Postbreakaway oxidation kinetics of two ferritic steels

The time needed to reach breakaway oxidation, weight gain at the breakaway point, and the postbreakaway oxidation rate are three important variables which, along with others, play an important role in causing boiler tube failure. In this paper an attempt has been made to study the behavior of two ferritic steels-21/4Cr-1Mo and 9Cr-1Mo. Postbreakaway oxidation kinetics of these two alloys have been studied at temperatures in the range 900–1100°C in pure oxygen for a short duration (maximum of 3 hr). No breakaway was observed at 900°C under these conditions. Postoxidation kinetics are linear at first, followed by a slower oxidation rate. The results have been substantiated by the postoxidation studies using SEM, EDAX, and X-ray diffraction.     

Detection of breakaway oxidation and spalling in the oxide scales of 2 1/4Cr-1Mo steel using acoustic emission technique

The onset of breakaway oxidation and in-situ cracking and spalling of the oxide layer formed on 2 1/4Cr-1Mo steel have been detected by the use of the acoustic emission (AE) technique. Various AE parameters, viz., acoustic emission event counts vs time, acoustic emission counts vs time, and voltage level vs time have been used to detect these phenomena. It has been found that variations in AE parameters are marginal during heating at 600, 700, and 800°C, and a sudden rise in these parameters occurs during cooling. Increase in AE activity during cooling has been related to spalling of the oxide layers. At 900 and 950°C, a considerable increase in AE parameters (except voltage level) has been detected after certain times at the respective temperatures. By comparing with corresponding thermogravimetric curves, this increase in AE parameters has been attributed to the onset of breakaway oxidation. A very large increase in AE parameters during cooling from these temperatures has been related to in-situ cracking.     

Influence of Silicon and Aluminum Additions on the Oxidation Resistance of a Lean-Chromium Stainless Steel

The effect of Si and Al additions on the oxidation of austenitic stainless steels with a baseline composition of Fe–16Cr–16Ni–2Mn–1Mo (wt.%) has been studied. The combined Si and Al content of the alloys did not exceed 5 wt.%. Cyclic-oxidation tests were carried out in air at 700 and 800°C for a duration of 1000 hr. For comparison, conventional 18Cr–8Ni type-304 stainless steel specimens were also tested. The results showed that at 700°C, alloys containing Al and Si, and alloys with only Si additions showed weight gains about one half that of the conventional type-304 alloy. At 800°C, alloys that contained both Al and Si additions showed weight gains approximately two times greater than the type-304 alloy. However, alloys containing only Si additions showed weight gains four times less than the 304 stainless. Further, alloys with only Si additions preoxidized at 800°C, showed zero weight gain in subsequent testing for 1000 hr at 700°C. Clearly, the oxide-scale formation and rate-controlling mechanisms in the alloys with combined Si and Al additions at 800°C were different than the alloys with Si only. ESCA, SEM, and a bromide-etching technique were used to analyze the chemistry of the oxide films and the oxide–base-metal interface, in order to study the different oxide film-formation mechanisms in these alloys.     

Behavior of oxide scales on 12Cr-1Mo steel during thermal cycling

Soot-blower operation leads to thermal-cyclic-oxidation conditions of heat-resistant steels in conventional power stations. The consequence may be failure of the protective oxide scales and increased corrosive attack. The behavior of protective oxide scales on 12Cr-1Mo steel was investigated under isothermal conditions at 650°C and under thermal cycling conditions between 650 and 300°C (200°C). The tests were performed in air, air + 0.5%SO₂, simulating the fire side, and Ar-5% H₂-50% H₂O, simulating the steam side. Complete heat-exchanger tubes were used as specimens. The main instrument for the detection of scale failure was acoustic-emission analysis. In air and air + 0.5% SO₂ the M₂O₃ scales with M = Fe, Cr were very thin and did not show significant failure either during isothermal or during cyclic oxidation. The thicker scales formed in Ar-5% H₂-50% H₂O, consisting of several partial layers, failed even during isothermal oxidation due to geometrically-induced growth stresses in the scale. Thus, in the thermal-cycling cooling periods there was only very little additional scale cracking. The scale behavior can be explained consistently by applying the existing quantitative models.     

Microstructure and mechanical properties of weld fusion zones in modified 9Cr-1Mo steel

Modified 9Cr-1Mo steel finds increasing application in power plant construction because of its excellent high-temperature properties. While it has been shown to be weldable and resistant to all types of cracking in the weld metal and heat-affected zone (HAZ), the achievement of optimum weld metal properties has often caused concern. The design of appropriate welding consumables is important in this regard. In the present work, plates of modified 9Cr-1Mo steel were welded with three different filler materials: standard 9Cr-1Mo steel, modified 9Cr-1Mo, and nickel-base alloy Inconel 182. Post-weld heat treatment (PWHT) was carried out at 730 and 760 °C for periods of 2 and 6 h. The joints were characterized in detail by metallography. Hardness, tensile properties, and Charpy toughness were evaluated. Among the three filler materials used, although Inconel 182 resulted in high weld metal toughness, the strength properties were too low. Between modified and standard 9Cr-1Mo, the former led to superior hardness and strength in all conditions. However, with modified 9Cr-1Mo, fusion zone toughness was low and an acceptable value could be obtained only after PWHT for 6 h at 760 °C. The relatively poor toughness was correlated to the occurrence of local regions of untransformed ferrite in the microstructure.     

Frequency analysis of AE signal obtained during breakaway oxidation and internal cracking of 2 1/4Cr-1Mo steel at 900°C

The acoustic emission (AE) signal obtained during breakaway oxidation and internal cracking of 2 1/4Cr-1Mo steel at 900°C was analysed over its frequency domain. Three regions, namely pre-breakaway, post-breakaway, and internal cracking were distinguished and confirmed by thermogravimetric analysis and scanning electron microscopy observations. The frequency analysis of the AE signal obtained in the three different regions showed three different characteristic patterns. Frequency spectra based on predominant frequencies were correlated with the physical phenomena happening during the course of oxidation.     

High temperature corrosion of nanoceria coated 9Cr-1Mo ferritic steel in air and steam

In this article, air and steam oxidation resistance of bare and nanoceria coated 9Cr-1Mo ferritic steel at 650 °C for 500 h is reported. The kinetics of the oxidation process and the changes in the morphology and the chemistry for both the specimens were analyzed by backscattered electron and EPMA mapping. The nanoceria coated 9Cr-1Mo ferritic steel showed better oxidation resistance than uncoated steel under similar conditions. The thickness of the oxide scales formed on the coated samples was significantly reduced as compared to the uncoated substrates. The effectiveness of such a thin and simple nanocoating to improve the oxidation resistance of 9Cr-1Mo ferritic steel compared to the pristine is attributed to the change in the migration of cations from outward to inward diffusion of oxygen as the major mechanism.     

Oxidation of Ti−14Al−21Nb in air and oxygen from 700 to 1000°C

The oxidation behavior of Ti–14Al–21Nb in air and in oxygen was determined over the temperature range 700 to 1000°C. Weight gains in both atmospheres were measured using thermogravimetric analysis. The resulting oxidation products were identified using X-ray diffraction, and oxide morphology was evaluated using electron microscopy and wavelength-dispersive X-ray analysis. Total weight gains in oxygen were up to four times higher than in air, and a higher percentage of the weight gain in oxygen was due to oxygen dissolution into the metal. Based on metallurgical examination of the oxidized specimens, it was concluded that the lower oxidation weight gains in air are due to the formation of a thin layer of TiN and TiAl at the oxide-metal interface which inhibits the diffusion of oxygen into the metal.     

Oxidation behavior of Fe-20Cr-5Al rare earth alloys in air and synthetic exhaust gas

Fe-20Cr-5Al alloy foils are used in automotive catalytic converters. This work examines oxidation behavior of four production-processed alloy foils in both air and synthetic exhaust gas environments. Oxidation tests were performed between 750° C and 1150° C for times to 96 hrs. Weight gain results in both atmospheres were similar, an indication that the same mechanism controls oxidation in both environments. At high temperatures (>-950° C) both atmospheres produce weight gains consistent with -alumina growth. Activation energies of 323 kJ/gmole and 271 kJ/gmole were calculated for oxidation in air and synthetic exhaust gas, respectively. At lower temperatures (<-850° C), accelerated weight gains can occur from growth of transition alumina. Despite similar weight gain results, the two atmospheres produce different oxide morphologies: at 950° C and above, air produces a rounded, porous oxide while synthetic exhaust produces a more compact, angular oxide. Unexpectedly, oxide spalling occurred on foils oxidized in synthetic exhaust at 1050° C and above.     

Effect of cold work on the oxidation resistance of2\tfrac{1}{4} Cr-1 Mo steel

The effect of cold work on the oxidation rate of 21/4 Cr-1 Mo steel in pure oxygen at 1 atm pressure at temperatures ranging from 400 to 950C has been studied for short exposure periods (max. 4 hr). The specimens had been cold worked up to 90% by cold rolling. The results indicate negligible effect of cold work on the oxidation kinetics up to 700C, beyond which there is general reduction in oxidation. The effect was pronounced at 900C. The increased resistance to oxidation has been attributed to the faster diffusion of chromium in the cold-worked material compared to the annealed one, leading to the formation of a chromium-rich spinel which helps in slowing down the oxidation of the alloy. The findings have been corroborated by the examination of all samples oxidized at 900C by optical, EPMA, SEM, and EDAX analyses.     

High temperature oxidation resistance of 1.25cr-0.5mo wt.% steels by zirconia coating

A zirconia coating was applied to improve the oxidation resistance of 1.25Cr-0.5 Mo wt.% steels. A 8 wt.% yttria-stabilized zirconia coating was deposited by sol-gel technique. One problem with this method is that the hydrolysis of the organometallic precursors is faster than condensation, and the formation of precipitates is favored. Ethyl acetate was used to slow the hydrolysis rate in order to obtain a more continuous layer. The air oxidation behavior of the coating was studied at 600 °C and 700 °C by the continuous measurement of the weight gain. The microstructural characterization was performed by optical and scanning electron microscopy, and the composition was determined by energy dispersive spectroscopy (EDS). The weight gain of the 1.25Cr-0.5 Mo wt.% was diminished by about 70% compared to uncoated samples.     

2.25Cr-1Mo与2.25Cr-1Mo-0.25V钢冷却转变对比分析

采用相变仪、光学显微镜及扫描电镜研究了2.25Cr-1Mo和2.25Cr-1Mo-0.25V钢冷却转变过程,并分析了钒对其冷却转变的影响。结果表明,与2.25Cr-1Mo钢的CCT曲线相比,2.25Cr-1Mo-0.25V钢铁素体转变区及贝氏体转变区均向右移,并且在现有试验条件下已无法观察到珠光体转变区;钒添加到2.25Cr-1Mo钢中,能够提高钢的淬透性,2.25Cr-1Mo-0.25V钢在48 000 ℃/h冷速下能生成部分马氏体;钒能够细化钢板显微组织,与2.25Cr-1Mo钢相比,2.25Cr-1Mo-0.25V钢在800 ℃/h冷速下组织更为细小。     

High-temperature oxidation and thermal cycling of aluminum-electroplated stainless steels

An alumina coating, produced from the oxidation of an aluminum-electroplated deposit, improved the oxidation resistance in air of a ferritic, AISI-type 446 stainless steel, Fe-24Cr-1.2Al containing 0.15% of mischmetal, and an austenitic AISI 321 stainless steel containing 0.53% Ti, at least up to 1100°C. In thermal-cycling tests from 1000°C to room temperature, the alumina coating was adherent on the ferritic and austenitic steels, for at least 1000 and about 700 cycles, respectively. The addition of rare earths to the ferritic steels and titanium to the austenitic, provided good adhesion between the coating and substrate. The porous nature of the coating was found to be very beneficial by causing the coating to be more resistant to thermal and growth stresses. Oxidation mechanisms are discussed in the light of results obtained from the thermogravimetric tests and metallographic observations by SEM-ED analysis.     

Observation of oscillating reaction rates during the isothermal oxidation of ferritic steels

Oscillating reaction rates have been observed in the steam oxidation of 21/4Cr-1MoNb and 9Cr-1Mo ferritic steels at 500–550°C. Changes in reaction rate are associated with the formation of a laminated, inner-oxide layer, made up of bands of fine and coarse-grain spinel oxide. The lowest reaction rates occur during growth of the fine-grain oxide. Coarse-grain oxide generally contains the same levels of Cr, Mo, and Si as the steel (after allowing for loss of Fe to the outer layer), while the fine-grain material contains three times these levels. Ni builds up in the metal and is present in the oxide as metallic particles (mostly associated with fine-grain oxide). A mechanism is proposed in which the highest reaction rates are controlled by diffusion of Fe ions through the oxide layer (as in normal parabolic oxidation) and the lowest rates by diffusion of Fe through the Ni-rich layer in the metal.     

The Influence of Temperature on Oxide-Scale Formation during Erosion--Corrosion

Four steels (Fe2.25Cr1Mo, Fe9Cr1Mo, AISI 304, 353 MA) and one Ni-based superalloy (Inconel 625) were exposed in an erosion--corrosion test rig at the temperatures 20, 350, 550 and 700°C for 1 week. The atmosphere was air and the particle velocity 1.2 m/s. The composition and thickness of the developed surface layers were determined by Auger electron spectroscopy and X-ray photoelectron spectroscopy. The ferritic and Ni-based alloys investigated show a minimum in wastage rate around 350°C due to the development of a particle strengthened/toughened composite layer on the surface. The greatest wastage rates were measured at 700°C. Rapid diffusion paths created in the oxide from the particle bombardment results in the growth of oxide nodules at the oxide/metal-interface causing protruding oxide flakes which are chipped away. At this temperature the ferritic steel Fe9Cr1Mo is degraded to a larger extent than the austenitic steels.     

Effect of alloying on the properties of 9Cr low activation martensitic steels

Two types of 9Cr low activation martensitic steels (named 9Cr-1 and 9Cr-2) were developed in University of Science and Technology Beijing. 9Cr-1 and 9Cr-2 were produced by vacuum induction melting method, and examinations of the microstructures were carried out with X-ray diffraction analysis, optical microscopy, scanning electron microscopy and transmission electron microscopy. The ultimate tensile strength and yield tensile strength were evaluated with tensile tests. The impact properties were characterized with Charpy impact experiments. The results indicated that 9Cr-1 and 9Cr-2 on as-received condition (95℃/30~min/water quenching plus 780℃/90~min/air cooling) were fully martensitic steels free ofβ-ferrite. The ultimate tensile strength of 9Cr-1 and 9Cr-2 were 695~MPa and 680~MPa, respectively. However, 9Cr-2 showed a fine grain size of 4.8 μm, and its value of ductile-brittle transition temperature (DBTT) was -90℃ under as-received condition. The additions of vanadium, titanium and boron accelerated the formation of MX precipitates and resulted in fine grains and precipitates. The fine grains effectively reduced the value of DBTT from -60℃ to -90℃ with identical upper shelf energy (USE). The decrease in silicon concentration of 9Cr-2 induced a slight reduction in ultimate tensile strength from 695 MPa to 680 MPa.     

High-temperature oxidation of Fe-2 1 4 Cr-1Mo in oxygen

The oxidation of a 2 ₁ ⁴ Cr-1Mo steel in dry flowing oxygen has been studied in the temperature range 550–700°C for periods of up to 100 hr. A detailed low-resolution microstructural investigation revealed a layered oxide consisting of a very fine-grained and finely pored innermost layer of doped spinel, a central columnar-grained relatively coarsely pored layer of magnetite, and an outer fine-grained hematite layer with fine pores and covered with whiskers of -Fe₂O₃. This structure is compared with previous results on Fe and model Fe-Cr alloys, as are the kinetics of the oxidation reaction.     

Codeposited chromium and silicon diffusion coatings for Fe-Base alloys via pack cementation

The simultaneous deposition of Cr and Si into plain carbon, low-alloy, and austenitic steels using a halide-activated pack-cementation process is described. Equilibrium partial pressures of gaseous species have been calculated using the STEPSOL computer program to aid in designing specific processes for codepositing the desired ratios of Cr and Si into a given alloy. The calculations indicate that NaCl-activated packs are chromizing, while NaF-activated packs deposit more Si with less Cr. The use of a dual activator (e.g., NaF+NaCl) allows for the deposition of both Cr and Si in the desired amounts. Single-phase ferritic coatings (150–250 microns thick) with a surface concentration of 20–35 wt.% Cr and 2–4% Si have been grown on AISI 1018, Fe-2.25 Cr-1.0Mo-0.15C, and Fe-0.5 Cr-0.5 Mo-0.2C steels using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF+NaCl activator, and a silica filler. Two-phase coatings (approximately 75 microns thick) containing 20–25 wt.% Cr and 2.0–2.4% Si have been obtained on 304 stainless steel using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF activator, and an alumina filler. The same pack chemistry allowed the diffusion of Cr and Si into the austenitic Incoloy 800 alloy without a phase change. A coated Fe-2.25 Cr-1.0 Mo-0.15 C coupon with a surface concentration of Fe-34 wt.% Cr-3Si was cyclically oxidized in air at 700°C for over four months and 47 cycles. The weight gain was very low (<0.2 mg/cm²) with no scale spalling detected. Coated coupons of AISI 1018 steel, and Fe-0.5 Cr-0.5 Mo-0.2C steel have shown excellent oxidation-sulfidation resistance in reducing, sulfur-containing atmospheres at temperatures from 400 to 700°C and in erosion and erosion-oxidation testing in air at 650 and 850°C.     

High-temperature oxidation of 1.25Cr-0.5Mo steel in SO2

The oxidation kinetics of 1.25Cr-0.50Mo steel under different thermal cycling conditions in the range of 520–700°C in a 2000 ppm SO ₂-argon gas mixture have been studied by thermogravimetric techniques. The influence of the number of thermal cycles experienced by the specimen has been determined on the oxidation process. Characterization of the scales was performed by scanning electron microscopy, and it was found that the nature of the scale is a function of oxidation temperature.     

Behavior of oxide scales on 2.25Cr-1Mo steel during thermal cycling. II. Scales grown in water vapor

The acoustic emission (AE) technique has been applied to identify scale cracking during thermal cycling of tubes of 2.25Cr-1Mo steel. The scale morphology and failure mode were investigated by light and electron optical methods. The scale formed at 600°C in water vapor consists of an outer magnetite and an inner, chromium-containing spinel layer. Cooling leads to tensile stresses in the scale that cause macro and microcrack formation in the scale. At constant-cycle parameters, a characteristic set of crack length and crack density is established. Changes in the cycle parameters also change the crack length and crack density. The experimental results can be described by a model developed by Hasselmann assuming a large number of noninteracting microcracks in a ceramic plate.