High-Temperature Behavior of a High-Velocity Oxy-Fuel Sprayed Cr3C2-NiCr Coating

High-velocity oxy-fuel (HVOF) sprayed coatings have the potential to enhance the high-temperature oxidation, corrosion, and erosion-corrosion resistance of boiler steels. In the current work, 75?pct chromium carbide-25?pct (nickel-20?pct chromium) [Cr₃C₂-NiCr] coating was deposited on ASTM SA213-T22 boiler steel using the HVOF thermal spray process. High-temperature oxidation, hot corrosion, and erosion-corrosion behavior of the coated and bare steel was evaluated in the air, molten salt [Na₂SO₄-82?pct Fe₂(SO₄)₃], and actual boiler environments under cyclic conditions. Weight-change measurements were taken at the end of each cycle. Efforts were made to formulate the kinetics of the oxidation, corrosion, and erosion-corrosion. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM)/energy dispersive spectroscopy (EDS) techniques were used to analyze the oxidation products. The coating was found to be intact and spallation free in all the environments of the study in general, whereas the bare steel suffered extensive spallation and a relatively higher rate of degradation. The coating was found to be useful to enhance the high-temperature resistance of the steel in all the three environments in this study.     

Effect of Cyclic Oxidation Exposure on Tensile Properties of a Pt-Aluminide Bond-Coated Ni-Base Superalloy

The tensile behavior of a directionally solidified (DS) Ni-base superalloy, namely, CM-247LC, was evaluated in the presence of a Pt-aluminide bond coat. The effect of the thermal cycling exposure of the coated alloy at 1373 K (1100 °C) on its tensile properties was examined. The tensile properties were evaluated at a temperature of 1143 K (870 °C). The presence of the bond coating caused an approximately 8 pct drop in the strength of the alloy in the as-coated condition. However, the coating did not appreciably affect the tensile ductility of the substrate alloy. The bond coat prevented oxidation-related surface damage to the superalloy during thermal cycling exposure in air at 1373 K (1100 °C). Such cyclic oxidation exposure (up to 750 hours) did not cause any further reduction in yield strength (YS) of the coated alloy. There was a marginal decrease in the ultimate tensile strength (UTS) with increased exposure duration. Because of the oxidation protection provided by the bond coat, the drastic loss in ductility of the alloy, which would have happened in the absence of the coating, was prevented.     

Characterization and High-Temperature Oxidation Behavior of Cold-Sprayed Ni-20Cr and Ni-50Cr Coatings on Boiler Steels

Microstructure and mechanical properties of cold-spray coatings are usually required in order to explore the potential industrial application of the latter. This article demonstrates the successful formulation of Ni-20Cr and Ni-50Cr coatings on two boiler steels, namely, SAE 213-T22 and SA 516 steel by cold-spray process. The microstructure, coating thickness, phase formation, and microhardness properties of the coatings were evaluated. The coatings were subjected to cyclic heating and cooling cycles at an elevated temperature of 1173.15 K (900 °C) to ascertain their high-temperature oxidation behavior. Moreover, these cyclic exposures can give useful information regarding the adhesion of the coatings with the substrate steels. Of all the coatings, the Ni-50Cr coating on SA 516 steel had a maximum average hardness value of 469 Hv. As observed from the surface field emission–scanning electron microscopy (FE-SEM) analysis, the coatings were found to have nearly dense microstructure with the sprayed particles in interlocked positions. It was concluded that the cold-spray process is suitable for spraying the preceding powders onto the given boiler steels to produce nearly dense and low oxide coatings. The coatings, in general, were found to follow the parabolic rate of oxidation and were successful in maintaining their surface contact with their respective substrate steels.     

Crack Progression during Sustained-Peak Low-Cycle Fatigue in Single-Crystal Ni-Base Superalloy René N5

Crack progression during compressive sustained-peak low-cycle fatigue (SPLCF) was examined in vapor phase aluminide coated single-crystal Ni-base superalloy René N5. Strain-controlled tests with a 120-second hold at compression were conducted at 1366 K (1093 °C) with A = –1 (R = –∞) and 0.35 pct total strain range, and were terminated at selected fractions of predicted life. Crack lengths on the surface and crack depth in longitudinal sections were examined for each specimen. All cracks appeared to have initiated at the coating surface. Failed specimens showed that cracks initially grew on (001), perpendicular to the stress axis, and then deflected to other crystallographic planes. Interrupted test specimens showed crevices initiated on the coating surface at less than 10 pct of the predicted life. The depths of crevices into the coating increased with cyclic exposure, but they did not penetrate into the substrate through the interdiffusion zone (IDZ) until about 80 pct of predicted life. Stress relaxation during compressive hold results in residual tension upon unloading. These results suggest that improving creep resistance of the substrate alloy and developing a coating system that can delay crack penetration into the substrate are keys for improved SPLCF life.     

Characterization of Tribological and Mechanical Properties of the Si3N4 Coating Fabricated by Duplex Surface Treatment of Pack Siliconizing and Plasma Nitriding on AISI D2 Tool Steel

Silicon nitride (Si₃N₄) coating was deposited on AISI D2 tool steel through employing duplex surface treatments—pack siliconizing followed by plasma nitriding. Pack cementation was performed at 650 °C, 800 °C, and 950 °C for 2 and 3 hours by using various mixtures to realize the silicon coating. X-ray diffraction analyses and scanning electron microscopy observations were employed for demonstrating the optimal process conditions leading to high coating adhesion, uniform thickness, and composition. The optimized conditions belonging to siliconizing were employed to produce samples to be further processed via plasma nitriding. This treatment was performed with a gas mixture of 75 pct H₂-25 pct N₂, at the temperature of 550 °C for 7 hours. The results showed that different nitride phases such as Si₃N₄-β, Si₃N₄-γ, Fe₄N, and Fe₃N can be recognized as coatings reinforcements. It was demonstrated that the described composite coating procedure allowed to obtain a remarkable increase in hardness (80 pct higher with respect to the substrate) and wear resistance (30 pct decrease of weight loss) of the tool steel.

     

The chemical and mechanical processes of thermal fatigue degradation of an aluminide coating

The influence of strain history on the oxidation and mechanical degradation of an aluminide coating was examined by induction heating of stepped-disk specimens. The coating was applied to a single-crystal Ni-base superalloy (RENè N4) by pack aluminization. The anisotropic elasticity of the single-crystal substrate allowed simultaneously subjecting the aluminide coating to different strain amplitudes. Two distinct modes of coating degradation were observed for tests performed in air between temperature limits of 520 °C and 1080 °C: scalloping (spatially periodic surface oxidation and roughening) and cracking. The degree of scalloping became more severe as the compressive strain imposed on the coating was increased. Six thousand cycles between peak strains of -0.20 and 0.007 pct produced uniform surface oxidation, without scalloping, whereas 6000 cycles between peak strains of -0.56 and 0.01 pct gave oxidation and scalloping to 80 pct of the coating thickness. Cracks along coating grain boundaries were observed after 6000 cycles between peak strains of -0.45 and 0.16 pct. The depth of scalloping was found to correlate approximately with peak compressive substrate strain. Based on this correlation, a mechanism for scallop initiation and growth involving cyclic breakdown of the surface oxide and irreversible cyclic creep of the coating is proposed. Cracking along coating grain boundaries is attributed to tensile strains applied below the transition temperature of the coating. The results obtained from this study indicate that cyclic strain history is an important variable which should be included when determining the oxidation rate of coatings and alloys.     

The chemical and mechanical processes of thermal fatigue degradation of an aluminide coating

The influence of strain history on the oxidation and mechanical degradation of an aluminide coating was examined by induction heating of stepped-disk specimens. The coating was applied to a single-crystal Ni-base superalloy (RENè N4) by pack aluminization. The anisotropic elasticity of the single-crystal substrate allowed simultaneously subjecting the aluminide coating to different strain amplitudes. Two distinct modes of coating degradation were observed for tests performed in air between temperature limits of 520 °C and 1080 °C: scalloping (spatially periodic surface oxidation and roughening) and cracking. The degree of scalloping became more severe as the compressive strain imposed on the coating was increased. Six thousand cycles between peak strains of -0.20 and 0.007 pct produced uniform surface oxidation, without scalloping, whereas 6000 cycles between peak strains of -0.56 and 0.01 pct gave oxidation and scalloping to 80 pct of the coating thickness. Cracks along coating grain boundaries were observed after 6000 cycles between peak strains of -0.45 and 0.16 pct. The depth of scalloping was found to correlate approximately with peak compressive substrate strain. Based on this correlation, a mechanism for scallop initiation and growth involving cyclic breakdown of the surface oxide and irreversible cyclic creep of the coating is proposed. Cracking along coating grain boundaries is attributed to tensile strains applied below the transition temperature of the coating. The results obtained from this study indicate that cyclic strain history is an important variable which should be included when determining the oxidation rate of coatings and alloys.     

Role of Silicon Carbide in Phase-Evolution and Oxidation Behaviors of Pulse Electrodeposited Nickel-Tungsten Coating

Silicon carbide (SiC) was reinforced in the pulse electrodeposited nickel-tungsten (Ni-W) coatings deposited on the steel substrate, and isothermal oxidation test was performed at 1273 K (1000 °C) for 24 hours. Addition of just 2 vol pct of SiC showed 26 pct increase in the relative oxidation resistance of Ni-W coating. The increased oxidation resistance was attributed to the phase evolution (SiO₂, Cr₂O₃, CrSi₂, Ni₂SiO₄, Cr₇C₃, Cr₃C₂, and Cr₃Si), which suppressed the spallation of the oxide scale in Ni-W-2 vol pct SiC. The presence of Fe₂O₃ phase in the oxidized Ni-W coating was mainly responsible for the major multiple spallations at the interface and in the bulk, which resulted in the degradation of oxidation resistance.

     

Influence of Electrolyte Chemistry on Morphology and Corrosion Resistance of Micro Arc Oxidation Coatings Deposited on Magnesium

In the present work, micro arc oxidation (MAO) coatings were synthesized on magnesium substrate employing 11 different electrolyte compositions containing systematically varied concentrations of sodium silicate (Na₂SiO₃), potassium hydroxide (KOH), and sodium aluminate (NaAlO₂). The resultant coatings were subjected to coating thickness measurement, energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), image analysis, and three-dimensional (3-D) optical profilometry. The corrosion performance of the coatings was evaluated by conducting potentiodynamic polarization tests in 3.5 wt pct NaCl solution. The inter-relationships between the electrolyte chemistry and the resulting chemistry and porosity of the coating, on one hand, and with the aqueous corrosion behavior of the coating, on the other, were studied. The changes in pore morphology and pore distribution in the coatings were found to be significantly influenced by the electrolyte composition. The coatings can have either through-thickness pores or pores in the near surface region alone depending on the electrolyte composition. The deleterious role of KOH especially when its concentration is >20 pct of total electrolyte constituents promoting the formation of large and deep pores in the coating was demonstrated. A reasonable correlation indicating the increasing pore volume implying the increased corrosion was noticed.     

Effect of thermal spray on the microstructure and adhesive strength of high-velocity oxy-fuel-sprayed Ni-Cr coatings on 9Cr-1Mo steel

Coatings of 80Ni-20Cr and 50Ni-50Cr on a 9Cr-1Mo steel substrate were produced by high-velocity oxy-fuel (HVOF) spraying to protect the steel against steam oxidation in ultrasupercritical (USC) boilers. The oxidation studies on the coated specimens showed good protection against the scale growth on the steel substrate. Both the 80Ni-20Cr and 50Ni-50Cr coatings formed a thin protective oxide film on the coating surface. The 80Ni-20Cr coating showed Fe diffusion from the substrate to the coating and nickel diffusion from the coating to the substrate during the oxidation process. In the case of 50Ni-50Cr coatings, the diffusion process was reduced, but a continuous layer of chromium carbide was observed at the coating/substrate interface during the oxidation. The adhesive/cohesive strength of these coatings was evaluated on aged specimens at 750 °C by using a simple tensile test. The results of the as-coated 80Ni-20Cr specimens showed an adhesive-strength value of 68 MPa. On extended aging, the strength of the coating increased beyond the detection limit of the resin. The nickel diffusion from the coating to the substrate and the iron diffusion from the substrate to the coating caused the increased adhesive strength. In the case of 50Ni-50Cr, the as-coated specimens showed an average adhesive strength of 76 MPa and showed a decreasing trend on the aged specimens. The formation of chromium carbide at the interface caused inferior values in the adhesive/cohesive strength of the 50Ni-50Cr coatings. The chromium carbide formed on the coating/substrate interface was identified as M₂₃C₆-type carbide.     

Selective Oxidation and Reactive Wetting during Galvanizing of a CMnAl TRIP-Assisted Steel

A transformation induced plasticity (TRIP)-assisted steel with 0.2 pct C, 1.5 pct Mn, and 1.5 pct Al was successfully galvanized using a thermal cycle previously shown to produce an excellent combination of strength and ductility. The steel surface chemistry and oxide morphology were determined as a function of process atmosphere oxygen partial pressure. For the 220 K (–53 °C) dew point (dp) + 20 pct H₂ atmosphere, the oxide morphology was a mixture of films and nodules. For the 243 K (–30 °C) dp + 5 pct H₂ atmosphere, nodules of MnO were found primarily at grain boundaries. For the 278 K (+5 °C) dp + 5 pct H₂ atmosphere, nodules of metallic Fe were found on the surface as a result of alloy element internal oxidation. The steel surface chemistry and oxide morphology were then related to the reactive wetting behavior during continuous hot dip galvanizing. Good wetting was obtained using the two lower oxygen partial pressure process atmospheres [220 K dp and 243 K dp (–53 °C dp and –30 °C dp)]. An increase in the number of bare spots was observed when using the higher oxygen partial pressure process atmosphere (+5 °C dp) due to the increased thickness of localized oxide films.     

High Temperature Erosion Performance of Nanostructured and Conventional TiAlN Coatings on AISI-304 Boiler Steel Substrate

According to modern design philosophy better overall performance can be obtained with the modification of the surface structure and their properties without damaging underlying bulk material or substrate. The surface engineering can be classified in two broad classes: surface modification and surface coating. In the present research TiAlN coating was deposited on AISI-304 grade boiler steel using three different techniques, out of which two were thin nano coatings deposited at different temperatures of 500 and 200 °C developed by Oerlikon Balzers rapid coating system machine under a reactive nitrogen atmosphere. One conventional coating of TiAlN was deposited by plasma spraying method. The coated samples were characterized relative to their coating thickness, microhardness, porosity and micro structure. The optical microscopy, the X-ray diffraction analysis and field emission scanning electron microscope (FESEM with EDAX attachment) analysis have been used to identify various phases formed after coating deposition on the surface of AISI-304 grade boiler steel. The erosion studies were conducted on uncoated as well as coated specimens in simulated coal fired boiler environment using an air jet erosion test rig at various impingement angles of 30°, 60° and 90°. The alumina particles of average size of 50 µm were used as erodent at a velocity of 35 m/s. The eroded samples were analysed with SEM/EDAX and optical profilometer. The main objective of this research work was to increase the life of boiler tubes by using nanostructured and conventional TiAlN coatings and at the same time to compare the performance of coatings with respect to bare AISI-304 grade boiler steel. The nanostructured TiAlN coatings has shown minimum erosion rate as compared to conventional TiAlN coating and uncoated AISI-304 grade boiler steel. Maximum erosion was observed at an angle of 30° as compared to 60° and 90° indicative ductile behaviour.     

Mechanical Properties of Uncoated and Aluminide-Coated René 80

Nickel-base superalloys such as René 80 are widely used in manufacturing aircraft turbine blades. They are usually coated in order to increase their wear, oxidation, erosion, and hot corrosion properties against environmental degradation. In this article, the mechanical behavior (tensile and low-cycle fatigue (LCF)) of uncoated and aluminide-coated (CODEP-B) René 80 has been studied at 871 °C and 982 °C. Experimental results show that the tensile properties of coated specimens are relatively lower than those of uncoated ones in the same conditions, but application of coating increases the LCF life of René 80 at T = 871 °C, 982 °C, R = (ε _min/ε _max) = 0, strain rate of 2 × 10⁻³ s⁻¹, and Δε _t  = 0.8 pct. Scanning electron microscopy (SEM) studies of coated specimens at N = Nf show that the nucleation of cracks occurs merely in substrate, but cracks start from the surfaces in uncoated specimens. Transmission electron microscopy (TEM) investigations have been performed on fractured uncoated specimens to evaluate the microstructures at different temperatures. The misfit dislocation, pair dislocations, and cutting of γ′ were observed at T = 871 °C and 982 °C. The TEM studies also showed that at 982 °C stacking fault was observed in γ′ particles.     

Abnormal Growth Transport in Oxide Scales on Fe-16Cr Steels in Water Vapor

The oxidation behavior of Fe-16Cr steels in N₂-12 vol pct H₂O was studied at 850 °C. The oxide scale was compact and had excellent adhesion to the substrate; moreover, there were three layers of different compositions existing in the scale. To gain an insight into the transport mechanism, two-stage oxidation was carried out in N₂-12 vol pct H₂¹⁶O and followed in N₂-12 vol pct H₂¹⁸O gas mixtures. The oxygen isotope profiles in oxide scales were determined by secondary ion mass spectrometry. The results showed that oxidation in water vapor proceeded by outward chromium transport, especially, the oxidation involved inward transport of water molecules.     

Roasting of Nickel Concentrates

The oxidation of three nickel concentrates from two Canadian smelters was studied by thermogravimetric analysis. Concentrate samples were heated to 1223 K (950 °C) in inert or oxidizing atmospheres to determine the reaction behavior. By recording the mass change as well as the SO₂ content in the outlet gas, the oxidation behaviors were quantified. Isothermal roasting tests were carried out on the concentrates over the temperature range of 673 K (400 °C) to 1123 K (850 °C). When heated in air, the samples gain mass as a result of sulfate formation at temperatures up to approximately 873 K (600 °C) to 973 K (700 °C), whereas at higher temperatures, the samples exhibit a large mass loss attributed to sulfate decomposition as well as direct SO₂ formation by oxidation. In a 4 pct O₂ gas atmosphere, significantly less sulfates were formed. Mixed reactions take place, in which some lead to mass loss and SO₂ generation, and others lead to mass gain and SO₂ consumption. The relative importance of the various reactions depends on the experimental conditions.     

Characteristics of Zinc Phosphate Coating Activated by Different Concentrations of Nickel Acetate Solution

Activation pretreatment with nickel acetate solution at various concentrations was performed prior to the phosphating step to enhance the corrosion resistance of carbon steel substrates. The activation solution was studied over various concentrations: 10, 50, and 100 g/L. The effects of these concentrations on surface characteristics and microstructural evolution of the coated samples were characterized by scanning electron microscopy and energy-dispersive spectroscopy. The electrochemical behavior was evaluated using potentiodynamic polarization curves, electrochemical impedance spectroscopy, and immersion test in a 3.5 pct NaCl solution. Significant increases in the nucleation sites and surface coverage of zinc phosphate coating were observed as the concentration of activation solution reached 50 g/L. The electrochemical analysis revealed that the activation treatment with 50 g/L nickel acetate solution significantly improved the protection ability of the zinc phosphate coating. The corrosion current density of activated phosphate coating with 50 g/L was reduced by 64.64 and 13.22 pct, compared to the coatings obtained with activation solutions of 10 and 100 g/L, respectively.

     

Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060 °C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-μm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750 °C and 925 °C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.     

Environment-Assisted Cracking in Custom 465 Stainless Steel

The influence of cold work and aging on the environment-assisted cracking (EAC) behavior and mechanical properties of Custom 465 stainless steel (SS) was studied. Four sets of specimens were made and tested. All specimens were initially solution annealed, rapidly cooled, and refrigerated (SAR condition). The first specimen set was steel in the SAR condition. The second specimen set was aged to the H1000 condition. The third specimen set was 60 pct cold worked, and the fourth specimen set was 60 pct cold worked and aged at temperatures ranging from 755 K to 825 K (482 °C to 552 °C) for 4 hours in air. The specimens were subsequently subjected to EAC and mechanical testing. The EAC testing was conducted, using the rising step load (RSL) technique, in aqueous solutions of NaCl of pH 7.3 with concentrations ranging from 0.0035 to 3.5 pct at room temperature. The microstructure, dislocation substructure, and crack paths, resulting from the cold work, aging, or subsequent EAC testing, were examined by optical microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The aging of the cold-worked specimens induced carbide precipitation within the martensite lath, but not at the lath or packet boundaries. In the aged specimens, as aging temperature rose, the threshold stress intensity for EAC (K_IEAC)_, elongation, and fracture toughness increased, but the strength and hardness decreased. The K_IEAC also decreased with increasing yield strength and NaCl concentration. In the SAR and H1000 specimens, the EAC propagated along the prior austenite grain boundary, while in the cold-worked and cold-worked and aged specimens, the EAC propagated along the martensite lath, and its packet and prior austenite grain boundaries. The controlling mechanism for the observed EAC was identified to be hydrogen embrittlement.