Search Results for magnetite

Advances in materials for power plants include not only new materials with higher-temperature capabilities, but also the use of current materials at increasingly higher temperatures. This latter activity builds on extensive experience of the performance of the various alloys, and provides a basis for identifying changes in alloy behavior with increasing temperature as well as understanding the factors that ultimately determine the maximum use temperatures of the different alloy classes. This paper presents results from an effort to model the exfoliation processes of steam-side oxide scales in a manner that describes as accurately as possible the evolution of strains in oxides growing inside small-diameter tubes subjected to large thermal gradients and to thermal transients typical of normal steam boiler operation. One way of portraying the results of such calculations is by plotting the evolving strains in a given oxide scale on an ‘Exfoliation Diagram’ (of the type pioneered by Manning et al. of the British Central Electricity Research Laboratory) to determine the earliest time at which the trajectory of these strains intersects a criterion for scale failure. Understanding of how such ‘strain trajectories’ differ among different alloys and are affected by the major variables associated with boiler operation has the potential to suggest boiler operating strategies to manage scale exfoliation, as well as to highlight the mode of scale failure and the limitations of each alloy. Preliminary results are presented of the strain trajectories calculated for alloys T22, T91, and TP347 subjected to the conditions experienced by superheaters under assumed boiler operating scenarios. For all three alloys the earliest predicted scale failures were associated with the increased strains developed during a boiler shut-down event; indeed, in the cases considered it appeared unlikely that scale failure would occur in any practically meaningful time due to strains accumulated during operation in a load-following mode in the absence of a shut down. The accuracy of the algorithms used for the kinetics of oxide growth appeared to be a very important consideration, especially for alloy TP347 for which large effects on oxide growth rate are known to occur with changes in alloy grain size and surface cold work.

Steam oxidation of a novel austenitic steel, of which composition is Fe-20Cr-30Ni-2Nb (at.%), has been conducted at 973 K to evaluate steam oxidation resistance based on detail analyses of scale morphology and scale growth. Two types of scale morphologies were observed in the solution treated sample, depending on the grain of the steel. Although thin duplex scale with the Cr-rich layer was formed in the early stage, most of the surface was covered with thick duplex scale which consists of magnetite as the outer scale and the mixture of Fe-Cr spinel and metallic Ni as the inner scale. On the other hand, surface morphology of the oxide scale was independent of grain of the steel and thick duplex scale as seen on the solution treated sample was formed on the pre-aged sample. Steam oxidation resistance of the steel is almost the same as that of commercial austenitic steels and it can be improved by the surface treatment such as shot peening. Based on the results, this steel has both enough creep rupture strength and good steam oxidation resistance for A-USC power plants.

The oxidation behavior of X20 steel in steam environments was studied isothermally between 580-640°C. Initially, the magnetite (Fe 3 O 4 ) phase formed on the surface. With increasing time and temperature, the hematite (Fe 2 O 3 ) phase formed. The oxide scale consisted of an inner layer divided from an outer layer by the original metal surface. A Cr-rich area was observed beneath the original metal surface. Oxide scales formed on a serviced boiler tube at 540°C for 7000h were also analyzed and found to be similar in oxide phase composition to those formed on X20 steel under laboratory conditions. However, differences existed in the microstructure and distribution of the Cr-rich area within the oxide scale. It was concluded that the oxidation mechanism under field conditions differs from that under laboratory conditions.

A new ferritic steel branded as Thor 115 has been developed to enhance high-temperature resistance. The steel design combines an improved oxidation resistance with long-term microstructural stability. The new alloy was extensively tested to assess the high-temperature time- dependent mechanical behavior (creep). The main strengthening mechanism is precipitation hardening by finely dispersed carbide (M 23 C 6 ) and nitride phases (MX). Information on the evolution of secondary phases and time-temperature-precipitation behavior of the alloy, essential to ensure long-term stability, was obtained by scanning transmission electron microscopy with energy dispersive spectroscopy, and by X-ray powder diffraction on specimens aged up to 50,000 hours. The material behavior was also tested in service conditions, to validate the laboratory results: Thor 115 tubing was installed in a HRSG power plant, directly exposed to turbine flue gasses. Tubing samples were progressively extracted, analyzed and compared with laboratory specimens in similar condition. This research shows the performance of Thor 115 regarding steam oxidation and microstructure evolution up to 25,000 exposure hours in the field. So far, no oxide microstructure difference is found between the laboratory and on field tubing: in both cases, the oxide structure is magnetite/hematite and Cr-spinel layers and the oxide thickness values lay within the same scatter band. The evolution of precipitates in the new alloy confirms the retention of the strengthening by secondary phases, even after long-term exposure at high temperature. The deleterious conversion of nitrides into Z phase is shown to be in line with, or even slower than that of the comparable ASME grade 91 steel.

High Cr ferritic steels have been developed for the large components of fossil power plants due to their excellent creep resistance, low thermal expansion, and good oxidation resistance. Development works to improve the operating temperature of these steels mainly focused on the high mechanical properties such as solid solution strengthening and precipitation hardening. However, the knowledge of the correlation between Laves phase precipitation and oxidation behavior has not clarified yet on 9Cr ferritic steels. This research will be focused on the effect of precipitation of Laves phase on steam oxidation behavior of Fe-9Cr alloy at 923 K. Niobium was chosen as the third element to the Fe- 9Cr binary system. Steam oxidation test of Fe-9Cr (mass%) alloy and Fe-9Cr-2Nb (mass%) alloy were carried out at 923 K in Ar-15%H 2 O mixture for up to 172.8 ks. X-ray diffraction confirms the oxide mainly consist of wüstite on the Fe-9Cr in the initial stage while on Nb added samples magnetite was dominated. The results show that the Fe-9Cr- 2Nb alloy has a slower oxidation rate than the Fe-9Cr alloy after oxidized for 172.8 ks

Oxide scale formation in the inner bore of steam tubing has been identified as a key metric for determining operational parameters and life expectancy of modern boiler systems. Grade 91 tubing is commonly used for the construction of key components within boiler systems designed for power generation operating in the temperature range of 500 to 650 °C. Standard laboratory test procedures involve grinding the surface of test coupons to homogenise their surface structure and improve experimental consistency, however, data presented here shows a discrepancy between laboratory and industrial practices that has long term implications on scale growth kinetics and morphological development. Microstructural analysis of both virgin and ex-service tubing reveals the presence of a pre-existing oxide structure that is incorporated into the inwardly growing scale and is implicated in the formation of multiple laminar void networks. These void networks influence thermal diffusivity across the scale and may function as regions of spallation initiation and propagation.

Both non-destructive and traditional microsectioning techniques have been used to measure the oxide thickness of steam grown oxides between two close contacting surfaces. Different power plant materials, nickel based alloys and ferritic-martensitic steels, were exposed to steam oxidation at temperatures ranging from 650 °C up to 750 °C and periods from 500 h to 3000 h. Ultrasonic measurements of thickness, based on the speed of sound in the oxide, were performed and compared to optical thickness measurements based on conventional metallographic microsectioning with promising results. Improvements on the measurement resolution have been practically demonstrated with oxides down to 65 μm thickness being measured successfully.

CWT (combined water treatment) was introduced in Japan in 1990 and over 50 power generation boilers are now in operation. However, the effect of oxygenated treatment on the steam oxidation of the ferritic-martensitic steels and austenitic stainless steels that are used for superheaters and reheaters is currently far from clear. In this study, laboratory tests were used to examine the effect of the oxygen level of the feed water on the scale growth and the scale exfoliation propensity of T91 ferritic-martensitic steel and 300-series austenitic stainless steels, as represented by TP316H and TP347H (coarse- and fine-grained, respectively). The oxygen level of the feed water had little effect on the steam oxidation rates of all the steels tested. Hematite (Fe 2 O 3 ) formed in the outer layer of the oxide scales on both the ferritic and austenitic steels and is considered to have been encouraged in the simulated CWT atmosphere. The adhesion strength of the oxide scale formed on T91 in the simulated CWT atmosphere, that is, scale in which hematite was present, was lower than that of the oxide scale formed in the simulated AVT (all volatile treatment) atmosphere. The oxidation rate of fine-grained TP347H was confirmed to be slower than that of coarse-grained TP316H. Hematite significantly influenced the scale exfoliation of the austenitic steels and the critical oxide thickness for exfoliation decreased with increasing proportion of hematite in the outer scale.

A model based on a concept of “fraction of exfoliated area” as a function of oxide scale strain energy was developed to predict the extent of exfoliation of steam-side scale from boiler tube superheater loops. As compared with the Armitt diagram, which can be used to predict when scale damage and exfoliation would be likely to occur, a “fraction of exfoliated area” approach provides an estimation of mass of scale released and the fraction of tube likely to be blocked by the exfoliation. This paper gives results for the extent of blockage expected in a single bend of a superheater loop was predicted as a function of operating time, bend geometry, and outlet steam temperature under realistic service conditions that include outages. The deposits of exfoliated scale were assumed to be distributed horizontally the tubes bends. Three types of bends were considered: regular bends, short bends, and hairpin bends. The progressive increase in steam and tube temperatures along a single loop of superheater tubing and the ensuing variation of oxide scale thickness are considered. Numerical simulation results for a superheater loop made of TP347H austenitic steel indicated that tube blockage fractions larger than 50% are likely to occur within the first two years of boiler operation (with regularly scheduled outages) for outlet tube temperatures of 540-570°C, which is consistent with practical experience. Higher blockage fractions were predicted for tubes with hairpin bends than for tubes with regular bends, of length that are larger than five internal tube diameters. Finally, the blockage model presented can be used with some confidence to devise operating schedules for managing the consequences of oxide scale exfoliation based on projections of time to some critical blockage fraction for specific boiler operating conditions.

The newly developed 12%Cr steel Super VM12 is characterized by excellent creep rupture strength properties (better than Grade 92) and enhanced steam oxidation resistance of 12%Cr steels such as VM12-SHC. Balanced properties profile of the new steel development in comparison to the existing well-established steels such as Grade 91 and Grade 92, opens opportunities for its application as construction material for components in existing or future high-efficiency power plants. In this study the oxidation behavior of typical 9%Cr steels was compared with the new steel development. The oxide scale morphologies and compositions of different oxide layers as function of temperature and exposure time in steam-containing atmospheres were characterized using light optical metallography, Scanning Electron Microscopy (SEM). Creep testing has been carried out in the temperature range between 525°C and 700°C. Selected creep specimens were investigated using the Transmission Electron Microscopy and the Atom Probe Tomography techniques.

A failure of the upper casing of the circulation pump led to a big damage in the PP Staudinger unit 5 on 12th of May 2014. According to the §18(2) BetrSichV an extensive root cause analysis (RCA) was started. From the beginning on different lines of activities were initiated to handle the situation with the required diligence. Decisions were made, taking into account safety regulations, possibility of repair and best practice engineering. Following the board decision to repair the unit 5, a lot of detailed work was done. All of the performed work packages were linked in different timelines and needed to meet in the key points. Consequently it was a challenge to achieve the agreed date of unit 5 restart on 15th of January 2015. The unit restart on the targeted date was a proof of the excellent collaboration between all involved parties. The presentation gives a summarizing overview about the damage, the main results of the RCA and the repair activities.

Traditional laboratory steam experiments are conducted at ambient pressure with water of variable chemistry. In order to better understand the effect of steam pressure and water chemistry, a new recirculating, controlled chemistry water loop with a 650°C autoclave was constructed. The initial experiments included two different water chemistries at 550° and 650°C. Two 500-h cycles were performed using oxygenated (OT, pH ~9 and ~100 ppb O 2 ) or all-volatile treated (AVT, pH ~9 and <10 ppb O 2 ) water conditions at each temperature. Coupons exposed included Fe-(9-11)%Cr and conventional and advanced austenitic steels as well as shot peened type 304H stainless steel. Compared to ambient steam exposures, the oxides formed after 1,000 h were similar in thickness for each of the alloy classes but appeared to have a different microstructure, particularly for the outer Fe-rich layer. An initial attempt was made to quantify the scale adhesion in the two environments.

In order to assist in developing mechanistic and computational models for understanding the performance of current Fe-base waterwall tubing, characterization has been performed on three field-exposed low alloy steel waterwall tubes. The waterside oxide thickness was characterized using standard metallographic techniques. Alloy and oxide chemical composition was characterized using electron microprobe analysis. Waterside scale thickness was measured as a function of location. Agreement between the measured and predicted values based on likely rate constants was poor.

Recently, single-phase flow accelerated corrosion (FAC) has been found extensively in Thailand, especially in single shaft combined cycle power plant heat recovery steam generators, the design of which are compact and cannot be easily accessed for service. This takes at least one week for repairing and costs at least half a million dollar per shutdown. In this paper, the investigation of the single-phase FAC in a high-pressure economizer of a combined cycle power plant is demonstrated. Water chemical parameters such as pH and dissolved oxygen are reviewed, the process simulation of the power plant is performed to capture risk areas for the FAC. A computational fluid dynamics study of the flow is done to understand the flow behavior in the damaged tubes next to an inlet header. Some modifications such as flow distributor installation and tube sleeve installation were performed for short-term solutions. Moreover, new economizer headers are designed with low alloy material to mitigate the problem. The installation process of the newly fabricated headers is finally described. The findings in this paper serve as a guideline for FAC risk assessment, FAC investigation and mitigation, and service in compact heat recovery steam generators.

Laboratory-scale tests are frequently used to generate understanding of high-temperature oxidation phenomena, to characterise and rank the performance of existing, future materials and coatings. Tests within the laboratory have the advantage of being well controlled, monitored and offer the opportunity of simplification which enables the study of individual parameters through isolating them from other factors, such as temperature transients. The influence of pressure on the oxidation of power plant materials has always been considered to be less significant than the effects of temperature and Cr content, but still remains a subject of differing opinions. Experimental efforts, reported in the literature, to measure the influence of steam pressure on the rate of oxidation have not produced very consistent or conclusive results. To examine this further a series of high pressure steam oxidation exposures have been conducted in a high pressure flowing steam loop, exposing a range of materials to flowing steam at 650 and 700 °C and pressure of 25, 50 and 60 bar. Data is presented for ferritic-martensitic alloys showing the effect of increasing pressure on the mass change and oxide thickness of these alloys in the flowing steam loop. In addition the effect observed on the diffusion of aluminium from an aluminised coating in these alloys is also presented and the differences in the extent of diffusion discussed.

The acceptance of materials for long-term, safety-critical power generation applications requires multiple testing stages and data generation. Initial screening involves short-term exposures under simplified, constant atmospheres and temperatures, which can eliminate unsuitable materials but fail to distinguish between those with broadly acceptable properties. Subsequent pilot plant testing, costing over £100K for month-long exposures, is typically required. An intermediate laboratory testing step that better replicates in-service conditions would offer a cost-effective approach to material selection and lifetime prediction. For steam oxidation degradation, key experimental parameters—such as water chemistry, pressure, steam delivery, and flow rate—must be tailored to produce oxide scale morphologies similar to those observed in actual plant conditions. This study examines the effects of these parameters through steam exposure tests on ferritic (P92), austenitic (Esshete 1250), and superalloy (IN740) materials. Results indicate that oxidation rates vary with dissolved oxygen levels in feed water, increasing for austenitic materials and decreasing for ferritic materials, while also influencing spallation tendencies. Additionally, steam pressure and delivery methods impact oxidation rates and scale morphology. A comparison with service-exposed materials revealed that traditional oxide scale morphologies were not adequately replicated, whereas cyclic oxidation tests provided a closer match to service-grown scales.

The steam generation systems (SGS) of concentrated solar power (CSP) plants employ multiple heat exchangers arranged in series to convert thermal energy collected from the sun via a heat transfer fluid (HTF) to produce superheated steam in the Rankine cycle. Common CSP plant designs are based either on parabolic trough or central tower technology. The major Rankine cycle components consist of preheaters, evaporators, steam drums, superheaters, steam turbines, and water/air-cooled condensers, all connected through steel piping. For CSP plants capable of reheating the steam for improved efficiency, reheaters are also included in the Rankine cycle. In central tower design with directly heated water as the HTF, the receiver can also be considered part of the Rankine cycle. Operating experiences of CSP plants indicate that plant reliability is significantly impacted by failures in various components of the Rankine cycle. Many damage mechanisms have been identified, which include corrosion, thermal fatigue, creep, and stress corrosion cracking, among others. Much of the damage can be attributed to poor water/steam chemistry and inadequate temperature control. While damage in the Rankine cycle components is common, there is generally lack of comprehensive guidelines created specifically for the operation of these CSP components. Therefore, to improve CSP plant reliability and profitability, it is necessary to better understand the various damage mechanisms experienced by linking them to specific operating conditions, followed by developing a “theory and practice” guideline document for the CSP operators, so that failures in the Rankine cycle components can be minimized. In a major research project sponsored by the U.S. Department of Energy (DOE), effort is being undertaken by EPRI to develop such a guideline document exclusively for the CSP industry. This paper provides an overview of the ongoing DOE project along with a few examples of component failures experienced in the Rankine cycle.

An international initiative is underway to develop the first standardized high-temperature solid particle erosion test method for steam turbine applications, addressing limitations of the current room-temperature ASTM G76 standard. Led by EPRI, this program involves laboratories from seven countries in a “Round Robin” testing program, aiming to establish consistent testing procedures for evaluating erosion resistance of materials used in Ultra Supercritical (USC) and advanced USC turbines. The proposed standard will use Type 410 stainless steel tested at 30 and 90-degree impingement angles with 50-micron alumina particles at 200 m/s, both at room temperature and 600°C, providing more relevant conditions for current and next-generation steam turbine applications.

Starting in 2010 a new generation of coal fired power plants in Europe operating at a steam temperature of up 620°C was commissioned. During that commissioning process many cracks occurred in welds of T24 material which was extensively used as membrane wall material in nearly all of the new boilers. The cracks were caused by stress corrosion cracking (SCC) only occurring in the areas of the wall being in contact to high temperature water during operation. The question which step of the commissioning process really caused the cracking was not answered completely even several years after the damage occurred. To answer this question and to define parameters which will lead to cracking in high temperature water many tests were conducted. Generally it was found that slow tensile tests in controlled environment are well suited to get information about materials SCC sensitivity in the laboratory. In the present paper, first the influence of the cracking of welded T24 material in acidic environment containing well-defined amounts of H2S is investigated to address the question if a chemical cleaning process prior to the testing might lead to hydrogen induced SCC. As a second step, cracking behaviour in high temperature water is being investigated. Here the influence of the temperature, the oxygen concentration of the water, the deformation speed of the sample, the heat treatment and the condition of the material on the SCC is analysed.

A research program has been initiated to develop the first predictive methodology for corrosion fatigue life in steam turbine blades, addressing a critical gap in current understanding despite extensive research into corrosion pitting and fatigue failure. The study focuses initially on dual-certified 403/410 12% Cr stainless steel, utilizing a newly developed test facility capable of conducting high-cycle fatigue tests in simulated steam environments at 90°C with controlled corrosive conditions. This testing platform enables the investigation of various steady and cyclic stress conditions, establishing a foundation for future testing of other blade steels and the development of comprehensive blade life estimation techniques.