Grain-Size Effects on the High-Temperature Oxidation Behaviour of Chromium Steels

Oxidation of two low-Cr (Cr content 1.5 wt% and 2.25 wt%) and three high-Cr (Cr content 9 wt%, 12 wt% and 18 wt%) boiler steels was investigated at temperatures between 550 °C and 830 °C in laboratory air. Thermogravimetry (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were applied for evaluation of the oxidation kinetics, oxides phase identification and scale structure analysis. Particular attention was paid to the phenomenon of inward oxidation and its relationship with the Cr content of the steels under investigation. The results showed that the inward oxidation of the Cr steels is governed by grain boundary diffusion. Two different Cr-depending tendencies concerning the effect of alloy grain size on the inward oxidation were observed. For low-Cr steels (less than 2.25 wt% Cr), an increase in the grain size improved the oxidation resistance, while steels with high Cr content (18 wt% Cr) can form a thin and protective chromia scale on the surface more easily at finer grain size. In the latter case an increase in grain size deteriorates the oxidation resistance by the formation of a thicker scale composed of both an outer and an inner Fe-oxide-based layer.     

Effect of Shot Peening on the Oxidation Resistance of TP304H and HR3C Steels in Water Vapor

Oxidation behaviors of TP304H and HR3C steels with and without shot peening were investigated at 650?°C in water vapor. The oxide scales were studied with XRD, SEM and EDX. The results showed that the oxidation resistance of the steels was effectively improved by shot peening, especially for TP304H. The oxidation resistance of shot peened TP304H was even better than shot peened HR3C. The shot peening process produced an ultra-fine surface microstructure and plenty of slip bands which resulted in an enhanced Cr diffusivity to form Cr-rich oxides. The shot-peening process also induced martensite at the surface, which enhanced Cr diffusion and significantly affected the oxidation behavior of the TP304H steel.     

Production of Nanocrystalline Ni-20Cr Coatings for High-Temperature Applications

Presynthesized nanocrystalline Ni-20Cr powder was deposited on SA 516 and T91 boiler steels by a high-velocity oxy-fuel spraying process. Ni-20Cr powder was synthesized by the ball milling approach. The high-temperature oxidation behavior of bare and coated samples was then studied under cyclic isothermal conditions at 900 °C for 50 cycles. The kinetics of oxidation was established using weight change measurements for the bare and coated boiler steels. Uncoated and coated samples of T91 steel were exposed to the superheated zone of a power plant boiler at 750 °C under cyclic conditions for 15 cycles. Each cycle consisted of 100 h of heating followed by 1 h of cooling. Attempts were made to study the kinetics of erosion–corrosion using weight change and thickness loss data for the samples. Different characterization techniques were used to study the oxidized and eroded–corroded samples, including x-ray diffraction, scanning electron microscopy/energy-dispersive spectroscopy, and x-ray mapping analyses. The Ni-20Cr alloy powder coating was found to offer excellent oxidation resistance to the base steels and was successful in reducing the weight gain of SA 516 steel by 98.5 % and that of T91 steel by 65 %. The coating was observed to reduce the erosion–corrosion rate of T91 steel by 86 % in terms of thickness loss. This indicates that the investigated nanostructured coating can be a better choice over conventional coating for erosion–corrosion control of boiler tubes.     

Cyclic Oxidation Behaviour of 1Cr–0.5Mo (T11) Boiler Tube Steel and Its Weldments in Air at 900 °C

Oxidation behaviour of weldments at elevated temperature has become an object of scientific investigation. Weldments were prepared using shielded metal arc welding and tungsten inert gas processes to weld together 1Cr–0.5Mo (T11) boiler tube steels. This paper reports the oxidation behaviour of welded and unwelded 1Cr–0.5Mo (T11) boiler tube steel specimens after exposure to air at 900 °C under cyclic condition. The thermogravimetric technique was used to establish kinetics of oxidation. X-ray diffraction and scanning electron microscopy/energy-dispersive analysis techniques were used to analyse the oxidation products. The unwelded steel showed a higher oxidation rate (in terms of weight gain) than that of welded steels.     

Selective Oxidation of Chromium by O2 Impurities in CO2 During Initial Stages of Oxidation

This study shows that the corrosion behaviour of 12 wt% Cr steel in CO₂ at 550 °C is determined in the first stage of oxidation by reaction with O₂ impurities. Depending on the amount of theses impurities and the thermal ramp rate, selective oxidation of chromium could lead to the formation of a protective chromium-rich oxide. An oxidation model describing qualitatively the nature of the oxide layer formed in the initial period of oxidation is presented. From these observations, surface engineering processes for protecting 9–12 wt% chromium steels from fast corrosion rate have emerged.     

Grain-Size Effects on the High-Temperature Oxidation of Modified 304 Austenitic Stainless Steel

The high-temperature oxidation behavior of modified 304 austenitic stainless steels in a water vapor atmosphere was investigated. Samples were prepared by various thermo mechanical treatments to result in different grain sizes in the range 8–30 μm. Similar Σ3 grain boundary fraction was achieved to eliminate any grain-boundary characteristics effect. Samples were oxidized in an air furnace at 700 °C with 20 % water vapor atmosphere. On the fine-grained sample, a uniform Cr₂O₃ layer was formed, which increased the overall oxidation resistance. Whereas on the coarse-grained sample, an additional Fe₂O₃ layer formed on the Cr-rich oxide layer, which resulted in a relatively high oxidation rate. In the fine-grained sample, grain boundaries act as rapid diffusion paths for Cr and provided enough Cr to form Cr₂O₃ oxide on the entire sample surface.     

Effect of Silicon on the Steam Oxidation Resistance of a 9%Cr Heat Resistant Steel

The steam oxidation of 9Cr–0.5Mo–1.8W steels containing 0.06 to 0.49%Si was investigated at 500°, 550°, 600°, 650° and 700°C. The steam oxidation rate of the steel decreased with increasing silicon content. The effect of silicon was most remarkable at 700°C. At 500°, 550° and 600°C, the effect was almost the same, and was smaller than that at 700°C. At 700°C, the formation of a protective amorphous-SiO₂ film reduced the oxidation rate considerably. On the other hand, at 600°C or less, silicon dissolved in the Fe–Cr spinel lattice with no evidence of SiO₂. At 650°C, although amorphous SiO₂ was observed, as at 700°C, at the scale–metal interface, the effect of silicon was the least within the test-temperature range. Thus, 650°C was a peculiar temperature for the effect of silicon on the steam oxidation of 9%Cr steels. The relatively small effect of silicon at 650°C is attributed to the formation of metastable FeO.     

Effects of Temperature and Straining on the Oxidation Behavior of Electrical Steels

The effect of Si content (in the range of 0.01–1.91 wt%) on scale formation of electrical steels in dry air at temperatures ranging from 850 to 1200 °C was investigated. The effect of applied tensile strain on oxidation behavior was also explored. A thermo-mechanical simulator (Gleeble machine) was employed to conduct the oxidation tests at different load conditions. The experimental results showed that at 1000 °C the oxidation rate decreased with increasing Si content in the steel. The formation of an inner scale, mainly consisting of amorphous silica, was responsible for the improved oxidation resistance. However, a substantial increase in oxidation rate due to the formation of molten eutectic fayalite (Fe₂SiO₄) was observed when the temperature was raised to 1200 °C. Under straining conditions at a very short oxidation time, the inner scale structure was slightly modified though the scale thickness remained almost unchanged for the steel containing 1.91 wt% Si.     

High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr₂O₃ healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr₂O₃ oxide film on HR3C was unstable and partly converted into a less protective MnCr₂O₄ with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr₂O₃ to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al₂O₃ and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.     

Development of Creep-Resistant and Oxidation-Resistant Austenitic Stainless Steels for High Temperature Applications

Austenitic stainless steels are cost-effective materials for high-temperature applications if they have the oxidation and creep resistance to withstand prolonged exposure at such conditions. Since 1990, Oak Ridge National Laboratory (ORNL) has developed advanced austenitic stainless steels with creep resistance comparable to Ni-based superalloy 617 at 800–900°C based on specially designed “engineered microstructures” utilizing a microstructure/composition database derived from about 20 years of radiation effect data on steels. The wrought high temperature-ultrafine precipitate strengthened (HT-UPS) steels with outstanding creep resistance at 700–800°C were developed for supercritical boiler and superheater tubing for fossil power plants in the early 1990s, the cast CF8C-Plus steels were developed in 1999–2001 for land-based gas turbine casing and diesel engine exhaust manifold and turbocharger applications at 700–900°C, and, in 2015–2017, new Al-modified cast stainless steels with oxidation and creep resistance capabilities up to 950–1000°C were developed for automotive exhaust manifold and turbocharger applications. This article reviews and summarizes their development and their properties and applications.     

Long-Term Oxidation of Candidate Cast Iron and Stainless Steel Exhaust System Alloys from 650 to 800 °C in Air with Water Vapor

The oxidation behavior of candidate cast irons and cast stainless steels for diesel exhaust systems was studied for 5,000 h at 650–800 °C in air with 10 % H₂O. At 650 °C, Ni-resist D5S exhibited moderately better oxidation resistance than did the SiMo cast iron. However, the D5S suffered from oxide scale spallation at 700 °C, whereas the oxide scales formed on SiMo cast iron remained relatively adherent from 700 to 800 °C. The oxidation of the cast chromia-forming austenitics trended with the level of Cr and Ni additions, with small mass losses consistent with Cr oxy-hydroxide volatilization for the higher 25Cr/20–35Ni HK and HP type alloys, and transition to rapid Fe-base oxide formation and scale spallation in the lower 19Cr/12Ni CF8C plus alloy. In contrast, small positive mass changes consistent with protective alumina scale formation were observed for the cast AFA alloy under all conditions studied. Implications of these findings for exhaust system components are discussed.     

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.     

The Influence of the Rare Earth Element Ce on the High-Temperature Oxidation Kinetics of Low-Cr Steels

The oxidation behavior of steels containing low-Cr concentrations (0.5-2.25 wt.%) has been studied in laboratory air in the temperature range of 400-550 °C. The oxidation rate of the steels was lower than that of pure iron, but higher than that of pure iron when a small amount of rare earth element cerium (0.03 wt.%) is added to the 2.25Cr1Mo steel. The mass change follows a nearly parabolic law for the case of pure iron and the steel without Ce addition, while linear behavior describes the oxygen uptake for the case of the 2.25Cr1Mo+0.03Ce steel. SEM cross-section observations and thermodynamic calculations confirm that there is no wustite (FeO) formation during oxidation of pure iron and low-Cr steels at 550 °C, whereas FeO might be formed in the oxide scale of 2.25Cr1Mo+0.03Ce at the same oxidation conditions (temperature, atmosphere, and exposure time). By investigating the temperature for FeO stability, this study reveals that the temperature for FeO formation on pure iron is 568 °C, for the 2.25Cr1Mo steel 589 °C, and 471 °C for the 2.25Cr1Mo+0.03Ce. This low value for the FeO stability temperature found for the steel 2.25Cr1Mo+0.03Ce steel explains why this steel oxidizes very fast at 550 °C.     

Internal Oxidation of Fe–Mn–Cr Steels,Simulations and Experiments

A multi-element and multi-phase internal oxidation model that couples thermodynamics with kinetics is developed to predict the internal oxidation behaviour of Fe–Mn–Cr steels as a function of annealing time and oxygen partial pressure. To validate the simulation results, selected Fe–Mn–Cr steels were annealed at 950 °C for 1–16 h in a gas mixture of Ar with 5 vol% H₂ and dew points of ? 30, ? 10 and 10 °C. The measured kinetics of internal oxidation as well as the concentration depth profiles of internal oxides in the annealed Fe–Mn–Cr steels are in agreement with the predictions. Internal MnO and MnCr₂O₄ are formed during annealing, and both two oxides have a relatively low solubility product. Local thermodynamic equilibrium is established in the internal oxidation zone of Fe–Mn–Cr steels during annealing and the internal oxidation kinetics are solely controlled by diffusion of oxygen. The internal oxidation of Fe–Mn–Cr steels follows the parabolic rate law. The parabolic rate constant increases with annealing dew point, but decreases with the concentration of the alloying elements.     

Steam oxidation and surface hardness of power plant valve materials under the ultra super critical steam condition

Current supereritical steam power plants operate at 3,600 psi and 1,000°F. If the steam temperature is raised from 1,000 °F (538 °C) to 1,150 °F (621°C), the efficiency increases by 2%. Therefore, study on the high temperature corrosion of power plant materials under ultra-superciritical conditions (USC) is necessary to protect the plant from corrosion. In this study, valve materials of 17% Cr martensitic steels (17Cr steel), Incoloy 901 (1901) and their surface nitrided specimens were exposed to USC of 621 °C and 3600 psi (255 kg/cm²) steam for 200 °C, 400 °C, and 800 h. The oxidation of both 17Cr steel and 1901 under the USC for 800 h is very small due to the formation of a protective thin oxide layer formation on the surface. The USC oxidation of both nitrided specimens were increased due to the decomposition and formation of active nitrogen from the non protective nitrides such as Fe₄N, Fe_2–3N, and CrN. The oxidation of nitrided 17Cr steel (n17Cr steel) is about two times higher compared to nitrided 1901 (n1901). The surface hardness is improved by more than two times near the surface by nitriding, and the degradation of hardness by USC oxidation is rapid for n17Cr steel, but slow for n1901.     

Si对625合金激光熔覆层高温氧化特性的影响

利用循环氧化法,研究了不同Si含量（0%,1%,3%,质量分数）的625合金熔覆层在700、800、900 ℃下氧化144 h后的高温氧化行为。用XRD分析了氧化物相。通过SEM/EDS研究了氧化物表面和截面的形貌、元素组成和氧化膜的厚度。结果表明,不同温度下试样的氧化动力学都保持抛物线规律,随着温度的升高,氧化增重逐渐增加。通过观察,在900 ℃时,0% Si含量的625合金熔覆层出现了氧化膜大面积剥落的情况,3% Si含量的合金熔覆层氧化膜保持完整。在700 ℃时,随着Si含量增加,氧化膜表面的氧化颗粒尺寸减小且更加致密,同时促进了Cr₂O₃氧化物的生成。在700 ℃下,0 % Si含量的试样出现了大片的内氧化区域;1% Si含量的试样基体部分出现了2处条状的含Ni,Cr,Mo的氧化物相区;而3% Si含量的试样氧化后由于生成了富Si的内氧化层,这阻止了内氧化的发生。外层Cr₂O₃氧化膜和内层SiO₂的联合作用既阻止了O阴离子的渗入也抑制了Fe等金属离子的扩散,提高了合金熔覆层的抗氧化性。     

Preparation of Self-Healing α-Al2O3 Films by Low Temperature Thermal Oxidation

This paper reports a new approach to lowering the temperature necessary for the preparation of α-Al₂O₃. Oxidation of Al–Cr alloys, with Cr contents of 18, 23 and 27 %, was performed at temperatures ranging from 620 to 720 °C in air for 100 h. The resulting oxide films were analyzed by SEM, EDS, XRD and XPS. The results showed that α-Al₂O₃ films were obtained following oxidation of the 18 and 23 wt% Cr alloy samples at 720 °C and that rough surfaces were conducive to the formation of α-Al₂O₃ such that peened surface samples showed significant α-Al₂O₃ growth while polished samples showed no oxide by XRD. A 23 wt% Cr sample with a roughened surface exhibited the formation of α-Al₂O₃ at a temperature of 670 °C. Conversely, only a very thin oxide film was observed on a 27 wt% Cr sample after oxidation at 720 °C.     

Performance of Detonation Gun-Sprayed Ni-20Cr Coating on ASTM A213 TP347H Steel in a Boiler Environment

Detonation gun-sprayed coatings are known for their high density, high bond strength, moderate substrate heating, superior surface finish, better wear/corrosion resistance, and low cost. In this study, detonation gun-spraying technique was used to deposit Ni-20Cr coating on a commonly used boiler steel ASTM A213 TP347H. The specimens with and without coating were subjected to cyclic oxidation testing at an elevated temperature of 700 °C in actual boiler environment to ascertain the usefulness of the coating. The mass change technique was used to establish the kinetics of erosion-corrosion. XRD and SEM/EDS techniques were used to analyze the exposed samples. The uncoated sample suffered from erosion, and a significant mass loss was recorded. It was observed that overall mass loss was reduced by 83% and thickness loss by 53% after the application of the coating. The detonation gun-sprayed Ni-20 Cr coating was found to be suitable to impart erosion resistance to the given steel in the actual boiler environment.     

The Effect of Temperature on the Formation of Oxide Scales Regarding Commercial Superheater Steels

This study addresses the surface changes of three commercial steels (a low alloy ferritic 10CrMo9-10 steel, a Nb-stabilized austenitic AISI347 steel, and a high alloy austenitic Sanicro 28 steel) by comparing the oxide scale thicknesses, chemical compositions, and surface morphologies of samples after pre-oxidation at 200, 500 and 700 °C with different exposure times (5 and 24 h) under humid or dry conditions. With all three steels, the oxide scale thickness increased as functions of temperature and exposure time, the effect of temperature being more prominent than the effect of exposure time. The presence of water resulted in thicker oxide scales at the studied low alloy ferritic steel, whereas in the two austenitic steels, the presence of water increased chromium diffusion to the oxide scale rather than the scale thickness. The oxide layers characterized and analyzed in this paper will be further studied in terms of their abilities to resist corrosion by exposing them under corrosive conditions. The results regarding the corrosion resistance of the steels will be published in a sequel paper.     

Effect of TiC particles on the oxidation behavior of 2Cr13 stainless steel in a simulated marine environment at 550°C

In this study, the oxidation behavior of 2Cr13 stainless steels with/without TiC particles was investigated in a simulated marine environment at 550°C. The results showed that TiC particles greatly accelerated the oxidation rate of 2Cr13 steel during the cyclic oxidation reaction. At the same time, the effect of acceleration was alleviated by the small size and uniform distribution of TiC particles. As galvanic corrosion in the vicinity of TiC particles occurred in 3.5% NaCl solution spray, more NaCl particles deposited on the alloy surface, especially in the area around TiC particles. At 550°C, some shell-like oxidation products were generated on the 2Cr13 steel surface due to TiC addition, and then the porous oxidation products provided poor protective ability. Besides, the defect and stress in the oxide scale was reduced due to the small size and uniform distribution of TiC particles in 2Cr13 steel.