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 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.     

Influence of water vapour on high temperature oxidation of steels used in petroleum refinery heaters

The oxidation behaviours of three different steels used in the construction of petroleum refinery heaters were investigated by thermogravimetric analysis (TGA) technique. C‐5, P‐11 and P‐22 steel samples were tested in two different environments: air and CO₂ + 2H₂O + 7.52N₂, a gas composition which simulates the combustion products of natural gas, at 450 and 500 °C. P‐22 steel had the best oxidation resistance at both temperatures in air. In CO₂ + 2H₂O + 7.52N₂ environment, the oxidations of all the steels were accelerated and C‐5 exhibited better oxidation resistance than P‐22 and P‐11. Analyses of oxidation products by optical microscopy, SEM‐EDX and XRD were carried out to correlate TGA results to oxide composition and morphology. The lower oxidation rate of P‐22 in air was explained with reference to the formation of a protective Cr‐containing oxide layer between the steel and the iron oxide scale. The higher oxidation rates of chromium containing steels in CO₂ + 2H₂O + 7.52N₂ environment were attributed to the depletion of protective Cr‐containing oxide scale, which was deduced from the lower Cr content of this layer than that formed in air oxidation, as a result of probably faster oxidation of Cr even inside the steel. Therefore, the oxidation mechanisms of Fe? Cr alloys with intermediate Cr contents at higher temperatures could also be valid for steels with low chromium contents such as P‐22 (2.25%) even at 450 and 500 °C.     

Effect of alloying elements and coiling temperature on the recrystallization behavior and the bainitic transformation in TRIP steels

The effects of alloying elements and coiling temperature on recrystallization behavior and bainitic transformation were investigated based on 0.07C-Mn-Cr-Nb steel with a low carbon equivalent. Based on the ferrite recrystallization behavior, the proper intercritical annealing temperature of all studied steels was suggested to produce TRIP steel with good strength and elongation balance. All steels coiled at 550 °C showed much faster ferrite recrystallization behavior than steels coiled at 700 °C. In addition to the coiling temperature, the effect of increasing carbon content on the ferrite recrystallization was minor at a coiling temperature of 550 °C, but much more prominent at a coiling temperature of 700 °C. The highest Mo added steel showed the best strength and elongation balance, and the highest carbon and Mo added steel showed the highest tensile strength at a coiling temperature of 550 °C. The steel containing a higher amount of elemental Al (0.7 wt.% Al) exhibited much better elongation than the lower Al added steel (0.04 wt.% Al) in TS 780 MPa grade, about 24 % and 19 %, respectively.     

Nickel-chromium plasma spray coatings: A way to enhance degradation resistance of boiler tube steels in boiler environment

Boiler tube steels, namely low carbon steel ASTM-SA-210-Grades A1 (GrA1), 1Cr-0.5Mo steel ASTM-SA213-T-11 (T11), and 2.25Cr-1 Mo steel ASTM-SA213-T-22(T22), were used as substrate steels. Ni-22Cr-10Al-1Y powder was sprayed as a bond coat 150 μm thick before a 200 μm final coating of Ni-20Cr was applied Coatings were characterized prior to testing in the environment of a coal fire boiler. The uncoated and coated steels were inserted in the platen superheater zone of a coal fired boiler at around 755°C for 10 cycles, each 100 h. Coated steels showed lower degradation (erosion-corrosion) rate than uncoated steels showed. The lowest rate was observed in the case of Ni-20Cr coated T11 steel. Among the uncoated steels, the observed rate of degradation was the lowest for the T22 steel.     

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.     

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.     

Effect of alloy composition on high-temperature bending fatigue strength of ferritic stainless steels

Exhaust manifolds are subjected to an environment in which heating and cooling cycles occur due to the running pattern of automotive engines. This temperature profile results in the repeated bending stress of exhaust pipes. Therefore, among high-temperature characteristics, the bending fatigue strength is an important factor that affects the lifespan of exhaust manifolds. Here, we report on the effect of the alloy composition, namely the weight fraction of the elements Cr, Mo, Nb, and Ti, on the high-temperature bending fatigue strength of the ferritic stainless steel used in exhaust manifolds. Little difference in the tensile strength and bending fatigue strength of the different composition steels was observed below 600 °C, with the exception of the low-Cr steel. However, steels with high Cr, Mo, or Nb fractions showed considerably larger bending fatigue strength at temperatures of 800 °C. After heating, the precipitates from the specimens were extracted electrolytically and analyzed using scanning electron microscopy energy dispersive spectrometry and transmission electron microscopy. Alloying with Cr and Mo was found to increase the bending fatigue strength due to the substitutional solid solution effect, while alloying with Nb enhanced the strength by forming fine intermetallic compounds, including NbC and Fe₂Nb.     

Oxidation of Uncoated and Aluminized 9-12% Cr Boiler Steels at 550-650?°C

In this paper, oxidation behavior of 9-12% Cr steels P91 and HCM12A is studied in air and in a mixture of air and water vapor. Comparison is made between these steels in uncoated condition and coated with aluminum diffusion coating by a slurry method. Oxidation tests were carried out at 550, 600, and 650 °C for a discontinued duration of 1000 h; every 250 h the specimens were slowly cooled to room temperature and weighed. SEM + EDS and XRD characterization were performed after 500 and 1000 h. The results showed that oxidation rate of uncoated P91 and HCM12A was significantly higher in the mixture of air and water vapor than in air. Oxidation resistance of the studied materials improved substantially when they were aluminized.     

Oxide scales characterization of micro-alloyed steel at high temperature

A micro-alloyed steel was oxidized at different temperature between 900 °C and 1200 °C in 80%N₂–15%CO₂–5%O₂ atmosphere. As compared with the pure iron, an additional layer between the matrix and the oxide scale was detected in the micro-alloyed steel. The micro-alloying elements, mainly Si and Mo, are found accumulated in the subscale in the form of Laves phases. As the Laves phases can impede the diffusion of iron ion and electron across the subscale, the micro-alloyed steel shows a lower oxidation rate than that of pure iron. Moreover, the existence of the subscale results in a different thickness ratio of hematite, magnetite and wustite in the micro-alloyed steel as compared with that in pure iron. It is also found that the surface roughness of oxide scale significantly depends on the content of hematite.     

Alloy carbide precipitation in tempered 2.25 Cr–Mo steel under high magnetic field

The influence of a high magnetic field on carbide precipitation during the tempering of a 2.25 Cr–Mo steel was investigated by means of transmission electron microscopy. As-quenched specimens were tempered at 200, 550 and 700 °C for various times in the absence and presence of a 12 T magnetic field. Experimental results indicate that the applied high magnetic field effectively promotes the precipitation of M₂₃C₆ carbides at low temperature (200 °C) and M₇C₃ and M₂₃C₆ carbides at intermediate temperature (550 °C). The increased Fe content in the M₂₃C₆ and M₇C₃ carbide significantly increases the magnetization. The magnetic Gibbs free energy, which influenced the alloy carbide precipitation behavior, was considered to be mainly determined by the intrinsic magnetization energy for M₂₃C₆ and M₇C₃ carbides. With the increase of the tempering temperature (700 °C), there was no pronounced effect of the high magnetic field on the precipitation sequence and the concentration of substitutional solute atoms in paramagnetic carbides. The investigation of alloy carbide precipitation under high magnetic fields could contribute to a better understanding of phase transformation of alloy carbides and to the heat treatment and fabrication of heat-resistant steels.     

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.     

Influence of Prior Austenite Grain Size on the Degree of Temper Embrittlement in Cr-Mo Steel

Reversible temper embrittlement has been frequently observed in many different low alloy steels serving at high temperature, e.g. order of 500 °C. This type of embrittlement can change the brittle transgranular fracture mode to intergranular decohesion with subsequent change in fracture stress and fracture toughness. The present paper deals with the influence of the prior austenite grain size and isothermal aging time on the degree of embrittlement of 2.25Cr-1Mo steel, which is very popular for its use in power generating and other petrochemical industries. In this research work, the specimens of 2.25Cr-1Mo steel were treated in three different austenitizing temperatures along with different isothermal embrittling time periods. Then the induced degree of embrittlement was characterized by the fracture stress values at −196 °C and area fraction of intergranular failure. The outcome of the experimental results shows that the increase in austenite grain size and/or isothermal embrittling time severely weakens the grain boundary cohesive strength leading to brittle intergranular failures and thus to a greater degree of temper embrittlement.     

Oxidation behaviour of Mo–Si–B–(Al,Ce) ultrafine-eutectic dendrite composites in the temperature range of 500–700 °C

A comparative study has been carried out to investigate the effects of Al and/or Ce additions on microstructure of Mo–Si–B alloys and their isothermal oxidation behaviour at 500 and 700 °C in laboratory air for 24 h. Microstructure of arc melted Mo–Si–B–(Al, Ce) alloys consists of bcc α-Mo dendrites embedded in ultrafine lamellar Mo₃Si and Mo₅SiB₂ eutectic matrix. Isothermal oxidation kinetics of ultrafine structured Mo–Si–B alloy at 500 °C has been found to show hardly any mass change during 24 h exposure. Addition of Al to Mo–Si–B alloy refines the microstructure, decreases the net mass loss at 700 °C by ～43%, whereas Ce does not bring about any significant change. The enhanced oxidation resistance of Mo–Si–B–Al alloy is due to the formation of Al–O rich inter-layer at the alloy/oxide interface along with the formation of a protective and dense Al₂(MoO₄)₃ outer layer, which reduces the sublimation of MoO₃ at 700 °C. Various transient/complex oxides formed on the alloys during their high temperature exposure have been examined to determine the oxidation mechanisms.     

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.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels Austenitic stainless steels (18% Cr, 12% Ni, Mo gradation between 0.5 to 3.6%) had been gas‐nitrided. By stepwise removal, samples could be prepared with various surface content of nitrogen up to 0.45%. The susceptibility against pitting corrosion of these samples had been tested by two methods: – determination of the stable pitting potential in 0.5 M NaCl at 25°C – determination of the critical pitting temperature in artificial sea water (DIN 81249‐4) The influence of nitrogen to both determined parameter can be described well by PRE = Cr + 3,3 · Mo + 25 · N That means for the investigated steel composition and the used corrosion system there is no influence of molybdenum on the effectiveness of nitrogen.     

Evaluation of tempering behaviour in modified 9Cr–1Mo steel by magnetic non-destructive techniques

Modified 9Cr–1Mo steels were normalized at 1050 °C/30 min and tempered over a wide range of temperatures to observe the effect of tempering temperature on the material properties. The material revealed mechanical softening when tempered at temperature less than the lower critical temperature (～810 °C: Ac₁). Beyond Ac₁ temperature the materials strength increased drastically due to the formation of fresh martensite. Magnetic Hysteresis Loop (MHL) and Magnetic Barkhausen Emissions (MBE) techniques were used to evaluate the magnetic properties of the materials with tempering. Magnetic softening was observed by tempering the material below the Ac₁ temperature where a decrease in coercivity and an increase in RMS voltage of the MBE were found. Tempering beyond Ac₁ magnetic hardening was observed by the increase in coercivity and the decrease in RMS voltage of MBE. Such results revealed that magnetic techniques could be a better tool for the evaluation of tempering of modified 9Cr–1Mo steel.     

Combined Effect of Co and W on Deformation Resistance of 12Cr Heat-Resistant Steel for USC Steam Turbines

12Cr heat-resistant steels with different concentrations of Co and W, with Mo equivalent (Mo + 1/2W) fixed at 1.6 wt.%, were prepared by arc-melting and hot rolling processes. Mechanical properties were evaluated by tensile tests conducted at a low strain rate 2 × 10⁻⁵ s⁻¹ at 575, 600, and 625 °C. Microstructure of the steels was investigated via optical microscopy (OM) and electron transmission microscopy (TEM). The results show that when the content of W is fixed, the steel with 3.1 wt.% Co and the steel with 3.8 wt.% Co are found to obtain the best deformation resistance values at 575, 600, and at 625 °C; when the Co content is fixed, the steel with 1.5 wt.% W shows better performance. The highest ultimate tensile strength (UTS) and yield stress (YS) were achieved for the steel when its W content is at 1.5 wt.% and Co content is at 3.1 wt.% or 3.8 wt.%. Deformation resistance is related to the initial dislocation density in the steels, which increases with increasing Co content and decreases with increasing W content. It is verified that the deformation mechanism of the tested steels during high-temperature tensile tests at a low strain rate is that of the recovery-controlled dislocation creep. Furthermore, the thermodynamic calculation result is in agreement with the experimental result, demonstrating that 0.85Mo-1.5W-3.8Co steel has the best deformation resistance at 625 °C. Therefore, 0.85Mo-1.5W-3.1Co steel is recommended as a potential candidate material for 600 °C class steam turbines, and 0.85Mo-1.5W-3.8Co steel is also a potential material for 625 °C class Ultra supercritical (USC) steam turbines.     

Effect of Shot-peening on the Oxidation Behaviour of Boiler Steels

The presence of short diffusion paths is very important for rapid diffusion processes which are involved in forming protective oxide layers against high temperature corrosion, e.g. on boiler steels. Rapid diffusion paths can be produced by applying cold work such as shot-peening to the surface of the boiler steels prior to oxidation. The effect of shot-peening on oxidation behaviour was tested experimentally on 12 wt% Cr martensitic steel and 18 wt% Cr austenitic steel. Isothermal oxidation tests were performed at 700 and 750 °C. The surface treatment proved to be very effective in improving oxidation protection at 700 °C. Shot-peening the surface prior to the oxidation has an influential effect in changing the diffusion mechanisms of the elements involved in oxidation and changes the oxidation kinetics substantially at the applied conditions in this study.     

Molten Salt Corrosion of Stainless Steels and Low-Cr Steel in CSP Plants

The corrosive effects of 60?% NaNO₃/40?% NaNO₃ have been tested at 390 and 550?°C, in order to simulate the working conditions in two principal concentrated solar power (CSP) plants, on stainless steel (AISI 304, 430) and on a low-Cr alloy steel (T22) containing 2.25?% Cr. The corrosion rates were determined by gravimetric tests, measuring the weight gain during 2,000?h, identifying the corrosion products via scanning electron microscopy (SEM) and X-ray diffraction (XRD). Thereby, Fe₂O₃ and Fe₃O₄ were the most prominent products formed from the tests performed at 390?°C, being observed also the formation of some stable compounds due to the impurities of the salt, as magnesium ferrite (MgFe₂O₄) and NaFeO₂. The study at 550?°C of stainless steels revealed a better behavior under corrosive environments than T22 steel, identifying the formation of FeCr₂O₄ protective spinels mainly. To complete the corrosion tests, thermo-physical studies were carried out in binary mixture, using DSC and TGA. Also the most important parameters of the salt before and after corrosion test were tested.