Boiler Tube Steel Research Articles

Studies on the behaviour of stellite-6 as plasma sprayed and laser remelted coatings in molten salt environment at 900 °C under cyclic conditions

Stellite-6 (St-6) coating was obtained on boiler tube steels namely ASTM-SA210-Grade A1 (GrA1), 1Cr–0.5Mo steel ASTM-SA213-T-11 (T11) and 2.25Cr–1Mo steel ASTM-SA213-T-22 (T22) through plasma spray process. Ni–Cr–Al–Y was used as a bond coat before applying St-6 coating. Nd:YAG laser has been used for the post-coating treatment. As sprayed and laser remelted steels were subjected to molten salt environment (Na 2SO 4–60%V 2O 5) at 900 °C under cyclic conditions. The samples were visually examined and subjected to weight change measurements at the end of each cycle of study. Techniques like XRD, SEM/EDAX and EPMA analysis have been used to analyse the oxide scale. The coating was found to be effective in decreasing corrosion rate of the boiler tube steels. Protection is higher when GrA1 steel was the substrate steel and lower for T22 base steel. Due to the formation of vertical cracks, laser remelted coatings have indicated slightly higher corrosion rate in the given molten salt environment.

Mechanical and microstructural properties of HVOF sprayed WC–Co and Cr 3C 2–NiCr coatings on the boiler tube steels using LPG as the fuel gas

The characteristics and the phase composition of high velocity oxy fuel (HVOF) sprayed WC–Co and Cr 3C 2–NiCr coatings applied onto boiler tube steel substrates using liquid petroleum gas (LPG) as the fuel gas, have been evaluated. The coatings were examined using metallography, SEM/EDAX and XRD techniques. An attempt has been made to describe the transformations that take place during HVOF spraying. The purpose of this study was to compare the microstructure, porosity, surface roughness and microhardness of HVOF sprayed WC–Co, Cr 3C 2–NiCr coatings deposited on boiler steels. The paper will also investigate if there is an application of the deposit in energy generation plants with a view to enhance the life of the component and reduce tube failures in such power plants.

Erosion-corrosion of plasma as sprayed and laser remelted Stellite-6 coatings in a coal fired boiler

Unacceptable levels of surface degradation of metal containment walls and heat exchanger tubing by a combined erosion-corrosion (E-C) mechanism have been experienced in some boilers. The recent use of coatings to protect the heat exchanger tubes of fluidized bed combustor from E-C problems has been suggested by many authors. The laser remelting of the surface coating is suggested as a promising technique to improve its physical properties. Aim of the present investigation is to evaluate the erosion-corrosion (E-C) behaviour of plasma as sprayed and laser remelted Stellite-6 (St-6) coatings on boiler tube steels in the actual coal fired boiler environment. The cyclic experimental studies were performed in the platen superheater zone of a coal fired boiler where the temperature was around 755 °C and the study was carried out upto 10 cycles each of 100 h duration followed by 1 h cooling at ambient temperature. Coated steels were found to possess higher resistance to E-C than the uncoated steels. The highest degradation resistance has been indicated by the T11 steel coated and subsequently laser remelted.

Degradation Behavior of Ni<SUB>3</SUB>Al Plasma-Sprayed Boiler Tube Steels in an Energy Generation System

Boiler steels, namely, low-C steel, ASTM-SA210-Grade A1 (GrA1), 1Cr-0.5Mo steel, ASTM-SA213-T-11 (T11) and 2.25Cr-1Mo steel, ASTM-SA213-T-22 (T22) were plasma sprayed with Ni3Al. The alloy powder was prepared by mixing Ni and Al in the stoichiometric ratio of 3 to 1. The Ni-22Cr-10Al-1Y alloy powder was used as a bond coat, with a 150 µm thick layer sprayed onto the surface before applying the 200 µm coating of Ni3Al. Exposure studies have been performed in the platen superheater zone of a coal-fired boiler at around 755 °C for 10 cycles, each of 100 h duration. The protection to the base steel was minimal for the three steels. Scale spallation and the formation of a porous and nonadherent NiO scale were probably the main reasons for the lack of protection. In the case of T22-coated steel, cracks in the coatings have been observed after the first 100 h exposure cycle.

High-Temperature Oxidation Behavior of NiCrA1Y Bond Coats and Stellite-6 Plasma-Sprayed Coatings

Ni–22Cr–10A1–1Y and Stellite-6 coatings were deposited on boiler-tube steels, namely ASTM-SA210-Grade A1, ASTM-SA213-T-11 and ASTM-SA213-T-22, by a shrouded plasma-spray process. The NiCrA1Y alloy powder was also sprayed as a bond coat before applying a Stellite-6 coating. The cyclic-oxidation behavior of the NiCrA1Y bond coats and Stellite-6 coating were evaluated at 900 °C. Visual observations and gravimetric data were measured at the end of each cycle. Finally, the scale was analyzed using X-ray Diffraction, Scanning-Electron Microscope, Energy-Dispersive X-Ray Studies and Electron-Probe Micro-Analyzer techniques. The coatings were found to be effective in increasing resistance to oxidation at the test temperature of 900 °C. The protection is higher in the case of the Stellite-6 coating. The protection is lesser for both coating types when T22 steel was the substrate and higher in the case of GrA1 steel. The deleterious behavior of Mo present in the T22 steel is perhaps responsible for conferring lesser protection.

An overheat boiler tube failure

Failure analysis of an electric resistance welded carbon steel boiler tube demonstrated that the mechanism of failure was long-term overheat due to heavy internal scaling, followed by short-term overheat due to localized heating during boiler shutdown.

Characterisations of plasma sprayed and laser remelted NiCrAlY bond coats and Ni3Al coatings on boiler tube steels

Abstract NiCrAlY and Ni3Al coatings were obtained on boiler tube steels namely ASTM-SA210-Grade A1, ASTM-SA213-T-11 and ASTM-SA213-T-22 through shrouded plasma spray process. NiCrAlY was also used as a bond coat before applying Ni3Al coatings. The coatings were subjected to metallography, XRD and SEM/EDAX analysis. The microhardness of the coatings has been measured along the cross-section. Considerable reduction in the hardness values has been observed after laser remelting in case of NiCrAlY bond coats. The laser remelting has resulted in significant dilution of coating due to inter diffusion of atoms. Very dense and homogeneous structures have been obtained by laser remelting of coatings.

X-ray investigation of carbide precipitation in 2·25Cr–1Mo steel for predicting remaining life of boiler components after extended service in fossil fuel fired power stations

X-ray diffraction (XRD) studies were carried out on 2·25Cr–1Mo steel boiler tubes from a thermal power station with an objective of developing a faster method of health assessment of this steel at elevated temperatures on a routine basis. Carbide phase transformations in virgin and service exposed boiler tubes (up to ∼100 000 h) were determined for plate specimens using high power X-rays. Iron and chromium rich carbides were detected by XRD at various stages of transformation towards final equilibrium. Compositional variations of equilibrium precipitate M23 C6 with service life or overheating in the specimens studied were monitored by a shift in the lattice parameter. The XRD results were validated by optical and electron microscopy, hardness measurements, and particle size measurements.

Lab-Scale Investigations of High-Temperature Corrosion Phenomena in Straw-Fired Boilers

The corrosion of superheater tube material in straw-fired boilers was investigated in the laboratory. Metal test elements of boiler tube steel (X20CRMV121 and AISI 347FG) were covered with synthetic (KCl and/or K2SO4) and real deposits, and exposed to a synthetic flue gas (6 vol % O2, 12 vol % CO2, 400 ppmv HCl, 60 ppmv SO2, balance N2) in 550 °C electrically heated ovens. Exposure times from 1 week to 5 months were used. The corrosion of the metal test elements was, in general, quite uniform, and the corrosion products consisted mainly of oxides of iron and chromium. All test elements covered with KCl suffered from minor internal attack, and some elements had severe pits with chlorine found in the pit. A dense layer of potassium sulfate and iron oxide was found adjacent to the metal oxide layers on all the metal test elements covered with a deposit containing KCl. The layer had a characteristic structure, with iron oxide threads in a dense potassium sulfate matrix. A mechanism for chlorine corrosion is s...

99/01412 High temperature corrosion of boiler tube steels in simulated coal-fired plant atmospheres containing water vapor : Quadakkers, W. J. et al. Corros. Its Control, Proc. Int. Conf. Corros., 1997 (Pub 1998), (1), 199–207. Edited by Khanna, A. S. et al.

99/01412 High temperature corrosion of boiler tube steels in simulated coal-fired plant atmospheres containing water vapor : Quadakkers, W. J. et al. Corros. Its Control, Proc. Int. Conf. Corros., 1997 (Pub 1998), (1), 199–207. Edited by Khanna, A. S. et al.

Studies of corrosion in power plant boiler tubes by measurement of oxygen isotopes and trace elements using secondary-ion mass spectrometry

Recent advances in secondary ionization mass spectrometry (SIMS) techniques allow the resolution of differences in the natural abundances of the isotopes of 18O and 16O, and their ratio in both conducting and insulating materials. These techniques have been used in this study to measure the oxygen isotope signatures and their spatial distribution in corrosion products formed during high-temperature oxidation of steel boiler tubes from fossil-fuel power plants. The data obtained in this study are interpreted in terms of oxygen isotope fractionation between the available oxygen reservoirs and oxides formed on or within the metal. Results are presented for three different corrosion scenarios: steamside/fireside corrosion, aqueous phosphate corrosion and corrosion due to H 2 damage. Constant, but isotopically depleted values observed in magnetites formed during steamside corrosion and H 2 damage are indicative of interaction with locally derived meteoric water (which constitutes the boiler feed water). In contrast, isotope distributions in maricite (NaFePO 4) suggest equilibrium fractionation between this phase and precursor magnetite. Oxygen isotope patterns in fireside magnetites exhibit a complex zoning that requires at least two isotopically distinct species with significantly different transport rates.

Critical variables in high temperature erosive wear

A study has been made of the critical variables of particle type and size, flux and velocity on the erosion of a 1 Cr 1 2 Mo boiler tube steel up to temperatures of 600 °C in a purpose-built air-blast laboratory rig. Whilst particle type has a slight effect on wastage, particle size, flux and velocity affect the magnitude of the erosion rate significantly.

A Study on the Variation of Mechanical Properties Due to Thermal Aging in 2.25Cr-1Mo Boiler Tube Steel

As recieved boiler tuve steel was aged artificially at and for various time duration to simulate the material deterioration which could be occurred during the operation of fossiol power plants. And the tensile tests, the microhardness tests and the characterization of carbides formed in the aging process were performed to asses the relationship between the mechanical properties and the effect of thermal aging. Furthernore, the amout of Mo-rich carbide were investigated by ondestructive method by noticing the fact that formation of Mo-rich carbide were investigated by ondestructive melthod by noticing the fact that formation of Mo-rich carbides() which stabilizes lastly affects the mechanical properties. It was known that the microhardness results of service exposed materials were similar to the ones which are aged at . The room temperature measurement showed small variation in the yield points and ultimate strength in materials aged at . Those properties at showed the abrupt decrease compared with as received material even if short aging time. And it was found that aging cause different effects on mechanical properties, although the temperature time parameters(LMP;Larson-Miller parameter) are same. And it was concluded that the aigng at is more appropriate to simulate the service exposed condition. Finally, the relationship between high temperature tensile properties and Ip values were established, which offers a potential way of reliability tests onthe power plant components.

The effect of circulating fluidized bed particle characteristics on erosion of 1020 carbon steel

A series of erosion tests were performed to determine the erosivity of several different circulating fluidized bed materials. The tests were conducted with 1020 steel as the target material at 400°C and particle velocities of 75 m s −1. The bed material erosivity was found by measuring the sample weight change with time and determining the steady-state erosion rate. It was found that for some conditions, the steel target showed a weight gain owing to soft constituents in the erodent forming a deposit of particle fragments on the surface. Two bed materials were then separated by particle size, ranging from less than 106 μm to greater than 1400 μm, and each size fraction was erosion tested. The shape of the particles was measured using a quantitative image analysis system and it was found that particle shape changed with the size fraction of the bed particles. Particle composition, which was also found to vary with particle size, was determined using elemental analysis. Based on these results, the erosion of 1020 boiler tube steel was correlated to the particle characteristics of size, shape, and composition.

Examination of embrittled carbon steel boiler tube by a scanning Auger microprobe

The embrittlement of carbon steel seamless tube of a power plant boiler after extended service of 34,000 h at 346 °C under 15.68 MPa has been studied by scanning Auger microprobe. The segregation of N at grain boundaries and also the segregation of P at grain boundaries and part of the cleavage planes after long term service caused the embrittlement of carbon steel and resulted in the explosion of the boiler feedwater pipes. The N Al atomic ratio can be used to determine the ageing embrittlement tendency of carbon steel during long time service at 350 °C, and when the N Al atomic ratio is approximately 1 or less, the embrittlement can be avoided.

High Temperature Oxidation Behavior of 2.25%Cr-1%Mo Steel Boiler Tubes in Long-Term Exposure to Superheated Steam

The ferritic 2.25%Cr-1%Mo steel tube (JIS STBA 24), which is extensively used for superheaters and reheaters in fossil fuel-fired power plant boilers, was subjected to long-term steam oxidation tests of up to 3.6 x 10 7 s (10000 h) to clarify its oxidation behavior in high-temperature steam atmosphere. An equation was also derived to estimate the oxidation temperature of the material from the scale thickness. The validity of this equation was tested with a 2.25%Cr-1%Mo steel reheater tube used in an actual boiler for 3.6x x 10 8 s (100000 h). The results of this study are as follows: (1) It was revealed that the scale which formed on the 2.25%Cr-1%Mo steel tube exposed in steam at 823 to 898 K for 1.08 x 10 6 to 3.6 x 10 7 s comprises two distinctively characterized layers: the outer scale composed of Fe 3 O 4 (including a slight amount of Fe 2 O 3 ) and the inner scale composed of (Fe, Cr) 3 O 4 , each having almost identical thickness. It was also found that the outer scale grows as a result of the outward diffusion of Fe ions and the inner scale as a result of the inward diffusion of O ions. (2) The metal temperature of boiler steel tube, T (K), can be expressed as a function of the scale thickness d (m) and the cumulative service time t (s) by the following equation: ...formule math.... where R is the gas constant (8.314 J.mol -1 .K -1 ). The estimated temperature of the 2.25%Cr-1%Mo steel tube, which had been used in an actual boiler for 3.6 x10 8 s (100000 h), agreed well with the service temperature of the steel tube measured with a thermocouple.

Corrosion and inhibition of aged 347 grade stainless steel boiler tubes

General and pitting corrosion of both service and laboratory aged 347 grade stainless steel superheater tubes have been investigated. SEM analysis showed the precipitate phases to have a double structure consisting of σ phase and carbide. EDA analysis of the precipitates indicated the [Cr]/[Fe] ratio to increase with exposure time. For both set of specimens, the general corrosion rate in 1m H2SO4 increased with aging whilst the pitting studies (in 3.5% NaCl) also indicated increased pitting susceptibility with aging. These trends are consistent with corrosion from the chromium depleted areas adjacent to the precipitates. Methoxypropylamine is effective as an inhibitor for pitting corrosion and the concentration required to prevent this form of attack increases with exposure time for both types of specimens.

A metallurgical mechanism for corrosion-fatigue (circumferential) crack initiation and propagation in CrMo boiler tube steels

The metallurgical mechanism responsible for the initiation and propagation of circumferentially oriented, fire-side cracks on waterwall boiler tube steels was investigated. The results showed that cracks were initiated by a sulfidation corrosion/thermal fatique interactive mechanism. Crack propagation occurs by an environmentally assisted thermal-fatigue mechanism.

The role played by ion chromatography in the assessment of amines for two-phase erosion corrosion control in nuclear electric's steam— water circuits

Two-phase erosion corrosion is a world-wide problem and has been experienced in steam-generating plants in both Europe and the USA. In the UK, carbon steel boiler tubes in a number of Magnox Power Stations, operated by Nuclear Electric, have suffered from failures caused by two-phase erosion corrosion. Following a comprehensive investigation of the phenomenon, Nuclear Electric has solved this problem by changing to amine-dosed feedwater in place of the original ammonia treatment. This investigation included laboratory and large-scale rig studies together with a plant trial, which lead to a successful outcome. This paper describes in detail the important part ion chromatography has played in this project. The analytical procedures are described and calibration curves are included. Examples of practical results obtained in the laboratory, in rig tests and in a plant trial are given. Further examples of cation chromatography will demonstrate application to product purity determination and thermal break-down behaviour under simulated and real plant conditions.

Electrochemical Polarization Method to Detect Aging Embrittlement of 321 Stainless Steel

Abstract Type 321 austenitic stainless (UNS S32100) steel boiler tubes used at 550°C for nearly 20 y in power plant are tested by electrochemical polarization. The second anodic current peak increases with service time. The second anodic current peak is caused by the formation of chromium-rich carbides and σ phase at grain boundaries. A normalized second current peak value Ip/Io read from electrochemical polarization curves is used to estimate the aging embrittlement degradation. A close relation between the normalized anodic current density value and the service time is observed. From these results, a nondestructive method for evaluating the degree of aging embrittlement of 321 stainless steel is derived.