Temperature field analysis and its application in hot continuous rolling of Inconel 718 superalloy

A coupled thermo-mechanical model containing metal flow and temperature field for calculating temperature variation has been developed on fourteen-pass hot continu-ous rolling of round rod for Inconel 718 alloy using 3D elastic-plastic finite element method(FEM).The temperature of characteristic analysis points in the intermedi-ate cross-section of the workpiece has been simulated at initial temperature ranging from 960 to 1000 °C and initial velocity in range of 0.15-0.55 m·s-1.Based on fi-nite element ana...     

Effect of Hf and Y alloy additions on aluminide coating performance

Iron- and Ni-base alloys, with and without Hf or Hf and Y alloy additions, were aluminized by chemical vapor deposition to study the potential for minor alloy additions to improve oxidation resistance of coated alloys. Compared to uncoated specimens, the coated specimens showed improved cyclic oxidation resistance at 1100° and 1150 °C. However, alumina scale spallation was observed at relatively short times and, particularly for the Ni-base alloy X, the aluminized lab-cast alloy with Hf tended to have poor coating performance compared to the commercial alloy without Hf. Internal oxidation of Hf at 1150 °C and rapid Al depletion in the relatively thin aluminide coatings contributed to the observed detrimental Hf effect. For the Ni-base alloys, the increased scale spallation could be attributed to much higher S contents (10-50 ppma) in the laboratory-cast alloys. Oxide scale spallation from the coating surface was minimized when Hf and Y were added to a casting and the [Y]/[S] content ratio was ∼ 1.     

Influence of a mixed Al2O3+Cr2O3 superficial coating on the non-isothermal oxidation behavior of Fe and Fe-Cr alloys

The non-isothermal oxidation behavior of electrolytic-grade iron and Fe-Cr alloys in dry air has been studied using linear heating rates of 6 K/min, 10 K/ min, and 15 K/min up to a final temperature of 1273–1473 K. Some of the iron and iron-chromium alloy samples were given a surface treatment by dipping them in an aqueous solution containing both Cr and Al ions before their oxidation studies. This pretreatment has resulted in improved oxidation resistance and scale adherence as depicted by no scale rupture even after a second thermal cycle. Mass changes were recorded gravimetrically, and scales have been characterized by SEM, EPMA, and x-ray diffraction analyses.     

The electrochemical polishing behavior of the Inconel 718 alloy in perchloric-acetic mixed acids

The electropolishing behavior of the Inconel 718 alloy was studied by using rotating disc electrode (RDE) in the HClO₄-CH₃COOH mixed acids with different HClO₄-concentrations. After electropolishing, surface morphologies of RDE specimens were examined with surface profiler, atomic force microscope and scanning electron microscope. According to the surface morphologies observed, three types of anodic dissolution behavior can be characterized in relation to the HClO₄-content in mixed acids; namely, leveling without brightening of the surface in the mixed acids with 10 and 20 vol% HClO₄, leveling and brightening of the surface in the mixed acids with 30 and 40 vol% HClO₄, and a matt and gray surface in the mixed acids with 50 vol% or more HClO₄. Anodic dissolution in the first and second dissolution types follows a mass-transfer controlled mechanism, in which a linear relationship between the reciprocal of limiting-current density and the reciprocal of square root of rotating speed of RDE specimen can be detected. Owing to precipitation of salt film on the polished surface of the Inconel 718 material, saturated dissolved metallic ions could be the chemical species for the mass-controlled mechanism. The salt film, in addition, could enhance the corrosion resistance of the Inconel 718 alloy.     

The high-temperature oxidation behavior of vanadium-aluminum alloys

The oxidation behavior in air of pure vanadium, V-30Al, V-30Al-10Cr, and V-30Al-10Ti (weight percent) was investigated over the temperature range of 700–1000° C. The oxidation of pure vanadium was characterized by linear kinetics due to the formation of liquid V₂O₅ which dripped from the sample. The oxidation behavior of the alloys was characterized by linear and parabolic kinetics which combined to give an overall time dependence of 0.6–0.8. An empirical relationship of the form: W/A=Bt + Ct^1/2 + D was found to fit the data well, with the linear contribution suspected to be from V₂O₅ formation for V-30Al and V-30Al-10Cr, and a semi-liquid mixture of V₂O₅ and Al₂O₃ for V-30Al-10Ti. The parabolic term is presumed related to the formation of a solid mixture of V₂O₅ and Al₂O₃ for V-30Al and V-30Al-10Cr, and TiO₂ for V-30Al-10TiThe addition of aluminum was found to reduce the oxidation rate of vanadium, but not to the extent predicted by the theory of competing oxide phases proposed by Wang, Gleeson, and Douglass. This was attributed to the formation of a liquid-oxide phase in the initial stages of exposure from which the alloys could not recover. Ternary additions of chromium and titanium were found to decrease the oxidation rate further, with chromium being the most effective. The oxide scales of the alloys were found to be highly porous at 900° C and 1000° C, due to the high vapor pressure of V₂O₅ above 800° C.     

Low-temperature formation and oxidation resistance of ultrafine aluminide coatings on Ni-base superalloy

Ultrafine aluminide coatings were successfully produced on Ni-18Fe-17Cr superalloy at 540-600 °C in a modified pack-aluminizing process. Repeated ball-impacts accelerated the formation of the aluminide coatings by a surface refining process, resulting in atomic diffusion occurring at a relatively low temperature. The effects of the operation temperature and the treatment duration on the formation of the coatings have been investigated. The coatings possessed a two-layer structure. The top layer, approximately 5 µm in thickness, exhibited equiaxial coarse grains and was dominated by NiAl₃, with small amounts of Fe₂Al₅ and CrAl₅. The bottom layer showed high density, homogeneous, ultrafine grains with diameters approximately 30-50 nm. High-temperature oxidation tests were carried out at 1000 °C. The oxidation kinetics and microstructure of the oxide scale were studied. The experimental results indicated that the coatings greatly enhanced the high-temperature oxidation resistance of Ni-18Fe-17Cr superalloy.     

Structure and cyclic oxidation resistance of Pt, Pt/Pd-modified and simple aluminide coatings on CMSX-4 superalloy

The coatings were prepared by the means of Pt and Pt/Pd galvanizing, followed by vapor phase aluminizing at 1050 °C. Microstructural and phase analysis revealed that all the investigated coatings consisted mainly of β-NiAl phase, however the Pt-modified aluminide coating also contained PtAl₂ phase and pure platinum precipitates. The cross-sectional microstructure of the coatings was zonal and composed of β-NiAl phase zone and the diffusion zone. The Pt modified aluminide coating's cross-section also incorporated an outermost zone consisting of β-NiAl and PtAl₂ phases. The concentration profiles proved that both Pt and Pd contents decrease gradually inwards the modified coatings. Cyclic oxidation tests at 1100 °C proved that Pt/Pd-modified aluminide coatings exhibit the best performance under cyclic conditions. The analysis of oxidation kinetics curves showed that the course of simple aluminide coating's oxidation is slightly different from that of Pt- and Pt/Pd-modified aluminide coatings.     

Studies concerning the effect of nitrogen on the oxidation behavior of TiAl-based intermetallics at 900°C

The oxidation behavior of the titanium aluminides Ti-50Al and Ti-48Al-5Nb has been investigated in Ar+20%O₂ and in air at 900°C. Thermogravimetric studies in combination with structural analyses using optical metallography, SEM/EDX and X-ray diffraction show a marked influence of nitrogen on the composition and growth rate of the oxide scales. For a more detailed study concerning the effect of nitrogen on the scale-growth kinetics, thermogravimetrical analyses were carried out during which the gas atmosphere was changed from air to Ar–O₂, and vice versa, without intermediate cooling of the specimen. The results show, that nitrogen adversely affects the formation of the initially formed alumina scale and that it enhances the growth rate of the rapidly growing Ti-rich oxide. This effect was observed in both alloys investigated, although the thermogravimetric results at first sight indicated an opposite effect for the Nb-containing alloy. This apparent contradiction is caused by internal oxidation which occurs in this alloy during exposure in Ar–O₂.     

The combined effect of refractory coatings containing reactive elements on high temperature oxidation behavior of chromia-forming alloys

The high temperature oxidation behaviors of chromia-forming alloys (F17Ti and Fe-30Cr alloys) have been studied at 1273 K under isothermal conditions and at 1223 K under cyclic conditions, in air under the atmospheric pressure. To extend the oxidation lifetime, coatings have been applied onto the alloy surfaces. Al₂O₃ and Cr₂O₃ films doped with Sm₂O₃ or Nd₂O₃ were prepared via the metal-organic chemical vapor deposition technique. Single Cr₂O₃, Al₂O₃, Nd₂O₃ and codeposited Cr₂O₃-Nd₂O₃, Al₂O₃-Nd₂O₃, Al₂O₃-Sm₂O₃ coatings drastically improved the chromia-forming alloy high temperature oxidation behavior, since they decreased the oxidation rate and enhanced the oxide scale adhesion. Results showed that a critical amount of reactive element (Nd or Sm) in chromia or alumina coatings (11-18 at.%) was needed to observe the most effective effect. The fast precipitation of NdCrO₃ or NdTi₂₁O₃₈ and the segregation of reactive elements at the chromia grain boundaries slowing down outward cation transport and consequently blocking the chromia grain growth, was supposed to be the main reasons of the beneficial effect ascribed to the reactive elements in chromia scales.     

Cyclic oxidation behavior and thermal barrier effect of YSZ-(Al2O3/YAG) double-layer TBCs prepared by the composite sol-gel method

Novel YSZ (6 wt.% yttria partially stabilized zirconia)-(Al₂O₃/YAG) (alumina-yttrium aluminum garnet, Y₃Al₅O₁₂) double-layer ceramic coatings were fabricated using the composite sol-gel and pressure filtration microwave sintering (PFMS) technologies. The thin Al₂O₃/YAG layer had good adherence with substrate and thick YSZ top layer, which presented the structure of micro-sized YAG particles embedded in nano-sized α-Al₂O₃ film. Cyclic oxidation tests at 1000 °C indicated that they possessed superior properties to resist oxidation of alloy and improve the spallation resistance. The thermal insulation capability tests at 1000 °C and 1100 °C indicate that the 250 μm coating had better thermal barrier effect than that of the 150 μm coating at different cooling gas rates. These beneficial effects should be mainly attributed to that, the oxidation rate of thermal grown oxides (TGO) scale is decreased by the “sealing effect” of α-Al₂O₃, the “reactive element effect”, and the reduced thermal stresses by means of nano/micro composite structure. This double-layer coating can be considered as a promising TBC.     

Ti6Al4V钛合金表面Al2O3颗粒改性微弧氧化涂层制备及性能

微弧氧化技术是一门新的表面处理技术,在很多领域有着广泛的应用前景,但其结构受限于电解液成分。本文通过在磷酸盐电解液中加入Al2O3陶瓷颗粒,使得在Ti6Al4V钛合金表面的微弧氧化涂层结构和性能得到改性。涂层的结构和性能通过扫描电镜和XRD衍射仪进行表征和测试,涂层的抗高温氧化性能和热震性能通过高温热循环氧化试验和热震试验进行测试。结果表明,通过在电解液中添加Al2O3陶瓷颗粒,涂层由Al2TiO5 and TiO2组成,涂层更为致密,表现出更为优异的抗高温氧化和热震性能。电解液中游动的Al2O3陶瓷颗粒在微弧氧化过程中被吸入到样品表面并进入涂层,涂层的结构和性能得到改性。     

The effect of particle size of alumina dispersions on the oxidation resistance of Ni-Cr alloys

The oxidation behavior of Ni-Cr alloys with various chromium concentrations and particle sizes of a dispersion of 10 vol.% Al₂O₃ was observed in 1 atm of oxygen at 1000°C. This study was intended to determine the critical chromium concentration to form a protective Cr₂O₃ oxide layer for different Al₂O₃ particle sizes. The oxidation rate of Ni-Cr alloys containing 10 vol.% Al₂O₃ followed a parabolic rate law and a Cr₂O₃ protective layer continuously formed when the oxidation rate decreased rapidly. Times to form a continuous and protective Cr₂O₃ layer during the initial oxidation shortened as the size of the dispersion decreased. The critical chromium concentration to form a protective Cr₂O₃ layer in the oxide scale was 69 wt.% and was related strongly to the particle size of the Al₂O₃ dispersion.     

Microstructure and high temperature oxidation behavior of hot-dip aluminized coating on high silicon ductile iron

High silicon ductile iron was coated by hot-dipping into an Al molten bath. The oxidation behavior of the aluminized alloy and the bare substrate was studied in air at 750 °C. The results showed that the coating layers consisted of three layers, in the sequence of Al, Fe-Al intermetallic and Si pile-up layers from the external topcoat to the substrate. The intermetallic layer was composed of outer FeAl₃ and inner Fe₂Al₅. The outer rod-shaped FeAl₃ dispersed in the aluminum topcoat, while the inner tongue-like Fe₂Al₅ formed in the metallic layer becoming the major phase in the aluminide coating layer. Those three layers of aluminum, Fe₂Al₅ and silicon pile-up layer exhibited hardness of HV 50, HV 1100 and HV 450, respectively. In this study, when the as-coated specimens were examined, Fe-Al-Si compounds could not be found. But the silicon pile-up at the interface between the substrate and the Fe-Al intermetallic layer could be seen in all the as-coated specimens. The graphite nodules were noticed in the substrate. The presence of graphite nodules in the substrate might be markers of hot-dipping. After hot-dipping in Al all the specimens contained graphite nodules in the aluminide layer.The oxidized graphite nodules resulted in cracks propagating in aluminide coating. Even though graphite nodules meant the existence of crack in the aluminide coating, the high temperature oxidation experiments indicated that the aluminide coating could prevent the oxidation of substrate effectively even at 750 °C.     

The effect of an Al2O3 dispersion on the adhesion of Al2O3 scales on aluminized Co-10% Cr alloys

The beneficial effect of dispersions of reactive-metal oxide particles on the adhesion of Cr₂O₃ and Al₂O₃ scales formed on heat-resisting alloys is wellknown. It has been shown that an Al₂O₃ dispersion in an alloy can improve the adhesion of a Cr₂O₃ scale, and it is of particular interest in assessing the various theoretical proposals for the effect to determine whether such a dispersion can affect the adhesion of an Al₂O₃ scale. In this investigation, a Co–10% Cr–1 % Al alloy was first internally oxidized to form an Al₂O₃ dispersion. This alloy was then aluminized so that on subsequent oxidation an Al₂O₃ scale developed. It was shown that the dispersion did indeed improve the scale adhesion. The implications of this result are discussed.     

氩弧重熔对球墨铸铁热浸镀铝层组织和性能的影响

将球墨铸铁在780℃热浸镀铝后进行氩弧重熔处理,利用扫描电镜和X射线衍射仪对热浸镀铝层和氩弧重熔层组织进行观察,测定了氩弧重熔前后试件截面的显微硬度。结果表明:球墨铸铁热浸镀铝后,镀层由富铝层及扩散层构成,富铝层包含降温形成的针状FeAl3相,扩散层由FeAl3、Fe2Al5相组成,扩散层形状呈柱状延伸到基体中。热浸镀铝层经氩弧重熔处理后,试件由重熔层和过渡层及基体构成。重熔层由Al、Fe、C和FeAl3相组成,过渡层组织为马氏体+残留奥氏体+少量球状石墨。氩弧重熔处理能明显提高热浸镀铝层的显微硬度。重熔层硬度值可达900 HV,在过渡层硬度下降缓慢,直至基体。     

不锈钢表面镀铝-热氧化处理制备氧化铝膜及其性能

采用无机熔融盐电镀铝、热浸镀铝工艺联合的方法在钢基材表面制备了一层平整、连续的铝镀层,研究了铝镀层在不同氧化时间下的氧化行为。利用X射线衍射仪(XRD)、扫描电镜(SEM)和能谱分析(EDS)对氧化层的形貌和成分进行分析,并考察了氧化层的硬度和耐腐蚀性能。结果表明:将无机熔融盐电镀铝法和热浸镀铝法相结合可获得良好的铝镀层;铝镀层在900℃下热处理20 h后,可获得连续致密的Al2O3膜;Al2O3层有效提高了钢基材的表面硬度和耐腐蚀性能。     

Microstructure,adhesion and growth kinetics of protective scales on metals and alloys

Currentunderstanding of the complex interrelationships among growth kinetics, microstructure, and adhesion of protective Cr₂O₃ and Al₂O₃ scales is critically reviewed. Similarities and differences in the behavior of these two systems are highlighted. The morphology of the alloy-scale interface appears to be a critical factor. Recent ideas are advanced to interpret the effect of oxygen-active elements on the development of a tortuous interface conducive to improved scale tenacity.This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.Deceased.on leave to Industrial Materials Research Institute, NRC, Montreal, Quebec, H4C 2K3 Canada.     

Preparation and hot corrosion behaviour of an Al-gradient NiCoCrAlYSiB coating on a Ni-base superalloy

A gradient NiCoCrAlYSiB coating was prepared on a Ni-base superalloy using arc ion plating (AIP) and subsequent gaseous phase aluminisation techniques. Hot corrosion of normal NiCoCrAlYSiB and the gradient coating in pure Na₂SO₄ and Na₂SO₄/NaCl (75:25, wt./wt.) salts was performed at 900 °C in static air. The corrosion results indicated an enhanced corrosion resistance to both salts for the gradient NiCoCrAlYSiB coating, which the improved performance of it should be attributed to the β aluminide ‘‘pool” at the surface layer. By partially sacrificing Al₂O₃ (i.e. Al), the gradient NiCoCrAlYSiB coating specimen behaved excellently in the two kinds of salts. The grain growth during the gaseous phase aluminisation and the corrosion mechanism, including the role NaCl played in the mixture salt corrosion, are discussed.     

Hot corrosion behaviour of plasma sprayed YSZ/Al2O3 dispersed NiCrAlY coatings on Inconel-718 superalloy

Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na₂SO₄ and V₂O₅ salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO₄ phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al₂O₃ coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.     

Characterizations and hot corrosion resistance of Cr3C2-NiCr coating on Ni-base superalloys in an aggressive environment

In the current study, Cr₃C₂-NiCr coating was deposited on the Ni-base superalloys by using high velocity oxyfuel (HVOF) process for high temperature corrosive environment applications. Optical microscopy (OM), x-ray diffraction (XRD), scanning electron microscopy/energy-dispersive analysis (SEM/EDAX), microhardness tester, and electro probe microanalyzer (EMPA) techniques were used to characterize the coating with regard to coating thickness, porosity, microhardness, and microstructure. The thermogravimetric technique was used to establish kinetics of corrosion. The hot corrosion behaviors of the bare and Cr₃C₂-NiCr coated superalloys were studied after exposure to aggressive environment of Na₂SO₄-60% V₂O₅ salt mixture at 900 °C under cyclic conditions. The structure of the as-sprayed Cr₃C₂-NiCr coating mainly consisted of γ-nickel solid solution along with minor phases of Cr₇C₃ and Cr₂O₃. Coating has porosity less than 1.5% and microhardness in the range of 850–900 Hv (Vickers hardness). Some inclusions, unmelted and semimelted powder particles were observed in the structure of the coatings. The Cr₃C₂-NiCr coating has imparted necessary resistance to hot corrosion, which has been attributed to the formation of oxides of nickel and chromium, and spinel of nickel-chromium. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.