Modified 9Cr-1Mo alloy steel has been developed over the last few decades and has since gained wide acceptance in the boiler industry for the production of a variety of pressure-critical components, including tubing, piping and headers. The properties of creep-strength enhanced ferritic steels such as grade 91 are critically dependent on manufacturing parameters such as steelmaking, hot deformation, heat treatment and welding. Since the applications for which this material is used impose strict requirements in terms of resistance, corrosion, and creep behavior, poor process control can severely compromise the service behavior. This work discusses the impact of total deformation during the rolling process, and heat treatment parameters on time-independent and time-dependent properties for grade 91. For this study, two heats with similar chemical composition were produced with different reduction ratios: to which, several normalizing and tempering combinations were applied. For each combination, the microstructure was characterized, including evaluation of segregation by metallographic examination, and analysis of secondary phase precipitates by means of X-ray powder diffraction. Mechanical testing and creep testing were performed. A comparison of results is presented, and recommendations on the optimal process parameters are provided to ensure reliable performance of grade 91 material.

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

It is required to reduce the lifetime cost of turbine blades. To achieve the cost reduction, a refining and recycling method of scrapped turbine blades is proposed. For the establishment of the method, desulfurization mechanism of Ni-base superalloy by solid CaO was studied. 500 g of superalloy containing sulfur was heated in a vacuum induction furnace and kept at 1600 °C. A CaO rod was inserted into the molten alloy and held for 600 s. After the experiment, sulfur content in the alloy decreased from 200 ppm to 54 ppm. On the surface of the CaO rod after the experiment, only Ca, O, Al, and S were found by EPMA analysis. Especially, Al and S were distributed at the surface and grain boundaries of the rod. By powder XRD analysis, CaO, CaS and 3CaO･Al 2 O 3 were identified as constituent phases on the rod. The desulfurization mechanism of superalloy at 1600 °C is supposed to be three steps: (1) Al and S in the alloy react with CaO to generate CaS and Al 2 O 3 , respectively. (2) Al 2 O 3 melts with CaO as liquid slag. (3) CaS is captured by the slag, therefore, sulfur is removed from the alloy.

The competitive effect of Nb and V additions on the high-temperature oxidation behavior of Ti- 30Al alloys were studied at 800°C in air. Oxidation performance increased with increasing Nb content, however, V additions eliminated the beneficial effect of Nb on oxidation performance, causing higher oxidation mass gains. In-situ high-temperature XRD by means of synchrotron source suggested dissolution of Nb 5+ but lower valence of vanadium ions in the TiO 2 oxide scale during oxidation. Dissolution of Nb and V ions with different valence in TiO 2 during oxidation could cause the beneficial and detrimental effects observed on the performance of high-temperature oxidation of Ti-30Al.

Either at higher temperatures or when a certain alloying element content is exceeded, γ-TiAl alloys contain the β phase (bcc) or its ordered derivate β o (B2). The relatively soft β phase can facilitate hot deformation, but β o is detrimental for creep strength and ductility. Thus, knowledge about β o →β phase transformation is desirable. Surprisingly, for the binary Ti-Al system it is under discussion whether the ordered β o phase exists. Also, the effect of alloying elements on the β phase ordering is still unclear. In the present work the ordering of the β phase in binary Ti-(39,42,45)Al and ternary Ti-42Al-2X alloys (X=Fe, Cr, Nb, Ta, Mo) which was experimentally investigated by neutron and high energy X-ray diffraction is compared with the results of first principles calculations using density functional theory. Except for Cr the experimentally determined and the predicted behavior correspond.

Up to now, the amount of supercritical boilers in China has ranked number one in the world. Many supercritical boilers have run for more than 100,000 hours. Creep becomes one of the main reasons for supercritical boiler tubes failure. In this article, the failure of superheater tubes in a supercritical boiler was analyzed, the microstructural evolution of austenitic stainless steel tubes were studied, a full investigation into the failure cause was carried out involving in visual examination, optical microscope, SEM, TEM and XRD. The results show, sigma phase precipitates in this austenitic steel with the extension of service time, sigma precipitates form at grain boundaries by continuous chain. Sigma precipitates are hard and brittle, weaken grain boundaries and cause microscopic damage, eventually lead to boiler tubes failure.

In order to evaluate long term creep strength of modified 9Cr ferritic steels, the system free energy of creep ruptured specimens at both 650 and 700 °C is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. Change ratio of the system free energy and creep stress showed the relationship with one master curve irrespective of creep conditions, indicating that the steel ruptures when the applied stress exceeds a limited stress depending on the microstructural state expressed by the change ratio of system free energy. Furthermore, it was found that dominant factor of the change ratio was the chemical free energy change. On the basis of these results, long term creep strength of the steel was evaluated at 700 °C, for example, 19MPa at 700 °C after 10 5 h. It is concluded that long term creep strength of modified 9Cr ferritic steels can be predicted by the system free energy concept using the ruptured specimens with various creep conditions.

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.

Prior to utilizing new advanced materials in coal power plants, a large number of experimental testing is required. Test procedures are needed in specialized high temperature laboratories with state of the art facilities and precise, accurate analytical equipment capable of performing tests at a variety of temperatures and environments. In this study, the results of a unique technique involving salt spray testing at high temperatures are presented. The Haynes 282 gamma – prime (γ’) strengthened alloy fabricated by means of three different manufacturing processes: HAYNES 282 WROUGHT alloy, Haynes 282-SINT alloy, and finally Haynes 282-CAST alloy have been tested. The materials have been exposed to a salt spray corrosion atmosphere using 1% NaCl - 1% Na 2 SO 4 . Post exposure investigations have included SEM, EDS and XRD examinations. The test using salt spray of 1% NaCl - 1% Na 2 SO 4 water solution at 550 °C for 500 hours indicted no influence on the corrosion products formation, where Cr 2 O 3 has been developed in all three alloys, whereas NiO has been found only in Haynes 282-CAST material. On the other hand, it has been found that the fabrication process of HAYNES 282 alloy strongly influences the corrosion products formation under the high temperature exposures.

This study investigates the impact of adding small amounts of rare earth (RE) elements on the properties and microstructures of SA335P91 steel welds. The RE elements were incorporated into the weld metal using a coating process. The researchers then proposed an optimal RE formula aimed at achieving improved properties and microstructures. To evaluate the effectiveness of this approach, various tests were conducted on both welds with and without RE additions. These tests included tensile testing (both at room and high temperatures), impact testing, metallographic analysis to examine the microstructure, determination of phase transformation points, scanning electron microscopy, and X-ray diffraction. The results revealed that the addition of RE elements has the potential to enhance the properties and modify the microstructure of SA335P91 welds.

Precipitation of Z-phase, Cr(V,Nb)N, is known to negatively affect creep properties of 9-12%Cr steels for power plant applications as it dissolves finely distributed MX particles, (V,Nb)N, especially in high Cr steels. As the Z-phase precipitates slowly as large particles, this causes a net drop in precipitation strengthening. Two model alloys containing 9 and 12%Cr, but otherwise having similar composition, were produced in order to quantify the difference in Z-phase precipitation speed at different Cr levels. The nitride precipitation behavior was followed at different temperatures using TEM and XRD, allowing for a quantification of the Z-phase precipitation. The Z-phase was found to precipitate 20-50 times faster in the 12%Cr steel compared to 9%Cr steel in the temperature range 600- 650°C. The transformation of MX into Z-phase was followed in a Ta containing alloy without V or Nb. In this alloy the Z-phase precipitates very quickly, and thus appears as finely distributed particles which have the same strengthening effect as MX particles. Investigations using atomic resolution microscopy showed how Cr diffuses from the matrix into the TaN MX particles and gradually transforms them both chemically and crystallographically into Z-phase CrTaN particles.

Prediction of long-term creep strength is an important issue for industrial plants operated at elevated temperatures, although the creep strength of high Cr ferritic steels depends on their microstructural evolution during creep. The state of microstructure in metallic materials can be expressed as numerical values based on a concept of system free energy. In this study, in order to evaluate long term creep strength of 9Cr ferritic steel containing B, change in the system free energy during creep of the steel is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. The system free energy decreases with creep time. Change in the energy is expressed quantitatively as a numerical formula using the rate constants which depend on applied stress. On the basis of these facts, long term creep strength of the steel can be evaluated at both 948K(675°C) and 973K(700 °C).

In this paper, the microstructural evolution of P92 steel were studied in the viewpoint of degradation mechanism based on the creep rupture experiment results obtained at elevated temperature by means of macroscopic, metallographic, electronic microscope, energy spectrum, XRD and TEM examination. The results show that the decrease of mechanical properties of P92 steel is mainly due to the change of microstructure and the transformation of carbides, and there is definite relationship between microstructure evolution, mechanical properties and life loss of P92 steel. The results are beneficial to the further study of mechanism of high temperature creep rupture strength and microstructural evolution of heat-resistant steel. It also has important instructive significance to quantitative identification of scientific selection of materials.

The microstructural evolution has been investigated for an 18Cr-12Ni stainless steel (347HFG) that has been subject to a thermo-mechanical treatment to obtain a fine grain size (ASTM 7-10). In particular, sigma phase precipitation and growth has been evaluated. Samples of 347HFG stainless steel have been isothermally heat treated to reproduce and accelerate the ageing conditions experienced in-service at temperatures between 600 and 750 °C for up to 10,000 hours. Results have shown that sigma phase is precipitated at triple points and along grain boundaries after as little as 1000 hours which is contrary to thermodynamic predictions. In addition X-ray diffraction (XRD) and image analysis has been carried out to semi-quantitatively measure the amount of sigma phase present. The area fraction of sigma has been found to be 2.77 and 2.23 percent at 700 and 750 °C respectively. This is a higher volume fraction of sigma phase than has been previously observed in regular 347H at these conditions. It is thought that this is due to the reduced grain size that has provided an increase in nucleation sites and diffusion paths that can enhance the precipitation and growth of sigma phase. The results from this study are discussed with regards to the effect of precipitation on the service life of a 347HFG stainless steel tube operating in advanced supercritical boilers.

In thermal power plants, weldments of all currently used martensitic 9% chromium steels are prone to Type IV cracking in the fine-grained region of the heat-affected zone (HAZ). Japanese researchers have introduced a new martensitic steel for ultra-supercritical (USC) steam conditions that demonstrates resistance to Type IV cracking. This study compares a modified version of this boron-nitrogen balanced advanced 9Cr-3W-3Co steel with CB2, the most promising 9% Cr steel developed through the European research initiative COST, in terms of weldability. The HAZ was analyzed using the "Heat-Affected Zone Simulation" technique with a Gleeble 1500 thermo-mechanical simulator. Basic optical microscopy was complemented by advanced electron microscopy techniques, including energy-filtered TEM (EFTEM), electron energy loss spectroscopy (EELS), electron backscatter diffraction (EBSD), and energy-dispersive X-ray analysis (EDX). Phase transformations in the HAZ were directly observed using in situ X-ray diffraction with synchrotron radiation at the Advanced Photon Source (APS) of Argonne National Laboratory, IL, USA. Although both steels exhibited similar transformation behavior, their resulting microstructures after the weld thermal cycle differed significantly. At peak temperatures above 1200°C, delta ferrite formed and remained stable down to room temperature due to rapid cooling in both steels. While CB2 exhibited conventional coarse-grained (CG), fine-grained (FG), and intercritical HAZ regions, the boron-nitrogen balanced 9Cr steel did not develop a fine-grained HAZ. Since Type IV cracking primarily occurs in the FGHAZ, this alloy shows strong potential for eliminating Type IV cracking as a major life-limiting factor in heat-resistant steel weldments.

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.

The goal of improving the efficiency of pulverized coal power plants has been pursued for decades. The need for greater efficiency and reduced environmental impact is pushing utilities to ultra supercritical conditions (USC), i.e. steam conditions of 760°C and 35 MPa. The long-term creep strength and environmental resistance requirements imposed by these conditions are clearly beyond the capacity of the currently used ferritic steels and other related alloys. Consequently, new materials based on austenitic stainless steels and nickel-base superalloys are being evaluated as candidate materials for these applications. In the present work, the nickel-base superalloys CCA617, Haynes 230 and Inconel 740, and an austenitic stainless steel Super З04H, were evaluated. The materials were aged for different lengths of time at temperatures relevant to USC applications and the corresponding microstructural changes were characterized by x-ray diffraction, optical, scanning and transmission electron microscopy, with particular attention being given to the structure, morphology and compositions of phases (including γ, γ’, carbides, ordered phases, etc.) and the nature, density and distribution of dislocations and other defects. The results are presented and discussed in light of accompanying changes in microhardness.