T91 Steel Research Articles

Revealing the oxidation mechanism of ferritic heat resistant steel in high-temperature flue gas

Oxidation behaviors of ferritic heat resistant steel T91 exposed to high-temperature flue gas for long term have been investigated in this study. Oxidation mechanism in high-temperature S-containing environment was newly understood and a novel metallurgical model was proposed. It suggested that sulphidation always preceded oxidation by forming CrS/Cr7S8 particles in the front of the internal oxide layer, accompanied by M23C6 dissolution and local matrix recrystallization. CrS/Cr7S8 nucleated around the MX phases at grain boundaries of recrystallized ferrite by diffusion. Sulfur-assisted microstructural evolution fundamentally affected the subsequent internal oxidation, controlling the oxidation mechanism.

Corrosion behavior of T91 steel in liquid lead-bismuth eutectic at 550 °C: Effects of exposure time and dissolved oxygen concentration

Long-term corrosion behavior of T91 steel was investigated in 550 °C static liquid lead-bismuth eutectic (LBE) at different dissolved oxygen (DO) concentration. It was found that the oxide film formed in the oxygen-saturated LBE consists of iron-lead composite oxide, Fe3O4, Fe-Cr spinel and an inner oxidation zone (IOZ). The IOZ is composed of matrix and FeCr2O4 which follows orientation relationships with the matrix. The thickness of oxide film significantly reduces at DO= 1.26 × 10−6 wt %. Dissolution corrosion and intergranular penetration of Pb-Bi were observed at DO≤ 1.41 × 10−8 wt %. The corrosion mechanism of T91 steel in liquid LBE is discussed.

Evaluation of flow accelerated corrosion and mechanical performance of martensitic steel T91 for a ternary mixture of molten salts for CSP plants

Thermal energy storage (TES) systems with molten salts involve different operating conditions causing corrosion problems in the materials due to the high operating temperature. In this work, the corrosion behavior was studied by means of flow accelerated corrosion (FAC) and static corrosion tests in contact with the ternary mixture (KLiNa)NO 3 at 550 °C. Both tests were performed by gravimetric tests (mass gain) and subsequently characterized by SEM and XRD. The results showed a substantial increase in mass gain due to the dynamic effect of the FAC test in relation to the stationary test. Consequently, the dynamic effect of the FAC test at high temperatures could cause stress corrosion cracking in the material, leading to a change in the mechanical performance of T91 steel. Therefore, the slow strain rate tensile test (SSRT) on molten salts was performed to evaluate the impact of temperature on the mechanical property performance of the material. A considerable reduction in the mechanical performance of the material was observed compared to testing the material in air under normal conditions. This research shows the real operating conditions under the corrosion effect of T91 steel and its relevance as a material for the solar thermal industry. • Corrosion of T91 steel in (KLiNa)NO 3 under FAC and static conditions are different. • Lithium compound oxides are formed due to oxidation and lithiumization phenomena. • There was a reduction in the mechanical performance of T91 steel under tensile tests.

Effect of Silicon on Dynamic/Static Corrosion Resistance of T91 in Lead-Bismuth Eutectic at 550 °C.

The 9–12% Cr ferritic–martensitic heat-resistant steel is the main candidate structural material for the Lead-cooled Faster Reactor. The lower Gibbs free energy change of Si oxide can promote the formation of a stable oxide layer, which can improve the corrosion resistance of the material. Therefore, it is of great significance to study the effect of silicon (Si) on the corrosion resistance of T91 steel in lead–bismuth eutectic (LBE). The corrosion resistance of T91 steel with Si contents of 0.5 wt.%, 1.3 wt.%, and 2.0 wt.%, both in dynamic and static LBE at 550 °C, was investigated. The microstructure was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM), while the oxide films were characterized by electron probe microanalysis (EPMA). Results show that the addition of Si is conducive to improving the corrosion resistance of T91 steel in LBE. T91 steel with high Si content has a thinner and more stable oxide film. The change of Si content can change the contact angle between the steel and LBE, and the contact angle is the largest when the Si content is 1.3 wt.%. The Si-rich oxide layer is usually located in the inner oxide layer, which promotes the formation of a Cr oxide layer located in the internal oxidation zone (IOZ). Si will not enter the precipitated phase, but only change the ferrite content. The oxidation model of T91 steel containing Si in LBE was also proposed.

A comparison study of change in hardness and microstructures of a Zr-added FeCrAl ODS steel irradiated with heavy ions

FeCrAl oxide dispersion strengthened (ODS) steel is regarded as one of the most promising candidate materials for nuclear fission reactors such as supercritical pressurized water reactor (SCPWR), sodium-cooled fast reactor (SFR), and lead bismuth-cooled fast reactor (LFR) and so on, since it has excellent irradiation resistance and good corrosion resistance of super-critical water and Pb–Bi coolants. In the present work, irradiation response of a Zr-added FeCrAl ODS steel (15Cr–4Al-0.1Ti-0.6Zr) together with an Al-free ODS (16Cr-0.1Ti) steel and a non-ODS steel T91 was studied. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were used to characterize the microstructures including grains, dislocations, and oxides in the pristine specimens and irradiation defects in the irradiated specimens. Nano-indentation tests were used to evaluate the change in hardness after irradiation. The oxides in the 15Cr–4Al-0.1Ti-0.6Zr specimen are slightly larger than those in the 16Cr-0.1Ti specimen, but with a similar number density. After irradiated with 9.45 MeV 83Bi35+ to 26 dpa at ambient temperature, hardening was observed in all the specimens. Irradiation hardening of the 15Cr–4Al-0.1Ti-0.6Zr specimen is slightly higher than the 16Cr-0.1Ti specimen but significantly lower than the non-ODS steel T91. The irradiation defects in the ODS steel specimens are apparently smaller than those in T91 with a similar number density. By calculating sink strength of the initial microstructures such as grains, dislocations and oxides, the evolution of irradiation defects was discussed. A linear relationship between the irradiation hardening and the sink strength was established. A similar linear relationship between the calculated irradiation hardening using the TEM data (based on dispersion barrier hardening model (DBH) and Friedel-Kroupa-Hirsch (FKH) model, respectively) and sink strength was also found. The calculated irradiation hardening is relatively lower than the experimental value, indicating the possible contribution of Cr segregation to hardening which needs further study.

Effect of Cr segregation on creep strength in grade 91 steels

ABSTRACT Several heats of T91 steel and P91 steel were evaluated, focusing on the distribution of segregation and its effect on creep strength. The difference between the maximum and minimum Cr content, ΔCr, at a constant distance (x) was estimated in the range of 1 mm along the wall thickness direction for T91 steels. The standard deviation of ΔCr was employed as an indicator of the degree of segregation. The standard deviation correlated with time to rupture at 650°C when the Cr content is relatively high. However, no clear correlation between them was observed when the Cr content is very low in the range of specification. In addition to Cr segregation, the Cr content itself should be considered as a cause of reduced creep strength when the Cr content is low. For P91 steel, the distribution of segregation was inhomogeneous as compared with T91 steel. For example, a strongly segregated area was confirmed at the position of 3/4 t from the outer surface of the pipe. On the other hand, no segregation band was observed in other areas of the same pipe.

Protoporphyrin Extracted from Biomass Waste as Sustainable Corrosion Inhibitors of T22 Carbon Steel in Acidic Environments

Carbon steel is one of the most employed materials in many industrial sectors due to its unique physical and mechanical properties. However, within a certain period of time, carbon steel-based materials are susceptible to corrosion under operating conditions and corrosion inhibitors are important to extending the limit of use of carbon steel. In this study, the influence of protoporphyrin from animal blood hemin as an eco-friendly corrosion inhibitor for T22 carbon steel in an acidic environment (0.5 M HCl) was conducted. The hemin isolated from animal blood extracts was modified to obtain the protoporphyrin. The dosage of protoporphyrin was varied between 40 and 200 ppm and the temperature influence were studied in the range of 298–318 K. The inhibition efficiency of protoporphyrin in 0.5 M hydrochloric acid reached up to 46.2% at a dose of 160 ppm at a temperature of 298 K. The inhibition efficiency (IE) value further decreases with increasing temperature, thereby showing the process exothermic in nature and the weakening of the inhibitor molecules to adsorb on the surface of the T22 carbon steel. The potentiodynamic polarization measurements indicate that protoporphyrin acts as a mixed-type inhibitor. The adsorption of protoporphyrin on the surface of T22 carbon steel obeys the Langmuir adsorption isotherm. The thermodynamic parameter of adsorption allows us to suggest the adsorption process was dominated by physical adsorption. Thus, these current results present a case study using protoporphyrin as a promising green inhibitor for carbon steel T22 in hydrochloric acid prepared from livestock waste.

Evaluation of oxide-scales of T22 steel weldment to characterize the kinetics of cyclic hot-corrosion in molten salt environment

Welding is the primary joining-process, produced the heat affected zone which is mostly damaged severely because of prolonged period particularly in corrosive environment. Most industrial components are joined by shielded metal arc welding (SMAW) process. The present article, therefore, investigated the performance of corrosion of weld-metal (WM) and heat-affected zones (HAZs) of weldment in ferritic T22 steel exposed in salt (Na2SO4–60 wt% V2O5) environment at 900 °C under thermal-cyclic conditions. During the experiments the weight-gain measurements were applied on these regions of weldment to observe the hot corrosion kinetics. The weight gain per unit area increased with the increased number of testing cycles. The parabolic growth rate was described the corrosion kinetics after few initial cycles. HAZs corroded at faster rates than WM due to more spallation and cracking in surface oxide-scale.

Effects of pre-exposure and simulated corrosion “pits” on liquid metal embrittlement of T91 steel exposed to liquid lead-bismuth eutectic at 350 °C

In this paper, the effects of pre-exposure and simulated uneven corrosion “pitting” on liquid metal embrittlement (LME) of T91 ferritic/martensitic (F/M) steel exposed to liquid lead-bismuth eutectic (LBE) at 350 °C have been studied by oxygen-specific pre-exposure test, tensile test and ANSYS stress simulation. The results show that a “pit” with a size of 0.8 mm in diameter and 0.4 mm in depth on a cylindrical tensile specimen with a diameter of 3 mm and a gauge length of 15 mm can greatly promote occurrence of LME, regardless of the oxygen concentration dissolved in LBE, leading to the formation of typical quasi-cleavage on the whole fracture surface. ANSYS stress simulation shows that the presence of such a “pit” can give rise to a strong stress concentration effect and the stress level at the “pit” region before yielding is thus increased by a factor of ∼1.7, which is responsible for the premature failure of oxide scales and earlier LME crack initiation. The results also alarm that the LME enhancement effect of uneven corrosion should not be ignored during reactor system design especially for thin-walled tubes and under transient conditions, when taking this steel as a candidate material.

High-Throughput Nanoindentation Mapping of Additively Manufactured T91 Steel

This work aims to adapt nanoindentation mapping combined with a k-means algorithm as a high-throughput technique to study the nano-scale spatial changes in mechanical properties for a heterogeneous material. This technique can also classify the individual data points based on their properties. Hundreds to thousands of indents were performed on additively manufactured T91 at room temperature, 300°C, 400°C, and 500°C across a square area with a side length of 120 μm to 400 μm. From this data, the hardness and reduced modulus at each point could be calculated and mapped. Using k-means clustering, we were able to arrange the data into three or four clusters corresponding roughly to the ferritic and martensitic phases as well as one or two intermediate clusters sampling both the phases. The hardness of these two phases appears to be quite stable as a function of temperature. Nanoindentation mapping and the k-means algorithm can therefore be used to rapidly assess the feasibility of heterogeneous materials under extreme conditions, such as nuclear reactor steels.

Solute segregation and precipitation across damage rates in dual-ion–irradiated T91 steel

Dual-ion irradiations using 5.0 MeV defocused Fe2+ ions and co-injected energy degraded 2.00 to 2.85 MeV He2+ ions were conducted on a Fe9CrMo ferritic-martensitic steel T91 to 17 dpa at a damage rate range of 5 × 10−5 dpa/s to 3 × 10−3 dpa/s at 445°C, followed by characterization of the microstructure using conventional and scanning transmission electron microscopy. Radiation induced Ni/Si clusters and radiation induced segregation were quantified using energy dispersive X-ray spectroscopy at each condition and were compared with the same material irradiated in the BOR-60 reactor and in the as-received condition. No significant Cr segregation was found at lath boundaries after dual-ion irradiation, while Ni and Si enrichments both decreased with increasing damage rate leading to a sharp decrease in the density of Ni/Si clusters with damage rate. Increased point defect recombination at higher ion damage rates likely reduced the Ni/Si cluster density compared with BOR-60. Although the overall vacancy concentration and diffusion are enhanced by the irradiation damage rate, the lack of time for thermal diffusion and ballistic displacements of solutes are significant limiting factors for Ni/Si cluster formation. This work demonstrates the effect of irradiation damage rate on elemental segregation and clustering when using ion irradiation to simulate reactor irradiation.

Stability and corrosion behavior of AlO<sub><i>x</i></sub> coating on T91 steel and SIMP steel in static liquid Pb-Bi eutectic at 600 ℃

Ferritic/martensitic steels, such as T91 steel and SIMP steel, are chosen as the main candidates of structural materials for the Generation IV lead-cooled fast reactors and accelerator driven system. However, the compatibility between container steel and liquid Pb-Bi eutectic (LBE) at high temperature limits their applications. The corrosion of ferritic/martensitic steels is serious in LBE at 600 ℃. In order to avoid corroding the ferritic/martensitic steels in LBE, it is proposed to coat AlO<sub><i>x</i></sub> (<i>x </i>< 1.5) on the steel surface. The AlO<sub><i>x</i></sub> coating is conducted on T91 steel and SIMP steel by magnetron sputtering. In this exploratory work, the corrosion results of AlO<sub><i>x</i></sub> coating steel are compared with the corrosion results of the uncoated steel in LBE with a saturated oxygen concentration at 600 ℃ for 300 h and 700 h. The results show that the AlO<sub><i>x</i></sub> coating can effectively prevent the iron chromium and oxygen from diffusing, so the oxide scale of the coated steel is thinner than that of the uncoated steel. However, the coating cracks after 700 h corrosion in LBE. Meanwhile, T91 steel and SIMP steel also suffer serious oxidative corrosion, indicating that the coating can protect the substrate from being corroded by 600 ℃ static LBE in a short time. However, the coating cannot keep stable for a long time in LBE at 600 ℃. This may be due to the weak film base bonding force of AlO<sub><i>x</i></sub> coating prepared under the experimental conditions, or a large number of metal aluminum and structural defects existing in AlO<sub><i>x</i></sub> coating. It is needed to further study the stability of AlO<sub><i>x</i></sub> coating in LBE at elevated temperature.

Investigation of strain-rate sensitivity of T91 Steel Using Small Punch Testing

The Small Punch Test (SPT) is a useful technique for deriving location-dependent mechanical properties using small-sized specimens. In this work, we have investigated the tensile and fracture behavior of T91 steel, a candidate material for fusion and Gen IV fission nuclear reactors, by the SPT technique. Firstly, the hot shear flow behavior of T91 steel was investigated by SPT in the temperature range of 400 – 550 °C under quasi-static strain rates (0.003 mm/min and 0.3 mm/min). Further, the hot deformation material parameters in the exponential power-law and hyperbolic sine constitutive equations have been evaluated. The effect of temperature and strain rate variation on stress exponent in power-law and activation energy from hyperbolic sine equation have been correlated.

Hot corrosion performance of bare and coated T91 steel under actual and simulated bio-fuel fired boiler environment

The conventional fuel has now been replaced by bio-fuels, due to an increasing demand of power supply which has only been met through the non-renewable resources so far. However, burning of bio-fuel creates corrosive atmosphere which is different from that of coal and oil firing. Therefore, in the present investigation, bare and coated boiler steel T91 substrates were exposed to real andsimulated bio-fuel fired boiler environment working at high temperature. It was noticed that dense and protective oxide was present on Cr3C2-NiCr coated T91 steel after 50hours of exposure in the simulated environment which depicts the protective behaviour of the coating and compatibility of coating with steel despite of variation in the composition. However, oxide so formed in actual bio-fuel fired boiler was porous. WC-Co coated T91 steel has undergone severe corrosion by showing the oxide spallation and delamination of the coating from the substrate under a simulated environment. Similar observations were noticed in the actual boiler working at the temperature of 750±50°C.

Capturing the Softening in T91 Steel Annealed with Molten Lbe Through Mechanical Interface Energy Terms

Capturing the Softening in T91 Steel Annealed with Molten Lbe Through Mechanical Interface Energy Terms

Cause analysis on tube brust of T23 steel water-wall in a ultra-supercritical unit

During the hydrostatic test of an ultra supercritical unit, the water wall burst occurred. The causes of leakage were analyzed by means of macro observation, chemical composition analysis, metallographic examination and hardness test. The results show that the formation of cracks is related to fatigue, extending from the inner wall to the outer wall, accompanied by corrosion and oxidation. Fatigue is the main cause of transverse crack formation.

Wetting Behavior of LBE on Corroded Candidate LFR Structural Materials of 316L, T91 and CLAM.

In this work, the wetting behaviors of lead-bismuth eutectic (LBE) on corroded 316L, T91, and CLAM surfaces were studied. The wettability of LBE on virgin and corroded surfaces were tested at 450 °C by using the sessile-drop (SD) method after immersing the samples in LBE with saturated oxygen concentration for 400, 800, and 1200 h at 450 °C. Additionally, the morphology, as well as element distribution of the corrosion structure, were characterized by scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). The results showed that the virgin samples of three materials are non-wetting to LBE, and the formation of corrosion structures further reduces the wettability. Besides, the thickness of the corrosion layer formed on the 316L surface grew more slowly than the other two steel, which results in better corrosion resistance of austenitic steel 316L than that of ferritic/martensitic steels T91 and CLAM at 450 °C. Meanwhile, the morphology and distribution of corrosion products are important factors affecting the wettability of the steel surface. The formation of corrosion products with high roughness as well as disorder results in a significant reduction in surface wettability.

Early-stage microstructural evolution and phase stability in neutron-irradiated ferritic-martensitic steel T91

A Fe-9Cr ferritic martensitic (F/M) steel T91 was neutron-irradiated in the Advanced Test Reactor up to 3.96 dpa in two temperature ranges, 466 °C to 534 °C and 571 °C to 632 °C. The microstructure evolution including dislocation loops, precipitation, segregation of elements and phase stability were studied using analytical scanning-transmission electron microscopy and atom probe tomography. The hardening induced by irradiation was measured by nanoindentation. Ni/Si/Mn clusters were identified in all conditions except the one irradiated around 600 °C to 3.23 dpa. The compositions of Ni/Si/Mn clusters were found to be converging to G phase stoichiometrically with increasing dose, with Mn partially substituted by Cu. Significant coarsening of this phase was observed in high temperature cases, with total dissolution of intragranular G phase after prolonged irradiation. A similar trend was also identified for dislocation loops. The results obtained in this experiment provide evidences that the absence of dislocation loops in some specimens irradiated to high dose level under high irradiation temperature range (typically above 500 °C) is not due to the suppression of nucleation and growth by high point defect recombination rate, but rather fast coalescence of dislocation loops. Hardness measurements show different dose dependences for two temperature ranges. For specimen irradiated around 600 °C significant hardening before 0.5 dpa followed by softening process was observed, while in lower temperature range (450 °C ~ 500 °C) the normal hardening pattern with increasing dose was observed.

High temperature erosion of T22 steel coated by Cr–Ni electroplating, HVOF sprayed stellite–6 and WC–12Co

In present study, erosion behavior of ASTM–A213–T22 steel (T22 steel) coated by different coating processes was evaluated at high temperature. Cr–Ni electroplating and high–velocity oxy–fuel (HVOF) spraying using Stellite–6 and WC–12Co powders were chose as coating processes. Average coating thicknesses of the Cr–Ni electroplated coating and the HVOF sprayed Stellite–6 and WC–12Co coatings were 40 µm, 226 µm and 200 µm respectively. Oxide scale formation on an uncoated T22 steel specimen and coated specimens was done by covered the specimens with a synthetic coal ash and then annealed under a synthetic flue gas at temperatures of 700 °C with annealing times of 24 h 72 h and 123 h. After oxide scale formation process, the specimens were eroded in erosion tester at temperature of 700 °C using SiC erodent particle with average particle size of 60 µm at impingement velocity of 40 m s−1 and impingement angles of 30° and 90°. The experimental results showed that erosion rate of the specimens dramatically increased with increasing of exposure time at 700 °C. Erodent impingement angle was unaffected to erosion rate of the uncoated T22 steel, the Cr–Ni electroplated and the HVOF sprayed Stellite–6 and WC–12Co coatings. Microcracks, debonding of coating as well as deep cavities were observed on eroded surface of the uncoated T22 steel, the Cr–Ni electroplated and the HVOF sprayed Stellite–6 and WC–12Co coatings. Erosion resistance of the HVOF sprayed Stellite–6 coating was highest compared with that of the Cr–Ni electroplated and the HVOF sprayed WC–12Co coating. The superior erosion resistance of the HVOF sprayed Stellite–6 coating was due to high hardness, more thickness and low porosity of the deposited coating.

Preliminary Studies on Rare Elements Addition and Effect on Oxidation Behaviour of Pack Cementation Coatings Deposited on Variety of Steels at High Temperature

The aim of the paper was to investigate the air oxidation behaviour of pack aluminised steels exposed at 650 °C for 1000 h in static natural air atmosphere. The pack coatings were doped by rare elements such as gadolinium (Gd), cerium oxide (CeO2), and lanthanum (La) in order to enhance the corrosion resistance and plasticity of the deposited layers. In this work, the following steels were used: 16M, T91, VM12, Super 304H, and finally SANICRO25. The results indicated a much higher corrosion resistance in the coated 16M, T91, and VM12 steels; the steels with a higher Cr content than 16 wt % Cr indicated a better behaviour in the uncoated state than in the coated state. However, the observed difference in mass gain between the uncoated and the coated austenitic steels was not enormous. Furthermore, the addition of RE elements to the coating showed some effect in terms of coating thicknesses and differences in the layer structures. The materials prior to testing and after the exposure were investigated using XRD, the SEM X-ray maps with an EDS instrument were used for particular samples to evaluate the phase identifications, element concentrations, microstructure, and chemical composition.