosphere. All alloys suffered from dissolution attacks with the presence of oxide islands or a porous oxide layer on the surface. AISI 316L, T91 steel, and tungsten specimens were submitted to corrosion tests in liquid cadmium at 500 °C for 120 h. The corrosion of AISI 316L and T91 stainless steel was predominated by chemical oxidation, with the additional occurrence of severe Ni dealloying and Cd penetration on AISI 316L. Destabilization of the Cr oxide layer by cadmium was discovered, resulting in the formation of CdCrO4. Tungsten only suffered from a dissolution attack at a rate of 0.50 mm/a.

Micro characteristics and macro performance of HR3C/T92 steel joints after 100000 hours service at high temperature

In this study, HR3C/T92 dissimilar steel joint samples were taken from ultra-supercritical 1000 MW units of domestic power plants, and the microstructures, tensile property, hardness, and antioxidant performance as well as safety assessment of the welded joints were studied using OMR, SEM, UTM, and other equipment. The results reflected that the micromorphology characteristics and hardness of HR3C/T92 dissimilar steel joints are unaffected after 100000 hours of service. Nevertheless, tensile and impact tests revealed that the joints are prone to brittle fracture. Tensile fracture occurred on the weld fusion line of T92 steel, and the impact absorption energy was lower than the standard requirements. HR3C steel exhibited superior oxidation resistance and corrosion resistance. Moreover, there was no observable oxide layer on the inner wall of the pipe, and the outermost layer of the outer wall had an intact structure. The inner wall of the T92 steel pipe had a 300-μm-thick oxide layer comprising Fe2O3 and (Fe, Cr) oxides. The extrapolated performance of the dissimilar steel joint samples after operation for 200000 hours at high temperatures also met the service requirements.

Experimental study on influence of shielding gas pressure on the quality of pulsed laser welding of thin walled T91 steel clad tube with end plug

Fabrication of metal fuel pins for test irradiation in fast breeder test reactor requires welding of thin wall clad tube with solid end plug of modified 9Cr-1Mo steel material under high pressure helium atmosphere. Autogenous pulsed Gas Tungsten Arc Welding [GTAW] technique is currently used for end cap welding of metal fuel pins inside the glove box. Pulsed laser welding of end plug with clad tube of modified 9Cr-1Mo steel material under inert atmosphere at 1.2 bar gas pressure is under development.Pulsed laser beam welding [PLBW] is considered as alternate welding technique to replace the pulsed tungsten inert gas welding.In the current study the influence of shielding gas pressure on the weld joint characteristics like depth of penetration, bead width, and heat affected zone were investigated to optimize laser welding parameters. Initially experiments were conducted under helium gas at atmosphere pressure to optimize the process parameters to obtain the desired depth of penetration and then repeated the experiment with the optimized parameters at 0.5 bar pressure to study the effect of shielding gas pressure. Laser beam welding parameters like peak power − 1800 to 2400 W, pulse frequency – 5 to 8 Hz and pulse duration – 6 to 10 ms were varied during welding sample coupons. Welded sample coupons were subjected to various NDT techniques and subsequently metallographic examination was carried out to measure the bead width, depth of penetration and delta ferrite distribution in the weld metal. Laser beam welding parameters were optimized at atmosphere pressure to meet the stringent acceptance criteria. Further study was carried out under helium gas atmosphere at 0.5 bar using optimized welding parameters and it reveals that the drastic reduction in heat input due to laser beam attenuation and absorption by quartz window.

Corrosion behavior of heat-resistant steel T91 in high-temperature supercritical carbon dioxide with impurity O2, SO2 or H2S

This study investigated corrosion of T91 in supercritical carbon dioxide (sCO2) with impurity of 100 ppm O2, SO2 or H2S at 550 ℃ and 15 MPa. O2 slightly increased mass gain of corroded T91, and the corrosion kinetics both in pure sCO2 and in sCO2 with O2 impurity followed parabolic law. Fe2O3 was formed in outer corrosion layer due to enhanced partial pressure of oxygen. SO2 or H2S remarkably exacerbated T91 corrosion. For test in sCO2 with SO2 impurity, outer corrosion product was magnetite, and the inner was composed of Fe-Cr spinel and FeS, different from that in pure sCO2. Addition of H2S caused an external FeS layer and internal Fe-Cr spinel. Introduction of all the three impurities reduced carburization in the underlying alloy substrate.

Study on the effect of different cooling conditions on microstructural and mechanical properties of heat-treated T92 martensitic steel

The T92 martensitic steel is utilized in thermal power plants because of its better mechanical properties, and it is needed to enhance the strength and improve the properties using heat treatment. The research aims to determine how the microstructure changes in T92 martensitic steel when normalized at 1000 °C for an hour and tempered at 760 °C for 2 hours using various cooling techniques, including air cooling, furnace cooling, and water quenching. It is characterized by using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), and optical microscopy (OM). Tensile strength, hardness and toughness measurements were also performed on the heat-treated T92 steel specimens. After tensile testing of heat-treated samples of T92 steel, the fracture surface was characterized using SEM to determine the fracture mode. The strength and hardness of T92 material increased with a normalizing of 1000 °C, whereas they decreased with a tempering of 760 °C. Compared to various material conditions, the tempering of 760 °C with air-cooled hold the tensile strength of 887 MPa, hardness of 267.42 HV and toughness of 135 J was shown to be the optimal combination.