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.

Steam-side oxidation and the resultant exfoliation of iron-based scales cause unplanned shutdowns at coal-fired power generation plants. Exfoliate removal is currently limited to frequent unit cycling to minimize the volume of exfoliated scale, upgrading a plant with a “blow down” system, or installing a higher alloy. This paper discusses the rate of steam-side oxidation on Type 304H stainless steel (304H) tube after shot peening the internal surface with commercially available techniques. Shot peening the ID of Type 304H austenitic stainless steel superheater tubes has been shown to improve the overall oxidation resistance in steam. Decreasing the oxidation rate directly impacts the volume of exfoliated scale. The adherent spinel scales are thinner and more robust than non-shot peened tubes of the same alloy. Most of the improved oxidation resistance can be attributed to the presence of a spinel oxide layer combined with a continuous chromia layer formed near the steam-touched surfaces. The presence of a continuous chromia layer vastly reduces the outward diffusion of iron and minimizes the formation of iron-based scales that exfoliate. This work showed that a uniform cold-worker layer along the tube ID has a profound effect on oxidation resistance. Incomplete coverage allows oxidation to proceed in the non-hardened regions at a rate comparable to the oxidation rate on unpeened Type 304H.

The growth behavior of oxide scale in a laboratory steam environment has been conducted for the shot-peened 18Cr-8Ni stainless steels differing in grain size. Both steels (fine grained and coarse grained) have demonstrated almost the same steam oxidation behavior reacted at 700°C for up to 2000h, which had excellent oxidation resistance due to formation of a protective Cr 2 O 3 scale. After the exposure of 4000h, however, nodule-like oxide occurred on the coarse grained steel, while the fine grained steel still remained the uniform Cr 2 O 3 scale. These behaviors well explained in terms of changes of the outward Cr flux due to recovery and recrystallization of the deformed structure. This result has proven that the shot-peened tube composed of fine grain structure is capable of combat against the steam oxidation at high temperatures.

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.

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.

For higher efficiency and competitive overall performance, it would be an advantage to be able to safely exceed the highest operational values, currently up to about 600-620°C/25-30 MPa in supercritical (SC) boiler plants. Under such operating conditions the oxidation resistance in SC water tends to limit the potential for further improvement of efficiency. The way to increase the oxidation resistance of traditional austenitic boiler tubes e.g. TP 347H is to do additional cold work on the boiler tube inner surface. In the current paper the effect of cold work on the oxidation resistance of TP347H and TP347HFG has been evaluated by shot peened samples with different parameters and subjecting those samples to supercritical oxidation exposure. The results show an improvement in the oxidation resistance of the alloys, especially in the large grained alloy TP347H. Also the uniformity of the deformation layer was seen to have an influence on the oxidation resistance, since the oxide nodules start to grow at locations with the thinnest or no deformation layer.

To improve the efficiency of fossil fuel power plants the operating temperatures and pressures need to be increased. However, at high temperatures the steam side oxidation resistance becomes a critical issue for the steels used especially at the final stages of superheaters and reheaters. Apart from the chemical composition of the material, surface condition is a major factor affecting the oxidation resistance in steam and supercritical water. In this paper, stainless boiler steels (UNS S34710, S31035, S31042, and S30942) are investigated for oxidation resistance in flowing supercritical water. Tests were conducted in an autoclave environment (250 bar, with 125 ppb dissolved oxygen and a pH of 7) at 625°C, 650°C and 675°C for up to 1000 h. Materials were tested with as-delivered, shot peened, milled or spark eroded and ground surface finish. The results show a strong influence of surface finish at the early stages of oxidation. Oxides formed on cold worked surfaces were more adherent and much thinner than on a spark eroded and ground surface. This effect was stronger than the influence of temperature or alloy composition within the tested ranges.

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.

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.

As part of the Boiler Materials for Ultrasupercritical Coal Power Plants program, sponsored by the United States (U.S.) Department of Energy (DOE) and the Ohio Coal Development Office (OCDO), the steamside oxidation and oxide exfoliation behavior of candidate alloys have been thoroughly evaluated in steam at temperatures between 620°C and 800°C (1148°F and 1472°F) for times up to 10,000 hours. The results from this test program indicate that the oxidation rates and oxide morphologies associated with steamside oxidation are a strong function of the crystallographic lattice structure and the chromium content of the material. Oxide exfoliation correlates to oxide thickness. The time required to reach the critical oxide thickness for exfoliation can be estimated based on oxidation kinetic relationships. For austenitic stainless steels, shot peening is effective in reducing steamside oxidation/exfoliation, but the efficacy of this technique is limited by the operating temperature. Nickel-based alloys exhibit very low oxidation/exfoliation rates, but have a propensity to form aluminum/titanium oxides along near surface grain boundaries.

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.

A 9Cr-3W-3Co-VNbBN steel, designated MARBN ( MAR tensitic 9Cr steel strengthened by B oron and N itrides), has been alloy-designed and subjected to long-term creep and oxidation tests for application to thick section boiler components in USC power plant at 650 o C. The stabilization of lath martensitic microstructure in the vicinity of prior austenite grain boundaries (PAGBs) is essential for the improvement of long-term creep strength. This can be achieved by the combined addition of 140ppm boron and 80ppm nitrogen without any formation of boron nitrides during normalizing at high temperature. The addition of small amount of boron reduces the rate of Ostwald ripening of M 23 C 6 carbides in the vicinity of PAGBs during creep, resulting in stabilization of martensitic microstructure. The stabilization of martensitic microstructure retards the onset of acceleration creep, resulting in a decrease in minimum creep rate and an increase in creep life. The addition of small amount of nitrogen causes the precipitation of fine MX, which further decreases the creep rates in the transient region. The addition of boron also suppresses the Type IV creep-fracture in welded joints by suppressing grain refinement in heat affected zone. The formation of protective Cr 2 O 3 scale is achieved on the surface of 9Cr steel by several methods, such as pre-oxidation treatment in Ar gas, Cr shot-peening and coating of thin layer of Ni-Cr alloy, which significantly improves the oxidation resistance of 9Cr steel in steam at 650 o C. Production of a large diameter and thick section pipe and also fabrication of welds of the pipe have successfully been performed from a 3 ton ingot of MARBN.

This study aims to examine the effects of grain boundary oxidation and creep on crack initiation and fracture behaviors in cold worked surface layer, under static tensile stresses in air. To determine these effects in relation to percent cold work and hardness scale, cold-rolled plates with a reduction ratios between 10% and 50% were prepared. Uniaxial constant load (UCL) tests were conducted at elevated temperature in air using smooth round bar specimen. UCL tests with a load of 0.9σy (926MPa) at 550°C show that rupture time for all cold- rolled materials were shorter than that of as-received material. From cross-sectional observation after UCL testing, surface crack at grain boundary and voids were observed in as-received material, whereas creep cracks were also observed in cold-rolled materials. This implied that crack initiation was assisted by cold working. Comparing test results with a load reduced to 0.8σy (823MPa), difference of rupture time was expected as a factor of 5 for as-received material, and measured as 2-3 for cold-rolled materials. It was suggested that cold worked layer was more sensitive to creep than base metal.

Early supercritical units such as American Electric Power (AEP) Philo U6, the world’s first supercritical power plant, and Eddystone U1 successfully operated at ultrasupercritical (USC) levels. However due to the unavailability of metals that could tolerate these extreme temperatures, operation at these levels could not be sustained and units were operated for many years at reduced steam (supercritical) conditions. Today, recently developed creep strength enhanced ferritic (CSEF) steels, advanced austenitic stainless steels, and nickel based alloys are used in the components of the steam generator, turbine and piping systems that are exposed to high temperature steam. These materials can perform under these prolonged high temperature operating conditions, rendering USC no longer a goal, but a practical design basis. This paper identifies the engineering challenges associated with designing, constructing and operating the first USC unit in the United States, AEP’s John W. Turk, Jr. Power Plant (AEP Turk), including fabrication and installation requirements of CSEF alloys, fabrication and operating requirements for stainless steels, and life management of high temperature components

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.

Solid particle erosion (SPE) and liquid droplet erosion (LDE) cause severe damage to turbine components and lead to premature failures, business loss and rapier costs to power plant owners and operators. Under a program funded by the Electric Power Research Institute (EPRI), nano­coatings are under development for application in steam and gas turbines to mitigate the adverse effects of PE and LPE on rotating blades and stationary vanes. Based on a thorough study of the available information, most promising coatings such as nano-structured titanium silicon carbo-nitride (TiSiCN), titanium nitride (TiN) and multilayered nano coatings were selected. TurboMet International (TurboMet) teamed with Southwest Research Institute (SwRI) with state-of-the-art nano-technology coating facilities with plasma enhanced magnetron sputtering (PEMS) method to apply these coatings on various substrates. Ti-6V-4Al, 12Cr, 17-4PH, and Custom 450 stainless steel substrates were selected based on the current alloys used in gas turbine compressors and steam turbine blades and vanes. Coatings with up to 30 micron thickness have been deposited on small test coupons. These are extremely hard coatings with good adhesion strength and optimum toughness. Tests conducted on coated coupons by solid particle erosion (SPE) and liquid droplet erosion (LDE) testing indicate that these coatings have excellent erosion resistance. The erosion resistance under both SPE and LDE test conditions showed the nano-structured coatings have high erosion resistance compared to other commercially produced erosion resistance coatings. Tension and high-cycle fatigue test results revealed that the hard nano-coatings do not have any adverse effects on these properties but may provide positive contribution.

Energy requirements and environmental concerns have promoted a development in higher-efficiency coal fired power technologies. Advanced ultra-super critical power plant with an efficiency of higher than 50% is the target in the near future. The materials to be used due to the tougher environments become therefore critical issues. This paper provides a review on a newly developed advanced high strength heat resistant austenitic stainless steel, Sandvik Sanicro 25, for this purpose. The material shows good resistance to steam oxidation and flue gas corrosion, and has higher creep rupture strength than any other austenitic stainless steels available today, and has recently obtained two AMSE code cases. This makes it an interesting option in higher pressures/temperature applications. In this paper, the material development, structure stability, creep strength, steam oxidation and hot corrosion behaviors, fabricability and weldability of this alloy have been discussed. The conclusion is that the Sanicro 25 is a potential candidate for superheaters and reheaters in higher-efficiency coal fired boilers i.e. for applications seeing up to 700°C material temperature.

Extended storage of spent nuclear fuel (SNF) in intermediate dry cask storage systems (DCSS) due to lack of permanent repositories is one of the key issues for sustainability of the current domestic Light Water Reactor (LWR) fleet. The stainless steel canisters used for storage in DCSS are potentially susceptible to chloride-induced stress corrosion cracking (CISCC) due to a combination of tensile stresses, susceptible microstructure, and a corrosive chloride salt environment. This research assesses the viability of the cold-spray process as a solution to CISCC in DCSS when sprayed with miniature tooling within a characteristic confinement in two different capacities: cleaning and coating. In general, the cold-spray process uses pressurized and preheated inert gas to propel powders at supersonic velocities, while remaining solid-state. Cold-spray cleaning is an economical, non-deposition process that leverages the mechanical force of the propelled powders to remove corrosive buildup on the canister, whereas the cold spray coating process uses augmented parameters to deposit a coating for CISCC repair and mitigation purposes. Moreover, both processes have the potential to induce a surface compressive residual stress that is known to impede the initiation of CISCC. Surface morphology, deposition analysis, and microstructural developments in the near-surface region were examined. Additionally, cyclic corrosion testing (CCT) was conducted to elucidate the influence of cold-spray cleaning and coating on corrosion performance.

The United States Department of Energy (U.S. DOE) Office of Fossil Energy and the Ohio Coal Development Office (OCDO) have been the primary supporters of a U.S. effort to develop the materials technology necessary to build and operate an advanced-ultrasupercritical (A-USC) steam boiler and turbine with steam temperatures up to 760°C (1400°F). The program is made-up of two consortia representing the U.S. boiler and steam turbine manufacturers (Alstom, Babcock & Wilcox, Foster Wheeler, Riley Power, and GE Energy) and national laboratories (Oak Ridge National Laboratory and the National Energy Technology Laboratory) led by the Energy Industries of Ohio with the Electric Power Research Institute (EPRI) serving as the program technical lead. Over 10 years, the program has conducted extensive laboratory testing, shop fabrication studies, field corrosion tests, and design studies. Based on the successful development and deployment of materials as part of this program, the Coal Utilization Research Council (CURC) and EPRI roadmap has identified the need for further development of A-USC technology as the cornerstone of a host of fossil energy systems and CO 2 reduction strategies. This paper will present some of the key consortium successes and ongoing materials research in light of the next steps being developed to realize A-USC technology in the U.S. Key results include ASME Boiler and Pressure Vessel Code acceptance of Inconel 740/740H (CC2702), the operation of the world’s first 760°C (1400°F) steam corrosion test loop, and significant strides in turbine casting and forging activities. An example of how utilization of materials designed for 760°C (1400°F) can have advantages at 700°C (1300°F) will also be highlighted.

Hardfacing alloys are commonly used for wear- and galling-resistant surfaces for mechanical parts under high loads, such as valve seats. Cobalt-based Stellite, as well as, stainless-steel-based Norem02 and Tristelle 5183 alloys show similar microstructural features that correlate with good galling resistance. These microstructures contain hard carbides surrounded by a metastable austenite (fcc) phase that transform displacively to martensite (hcp or bcc or bct) under deformation. As a result, the transformed wear surface forms a hard layer that resists transition to a galling wear mechanism. However, at elevated temperature (350°C), the stainless steel hardfacing alloys do not show acceptable galling behavior, unlike Stellite. This effect is consistent with the loss of fcc to bcc/bct phase transformation and the increase in depth of the heavily deformed surface layer. Retention of high hardness and low depth of plastic strain in the surface tribolayer is critical for retaining galling resistance at high temperature.