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.

This paper focuses on the key properties of newly developed high-strength, heat-resistant steels for application in ultra-supercritical (USC) boilers. For some ferritic steels, improvements made to enhance their resistance to steam oxidation are highlighted. The latest welding techniques employed for these steels are introduced. Additionally, the high-temperature strength and weldability of Alloy 617 (52Ni-22Cr-13Co-9Mo-Ti-Al), a potential candidate material for the next generation of 700°C USC boilers, are described. The paper provides insights into the materials and welding technologies crucial for the development of advanced USC boilers operating at higher temperatures.

‘Chromium-Molybdenum’ ferritic steels are the most popular materials for construction of steam generators. Microstructure of ‘Cr-Mo’ steels is highly susceptible to changes due to heat- treatments. This susceptibility is often exploited for improving creep strength of these alloys, via suitable heat treatments, that are employed in order to incorporate changes in grain size and secondary precipitates. This paper presents a general overview of the role of microstructure in various aspects of gaseous oxidation, and the synergistic role of the alloy microstructure and oxidation in deterioration of mechanical properties. In the light of this overview, the paper also presents oxidation kinetics and oxidation-assisted microstructural degradation in the microstructurally different regions of weldments of a common variety of ‘Cr-Mo’ steel, viz., 2.25Cr-1Mo steel.

This paper explores the low cycle fatigue (LCF) and creep-fatigue properties of a hot-forged, normalized, and tempered 9Cr-1Mo ferritic steel. This steel offers good performance in high-temperature applications (up to 873K) in power plants and reactors. The steel was forged into 70 mm diameter rods and then heat-treated with normalizing (1313K for 1 hour, air cooling) and tempering (1033K for 1 hour, air cooling). LCF tests were conducted at 300-873K with varying strain amplitudes and strain rates to understand the influence of both factors. Additionally, some specimens were aged at different temperatures for 10,000 hours before testing. Finally, creep-fatigue interaction tests were performed at 823K and 873K using tensile hold times ranging from 1 to 30 minutes.

With the desire for higher operating temperatures and pressures to improve the thermal efficiency of new power generating plant there have been significant changes in the materials used. For operation up to 620°C, a new range of ferritic steels with 9-13%Cr has been developed. With proper control of composition and heat treatment these materials, including Grades 91 and 92,exhibit predominantly martensitic microstructures and a good balance between strength and ductility. However, fabrication processes such as welding and bending, normally combined with extreme operating conditions have resulted in in-service damage. Examples of factors leading to accelerated creep, creep fatigue and oxidation damage are described.

Current state-of-the-art coal-fired supercritical steam power plants operate with high-pressure turbine inlet steam temperatures close to 600°C. The best of the recently developed and commercialized advanced 9-12Cr martensitic-ferritic steels may allow prolonged use at temperatures to about 620°C, but such steels are probably close to their inherent upper temperature limit. Further increase in the temperature capability of advanced steam turbines will certainly require the use of Ni-based superalloys and system redesign, as seen in the European programs that are pioneering advanced power plants capable of operating with 700°C steam. The U.S. Department of Energy (DOE) has recently undertaken a concerted effort to qualify ultra-supercritical boiler tubing and piping alloys for 720/760°C steam for increased efficiency and reduced emissions. It is, therefore, necessary to develop the corresponding USC steam turbine, also capable of reliable operation at such conditions. This paper summarizes a preliminary assessment made by the Oak Ridge National Laboratory (ORNL) and the National Energy Technology Laboratory (NETL) of materials needed for ultra-supercritical (USC) steam turbines, balancing both technical and business considerations. These efforts have addressed an expanded portfolio of alloys, that includes austenitic stainless steels and alloys, in addition to various Ni-based superalloys for critical turbine components. Preliminary input from utilities indicates that cost-effective improvements in performance and efficiency that do not sacrifice durability and reliability are prime considerations for any advanced steam turbine technology.

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.

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.

A new 12%Cr steel, VM12, has been developed with the combined strength and oxidation resistance characteristics desired for high-temperature applications. The steel increases chromium content by around 0.2% to improve oxidation properties while alloying with other elements such as cobalt, tungsten, and boron to meet a range of requirements, including extending fatigue life. The steel is designed to have the same creep strength as T/P92 but with better oxidation resistance due to the higher chromium content. It has about a 0.2% increase in mechanical properties compared to T/P92 steel. Results are presented for tubes and pipes cast with a variety of surface conditions. In addition, detailed results are provided on the effects of alloying elements on creep and oxidation resistance.

Dissimilar joints between modern 10% chromium steels and low-alloy steels are unavoidable in new installations or upgrades of steam turbine components. Welds between 10CrMo9-10 (P22) and X10CrMoVNb9-1 (P91) steel pipes are often required. This paper studies this heterogeneous weld from a steam turbine manufacturer's practical perspective. Two types of filler materials were used: P22- and P91-based weld metals. The integrity and mechanical properties of the prepared heterogeneous welds were evaluated according to the welding standard EN 288-3. Both approaches yielded satisfactory results. Additionally, creep rupture strength was evaluated. The creep rupture strength of both joints fell within the -20% scatter band of the P22 base material's creep rupture strength. The weld design with P91 filler material appeared to slightly outperform the P22-based approach for longer exposure times.

A major cost contributor of P91 pipe welding is the vital requirement of ensuring proper protection of the root or first pass of the weld from oxidation through the use of an inert gas blanket, i.e. backing gas. The necessity for oxidation protection negatively impacts the cost of both weld set-up and the actual welding process of P91 pipe fabrication. In an effort to decrease the associated costs of welding P91, Fluor Corporation has invested in significant research and extensive field-testing to develop the wire/gas mixture that contributes to the breakthrough in welding P91 with “No Backing Gas (NBG)”. Combining this novel technique with the semiautomatic GMAW-S (using inverter technology with a controlled transfer) eliminates all cost associated with the need to provide a backing gas, including installation of purge dams, backing gas, and man-hours associated with implementing these activities.

An iron aluminide (Fe 3 Al) intermetallic coating was deposited onto F22 (2.25Cr-1Mo) steel substrate using a JP-5000 high velocity oxy-fuel (HVOF) thermal spray system. The as-sprayed coating was characterized by electron microscopy, X-ray diffraction, oxidation, and adhesion. Fe 3 Al coated steel specimens were exposed to a mixed oxidizing/sulfidizing environment of N 2 -10%CO-5%CO 2 -2%H 2 O-0.12%H 2 S (by volume) at 500, 600, 700, and 800°C for approximately seven days. All specimens gained mass after exposure, inversely proportional to temperature increases. Representative cross-sectioned specimens from each temperature underwent scanning electron microscopy (SEM) and X-ray mapping examination. Results are presented in terms of corrosion weight gain and product formation. The research evaluated the effectiveness of an HVOF-sprayed Fe 3 Al coating in protecting a steel substrate exposed to a fossil energy environment.

Six types of solution treated Ni-based alloy plates having a thickness of 25mm, namely Alloy 617, Alloy 263, Alloy 740, Alloy 141, HR6W (45Ni-23Cr-7W) and HR35 (50Ni-30Cr-4W-Ti) for advanced-USC boilers, were subjected to corrosion testing. In addition, three types of conventional ferritic and five types of conventional austenitic stainless tubes were also tested to compare their corrosion properties. Hot corrosion tests were conducted in order to assess the effects of temperature, material composition and coal ash composition on hot corrosion. The maximum average metal loss and the maximum corrosion rate were observed under 700°C test conditions. Cr content in the materials played an important role in the corrosion rate, with higher Cr content materials tending to show lower rates. However, Ni-based alloy materials showed slightly greater corrosion rates than those of stainless steels having equivalent Cr content in the over-700°C test condition. It was considered that rich Ni in the alloys easily reacted with sulfur, thus forming corrosion products having low melting points, such that corrosion was accelerated. The concentration of Fe 2 O 3 and NiO in the synthetic coal ash was also observed to affect the corrosion rate.

This paper reports on a study that investigated how low cycle fatigue (LCF) and fatigue crack propagation (FCG) properties of P92, P122, and P23 steels vary between 600°C and 700°C depending on the location relative to a cross weld. Microstructure analysis was also performed on fractured specimens. Due to its higher yield strength, P122 exhibited the best continuous LCF life. However, creep-fatigue interaction (CFI) in the weld heat-affected zone (HAZ) of P122 and P23 steels significantly reduced their lifespans compared to continuous LCF tests. This reduction is attributed to the effect of weld thermal cycles on fine precipitates. FCG tests revealed that the base metal consistently outperformed the HAZ in all tested steels and temperatures. P92 and P122 showed similar FCG rates except for P92's behavior at 600°C, which resembled P23. In both steels, the HAZ exhibited faster FCG rates at 600°C and 700°C compared to the base metal, particularly at lower stress intensity factor ranges (ΔK). Within the HAZ, the region 1 mm from the fusion line displayed the slowest FCG rates, followed by the base metal, while the fusion line and the region 2 mm from it showed the fastest. Fracture surfaces near the fusion line displayed cleavage-like features, while the region 1 mm away exhibited features associated with higher crack growth resistance.

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

Super Duplex stainless steels (SDSS) are alloys based on the Fe-Cr-Ni-N system. The chemical composition is tailored to achieve a balanced microstructure of 50% ferrite and 50% austenite. Hyper Duplex Stainless Steels (HDSS) are also duplex materials distinguished by their remarkable yield strength (≥700 MPa) and corrosion resistance (PREN>48). They have been developed as an alternative to the well-established SDSS when superior mechanical and corrosion performance is required. This enhanced performance is attributed to alloying additions, primarily Cr, Mo, and N. In this study, a comparison is conducted between filler metals of SDSS and HDSS for the root welding of SDSS plates. The gas tungsten arc welding (GTAW) process was used to carry out root welding passes and Gas Metal Arc Welding (GMAW) for filling passes on SDSS substrates arranged in a V groove to simulate a repair scenario. The heat input was controlled in both processes, keeping it below 2.0 kJ/mm in the GTAW and 1.2 kJ/mm in the GMAW. GTAW with constant current was used and the parameters achieved producing full penetration welds with SDSS and HDSS. In this case, Nitrogen was used as backing gas to avoid oxidation of the root. Thus, a special GMAW-Pulsed version was applied to achieve good wettability and defect-free joints. ASTM G48 tests were performed to evaluate the corrosion resistance through Critical Pitting Testing (CPT) analysis on the root pass, microstructural analysis via optical microscopy, and impact toughness. Consequently, a comprehensive examination of the welded joints outlines manufacturing conditions, limitations, and the applications of SDSS and HDSS filler metals.

Coal ash corrosion testing was conducted on six solution-treated nickel-based alloy plates (Alloy 617, Alloy 263, Alloy 740, Alloy 141, HR6W [45Ni-23Cr-7W], and HR35 [50Ni-30Cr-4W-Ti]) intended for advanced-USC boilers, along with conventional ferritic and austenitic stainless tubes for comparison. Tests used synthetic coal ash (Na 2 SO 4 , K 2 SO 4 , Fe 2 O 3 ) with varying SO 2 concentrations (0.02-1.00 vol%). Results showed maximum metal loss at 700°C, with higher SO 2 levels causing increased corrosion. Materials with higher chromium content demonstrated better corrosion resistance, suggesting chromium content is a crucial factor in material selection for these applications.

High-performance Ferritic (HiperFer) steels represent a promising materials innovation for next-generation thermal energy conversion systems, particularly in cyclically operating applications like concentrating solar thermal plants and heat storage power plants (Carnot batteries), where current market adoption is hindered by the lack of cost-effective, high-performance materials. HiperFer steels demonstrate superior fatigue resistance, creep strength, and corrosion resistance compared to conventional ferritic-martensitic 9-12 Cr steels and some austenitic stainless steels, making them potentially transformative for future energy technologies. This paper examines the microstructural mechanisms underlying HiperFer’s enhanced fatigue resistance in both short and long crack propagation, while also presenting current findings on salt corrosion properties and exploring potential alloying improvements for fusion reactor applications, highlighting the broad technical relevance of these innovative materials.

Research has demonstrated that creep damage in power plant steels is directly linked to grain boundary precipitates, which serve as nucleation sites for cavities and micro-cracks. The formation of M 23 C 6 carbides along grain boundaries creates chromium-depleted zones vulnerable to corrosion and significantly reduces creep life due to rapid coarsening. Through combined Monte Carlo grain boundary precipitation kinetics and continuum creep damage modeling, researchers have predicted that increasing the proportion of MX-type particles could enhance creep performance. This hypothesis was tested using hafnium-containing steel, which showed improved creep and corrosion properties in 9% Cr steels. Ion implantation of Hafnium into thin foils of 9 wt% Cr ferritic steel resulted in two new types of precipitates: hafnium carbide (MX-type) and a Cr-V rich nitride (M 2 N). The hafnium carbide particles, identified through convergent beam diffraction and microanalysis, appeared in significantly higher volume fractions compared to VN in conventional ferritic steels. Additionally, Hafnium was found to eliminate M 23 C 6 grain boundary precipitates, resulting in increased matrix chromium concentration, reduced grain boundary chromium depletion, and enhanced resistance to intergranular corrosion cracking.

More efficient, sustainable, flexible and cost-effective energy technologies are strongly needed to fulfil the new challenges of the German “Energiewende”. For a reduction of consumed primary resources higher efficiency steam cycles with increased operating parameters, pressure and temperature, are mandatory. Hence, advanced materials are needed. The present study focuses on stainless, high strength, ferritic (non-martensitic) steel grades, regarding thermal treatment effects on particle evolution. The heat treatment includes variations, e.g. a two phase pre heat treatment. Effects of the treatment were analysed and connected to creep performance. Experiments at differently heat treated materials show promising improvement of creep performance. These results can be linked to the stability and evolution of strengthening Laves phase particles.