High-Temperature Structural Stabilities of Ni-Based SingleCrystal Superalloys Ni–Co–Cr–Mo–W–Al–Ti–Ta with Varying Co Contents

It has been recently pointed out that the compositions of industrial alloys are originated from cluster-plus-glueatom structure units in solid solutions. Specifically for Ni-based superalloys, after properly grouping the alloying elements into Al, Ni-like(■), r-forming Cr-like(■) and c-forming Cr-like(■), the optimal formula for single-crystal superalloys is established [Al–Ni_(12)](Al_1■~_(0:5) ■_(1:5)). The Co substitutions for Ni at the shell sites are conducted on the basis of the first-generation single-crystal superalloy AM3, formulated as [Al–■_(12)Co_x](Al_1Ti_(0.25)Ta_(0.25)Cr_1W_(0.25)Mo_(0.25)), with x = 1.5, 1.75, 2 and 2.5(the corresponding weight percents of Co are 9.43, 11.0, 12.57 and 15.71, respectively). The900 ℃ long-term aging follows the Lifshitz–Slyozov–Wagner theory(LSW theory), and the Co content does not have noticeable influence on the coarsening rate of c0. The microstructure and creep behavior of the four(001) single-crystal alloys are investigated. The creep rupture lifetime is reduced as Co increases. The alloy with the lowest Co(9.43 Co) shows the longest lifetime of about 350 h at 1050 ℃/120 MPa, and all the samples show N-type rafting after creep tests.     

Liquid phase sintering and oxidation resistance of WC–Ni–Co–Cr cemented carbides

Fully dense WC–Ni–Co–Cr alloys have been consolidated via sinter HIP processing. Dilatometric tests show that shrinkage undergoes several accelerations and decelerations during heating, a phenomenon likely associated to the heterogeneous distribution of Cr in the binder phase. WC grain growth follows trends similar to those described for WC–Co hardmetals, increasing with the C activity and the amount of liquid phase of the cermets. Finally, the oxidation resistance of WC–Ni–Co–Cr cemented carbides is observed to improve as the metal content increases and the C content decreases. In both cases, the oxide layers present a higher proportion of (Co, Ni)WO₄ tungstates. The oxide scales formed on compositions with low metal content contain a higher amount of WO₃ oxide.     

High temperature oxidation of 9–12% Cr ferritic/martensitic steels under dual-environment conditions

ABSTRACT

In normal operations, the opposite surfaces of the power plant components are exposed to two different environments, i.e. air/flue gas on the one side and steam on the other side. Exposure under such dual-environment can lead to accelerated corrosion of the components on the air side. The oxidation behaviour of ferritic/martensitic steel T92 was investigated under dual-environment in a specially designed test equipment. The samples were exposed to dry oxyfuel flue gas (CO₂–27%N₂–2%O₂–1%SO₂) on one side and to steam on the other side up to 1000?h at 650°C. The formation of oxide scales was characterised by optical microscopy and scanning electron microscopy with attached energy-dispersive spectroscopy. Oxidation rate of specimens under dual-environment condition was almost three times higher than that in single-environment condition. This is explained based on hydrogen transport through the bulk alloy from the steam side to the flue gas side.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

Comparison between oxidation kinetics of HVOF sprayed WC–12Co and WC–10Co–4Cr coatings

In this study, the high temperature oxidation behavior of HVOF-sprayed WC–12Co and WC–10Co–4Cr coatings were investigated. To explore the oxidation mechanism, thermo-gravimetric analysis (TGA) was applied for isothermal treatments in the range of 500–800 °C for 3 h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the structural changes and microstructural evolutions during oxidation tests. The TGA experiments showed negligible oxidation mass gains at 500 °C for both coatings. At higher temperatures, i.e. 700 and 800 °C, the oxidation mass gains of WC–12Co were found to be much higher than those for WC–10Co–4Cr coating, respectively. The higher oxidation resistance of WC–10Co–4Cr coating probably results from the formation of compact chromium oxide layers and higher MWO₄ type tungstate (M: Co and/or Cr) to tungsten trioxide (WO₃) ratios which provide lower porosity and consequently more efficient passivation effect against oxidation. The time dependent mass gain of WC–12Co coating obeys the linear law within temperature range of 600–800 °C with apparent oxidation activation energy of ~ 104 kJ/mol. As for the oxidation of WC–10Co–4Cr coating, a negligible deviation from linear law was observed possibly due to the presence of chromium oxide and higher tungstate to tungsten trioxide ratio which hinders the diffusion process through the scales compared with WC–12Co coating. The apparent activation energy for oxidation of the WC–10Co–4Cr coating was found to be ~ 121 kJ/mol.     

Structure and oxidation resistance of W1–xAlxN composite films

A series of W_1–xAl_xN films (0<x<38.6%, mole fraction) were deposited by reactive magnetron sputtering. The composition, microstructure, mechanical properties and oxidation resistance of the films were characterized by EPMA, XRD, XPS, nano-indentation, SEM and HRTEM. The effect of Al content on the microstructure and oxidation resistance of W_1–xAl_xN films was investigated. The results show that WN film has a face-centered cubic structure. The preferred orientation changes from (111) to (200). The W_1–xAl_xN films consist of a mixture of face-centered cubic W(Al)N and hexagonal wurtzite structure AlN phases. The hardness of the W_1–xAl_xN films first increases and then decreases with the Al content increasing. The maximum hardness is 36 GPa, which is obtained at 32.4% Al (mole fraction). Compared with WN film, the W_1-xAl_xN composite films show much better oxidation resistance because of the formation of dense Al₂O₃ oxide layer on the surface.     

Transient oxidation of Mo–Si–B alloys: Effect of the microstructure size scale

The composition and phase constituency of Mo–Si–B alloys are known to be important parameters in determining the oxidation response. For three phase Mo + T₂ + Mo₃Si alloys with constant composition and phase constituency, it is observed that a refined microstructure scale provides superior oxidation resistance. The transient stage of oxidation is shortened and the recession of the alloy is decreased with microstructural refinement. In order to identify the phase interaction during the transient stage, oxidation of each of the three alloy phases, Mo, Mo₃Si (A15) and Mo₅SiB₂ (T₂) has been investigated separately. Quantification of the separate phase size distributions by image analysis was coupled with the individual phase oxidation response to evaluate the overall oxidation behavior and phase interaction effects. A kinetic model for oxidation of Mo–Si–B alloys is proposed that incorporates the key role of microstructure scale on the transient stage and provides guidance for microstructure design.     

The influence of the content of certain incidental elements on the cyclic oxidation resistance of 12–13% chromium steels

Cyclic oxidation tests within the temperature range 500–900°C have been carried out on a series of 12–13% chromium steels produced experimentally using pure raw materials, the objective being to determine the effects of the presence of Si, Mn, Ni, S and P in the contents typical of commercially produced steel. A “pure” steel was found to behave in a manner considerably inferior to that of commercially produced steels but the additions of a small amount of silicon (0·16%) gave improvement in oxidation behaviour to the level of that of the commercial steels. The other elements had no marked effect on results although the presence of phosphorus (0·018%) in otherwise “pure” steel proved to be slightly detrimental.     

Synthesis of Fe–Cr–Mo–C–B–P bulk metallic glasses with high corrosion resistance

Bulk metallic glasses with a maximum thickness (t_max) of 1.0–2.7 mm were synthesized in the Fe₄₃Cr₁₆Mo₁₆(C, B, P)₂₅ system over a wide composition range by copper mold casting. They exhibit a large supercooled liquid region (ΔT_x) of 40–90 K and a high reduced glass transition temperature (T_g/T_l) of 0.54–0.60, indicating high glass-forming ability (GFA) and high thermal stability of the supercooled liquid. The critical cooling rate for glass formation was evaluated to be of the order of 10² K s⁻¹. The bulk metallic glasses exhibited high corrosion resistance in aggressive HCl solutions. The alloying element P has a beneficial effect on corrosion resistance.     

High-temperature oxidation behavior of Mo–Si–B-based and Co–Re–Cr-based alloys

Improving the efficiency of aerospace gas turbine engines requires materials that can be used at increasingly higher temperatures in aggressive environments. This paper summarizes the current stage of alloy development of Mo–Si–B-based and Co–Re–Cr-based alloys regarding the high-temperature oxidation resistance. Since refractory metals, such as Mo and Re, suffer from catastrophic oxidation, the main task of research is to find alloying elements that improve the oxidation behavior of these alloys. For Mo–Si–B-based alloys, it was observed that an addition of Zr has a significant positive influence on the oxidation resistance by reducing the time necessary for the formation of a protective borosilicate layer. An addition of 0.2 at.% Y improves the viscous properties of the borosilicate increasing the protectiveness of the oxide scale. Macroalloying with Ti yields a strong positive effect on the oxidation behavior and, in addition, notably reduces the density of Mo–Si–B-based alloys. In Co–Re–Cr-based alloys, Cr is included to achieve favorable mechanical properties and to form a protective chromia layer during oxidation. As a consequence of the synergetic effect of Cr and Si, an addition of 2 at.% Si significantly improves the oxidation behavior of the alloy. Al addition further promotes the formation of the protective chromia layer at intermediate temperatures and exhibits the potential of the formation of a protective alumina scale suitable for applications at very high temperatures. The critical evaluation of the complex oxidation behavior of both metallic systems in a broad temperature range gives insight into the underlying fundamental mechanisms, reveals the potentials of particular alloying elements and, thus, guides future development of these material classes.     

Evolution of surface chemistry and morphology of oxide scale formed during initial stage oxidation of modified 9Cr–1Mo steel

Initial stage oxidation characteristics of the modified 9Cr–1Mo steel in ambient air at 650 °C have been investigated, for exposure times ranging from 5 to 500 h. Oxygen flux from the gas phase causes high initial oxidation rate, but the growth kinetics do not follow parabolic law. In “as-received” condition, binary oxides of Fe and Cr were found as native oxides. Upon oxidation, segregation of Mn resulted in the formation of MnCr₂O₄ along with FeCr₂O₄ and binary oxides of Fe, Cr and Mn. Thus, the initial oxide scale constitutes multiple oxides with delineated interface, unlikely to have a layered structure.     

Oxidation pre-treatment to improve the mechanical property and oxidation resistance of Si–Mo–Cr coated C/C composites

A transition layer with an interlocking structure was obtained at the interface between C/C composites and Si–Mo–Cr coating by oxidation pre-treatment and pack cementation process. The effects of pre-oxidation time on the microstructures, mechanical property and oxidation behavior of the coated C/C samples were investigated. After oxidation pre-treatment at 1173 K in air for 6 min, the flexural strength of the coated samples increases by 71.7% and the mass loss rate decreases by 77.4%. The improvements of the mechanical property and oxidation resistance are primarily attributed to the formation of the moderate transition layer between C/C and Si–Mo–Cr coating.     

Cold resistance of low-carbon Mn−Ni−Mo−V steels for pressure vessels

Conclusions  

Microstructure and oxidation resistance of Si–Mo–B coating for C/SiC coated carbon/carbon composites

A self-sealing Si–Mo–B oxidation resistance coating was prepared on C/SiC coated carbon/carbon (C/C) composites by slurry and high temperature treatment method. The oxidation resistance of the coating increases at 1173 K and first increases then decreases at 1873 K with the increase of B content from 0 to 20 wt.%. The C/SiC/gradient Si–Mo–B multilayer coating can protect C/C composite from oxidation for 100 h at 1173 K and 125 h at 1873 K. The good oxidation resistance of the coating in broad temperature range could be attributed to its good self-sealing property.     

Correlation of precipitate stability to increased creep resistance of Cr–Mo steel welds

An order of magnitude decrease (from 16.0 × 10^?4 to 4.1 × 10^?4% h^?1) in steady-state creep rate was observed in the fine-grained heat-affected zone (HAZ) of a Cr–Mo steel weld by the reduction of the pre-weld tempering temperature from 760 °C (HTT) to 650 °C (LTT). The microstructure during each stage of the manufacturing path, including pre-weld temper, thermal cycling and post-weld heat treatment, was characterized using a suite of characterization techniques. The techniques included simulated thermal cycling, dilatometry and electron microscopy, as well as time-resolved X-ray diffraction using Synchrotron radiation. Both LTT and HTT steels before welding contain M₂₃C₆ (M = Cr, Fe) and MX (M = Nb, V; X = C, N) precipitates in a tempered martensite matrix. During simulated HAZ thermal cycling with different peak temperatures, changes in M₂₃C₆ carbide characteristics were observed between the HTT and LLT conditions, while MX precipitates remained stable in both conditions. Simulated post-weld heat treatment samples show larger M₂₃C₆ in the HTT condition than in the LTT condition. The results provide a solution to extending the life of Cr–Mo steel welded structures used in power plants.     

The resistance to stress corrosion cracking of some Cr–Ni–Fe austenitic steels and alloys

Abstract

Stress-corrosion cracking testing by a variety of methods has been carried out in chloride and caustic environments on a series of Cr–Ni–Fe austenitic steels and alloys containing between 10 and 25 % of chromium and 15 and 45% of nickel. Limited testing has also been carried out on alloys containing additions of molybdenum and copper. The tests have confirmed that increasing the nickel content reduces the susceptibility of Cr–Ni–Fe alloys to stress-corrosion cracking in chloride solutions. Chromium content also affects cracking susceptibility but to a lesser degree. Stress corrosion susceptibility in caustic solutions is affected by these alloying elements in a different way. The results are discussed in relation to currently proposed theories of stress-corrosion cracking.     

Thermodynamic stability of Co–Al–W L12 γ′

Co-based superalloys in the Co–Al–W system exhibit coherent L1₂ Co₃(Al,W) γ′ precipitates in an face-centered cubic Co γ matrix, analogous to Ni₃Al in Ni-based systems. Unlike Ni₃Al, however, experimental observations of Co₃(Al,W) suggest that it is not a stable phase at 1173 K. Here, we perform an extensive series of density functional theory (DFT) calculations of the γ′ Co₃(Al,W) phase stability, including point defect energetics and finite-temperature contributions. We first confirm and extend previous DFT calculations of the metastability of L1₂ Co₃(Al_0.5W_0.5) γ′ at 0 K with respect to hexagonal close-packed Co, B2 CoAl and D0₁₉ Co₃W using the special quasi-random structure (SQS) approach to describe the Al/W solid solution, employing several exchange/correlation functionals, structures with varying degrees of disorder, and newly developed larger SQSs. We expand the validity of this conclusion by considering the formation of antisite and vacancy point defects, predicting defect formation energies similar in magnitude to Ni₃Al. However, in contrast to the Ni₃Al system, we find that substituting Co on Al sites is thermodynamically favorable at 0 K, consistent with experimental observation of Co excess and Al deficiency in γ′ with respect to the Co₃(Al_0.5W_0.5) composition. Lastly, we consider vibrational, electronic and magnetic contributions to the free energy, finding that they promote the stability of γ′, making the phase thermodynamically competitive with the convex hull at elevated temperature. Surprisingly, this is due to the relatively small finite-temperature contributions of one of the γ′ decomposition products, B2 CoAl, effectively destabilizing the Co, CoAl and Co₃W three-phase mixture, thus stabilizing the γ′ phase.     

The hardening mechanism of Cr−Ni and Ni maraging steels

Metal Science and Heat Treatment -     

First Principles Study of Structural Stability and Magnetic Property of Non-equilibrium Co–Mo Alloys

First principles calculations are carried out to investigate the structural stability of several non-equilibrium intermetallic phases in the cobalt(Co)–Mo system using spin polarized projected augmented-wave potentials. It is revealed that the Co3Mo, CoMo, and CoMo3 alloys are energetically favored to be in D019, B11, and A15 structures, respectively,and that the magnetic moments of Co atoms would decrease rapidly with an increasing percentage of Mo content and would most probably disappear when the content of Mo is no less than 50 at%. Generally, the calculated results in the present work match well with the available experimental observations.     

Evaluation of repair welding procedures for 2.25Cr–1Mo and 9Cr–1Mo steel welds

Abstract

Chromium–molybdenum steels are extensively used in the steam generator circuits of power plants. These components may require welding of the cracks that can develop during fabrication, storage, and transportation stages, or during the service life of the plant. This investigation compares repair welding methods for Cr–Mo steels, using 2.25Cr–1Mo and 9Cr–1Mo materials. To simulate aging during service, welds were heat treated at 873 K for 5000 h. Simulated repair welding of the aged welds was carried out at the weld/base metal interface, i.e. at the location at which cracks are usually reported to occur during service. Two repair welding methods (half bead and butter bead temper bead methods) conforming to the ASME Boiler and Pressure Vessel Code were used. Tensile properties, hardness profiles, and X-ray diffraction based residual stress distributions were determined for both the Cr–Mo steel welds to evaluate the simulated repair welds. Analysis of the test results showed that both the repair welding methods can be used for 2.25Cr–1Mo steel welds, although the butter bead temper bead method is much more suitable for both the 2.25Cr–1Mo and 9Cr–1Mo steel welds.     

Microstructure and oxidation resistance of Mo–Si–B coating on Nb based in situ composites

The development of robust high temperature oxidation resistant coatings for Nb–Si based alloy was evaluated for a Mo–Si–B coating system that was applied by a two step process. It is observed that the coating is composed of an outer layer of MoSi₂ containing boride dispersoids and an inner layer of unreacted Mo. The mass gain of substrate and Mo–Si–B coating is 190.08 and 1.28 mg cm⁻² after oxidation at 1250 °C in dry air for 100 h, respectively. The good oxidation resistance of the coating is attributed to the formation of a continuous borosilicate glass coverage.