Oxidation behavior of a fine-grained rapidly solidified 18-8 stainless steel

The cyclic oxidation behavior of a fine-grained, rapidly solidified 303 stainless steel was determined at 900 °C in pure oxygen. The rapidly solidified alloy exhibited superior resistance to oxidation compared with that of a wrought 304 stainless steel; its oxidation resistance was as good as that of a wrought 310 stainless steel, even though the latter alloy contained more Cr and Ni. The matrix of the rapidly solidified steel contained a uniform dispersion of fine MnS precipitates (0.2 to 0.5 μm), which were effective in inhibiting grain growth at elevated temperatures. The enhanced resistance to oxidation of the rapidly solidified alloy is attributed to two factors: (1) the formation and growth of protective Cr₂O₃ and SiO₂ scales were promoted by the fine alloy grain size (5 to 8 =gmm) and by the presence of the MnS dispersion, and (2) the adherence of the scale was increased by the formation of intrusions of SiO₂ from the external scale into the alloy, which formed around MnS precipitates and along closely-spaced alloy grain boundaries, and which acted to key the scale mechanically to the alloy.     

Differences in oxides on large-and small-grained 304 stainless steel

The oxides formed on large-grained (∼40 °m) and small-grained (∼4 μm) 304 stainless steel oxidized in air at 800 °C have been examined and compared by Auger electron spectroscopy to learn more about the role of grain boundaries in the oxidation of the materials. For vacuum preannealed specimens, relatively thick iron oxides formed over the grains and thin, chromium-rich oxides formed over grain boudaries of large-grained material. The oxide formed over the entire surface of the small-grained material was a thin chromium-rich layer similar to that formed over grain boundaries of the large-grained samples. The oxidation of both small- and large-grained samples was consistent with selective formation of Cr₂O₃ at grain boundaries followed by a lateral diffusion of Cr and spreading of Cr₂O₃. A protective Cr₂O₃ layer formed readily on small-grained material but not on large-grained material. In contrast to the differences in oxide morphology for small- and large-grained preannealed specimens, oxide morphologies were similar for small- and large-grained material when the outer surface layer was removed by polishing after annealing and before oxidation tests. Surface differences, not adequately defined by Auger and SEM studies, caused marked changes in oxide morphologies for large-grained material. The difference in oxidation behavior before and after polishing was attributed to enhanced oxidation at grain boundaries during the vacuum preannealing treatment and to differences in defect concentrations in the surface region.     

低铬和中铬含钛铁素体不锈钢高温氧化行为

通过对低铬（409L）和中铬（439）含钛铁素体不锈钢在900 ℃下空气中开展恒温氧化试验,获得了两种钢的氧化动力学曲线,并结合EPMA、SEM和XRD等方法对氧化皮的结构进行了分析。结果表明,409L和439氧化动力学曲线均遵从抛物线规律,但409L抛物线速度常数远高于439;409L氧化80 h后氧化层结构由外侧Fe2O3和内侧(Fe,Cr,Mn)3O4复合尖晶石组成;439氧化80 h后氧化层外侧则由Cr2O3和(Cr,Mn)3O4尖晶石组成,内侧为一层SiO2,基体内还伴有钛的内氧化;较高的铬含量能保证TiO2内氧化颗粒稳定存在,而TiO2内氧化颗粒阻止离子迁移,显著降低氧化速度。     

Protective Coatings La–Mn–Cu–O for Stainless-Steel Interconnector 08Х17Т for SOFC,Obtained by the Electrocrystallization Method from Non-Aqueous Solutions

A novel method for the formation of the protective layer on stainless steel interconnectors for solid oxide fuel cells was developed. The method was based on the electrocrystallization of metals from non-aqueous solutions on the stainless-steel interconnector with consecutive thermal treatments. Suggested method was applied for the stainless-steel 08X17T. Chemical composition of the electrolyte for the electrocrystallization was made in order to obtain the oxide protective layer of the stainless-steel interconnector of the following composition: LaMn_0.9Cu_0.1O₃. As a result, a uniform oxide layer was formed on the stainless-steel interconnector surface, protected the stainless-steel from the high-temperature oxidation leading to degradation of the functional properties of the interconnector. Forming coatings were characterized by means of grazing incidence X-rays diffraction, X-rays photoelectron spectroscopy and scanning electron microscopy. Elemental analysis and phase composition have shown that the main components of the protective coatings are found to be compounds with perovskite and spinel structures. The protective coating in the contact with cathode material based on lanthanum-strontium manganite shows significant decrease of chromium propagation from the stainless steel as a result of the diffusive firing in comparison with the sample of the stainless steel without the protective coating. Electrical resistance of the interconnector with the protective coating does not show noticeable degradation during at least 500 h at the temperature 850°C in ambient air.     

Grain-boundary penetration of type 316 stainless steel exposed to cesium or cesium and tellurium

When 20 pct cold-worked Type 316 stainless steel is exposed to Cs at 700°C under controlled oxygen-chemical potential environment, Cs penetration into the stainless steel grain boundaries occurs at oxygen potentials ΔG_o2 ≥ -96 kcal per mole. At lower oxygen potentials (~ΔG_o2 ≤ —110 kcal per mole), no corrosion occurs. Under the same experimental conditions, when the stainless steel is exposed to Cs:Te (2:1, atomic), corrosion occurs and penetration morphology appears to depend strongly on the oxygen-potential environment. The stainless steel suffers intergranular corrosion by Te (in the presence of Cs-Te) under conditions where chromium oxidation is not expected to occur. The kinetics of grain-boundary penetration by Te have been studied at temperatures between 550 and 700°C. The depth of the penetrated zone varies as (time)^1/2, and the process has an activation energy of 34 kcal per mole. The results are discussed, and the effects of stainless steel microstructure and externally applied stress on corrosion reactions are also described.     

Discontinuous creep deformation in a type 316 stainless steel casting

A static cast ingot of type 316 stainless steel was tested at 811, 866, and 922 K. Creep curves for this material under certain temperature and stress conditions show instantaneous increase in strain. In the open literature this increase has been called “strain burst” or “jump”. The number of “bursts” per curve varied from 0 to 3, and the instantaneous strain was as large as 2 pct. The bursts generally occurred at high stresses and at the two lower test temperatures, 811 and 866 K. The rupture time for specimens showing bursts was lower than for specimens without them. The creep-rupture strength of the static casting was approximately 40 MPa lower than that for wrought material. The rupture time of the casting was a strong function of specimen location. The fine grained structure near the ingot edge produced longer rupture time compared with the coarse grain structure. Microprobe analysis showed significant variation in chromium, nickel, and molybdenum across the dendritic regions. The inhomogeneous distribution of the potent creep-strengthening element molybdenum has been suggested as responsible for the strain bursts.     

Activity of Chromium Oxide in CaO‐SiO2 based Slags at 1873 K

Thermodynamic properties of chromium oxides in molten slags are very important for optimization of stainless steel refining processes as well as reduction processes of chromium ores. The solubility of chromite into molten slags has been found to vary drastically with oxygen partial pressure and slag composition in the former studies by the authors. In the present study, activity data and redox equilibria of chromium oxides measured under moderately reducing conditions, P_O2= 6.95×10^?11 atm, at 1873 K are summarized. For the CaO‐SiO₂‐CrO_x system, the activity coefficient of chromium oxide increased with increasing basicity and the optimized slag composition for stainless steel refining is assessed as that saturated with CaCr₂O₄ and Cr₂O₃ using the phase relations determined. On the other hand, the presence of MgO and Al₂O₃ brings about different behaviour of chromium oxide activity and redox equilibria and the 44 mass per cent CaO ‐ 39 mass per cent SiO₂ ‐11 mass per cent Al₂O₃ ‐ 6 mass per cent MgO slag is recommended to reduce the chromium oxidation loss in the practical stainless steel refining process at 1873 K.     

Oxidation of Alloy 600 and Alloy 690: Experimentally Accelerated Study in Hydrogenated Supercritical Water

The objective of this study is to determine whether the oxidation of Alloys 600 and 690 in supercritical water occurs by the same mechanism in subcritical water. Coupons of Alloys 690 and 600 were exposed to hydrogenated subcritical and supercritical water from 633 K to 673 K (360 °C to 400 °C) and the oxidation behavior was observed. By all measures of oxide character and behavior, the oxidation process is the same above and below the supercritical line. Similar oxide morphologies, structures, and chemistries were observed for each alloy across the critical point, indicating that the oxidation mechanism is the same in both subcritical and supercritical water. Oxidation results in a multi-layer oxide structure composed of particles of NiO and NiFe₂O₄ formed by precipitation on the outer surface and a chromium-rich inner oxide layer formed by diffusion of oxygen to the metal-oxide interface. The inner oxide on Alloy 600 is less chromium rich than that observed on Alloy 690 and is accompanied by preferential oxidation of grain boundaries. The inner oxide on Alloy 690 initially forms by internal oxidation before a protective layer of chromium-rich MO is formed with Cr₂O₃ at the metal-oxide interface. Grain boundaries in Alloy 690 act as fast diffusion paths for chromium that forms a protective Cr₂O₃ layer at the surface, preventing grain boundary oxidation from occurring.     

Oxidation Mechanism of Ni-0.5Y Microcrystal CoatingSputtered by Magnetron at 1 000 ℃

The isothermal and cyclic oxidation behaviors of bulk pure nickel and its Ni-0.5Y microcrystal coating sputtered by magnetron at 1 000 ℃ in air were studied. The scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were used to examine the structures of the coating and the NiO oxide films. The laser Raman spectrum was also used to measure the stress level in NiO films formed on bulk nickel and the coating. It is found that the Ni-0.5Y microcrystal coating has lower oxidation rate, and the grain size of NiO formed on Ni-0.5Y coating is also relatively smaller than that formed on bulk nickel. Meanwhile, the compressive stress level of oxide film formed on Ni-0.5Y coating was lower than that formed on bulk nickel, and the high temperature plasticity of oxide film was much improved in coating case. The improvements of anti-oxidation properties of the sputtered Ni-0.5Y coating are due to the microcrystal structure and yttrium.     

Flame Pyrolysis Spraying of Alumina-Yttria Ceramic Coating

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High-Temperature Oxidation Resistance of a Nanoceria Spray-Coated 316L Stainless Steel Under Short-Term Air Exposure

Nanoceria coatings using a spray method were implemented on a 316L stainless steel (SS). Coated and uncoated coupons were exposed to dry air at 1073 K to 1273 K (800 °C to 1000 °C) for short time periods (up to 24 hours) and in situ measurements of oxidation were carried out using a highly sensitive thermogravimetric balance. From the experimental outcome, activation energies were determined in both, coated and uncoated 316 SS coupons. The estimated exhibited activation energies for oxidation in the coated and uncoated conditions were 174 and 356 kJ/mol, respectively. In addition, the developed scales were significantly different. In the coated steel, the dominant oxide was an oxide spinel (Fe, Mn)₃O₄ and the presence of Fe₂O₃ was sharply reduced, particularly at 1273 K (1000 °C). In contrast, no spinel was found in the uncoated 316L SS, and Fe₂O₃ was always present in the scale at all the investigated oxidation temperatures. The coated steels developed a highly adherent fine-grained scale structure. Apparently, the nanoceria particles enhanced nucleation of the newly formed scale while restricting coarsening. Coarse grain structures were found in the uncoated steels with scale growth occurring at grain ledges. Moreover, the oxidation rates for the coated 316L SS were at least an order of magnitude lower than those exhibited by the steel in the uncoated condition. The reduction in oxidation rates is attributed to a shift in the oxidation mechanism from outward cation diffusion to inward oxygen diffusion.     

Grain-boundary penetration of type 316 stainless steel exposed to cesium or cesium and tellurium

When 20 pct cold-worked Type 316 stainless steel is exposed to Cs at 700°C under controlled oxygen-chemical potential environment, Cs penetration into the stainless steel grain boundaries occurs at oxygen potentials ΔG_o2 ≥ -96 kcal per mole. At lower oxygen potentials (~ΔG_o2 ≤ —110 kcal per mole), no corrosion occurs. Under the same experimental conditions, when the stainless steel is exposed to Cs:Te (2:1, atomic), corrosion occurs and penetration morphology appears to depend strongly on the oxygen-potential environment. The stainless steel suffers intergranular corrosion by Te (in the presence of Cs-Te) under conditions where chromium oxidation is not expected to occur. The kinetics of grain-boundary penetration by Te have been studied at temperatures between 550 and 700°C. The depth of the penetrated zone varies as (time)^1/2, and the process has an activation energy of 34 kcal per mole. The results are discussed, and the effects of stainless steel microstructure and externally applied stress on corrosion reactions are also described.     

BN1TL不锈钢连铸坯组织对鳞折缺陷形成影响及试验验证

节镍奥氏体不锈钢热轧板及冷轧板表面的鳞折问题严重影响了产品的合格率。以BN1TL不锈钢连铸坯作为研究对象,分析了BN1TL不锈钢凝固组织的特点,并通过冷装炉方式下的热轧模拟试验验证了连铸坯组织与鳞折缺陷的相关性。试验结果表明,BN1TL不锈钢的凝固组织以柱状晶为主,晶粒尺寸由表层至芯部逐渐增大,柱状晶晶界存在铬的碳化物析出。同时该特点的凝固组织由于存在大量晶间析出,导致晶界弱化严重,经过热轧后易生成鳞折缺陷,并且影响整个轧制工艺流程。热轧模拟试验结果表明,材料在粗轧时即产生鳞折缺陷,精轧时缺陷得到扩展。减小在粗轧时的道次压下量可以减少鳞折缺陷的生成,改善板坯表面质量。     

The aging behavior of homogenized type 308 and 308CRE stainless steel

The aging behavior of type 308 stainless steel and type 308 stainless steel with controlled residual elements (308CRE) was studied. The alloys were initially homogenized, resulting in a fully austenitic structure in the type 308 alloy and a duplex, austenite plus ferrite structure in the type 308CRE alloy. The materials were subsequently aged at 550, 650, 750, and 850 °C for times up to 10,000 hours. Aging of the type 308 steel resulted in precipitation of chromium-rich M₂₃C₆, primarily along grain boundaries, followed by sigma-phase formation after long aging times (≥5000 hours). The aging response of the duplex type 3O8CRE steel was somewhat more complicated. Although the titaniumrich carbides, nitrides, and sulfides produced during homogenization remained stable during subsequent aging, some additional precipitation was found. G-phase formed within the residual ferrite but eventually redissolved. The ferrite partially dissolved during early aging and later transformed to sigma phase. The sigma-phase formed significantly faster in the aged type 308CRE alloy. The results are summarized in the form of TTT diagrams. A comparison is made with a similar aging study on the same alloys but starting with the as-welded condition.     

Influence of Nanometric Ceria Coating on Oxidation Behavior of Chromium at 900 ℃

Isothermal and cyclic oxidation behaviors of chromium samples with and without nanometric CeO2 coating were studied at 900℃ in air. Scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and high-resolution electron microscopy （HREM） were used to examine the morphology and microstructure of the oxide film. It was found that ceria coating greatly improved the oxidation resistance of Cr both in isothermal and cyclic oxidizing experiments. Acoustic emission （AE） technique was used in situ to monitor the cracking and spalling of oxide film, and AE signals were analyzed in time-domain and number-domain according to related oxide fracture model. Laser Raman spectrometer was also used to study the stress of oxide film formed on Cr with and without ceria. The improvement in oxidation resistance of chromium is believed mainly due to that ceria greatly reduced the growth speed and grain size of Cr2O3. This fine grained Cr2O3 oxide film might have better high temperature plasticity and could relieve parts of the compressive stress by means of creeping and maintained ridge character and relatively lower level of internal stress. Meanwhile, ceria application reduced the size and number of interfacial defects, remarkably enhanced the adhesive property of Cr2O3 oxide scale formed on Cr substrate.     

Microstructural analysis and oxidation behavior of laser-processed Fe-Cr-AI-Y alloy coatings

Dense crack-free coatings of Fe-Cr-Al-Y quaternary alloy were produced on stainless steel 316L substrates using a continuous wave Nd-YAG solid-state laser coupled with a fiber optic beam delivery system. Experiments were performed at a laser power between 0.6 and 2.4 kW, process speed in the range 0.053 to 0.423 cm/s, powder feed rate fixed at 0.083 g/s, and focused multimode laser beam with a diameter of 0.5 cm. Various microanalysis techniques demonstrated that the coatings were metallurgically bonded to the substrate and possessed thicknesses between 0.35 and 4.64 mm, refined columnar microstructures with grain sizes of 15 to 150 μxm, increased concentration of key alloying elements, and appreciably high microhardness up to 409 kg/mm². The laser-processed microstruc-tures comprised a body-centered cubic (bcc) ferrite(α phase) crystal structure with a relatively large lattice parameter compared to α-Fe due to the enhanced dissolution of varying amounts of Cr, Al, Ni, and Y, depending on the dilution from the substrate material. Oxidation tests conducted in air at temperatures of 1100 ° to 1200 ° for 95 hours revealed the formation of an approximately 5-μm-thick dense α-Al₂O₃ oxide scale of a rhombohedral (hexagonal) crystal structure. The α-Al₂O₃ scale exhibited remarkable high-temperature resistance and strong adherence to the coating surface. Extensive oxidation of the uncoated stainless steel substrate produced a porous and heavily spalled alloy oxide scale about 60-μm thick consisting of FeCr₂O₄ and NiCr₂O₄ with cubic and tetragonal crystal structures, respectively. The retention of the bcc α phase and the insignificant grain growth after oxidation are indicative of the thermal stability of the laser-processed coating microstructures. The obtained results demonstrate that Fe-Cr-Al-Y alloy coatings exhibiting fine-grained hard mi-crostructures, high-temperature oxidation resistance, and strong adherence to stainless steel can be developed by means of laser processing.     

00Cr25Ni22Mo2N奥氏体不锈钢的高温氧化行为

采用静态增重法研究了00Cr25Ni22Mo2N奥氏体不锈钢在不同温度下的循环氧化动力学曲线.结果表明,00Cr25Ni22Mo2N不锈钢在700℃时的氧化动力学曲线是一条平行于时间轴的直线,这说明其在该温度下氧化反应非常微弱;在800和900℃时的氧化动力学曲线遵循抛物线氧化规律,具有良好的抗氧化性能.利用扫描电镜、能谱分析和X射线衍射对氧化膜的表面形貌、截面形貌和相组成进行研究,发现该钢在3个温度下都生成了致密和黏附性好的保护性氧化膜,700℃生成的氧化膜主要由Cr2O3和Fe2O3组成,氧化膜很薄;800℃生成的氧化膜由Cr2O3、Fe2O3和少量的NiCr2O4、FeCr2O4组成,氧化膜厚度增加;900℃生成的氧化膜由Cr2O3、Fe2O3和NiCr2O4组成,氧化膜厚度进一步增加.认为氧化膜组成和结构是00Cr25Ni22Mo2N不锈钢具有良好抗高温氧化性能的重要原因.     

Influence of Protective Coating at High Temperature on Surface Quality of Stainless Steel简

A ceramic matrix coating for minimizing steel loss of stainless steel at high temperatures was prepared by handled air-spraying technique, and the influence of coating on surface quality of stainless steel was mainly investiga ted in laboratory. Experimental results showed that the protective coating reduced the oxidation of stainless steel by more than 91% and minimized high temperature scaling and also enhanced steel surface quality. The scales of coated specimen were removed completely and the scales of uncoated specimen were partly residual on the surface after cool ing process. Mn-rich and Fe-rich zones were found in the oxides. The Cr2 O3 found in scales came from the underlying stainless steel and formed a Cr rich layer along the spalled surface.     

Hot working and recrystallization of As-Cast 316L

Stress-strain behavior and microstructure evolution during hot working of as-cast austenitic stainless steel alloy 316L were investigated by uniaxial compression of cylindrical specimens at a strain rate of 1 s⁻¹ over the temperature range 1000 °C to 1150 °C and up to a strain of one. The measured flow curves showed monotonic hardening, indicating that dynamic recrystallization was not important in microstructural evolution. Static recrystallization was observed to nucleate preferentially at the delta ferrite-austenite interphase boundaries. The recrystallization kinetics of the as-cast material was compared to a relatively fine-grained wrought 316L material and found to be somewhat slower. However, the difference between the two material conditions was not nearly as great as previously reported for as-cast and wrought 304L alloy. The difference in behaviors between 316L and 304L is attributed to the relatively large amount and vermicular morphology of the delta ferrite phase in the 316L, resulting in a relatively fine effective grain size, compared to the existing coarse columnar structure, and concomitant enhancement of recrystallization. Compared to wrought 316L, the recrystallization rate of the as-cast material was relatively sluggish, despite a relatively fine effective grain size. The difference is associated with the 100 orientations of the columnar grains with respect to the compression axis, producing a soft orientation and a reduced rate of accumulation of dislocation density in the substructure. Also, compared to wrought 316L, the recrystallization rate of the as-cast material tends to decrease with time, the drop occurring concurrently with spheroidization and dissolution of the ferrite. It is suggested that (1) movement of the delta ferrite-austenite interphase boundary during spheroidization may poison incipient recrystallization and (2) dissolution of delta ferrite can locally enrich the austenite matrix in Mo and Cr, raising the local stacking fault energy and lowering grain boundary mobility to favor recovery over recrystallization in the vicinity of the ferrite-austenite boundary. A kinetic model for recrystallization was developed and used to simulate evolution of the first cycle of recrystallization during various thermal-mechanical treatment schedules typically employed during the primary breakdown of as-cast material.