Ferritic Heat Resistant Steels Research Articles

The Influence of Groove Geometry on the Creep Fracture Behavior of Dissimilar Metal Welds between Ferritic Heat-Resistant Steels and Nickel-Based Alloys

This study investigates the influence of groove geometry on the high-temperature creep life and fracture behavior of Dissimilar Metal Welds (DMWs) between low-alloy steel 2.25Cr1Mo and austenitic stainless steel 347H using Inconel 82 nickel-based filling metal. This research aims to reveal the effect of groove geometry, especially the stepped groove, on creep crack propagation path and creep life, through a combined approach of finite element simulation considering stress triaxiality and experimental validation. The study reveals that the stepped groove alters the creep crack propagation path, enhancing the endurance life by deflecting cracks away from the weld/heat-affected zone (HAZ) interface and directing them into regions with higher creep resistance. The experimental results verify the simulation findings, revealing that the stepped groove joints exhibited longer creep life with changes in failure location and mechanism compared to the V-groove joints. However, it was found that the stepped groove intensified the stress concentration at the early creep stage. Thus, a good balance should be achieved between the negative (stress concentration at interface) and positive (changing crack paths) effects of the stepped groove to extend the creep life of DMWs.

Search for high-creep-strength welding conditions considering HAZ shape factors for 2 1/4Cr–1Mo steel

The creep rupture life of ferritic heat-resistant steel weld joints is limited by Type IV cracking that occurs in the heat-affected zone (HAZ), whose shape affects creep damage accumulation. In this study, we address the inverse problem of extending the creep rupture life of weld joints by controlling HAZ shape via welding conditions. As reported separately, we have developed a workflow that predicts weld joint creep rupture life from the predicted HAZ shape from welding conditions and have implemented it in the material design system. Using this workflow, we presented a tandem Bayesian model for predicting the creep rupture life from welding conditions via the geometric features of HAZ shapes (HAZ shape factors), which are considered to determine the creep rupture life. The prediction model of a HAZ shape factor from welding conditions was formed by Gaussian process regression. The prediction model of the creep rupture life was formed by Bayesian linear regression. These models were probabilistically connected by Bayesian statistical mathematics. An algorithm to increase the creep rupture life was developed to search for welding conditions. This method was applied to a 2 1/4Cr–1Mo heat-resistant steel weld joint simulated with a plate I-bevel three-layer gas tungsten arc welding. The number of welding conditions combination reaches 78=5764801\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${7}^{8}=5764801$$\\end{document}. Start from 49 initial HAZ shape factors and 22 creep rupture life data, we performed forward calculations of 20 rupture lives to find welding conditions that can improve the creep rupture life by 12% over the initial data.

Effect of Interface Form on Creep Failure and Life of Dissimilar Metal Welds Involving Nickel-Based Weld Metal and Ferritic Base Metal

For dissimilar metal welds (DMWs) involving nickel-based weld metal (WM) and ferritic heat resistant steel base metal (BM) in power plants, there must be an interface between WM and BM, and this interface suffers mechanical and microstructure mismatches and is often the rupture location of premature failure. In this study, a new form of WM/BM interface form, namely double Y-type interface was designed for the DMWs. Creep behaviors and life of DMWs containing double Y-type interface and conventional I-type interface were compared by finite element analysis and creep tests, and creep failure mechanisms were investigated by stress-strain analysis and microstructure characterization. By applying double Y-type interface instead of conventional I-type interface, failure location of DMW could be shifted from the WM/ferritic heat-affected zone (HAZ) interface into the ferritic HAZ or even the ferritic BM, and the failure mode change improved the creep life of DMW. The interface premature failure of I-type interface DMW was related to the coupling effect of microstructure degradation, stress and strain concentrations, and oxide notch on the WM/HAZ interface. The creep failure of double Y-type interface DMW was the result of Type IV fracture due to the creep voids and micro-cracks on fine-grain boundaries in HAZ, which was a result of the matrix softening of HAZ and lack of precipitate pinning at fine-grain boundaries. The double Y-type interface form separated the stress and strain concentrations in DMW from the WM/HAZ interface, preventing the trigger effect of oxide notch on interface failure and inhibiting the interfacial microstructure cracking. It is a novel scheme to prolong creep life and enhance reliability of DMW, by means of optimizing the interface form, decoupling the damage factors from WM/HAZ interface, and then changing the failure mechanism and shifting the failure location.

Enhancing oxidation resistance of ferritic heat-resistant stainless steel in air atmosphere at 1100 °C by self-repairing oxide layer

The oxidation behaviors of Al-free and 1Al ferritic heat-resistant stainless steels (FHSSs) at 1100 °C were investigated. After oxidation for 100 h, the Al-free FHSS had triple oxide layers of a first layer Fe2O3 + MnCr2O4, a second layer MnCr2O4 + Cr2O3 + MnO, a third layer SiO2 and oxides of Nb and Ti. The 1Al FHSS had double oxide layers of a first layer Ti-bearing (Al, Cr)2O3 particles, a second layer dense (Al, Cr)2O3. The (Al, Cr)2O3 layer with self-repairing feature can reduce the thickness of oxide layer by 515 times, and reduce the oxidation weight gain rate of the FHSS by 5 orders of magnitude.

Creep Life Predictions by Machine Learning Methods for Ferritic Heat Resistant Steels

Creep Life Predictions by Machine Learning Methods for Ferritic Heat Resistant Steels

Evolution mechanism of interfacial microstructure and its effect on failure in dissimilar metal welds containing ferritic heat resistant steels

This study investigated the behaviour and mechanism of interfacial microstructure evolution during long term high temperature exposure and the effect of interface structure on creep failure in the dissimilar metal welds (DMWs) involving ferritic heat resistant steels with nickel-based filler metal. The welding of nickel-based and ferritic materials led to the formation of interfacial martensite layer in the ferritic steel side of DMW. During long term high temperature exposure, interfacial martensite transformed to ferrite due to C diffusion and migration in the micro-region near the BCC/FCC interface between the martensite layer and the weld metal. The martensite-ferrite transformation was attributed to the large C chemical potential gradient caused by chemical composition gradient. Under creep condition, the failure of DMW was prone to occur along the weld metal/ferritic steel interface, which was related to strain concentration around the interfacial ferrite formed during service. Hence, the relationships between interface structures before and after service and interfacial failure in the DMWs were clarified.

Effect of Al Addition on Thermal Fatigue Deformation Morphology in High Heat-resistant Ferritic Stainless Steel SUS444

Effect of Al Addition on Thermal Fatigue Deformation Morphology in High Heat-resistant Ferritic Stainless Steel SUS444

Nondestructive Ultrasonic Evaluation of Ferritic CrW Steel upon Long-term Aging

The microstructural evolution of heat-resistant ferritic CrW steel during long-term aging was characterized by ultrasonic wave velocity measurement. Long-term aging was conducted at 700°C for 4000 h. The ultrasonic wave velocity increased monotonically upon long-term aging. The ultrasonic wave velocity increased rapidly during the initial 1000 h of aging and moderately thereafter. The MSUB23/SUBCSUB6/SUB carbide and Fe2W Laves phases underwent coarsening through the Ostwald ripening mechanism, causing a decrease in Cr and W solute atom concentration of the iron matrix during 4000-h aging. The dislocation density also decreased because of the heat treatment. The width of martensite lath increased owing to dislocation recovery at the lath boundaries. The ultrasonic wave velocity depends on the Young’s modulus of the material, which may influence microstructural features such as dislocation substructure, precipitates, solute atom concentration, and martensite lath boundaries. This study utilizes ultrasonic wave velocity to quantify the damage level and microstructural changes during the long-term aging of ferritic CrW steel.br/

Grain Size Dependence of Secondary Creep Rate in Pure Metals and Single-phase Alloys

Grain size dependence of secondary creep rate in pure metals and single-phase alloys and the mechanisms/models to interpret the dependence have been reviewed. Two types of the grain size dependence were reported, both of which show a negative dependence approximately below 100 μm. The model proposed by Garofalo in 1960s assumes that the density of dislocations generated at grain boundaries and sub-boundaries determines the secondary creep rate, which is not experimentally supported. Mclean’s model considers preferential subgrain growth near grain boundaries, which might be important in practical steels and alloys with sub-grains such as high Cr ferritic heat resistant steels. Grain boundary sliding (GBS) and its accommodation process in grain is considered as a source of the negative grain size dependence. A finite element modeling performed by Crossman and Ashby, which simulated a deformation process where GBS is accommodated by the power law creep process in the grain interior, indicates that the negative grain size dependence cannot be interpreted by the accommodation process. Based on substructure observation and internal stress measurements, Terada established a “core and mantle” model where the mantle region near grain boundary has no internal stress. This model reasonably interprets the negative grain size dependence.

Creep Deformation Behavior and Microstructure of Laves-strengthened Ferritic Heat-resistant Steels Containing Nitrogen or Carbon

The creep deformation behavior and microstructure of a N-containing steel expected to exhibit high creep strength and excellent oxidation resistance were investigated. Even for steel with a high W content, it was possible to form a martensitic microstructure by adding a sufficient amount of N. Comparison of the microstructures of the N-containing steel and a C-containing steel confirmed that the two steels have the same crystal orientation relationship. The N-containing steel precipitated with the Laves phase as a strengthening phase displayed a higher creep strength than conventional steel under relatively high stress. However, the superiority of the creep strength of the N-containing steel relative to the conventional steel decreased under low stress. The stress exponent of the N-containing steel was different from those of the C-containing steel and the conventional steel. This deference considered to be ascribed to the difference of variation behavior of dislocation density during creep deformation.

An overview on the novel heat-resistant ferritic stainless steels

An overview on the novel heat-resistant ferritic stainless steels

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.

Correlation of deformation with damage progression behavior around a notch tip under creep and fatigue conditions for W-added 9Cr steel including weld joint

Research concerning heat-resistant steels for the application in fossil-fired power plants has progressed remarkably during the past 60 years. This has resulted in improvements in the electrical efficiency of fossil-fired power plants. Currently, there are plans and programs to develop ultra-supercritical plants designed to operate at steam temperature and pressure conditions of 600/650 °C and 32 MPa. The W-added 9%Cr ferritic heat-resistant steel, that is, ASME grade P92, has been developed as a boiler material for this ultra-supercritical plant. Boiler materials, whose performance is critical for ultra-supercritical plant, are required to possess high creep resistant properties. In addition, these materials are exposed to fatigue induced by thermal stresses, that is, they are operated under creep-fatigue interacting conditions. In this study, mechanical tests under the condition of high temperature creep-fatigue interaction were conducted for P92 steel under stress-controlled and various load frequency conditions using the in-situ observational creep-fatigue testing machine to observe the damage formation behavior around a notch tip composed of voids in mesoscale. On the basis of these results, the effects of damage formation behavior on crack growth life were clarified. Furthermore, for the case of creep deformation, the numerical analyses of vacancy diffusion and concentration around a notch tip were conducted using our proposed numerical method of local stress-induced vacancy diffusion behavior, which is a nanoscale phenomenon to relate these behaviors to the damage formation behavior in mesoscale (μm scale).

Effect of Al content on the δ→γ transformation and M23C6 precipitation behavior in 18Cr-xAl-Si ferritic heat-resistant stainless steel during solidification process

Aimed at clarifying δ→γ transformation and M23C6 precipitation behavior in18Cr-xAl-Si (x = 0, 0.43wt.%, 0.91wt.% and 1.82wt.%) ferritic heat-resistant stainless steel (FHSS), high temperature confocal scanning laser microscopy (CSLM) was used for in-situ observation during solidification. The results demonstrated that increasing of Al content reduced the nucleation temperature of δ-ferrite, and at the same time reduced the free energy difference between δ-ferrite and γ-austenite, which increased the difficulty of δ→γ transformation, in turn reduced the temperature of δ→γ transformation. The γ-austenite preferentially precipitated at the δ-ferrite phase boundaries and gradually along the sides of the δ-ferrite phase boundaries as the temperature decreased, and part of the martensite transformed inside the γ-austenite phases. M23C6 carbides mainly precipitated at the phase boundaries between δ-ferrite and γ-austenite/martensite. With the increase of Al content, the amount of γ-austenite/martensite and M23C6 carbides decreased. In addition, the solidification mechanism of the complex nucleus was also discussed.

Influence of cooling rate on δ-ferrite/γ-austenite formation and precipitation behavior of 18Cr–Al–Si ferritic heat-resistant stainless steel

The δ-ferrite/γ-austenite formation and precipitation behavior of 18Cr–Al–Si FHSS (ferritic heat-resistant stainless steel) during solidification has been investigated by a high temperature confocal scanning laser microscopy (CSLM) at the cooling rates from 5 °C min−1 to 100 °C min−1. The results have demonstrated that the increase of cooling rate will decrease nucleation temperature of δ-ferrite phase, which will affect δ-ferrite/γ-austenite formation process, and the δ→γ transformation process all occurred at the texted cooling rates. The γ-austenite phase first precipitated at δ-ferrite phase boundaries and then appeared within δ-ferrite phase, and in the subsequent cooling process, part of the γ-austenite begins to transform into martensite. The content of γ-austenite/martensite phase and M23C6 carbides increased with the increment of cooling rate. In addition, the solidification mechanism of the complex nucleus at the cooling rates of 5 °C min−1 and 100 °C min−1 were also discussed.

Determination of Phase Equilibria among δ-Fe, γ-Fe and Fe2M Phases in Fe-Cr-M (M: Hf, Ta) Ternary Systems

Phase equilibria among δ-Fe, γ-Fe, and Fe2M phases in the Fe-Cr-M (M: Hf, Ta) ternary systems were determined using bulk alloys heat-treated at high temperatures. The final goal of this study is to develop novel ferritic heat resistant steels strengthened by precipitation of Fe2M phase on the eutectoid type reaction path: δ → γ + Fe2M. The phases present in heat-treated samples were identified by microstructural characterization and X-ray diffraction pattern analysis. The chemical compositions of the phases were analyzed by energy dispersive spectroscopy. A pseudo-eutectoid trough (δ → γ + Fe2M) exists at ~1220 °C at a Hf content of 0.1% and at ~1130 °C at a Ta content of 0.6% on the vertical section at a Cr content of 9.5% in each ternary system, respectively. A thermodynamic calculation with a database that reflects reported binary phase diagrams and the present study indicates that an increase in the Cr content decreases the temperature and the Hf/Ta contents of the pseudo-eutectoid troughs. The determined phase equilibria suggest that the supersaturation of Hf/Ta for the formation of γ phase is higher in the Hf doped system than in the Ta doped system, which is probably an origin of a much slower kinetics of precipitation on the eutectoid path in the latter system.

機械学習によるフェライト系耐熱鋼のクリープ破断寿命予測

機械学習によるフェライト系耐熱鋼のクリープ破断寿命予測

Oxidation Behaviors of Different Grades of Ferritic Heat Resistant Steels in High-Temperature Steam and Flue Gas Environments

Oxidation Behaviors of Different Grades of Ferritic Heat Resistant Steels in High-Temperature Steam and Flue Gas Environments

Protective Performance of a Novel Inorganic Composite Coatings on CB2 Ferritic Heat Resistant Steel at 650℃ in Oxygen Flow with Water Vapor

Protective Performance of a Novel Inorganic Composite Coatings on CB2 Ferritic Heat Resistant Steel at 650℃ in Oxygen Flow with Water Vapor

Analysis of the Effects of Al on the Ductile-to-Brittle Transition Behavior of Ferritic Heat-Resistant Stainless Steels

Analysis of the Effects of Al on the Ductile-to-Brittle Transition Behavior of Ferritic Heat-Resistant Stainless Steels