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.

This paper investigates the combined effect of shot peening and pre-oxidation treatment in air on the subsequent steam oxidation resistance of Modified 9Cr-1Mo steel with different sulfur contents. Cast steel balls (50-180 μm diameter) and pure Cr (50-230 μm diameter) were used for shot peening durations of 5-50 seconds. After shot peening, pre-oxidation was performed in air at 973K for 3.6ks. Then, oxidation testing was conducted in steam at 923K for up to 3.6Ms. Only the combination of Cr shot peening and pre-oxidation treatment facilitated the formation of a protective Cr-rich oxide scale on the specimen surface during pre-oxidation. This Cr-rich oxide scale remained stable during subsequent steam oxidation, resulting in excellent oxidation resistance of the steel.

Because of the problems experienced with steam-side oxidation in commercial power plants, there has been continuing interest in better understanding the steam oxidation behavior of creep strength enhanced ferritic steels such as grades 23, 24 and 91 as well as 300-series stainless steels such as 347H and 304H. Analysis of field-exposed tubes has provided information on the oxidation reaction products but relatively few specimens are available and there is limited information about the kinetics. Specimens have included tube sections with a shot peened surface, a treatment that is now widely used for austenitic boiler tubes. To complement this information, additional laboratory studies have been conducted in 1bar steam at 600°-650°C on coupons cut from conventional and shot-peened tubing. Exposures of 1-15 kh provide some information on the steam oxidation kinetics for the various alloys classes. While shot-peened type 304H retained its beneficial effect on oxidation resistance past 10,000 h at 600° and 625°C, the benefit appeared to decline after similar exposures at 650°C.

A new martensitic steel for power generation applications was developed: Tenaris High Oxidation Resistance (Thor) is an evolution of the popular ASTM grade 91, offering improved steam oxidation resistance and better long-term microstructural stability, with equal or better creep strength. Thanks to its design philosophy, based on consolidated metallurgical knowledge of microstructural evolution mechanisms, and an extensive development performed in the last decade, Thor was engineered to overcome limitations in the use of ASTM grade 91, above 600 °C, particularly related to scale growth and liftoff. After laboratory development, Thor was successfully validated at the industrial level. Several heats up to 80 metric tons were cast at the steel shop, hot rolled to tubes of various dimensions, and heat treated. Trial heats underwent extensive characterization, including deep microstructural examination, mechanical testing in the as-received condition and after ageing, long-term creep and steam oxidation testing. This paper presents an overview of metallurgical characterization performed on laboratory and industrial Thor material, including microstructural examination and mechanical testing in time-independent and time-dependent regimes. Data relevant to the behavior and the performance of Thor steel are also included.

The A-USC technology is still under development due to limited number of materials complying with the requirements of high creep strength and high performance in highly aggressive corrosion environments. Development of power plant in much higher temperatures than A-USC is currently impossible due to the materials limitation. Currently, nickel-based superalloys besides advanced austenitic steels are the viable candidates for some of the A-USC components in the boiler, turbine, and piping systems due to higher strength and improved corrosion resistance than standard ferritic or austenitic stainless steels. The paper, presents the study performed at 800 °C for 3000 hours on 3 advanced austenitic steels; 309S, 310S and HR3C with higher than 20 Cr wt% content and 4 Ni-based alloys including: two solid-solution strengthened alloys (Haynes 230), 617 alloy and two (γ’) gamma - prime strengthened materials (263 alloy and Haynes 282). The high temperature oxidation tests were performed in water to steam close loop system, the samples were investigated analytically prior and after exposures using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectrometry (EDS), and X-Ray Diffractometer (XRD). Mass change data have been examined every 250 hours.

The steam oxidation behaviour of boiler tubes and steam piping components is a limiting factor for improving the efficiency of the current power plants. Spallation of the oxide scales formed during service can cause serious damage to the turbine blades. Vallourec has implemented an innovative solution based on an aluminum diffusion coating applied on the inner surface of the T/P92 steel. The functionality of this coating is to protect the tubular components against spallation and increase the actual operating temperature of the metallic components. In the present study, the newly developed VALIORTM T/P92 product was tested at the EDF La Maxe power plant (France) under 167b and 545°C (steam temperature). After 3500h operation, the tubes were removed and characterized by Light Optical Metallography (LOM), Scanning Electron Microscopy (SEM), with Energy Dispersive X-ray spectrometry (EDX) and X-Ray Diffraction (XRD). The results highlight the excellent oxidation resistance of VALIORTM T/P92 product by the formation of a protective aluminum oxide scale. In addition, no enhanced oxidation was observed on the areas close to the welds. These results are compared with the results obtained from laboratory steam oxidation testing performed on a 9%Cr T/P92 steel with and without VALIORTM coating exposed in Ar-50%H 2 O at 650°C.

To save fossil fuel resources and to reduce CO 2 emissions, considerable effort has been directed toward researching and developing heat-resistant materials that can help in improving the energy efficiency of thermal power plants by increasing their operational temperature and pressure conditions. Instead of conventional 9-12Cr ferritic heat-resistant steels with a tempered martensitic microstructure, we developed “Precipitation Strengthened 15Cr Ferritic Steel” based on a new material design concept: a solid-solution treated ferrite matrix strengthened by precipitates. Creep tests for 15Cr-1Mo-6W-3Co-V-Nb steels with ferrite matrix strengthened by a mainly Laves phase (Fe 2 W) showed that the creep strengths of 15Cr ferritic steel at temperatures ranging from 923 K to 1023 K were twice as high as those of conventional 9Cr ferric heat-resistant steel. 15Cr steels have higher steam oxidation resistance than that of conventional steel in the same temperature range as the creep tests. Thus, the new material design concept of heat-resistant steel pro- vides improved creep strength and steam oxidation resistance. We are attempting to determine the optimum compositions, especially that of carbon, in order to improve the high-temperature creep strength.

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

Sanicro 25 material is approved for use in pressure vessels and boilers according ASME code case 2752, 2753 and VdTUV blatt 555. It shows higher creep rupture strength than any other austenitic stainless steels available today. It is a material for superheater and reheaters, enabling higher steam parameters of up to about 650 °C steam (ie about max 700 °C metal) without the need for expensive nickel based alloys. The aim of the present study is the investigation of the steam oxidation resistance of the Sanicro 25. The long term test was conducted in the temperature range 600 -750 °C up to 20 000 hours. The morphology of the oxide scale and the microstructure of the bulk material were investigated. In addition, the effect of surface finish and pressure on the steam oxidation were also studied.

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.

To stay competitive in today’s dynamic energy market, traditional thermal power plants must enhance efficiency, operate flexibly, and reduce greenhouse gas emissions. This creates challenges for material industries to provide solutions for harsh operating conditions and fluctuating loads. Higher efficiency demands steels with excellent steam oxidation resistance, favoring ferritic steels for cycling operation due to their limited thermal expansion. This paper presents a study modeling a combined cycle power plant using GE 9HA0.2 GT technology. The analysis compares different maximum live steam temperatures (585°C, 605°C, 620°C) and four alloys (grades 91 and 92, stainless S304H, and Thor 115) for heat exchangers exposed to steam oxidation. Results indicate that Thor 115, a creep strength enhanced ferritic (CSEF) steel, is a viable alternative to stainless steel for live steam temperatures above 600°C, offering improved oxidation resistance with minimal weight increase. Modern CSEF steels outperform stainless steel in power plants with lower capacity factors, reducing thermal fatigue during load changes. Increasing the live steam temperature boosts plant efficiency, leading to significant CO 2 savings for the same power output.

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.

Because of the problems experienced with steam-side oxide scale exfoliation in commercial power plants, there has been increased interest in understanding the steam oxidation resistance of 300- series stainless steels such as 347H and 304H. Model alloys were used in an attempt to understand the effect of varying Ni (9-12%) and Cr (16-20%) on steam oxidation resistance at 650°C. However, the model alloys generally showed superior oxidation resistance than commercial alloys of similar composition. Several surface engineering solutions also were investigated. The commercially favored solution is shot peening. Laboratory steam testing at 650°C found that annealing temperatures of ≥850°C eliminated the benefit of shot peening and a correlation was observed with starting hardness in the peened region. This effect of annealing has implications for the fabrication of shot peened tubing. Another route to improving oxidation resistance is the use of oxidation resistant diffusion coatings, which can be deposited inexpensively by a vapor slurry process. Uniform coatings were deposited on short tube sections and annealed at 1065°C to retain good 650°C creep properties. The coating was thicker than has been investigated in laboratory processes resulting in increased brittleness when the coating was assessed using 4-point bending.

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.

This study investigates the growth kinetics and spallation behavior of oxide scales formed under steam environments on alloys used in high-temperature plants. The influence of alloy composition is analyzed using two approaches: an empirical model based on the concept of “chromium equivalent” and a neural network model. Both models demonstrate a good correlation with experimental results when sufficient data is available to generate the model parameters. However, there is insufficient data on scale spallation to develop similar models describing the influence of alloy composition on this phenomenon.

The presence of sulfur at an impurity level in heat resistant steels could improve remarkably the steam oxidation resistance. As is well known, sulfur tends to form sulfides, in particular, chromium sulfides when the steel contains chromium. Therefore, there are two possibilities of sulfur states in the steel. One is in atomic sulfur state as a solid solution, and the other is in sulfide state as a precipitate. However, it still remains unclear which sulfur state contributes largely to the improvement of the steam oxidation resistance of the steels. In order to elucidate the sulfur state operated more effectively in improving the oxidation resistance, the steam oxidation resistance was investigated with high chromium ferritic steels, Fe-10mass%Cr-0.08mass%C-(0~0.015)mass%S, with controlling the sulfur states in them by proper heat treatments. From a series of experiments, it was found that the sulfide state played a more important role in improving the steam oxidation resistance than the atomic sulfur state. Furthermore, this sulfur effect worked significantly in the steam oxidation test performed at the temperatures above 873K which corresponded to the temperature for the chromium sulfide to dissolve and instead for the chromium oxide to form in the steels. This result indicates that the beneficial effect of sulfur in improving the steam oxidation resistance is related closely to the sulfide stability against the oxide in the steels.

This paper describes the steam oxidation behavior of two 18Cr-8Ni austenitic fine-grained stainless steels, TP347HFG and SUPER304H, which have been developed for ultra-supercritical (USC) boilers. A field exposure test was conducted by installing these tubes, along with comparative materials, in the tertiary superheater of a utility power boiler. After periodic service, the fine-grained tubes were removed to examine their steam oxidation behavior. Examination of the steam oxidation scale on the inner surface of the tubes indicated an extremely low scale growth rate for the fine-grained steels, even after 10 years of service. The oxidation structure is discussed and compared with conventional materials, TP321H and TP347H. Accelerated steam oxidation tests were conducted using an oxidation test with saturated dissolved oxygen concentration. The combination of fine-grained steel and a shot-peening layer exhibits high steam oxidation resistance at 700°C or higher temperatures.

The oxidation resistance of 9-12% chromium steels in steam-containing environments simulating the service conditions of steam power plant has been investigated for exposure times ranging from 1 h up to 10 000 h. For characterizing the oxidation behavior, the results of gravimetric studies were combined with data obtained from a number of analysis techniques, such as optical metallography, SEM/EDX and LRS. Different mechanisms of oxidation were observed for the various steels in different temperature regimes, exposure times and exposure conditions. The cracking and spallation of scales was correlated with the type, morphology and growth of pores and voids in the scale and could be influenced by the steel microstructure. For some steels, the steam oxidation resistance increased with increasing exposure temperature. The oxidation rates only slightly depend on the exact water vapor content in the test gas but can be strongly affected by the gas flow rates.

Most effective method to increase the boiler efficiency and decrease emissions is to increase the steam temperature of modern coal-fired power plants. The increase in the steam temperature of the AUSC power plants will require higher grade heat-resistant materials to support the long-term safety and service reliability of power plants. The corrosion resistance of alloys is one of the most important factors for the application in AUSC power plants.