Nb Wt Research Articles

Development of Ultra-high Purity (UHP) Fe-Based Alloys with High Creep and Oxidation Resistance for A-USC Technology

The design of ultra-high purity (UHP) Fe-based model alloys for advanced ultra-supercritical (A-USC) technology is attempted in this work. Creep testing has been performed in air at 700 °C and a stress level of 150 MPa. Analysis of the fracture surface and cross section of the crept specimen was performed. To evaluate the oxidation resistance in A-USC conditions, oxidation testing was performed in supercritical water (SCW) at 700 °C and 25 MPa. Weight gain (WG) measurements and meticulous characterization of the oxide scale were carried out. Based on thermodynamics and density functional theory calculations, some reactive elements in the Fe-Cr-Ni system were designated to promote precipitation strengthening and to improve the hydrogen-accelerated oxidation resistance. The addition of a 2 wt pct Mo into Fe-22Cr-22Ni-0.6Nb wt pct-based matrix did not significantly improve the creep resistance. The addition of 0.26 wt pct Zr coupled with cold working was effective for improving creep properties. The Mo-modified model alloy showed almost the same WG value as SUS310, while the Zr-modified alloy showed a higher WG value. Meanwhile, a Cr-enriched continuous oxide layer was formed at the oxidation front of the Zr-modified alloy and SUS310S after exposure to SCW conditions.

Effect of NbC Addition on the High-Temperature Oxidation Resistance of Co- and Ni-Based Chromium-Rich Alloys

Two {M–25Cr}-based alloys, with M = Co or Ni, containing 0.5C and 3.9Nb (wt%) to favor the crystallization of NbC carbides during solidification were prepared by casting and then subjected to oxidation in air at 1000, 1100 and 1200 °C for 46 h. The results were studied by comparison with recent similar ones obtained with Nb-free ternary alloys issued from the same preparation protocol and tested in oxidation in the same conditions. In both cases interdendritic script-like-shaped NbC carbides were obtained as expected, however, not as single carbides in the nickel alloy which also contain chromium carbides. Preliminary exposure to 1200 °C having shown a morphological evolution of the NbC, as-cast and preliminarily aged versions of both alloys were tested in oxidation. It appeared first that, whatever the aged state, the cobalt alloy behaved much worse than the nickel one, that the presence of Nb deteriorated the behavior in both cases and that aging the alloys, which resulted in NbC fragmentation, was also detrimental for the high-temperature oxidation behavior.

Oxidation Behavior of UHP Fe-Based Model Alloy and SUS310S Steel in Superheated Steam and Supercritical Water Conditions

The quest for improvements in cycle efficiency of energy production plants leads to progressively higher operating temperatures and pressures in steam boiler. For this reason, the oxidation resistance of structural materials is the key issue, and hence, the consequences of increased growth rate of the oxides pose serious concern, such as oxide exfoliation. The oxidation behavior of ultra-high-purity (UHP) Fe–23Cr–23Ni–2Mo–0.6Nb wt% model alloy and SUS310S in steam and super critical water at atmospheric pressure and 25 MPa, respectively, and at 973 K (700 °C) is investigated. To elucidate the oxidation behavior, weigh gain measurement and characterization of the resulting oxide were performed. The critical chromium content to form and maintain protective oxide increased with increasing the pressure, 23 wt% in steam and 26 wt% in SCW conditions. Grain refinement was efficient for enhancing the oxidation resistance of SUS310S.

Ti-Al-Nb Material for High-Temperature Applications: Combustion Synthesis from Oxide Raw Materials

This work is focused on preparation of Nb-doped Ti–Al material (Ti–29.3Al–17.6Nb wt %) by centrifugal SHS. Results of studying of combustion and synthesis regularities of intermetallic material on γ-TiAl base from oxide materials under high gravity are presented in the article. For the first time is used a reduction mixture on Al/Ca base which markedly increases the yield of Ti–Al–Nb and decreases the amount of non-metallic impurities (oxygen, nitrogen, carbon) in target product.

Relation between microstructure and formability of quenching-partitioning-tempering martensitic steel

Investigation of the mechanical properties and forming limit diagrams (FLDs) of Fe-0.20C-1.49Mn-1.52Si-0.58Cr-0.05Nb (wt%) steel treated by conventional quenching and tempering (Q&T) and a novel quenching-partitioning-tempering (Q-P-T) process has revealed that the latter produces more retained austenite (10.8%) than the former (less than 3%) and improves the formability. The better formability of the Q-P-T martensitic steel is attributed to a higher strain hardening exponent (n¯=0.0958) and true uniform elongation (ε¯u=10.3%) than the Q&T martensitic steel (n¯=0.0410,ε¯u=3.6%), which leads to a lower yield ratio (σs/σb) and higher plastic strain ratio (r). The high value of n is found through microstructural characterization to stem from the high strain hardening rate of the retained austenite during deformation, as well as a low dislocation density in the martensitic matrix prior to deformation due to carbon partitioning into the retained austenite during the Q-P-T process. The high εu is attributed to the dislocation absorption by retained austenite (DARA) effect, which effectively enhances the deformability of the martensitic matrix, as well as the transformation induced plasticity of retained austenite during deformation. Since there is no carbon-depletion in the single-phase martensitic matrix of the Q&T steel, it exhibits reduced formability due to a lower value of n and εu.

Phase transformation and shape memory effect of a Cu-Al-Ni-Mn-Nb high temperature shape memory alloy

The microstructure, phase transformation and shape memory effect of the high temperature shape memory alloy Cu-11.4Al-3.2Ni-3Mn-0.5Nb (wt%) were investigated for the first time. The shape recovery ratio can achieve 73% when submitted to 690MPa and the compressive stress of fracture of the alloy is 1560MPa.

Effect of retained austenite on the dynamic tensile behavior of a novel quenching-partitioning-tempering martensitic steel

The dynamic tensile test with a strain rate of 500s−1 and the quasi-static tensile test with a strain rate of 5.6×10−4s−1 were performed for a novel Fe-0.20C-1.49Mn-1.52Si-0.58Cr-0.05Nb (wt%) quenching-partitioning-tempering (Q-P-T) martensitic steel with high amount of retained austenite, respectively. This low carbon steel was also treated by the traditional quenching and tempering (Q&T) process, and the same experimental tests were performed for the low carbon Q&T martensitic steel with little retained austenite to understand the effect of the retained austenite on the dynamic tensile behavior. The results indicate that compared with the quasi-static tensile test, the high strain rate in the dynamic tensile test raises the strength of the Q-P-T steel. However, the elongation slightly decreases. These results differ from the enhancement in both the strength and elongation of the Q&T steel in the dynamic tensile test. The increase in the strength of the Q-P-T steel in the dynamic tensile test is attributed to the strain rate hardening effect. The slight decrease in the elongation stems mainly from that the suppression of the dislocation absorption of the retained austenite (DARA) effect existing in the quasi-static tensile test, moreover, such a suppression is not effectively complemented by the adiabatic softening of the martensitic matrix in dynamic tensile test. The marked increase in the elongation of the Q&T steel in the dynamic tensile test is only attributed to the adiabatic softening of the martensite matrix because there is no DARA effect in the Q&T steel with little retrained austenite.

The numerical and experimental evaluations in the weld mushy zone of modern third generation nickel-based superalloys

The goal of the current research is to investigate the weld mushy zone in the newly designed third-generation superalloys numerically and experimentally, in order to predict the weld mushy zone solidification temperature range and size and predict the weldability. The mathematical equations for interpretation of weld mushy zone temperature have been obtained. Also, the equations which calculate the distance of the weld mushy zones are extracted. The equations correlate the retained liquid fraction to the temperature and longitudinal coordination of weld centerline. The calculations indicated that welds containing high amount of Ti and Nb simultaneously display wider weld mushy zone temperature range and size, while welds containing only one of Nb or Ti exhibit shorter weld mushy zone temperature range and size. The mushy zone temperature range of weld containing 5.6 wt.% (Ti + Al), 2.7 wt.% Nb reaches to a maximum of 212 °C. The output of the experimental observations demonstrates an appropriate validity for the mushy zone modelling. According to the numerical and experimental results, the composition ranges of {5.2 ≤ Nb wt. % ≤ 5.6} /{1.7 ≤ (Ti + Al) wt. % ≤ 2.4}, and{0 ≤ Nb wt. % ≤ 0.5} /{9.3 ≤ (Ti + Al) wt. % ≤ 10 } offer the best weldability for the newly designed superalloys.

Dynamic Compression Behavior and Microstructure of a Novel Low-Carbon Quenching-Partitioning-Tempering Steel

A 0.2C-1.5Mn-1.5Si-0.6Cr-0.05Nb (wt%) steel is treated respectively by novel quenching-partitioning-tempering (Q-P-T) process and traditional quenching and tempering (Q&T) process for comparison. X-ray diffraction analysis indicates that Q-P-T steel has about 10% retained austenite, but Q&T steel hardly has one. With the increase of compression strain rate from 7 × 102 to 5 × 103 s−1, the flow stress of Q-P-T steel increases, which demonstrates the positive strain rate effect, but does not exist in Q&T steel. The characterization of scanning electron microscopy indicates that a large number of long, straight martensite laths in Q-P-T steel will bend or be destroyed by large compressive strain of 35% at 5 × 103 s−1. However, relative small compressive strain of about 5% at 7 × 102 s−1 almost does not have any effect on the original lath morphology. The characterization of transmission electron microscopy further reveals the origin of the positive strain rate effect and the microstructural evolution during dynamic compressive deformation.

Corrosion of alumina‐forming austenitic steel in molten nitrate salts by gravimetric analysis and impedance spectroscopy

In recent years, the study of renewable energies and its practical application has increased significantly. Solar energy feasibility entails the development of energy storage systems since solar power plants need to be working in unfavorable weather or night periods. The main heat transfer fluid (HTF) used on these plants is a salt mixture of 60% NaNO3/40% NaNO3 which must be kept above 220 °C to prevent freezing. This high operating temperature causes corrosion problems for steels in contact with the HTF, reducing the lifetime of the solar plants. The present research studies the potential of an alumina‐forming austenitic (AFA) stainless steel (OC‐4, Fe‐25Ni‐14Cr‐3.5Al‐2.5Nb wt% base) as a candidate material for solar plant heat exchangers and pipes. Corrosion behavior of OC‐4, relative to 304 stainless steel and T22 steel, was studied by gravimetric analysis and electrochemical impedance spectroscopy (EIS). The AFA OC‐4 exhibited better corrosion resistance in HTF at 390 °C than the currently used 304 austenitic stainless steel.

Tensile and impact properties of low nickel maraging steel

This short communication presents a research update of a new low nickel maraging steel, Fe–12.94%Ni–1.61%Al–1.01%Mo–0.23%Nb (wt%). Its yield stress and the tensile strength are 1080MPa and 1180MPa, respectively, after ageing treatment. Tensile specimens show ductile fracture. Fractography demonstrated deep dimples. Impact energy is 22J on half-size specimens.

Effect of deformation temperature on niobium clustering, precipitation and austenite recrystallisation in a Nb–Ti microalloyed steel

The effect of deformation temperature on Nb solute clustering, precipitation and the kinetics of austenite recrystallisation were studied in a steel containing 0.081C–0.021Ti–0.064Nb (wt%). Thermo-mechanical processing was carried out using a Gleeble 3500 simulator. The austenite microstructure was studied using a combination of optical microscopy, transmission electron microscopy, and atom probe microscopy, enabling a careful characterisation of grain size, as well as Nb-rich clustering and precipitation processes. A correlation between the austenite recrystallisation kinetics and the chemistry, size and number density of Nb-rich solute atom clusters, and NbTi(C,N) precipitates was established via the austenite deformation temperature. Specifically, we have determined thresholds for the onset of recrystallisation: for deformation levels above 75% and temperatures above 825°C, Nb atom clusters <8nm effectively suppressed austenite recrystallisation.

Deformation Temperature Dependence of Mechanical Properties and Microstructures for a Novel Quenching–Partitioning–Tempering Steel

The present investigation on a designed high strength Fe–0.25C–1.48Mn–1.20Si–1.51Ni–0.05Nb (wt%) steel treated by a novel quenching–partitioning–tempering (Q–P–T) process was focused on deformation temperature dependence of mechanical properties and microstructures. The results indicate that the Q–P–T steel deformed at various deformation temperatures from −70 to 300 °C exhibits superior mechanical properties due to excellent thermal stability of retained austenite. The microstructural characterization by transmission electron microscopy (TEM) reveals that the high strength of the Q–P–T steel results from dislocation-type martensite laths and fcc NbC carbides or/and hcp ɛ-carbides precipitated dispersively in martensite matrix, while excellent ductility is attributed to the significant transformation induced plasticity (TRIP) effect produced by considerable amount of retained austenite. The relationship between mechanical properties and microstructures at different deformation temperatures was clarified.

Effect of Bake-Hardening Treatment on the Mechanical Properties of High Strength Bainitic Steel Produced by Thermomechanical Processing

The influence of pre-straining and bake-hardening on the mechanical properties of thermomechanically processed 0.2C-1.5Si-1.5Mn-0.2Mo-0.004Nb (wt%) steel was analysed using tensile test, transmission electron microscopy (TEM) and atom probe tomography (APT). This steel after processing had high strength (~1200MPa) and good ductility (~20%) due to the formation of fully bainitic microstructure with nanolayers of bainitic ferrite and retained austenite. The bake hardening (BH) of pre-strained (PS) samples increased the yield strength of steel up to 690MPa and showed the bake-hardening response of 220MPa due to the operation of several strengthening mechanisms such as transformation induced plasticity during pre-straining and pinning the dislocations by carbon during bake-hardening treatment. The carbon content of the bainitic ferrite and retained austenite before and after bake-hardening treatment, the solute distribution between these phases and the local composition of fine Fe-C clusters and particles formed during bake-hardening treatment was calculated using APT. The bainitic ferrite and retained austenite microstructural characteristics such as thickness of the layers and their dislocation density before and after bake-hardening treatment were studied using TEM.

The Mechanism of High Strength-Ductility Steel Produced by a Novel Quenching-Partitioning-Tempering Process and the Mechanical Stability of Retained Austenite at Elevated Temperatures

The designed steel of Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt pct) treated by a novel quenching-partitioning-tempering (Q-P-T) process demonstrates an excellent product of strength and elongation (PSE) at deformed temperatures from 298 K to 573 K (25 °C to 300 °C) and shows a maximum value of PSE (over 27,000 MPa pct) at 473 K (200 °C). The results fitted by the exponent decay law indicate that the retained austenite fraction with strain at a deformed temperature of 473 K (200 °C) decreases slower than that at 298 K (25 °C); namely, the transformation induced plasticity (TRIP) effect occurs in a larger strain range at 473 K (200 °C) than at 298 K (25 °C), showing better mechanical stability. The work-hardening exponent curves of Q-P-T steel further indicate that the largest plateau before necking appears at the deformed temperature of 473 K (200 °C), showing the maximum TRIP effect, which is due to the mechanical stability of considerable retained austenite. The microstructural characterization reveals that the high strength of Q-P-T steels results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp e-carbides in martensite matrix, while excellent ductility is attributed to the TRIP effect produced by considerable retained austenite.

High strength-elongation product of Nb-microalloyed low-carbon steel by a novel quenching–partitioning–tempering process

Abstract In this article, a novel quenching–partitioning–tempering (Q–P–T) process was applied to a Fe–0.25C–1.5Mn–1.2Si–1.5Ni–0.05Nb (wt%) hot-rolled steel, and its optimized parameters were obtained by a Gleeble-3500 thermal simulator and salt baths, respectively. Mechanical property results of the as-treated Q–P–T samples show that the Nb-microalloyed low-carbon steels subjected to Q–P–T processes cover a wide spectrum of strength (1200–1500 MPa) and elongation (14–18%), and exhibit excellent product of strength and elongation (21,000–22,000 MPa%). Microstructural characterization indicates that high strength results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp ɛ-carbides in martensite matrix and good ductility is attributed to transformation induced plasticity (TRIP) effect from plenty of retained austenite flakes between martensite laths.

Incorporation of Nb(V) into BEA zeolite investigated by XRD, NMR, IR, DR UV–vis, and XPS

Nb x SiBEA ( x = 1.0, 1.5 Nb wt%) zeolites are prepared by a two-step postsynthesis method involving first the creation of vacant T-atom sites associated with silanol groups by dealumination of BEA zeolite by nitric acid solution and then by impregnation of the resulting SiBEA zeolite with the niobium ethoxide Nb(OC 2H 5) 5 precursor. The incorporation of Nb into the framework of SiBEA as isolated mononuclear Nb(V) species is evidenced by XRD, 29Si MAS NMR, 1H– 29Si CP-MAS NMR, IR and 1H MAS NMR. DR UV–vis investigation shows that two types of framework mononuclear Nb(V) ions are present, evidenced by two bands at 222 and 236 nm, the intensity of which increases with Nb content. XP spectra with a doublet peaks at BE of 209.6–209.9 eV for Nb 3d 3/2 and of 206.9–207.5 eV for Nb 3d 5/2 show that niobium is present in the framework BEA zeolite as Nb(V).

The Influence of Niobium Microalloying on Austenite Grain Coarsening Behavior of Ti-modified SAE 8620 Steel

The potential for suppressing unacceptable austenite grain growth during carburizing by Nb microalloying additions in the range of 0.02 to 0.11 wt% to a Ti-modified SAE 8620 carburizing steel were evaluated. Alloys, were designed based on fundamental equilibrium thermodynamic analyses, as part of an extensive study on the effects of alloy composition, thermomechanical history, and pseudo-carburizing conditions on austenite grain coarsening behavior. Laboratory samples were produced to simulate both conventional hot rolling and controlled rolling practices designed to produce different initial precipitate distributions. Pseudo-carburizing heat treatments, i.e. without a carburizing atmosphere, were performed in the temperature range of 950 to 1100°C for holding times of 30 to 360 min. Precipitate distributions, including size, number density, morphology, distribution, and chemical composition in selected samples from the as-rolled and pseudo-carburized conditions were evaluated with transmission electron microscopy on extraction replicas. Results showed that increasing Nb additions to the Ti-modified SAE 8620 steel restrained austenite grain coarsening, and increased the grain coarsening time, especially at temperatures below 1050°C. The Nb-free (Ti-modified) steel yielded either severely duplex grain structures or pseudo-normal grain growth (with very large mean grain diameter). However, holding a Ti–Nb-modified steel (e.g. 0.06 Nb wt%) at 950°C for 6 h or at 1000°C for 4 h. produced fine and uniform austenite grain structures (with a mean grain diameter less than 20 μm). The finer grain sizes observed in the Ti–Nb-modified steels were due to the presence of Nb-rich precipitates that hinder austenite grain coarsening, and precipitate distributions and grain growth behaviors are also influenced by the steel rolling history. The results indicate that Nb can successfully be used to suppress grain growth in carburizing steels.

Effect of Annealing Conditions on the Microstructure and Corrosion Characteristics of Zr-xNb Alloys

Most of the advanced Zr-based alloys contain Nb for improving the corrosion resistance. However, the Nb effect on the corrosion behavior of Zr alloys has not been established. For developing fuel cladding materials, it is essential to investigate the effect of the Nb-content and annealing conditions after beta quenching on the microstructure and the corrosion of Zr-xNb alloys. In this study, a systematic investigation to obtain the optimized annealing condition and Nb content were performed for Zr-xNb alloys (x = 0.1~2.0 wt.%). The corrosion resistance of 0.1 and 0.2 wt.% Nb alloys where Nb existed in an equilibrium soluble state in the matrix was excellent in the quenched and annealed conditions. For the high Nb-containing alloys, the corrosion rates were very sensitive to the annealing condition, and it took about 50 hours at 570 to reach the corrosion rates comparable to those of the low Nb wt.% alloys. The corrosion resistance was closely related to the stabilization of the tetragonal ZrO2 and the columnar oxide structure when the Nb concentration in the matrix was reduced to the equilibrium soluble limit.

Anelastic Relaxation due to Interstitial Solutes Diffusion in Nb and Nb-1 wt% Zr

Metals and alloys containing solute atoms dissolved interstitially often show anelastic behavior due to a process know as stress-induced ordering. The application of mechanical spectroscopy measurements to diffusion studies in body-centered cubic metals has been extensively used in the last decades. However the kind of preferential occupation of interstitial solutes in bodycentered cubic metals is still controversial. The anelastic properties of the Nb and Nb-1 wt% Zr polycrystalline alloys were determined by internal friction and oscillation frequency measurements using a torsion pendulum inverted performed between 300K and 650K, operating in a frequency oscillation in the hertz bandwidth. The interstitial diffusion coefficients of oxygen and nitrogen in Nb and Nb-1 wt% Zr samples were determined at two distinct conditions: (a) for low concentration of oxygen and (b) for high concentration of oxygen.