The delivery state of austenitic heat resistant steel boiler tubes is paramagnetic, such as TP304H, TP347H and S30432, the material state, however, appears obviously magnetic after long-time high-temperature service. Vibrating Sample Magnetometer (VSM) has been employed to test the magnetism difference after high-temperature service, and XRD, SEM, TEM, SAED and EDS has been adopted to observe and analyze their microstructure, phase structure and composition. The research results show that compared with the delivery state, the lath α´-Martensite and sometimes the lamellar ε-Martensite will occur in areas adjacent to grain boundaries due to martensite transformation in the microstructure of austenitic heat resistant steel boiler tube after high temperature service. There are high density dislocations tangled together in the substructure of α´-Martensite, and lamellar stacking faults arrayed orderly by a large number of dislocations in the substructure of ε-Martensite. The magnetism of α´-Martensite, its internal stress and carbides is the reason why the austenitic heat resistant steel boiler tubes appear obviously magnetic after high temperature service, and the α´-Martensite plays a major role.

Qualifying welding procedures for repair of components in high temperature service requires careful consideration of factors including identification of the materials involved, existing mechanical properties and service operating parameters such as temperature, pressure and environment. Selection of weld metals to match, under match or overmatch base material as well as direct and indirect consequences on the heat-affected zone also require evaluation. Application of post weld heat treatment and ramifications where dissimilar base materials are involved are discussed plus the necessity of conducting tests at the operating temperatures and conditions where information is not available from the literature. Each of these factors is discussed and examples provided.

The creep degradation/life assessment for high temperature critical component materials is absolutely needed to assure the long-term service operation and there is little experience with the service exposure of the high temperature components made of newly developed Ni-based alloys. In this study, therefore, the creep degradation assessment study on the Ni-based alloys, Alloy 617 and HR6W was conducted based on the hardness method, because the hardness measurement is a useful and simple technique for the materials characterization for any kind of high temperature-serviced steels and alloys. As the result, it was found that the hardness was increased by not only precipitation due to thermal aging but also creep stress/strain, and there existed linear relationship between the applied stress and creep-induced hardness increase. Also the hardness scatter measured was increased along with the progress of creep hardening and damage progressing in terms of creep life consumed. Those findings suggested that the creep life assessment of Ni-based alloys would be possible by means of hardness measurement. The paper also deals with the role and perspective development of non destructive damage detecting techniques, and life assessment issues on Ni-based alloys for A-USC power applications.

The long-term performance of superheater super 304h tube during the normal service of an ultra-supercritical 1000mw thermal power unit was tracked and analyzed, and the metallographic structure and performance of the original tube sample and tubes after 23,400h, 56,000h, 64,000 h, 70,000 h and 80,000 h service were tested. The results show that the tensile strength, yield strength and post-break elongation meet the requirements of ASME SA213 S30432 after long-term service, but the impact toughness decreases significantly. The metallographic organization is composed of the original complete austenite structure and gradually changes to the austenite + twin + second phase precipitates. With the extension of time, the number of second phases of coarseness in the crystal and the crystal boundary increases, and the degree of chain distribution increases. The precipitation phase on the grain boundary is dominated by M 23 C 6 , and there are several mx phases dominated by NbC and densely distributed copper phases in the crystal. The service environment produces a high magnetic equivalent and magnetic induction of the material, the reason is that there are strips of martensite on both sides of the grain boundary, and the number of martensite increases with the length of service.

New Monte Carlo models have recently been developed to predict microstructural evolution in steels and aluminum alloys during heat treatment and high-temperature service. These models can control precipitate type and size distribution, distinguishing between pure lattice and grain boundaries. Consequently, they can forecast the precipitate size distribution within grains and on grain boundaries as a function of time. This paper describes the model validation for ferritic Fe-9Cr P92 steels. The model provides new information over a range of time intervals adding up to the total plant lifetime in an ultra-supercritical plant. This information can be incorporated into continuum damage mechanics models for predicting creep rate and stress rupture life. The paper discusses how this technique is used as a materials development tool to forecast necessary compositional modifications for improving creep properties in ferritic steels.

Dissimilar metal welds between T91 ferritic steels and TP347H austenitic alloys are commonly used in fossil power plants in China. Premature failure of such dissimilar welds can occur, resulting in unplanned plant outages that can cause huge economic losses. In this article, microstructural evolution of T91/TP347H dissimilar welds after different service conditions were studied, mechanical properties before and after service were also analyzed, a full investigation into the failure cause was carried out. The results show, the dissimilar metal welds in the as-welded condition consists of a sharp chemical concentration gradient across the fusion line, failure is attributed to the steep microstructural and mechanical properties gradients, formation of interfacial carbides that promote creep cavity formation.

Ultrasupercritical (USC) coal-fired boilers, currently under study, will be required to utilize a variety of new, high strength alloys. These alloys will have improved mechanical properties compared with more traditional boiler materials and so will be suitable for operation in higher temperature service. However, environmental resistance, i.e. internal steam oxidation and external coal-ash corrosion, will be a factor limiting application of some materials under consideration. In those cases, the operating range of lower-cost alloys can be significantly extended by the use of surface modification techniques. This paper will review potential surface modification techniques and report on early test results of some laboratory evaluations.

Welded joints of Ni-base alloys are often the critical part of components operated under high temperature service conditions. Especially welds in thick-walled structures are susceptible to various crack phenomena. Creep rupture and deformation behavior of different similar welds of Alloy 617B, both circumferential and longitudinal, were determined in many research German projects with the aim to qualify the nickel alloys and its welded joints for the use in highly efficient Advanced Ultra Supercritical (AUSC) power plants. Damage mechanisms and failure behavior have also been investigated within these projects. In order to reduce the welding residual stresses in thick-walled components a post weld heat treatment (PWHT) for Alloy 617B is recommended after welding. This PHWT reduces not only residual stresses but causes changes in the damage mechanisms and failure behavior of welded joints of Alloy 617B. Improving effects of PWHT have been investigated in this study and results of microstructural investigations were correlated with the material behavior.

The microstructures and mechanical properties of T122 steel used for superheater tube of the boiler in a 1000 MW ultra supercritical power plant after service for 83,000h at 590℃ were investigated, and compared with data of that served for 56,000h in previous studies. The results show that compared with T122 tube sample service for 56,000h, the tensile properties at room temperature and the size of precipitated phase exhibit few differences, but the lath martensites features are apparent, and the Brinell hardness value are obviously higher. SEM and TEM experiments show that the substructure is still dominated by lath martensite. A few lath martensites recover, subgrains appear and equiaxe, and the dislocation density in grains is relatively low. A large number of second-phase particles precipitated at boundaries of original austenite grains and lath martensite phases, which are mainly M 23 C 6 and Laves phases.

Metallographic tests, micro-hardness tests, mechanics performance tests and Energy Dispersion Spectrum (EDS) were conducted for a 2.25Cr-1Mo main steam pipe weldment served for more than 32 years. Microstructural evolution of the 2.25Cr-1Mo base metal and weld metal was investigated. Degradation in micro-hardness and tensile properties were also studied. In addition, the tensile properties of subzones in the ex-service weldment were characterized by using miniature specimens. The results show that obvious microstructural changes including carbide coarsening, increasing inter lamella spacing and grain boundary precipitates occurred after long-term service. Degradation in micro-hardness is not obvious. However, the effects of long term service on tensile deformation behavior, ultimate tensile strength and yield stress are remarkable. Based on the yield stress of micro-specimens, the order of different subzones is: WM>HAZ>BM, which is consistent with the order of different subzones based on micro-hardness. However, the ultimate tensile strength and fracture strain of HAZ are lower than BM.

This paper presents the creep and creep-fatigue crack growth behaviors of 30Cr1Mo1V turbine rotor steel which had been in service for 16 years. Two typical sections of the rotor, i.e. high and low temperature sections, are examined at 538°C, with crack initiation and propagation monitored by D.C. potential drop method in a compact tension (CT) specimen. The material of the high temperature section has the lower resistance to creep and creep-fatigue crack growths than the low temperature section. The creep crack initiation (CCI) time decreases with the increase of initial stress intensity factor. The creep-fatigue crack growth (CFCG) is dominated by the cycle-dependent fatigue process when the hold time at the maximum load is shorter, but it becomes dominated by the time-dependent creep process when the hold time becomes longer. The high temperature section shows a larger influence of time-dependent creep behavior on CFCG than the low temperature section.

Increasing demand for reliable design of all kinds of structures requires materials properties evaluated under the conditions as close to real service conditions as possible. Presently resolved project dealing with development of new turbine blades geometry requires better understanding of the material behavior under service conditions. Service conditions of turbine blades are cyclic loading at high temperatures under superheated steam conditions and complex mechanical loading. There are not commercially available testing systems providing such functionality and thus the system allowing samples testing under considered conditions was developed. The system allows cyclic loading at temperatures up to 650°C under superheated steam conditions. Typical blade steel is investigated here and experimental approach considering complex mechanical loading as well as thermal and corrosion is shown here. The results of high cycle fatigue tests in superheated steam corrosive environment are shown here.

Conventional time-temperature-parameter (TTP) methods often overestimate long-term creep rupture life of creep strength enhanced high Cr ferritic steels. The cause of the overestimation is studied on the basis of creep rupture data analysis on Gr.91, 92 and 122 steels. There are four regions with different values of stress exponent n for creep rupture life commonly in stress-rupture data of the three ferritic steels. Activation energies Q for rupture life in the regions take at least three different values. The values of n and Q decrease in a longer-term region. The decrease in Q value is the cause of the overestimation of long-term rupture life predicted by the conventional TTP methods neglecting the change in Q value. Therefore, before applying a TTP method creep rupture data should be divided into several data sets so that Q value is unique in each divided data set. When this multi-region analysis is adopted, all the data points of the steels can be described accurately, and their long-term creep life can be evaluated correctly. Substantial heat-to-heat and grade-to-grade variation in their creep strength is suggested under recent service conditions of USC power boilers.

The A286 is one of the earliest superalloys developed. It has been used for manufacturing different components of turbo machineries because of its balanced high temperature properties. These components include shafts, discs, spacers, blades and fasteners. This paper reviews some of the issues and recent field experiences related to metallurgy, fabrication, in-service evaluation and failure of some of these components. The fabrication aspects including the effects of alloy melting processes, forging parameters and different types of heat treatments on the processed parts are discussed. The importance of these factors on the microstructure and properties of A286 are highlighted. The effects of service exposure on some of these parts are also discussed. In service evaluation involves checking for service induced damage or changes in microstructures and properties so that the suitability of the part for continued service can be determined. The failure analysis section of the paper briefly discusses failures of two expander wheels and an expander disc made out of A286 to pinpoint some of the salient features of damage accumulation and fracture during service.

In the late 1980s, the domestic utility industry experienced failures in dissimilar metal welds (DMWs) between low-alloy ferritic tubing and austenitic tubing in superheaters and reheaters. Extensive research by EPRI found that nickel-based filler metals provided significant service life improvements over 309 stainless steel filler metals. Improved joint geometries and additional weld metal reinforcement were determined to extend service life further. A new nickel-based filler metal was also developed, exhibiting thermal expansion properties similar to the low-alloy base metal and a low chromium content that would result in a smaller carbon-depleted zone than currently available fillers. However, this new filler metal was never commercialized due to a tendency for microfissuring, resulting in less than desired service life. This paper discusses further investigation into the filler metal microfissuring issue and examines long-term testing to determine the filler's suitability for high-temperature applications.

This work concerns a study into the design of creep resistant precipitation hardened austenitic steels for fossil fuel power plants using an integrated thermodynamics based model in combination with a genetic algorithm optimization routine. The key optimization parameter is the secondary stage creep strain at the intended service temperature and time, taking into account the coarsening rate of MX carbonitrides and its effect on the threshold stress for secondary creep. The creep stress to reach a maximal allowed creep strain (taken as 1%) at a given combination of service temperature and time is formulated and maximized. The model was found to predict the behavior of commercial austenitic creep resistant steels rather accurately. Using the alloy optimization scheme three new steel compositions are presented yielding optimal creep strength for various intended service times up to 105 hours. According to the evaluation parameter employed, the newly defined compositions will outperform existing precipitate strengthened austenitic creep resistant steels.

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.

The global electric power production is largely dependent on the operation of fossil-fired generation units. Many coal-fired units are exceeding 300,000 hours, which is beyond the expected design life. This has caused a continuous need to inspect steam touched components operating at high temperature and pressure. State-of-the-art coal and combined cycle gas units are specifying ever-greater amounts of the Creep Strength Enhanced Ferritic (CSEF) steels such as Grade 91 or Grade 92. The martensitic 9%Cr CSEF steels were developed to provide greater strength than traditional low alloy power plant steels, such as Grades 11, 12 and 22. The enhanced strength allows for a reduction in overall wall thickness in new or replacement components. Extensive research in both service failures and laboratory testing has shown that time-dependent creep damage can develop differently in Grade 91 steel when compared to low alloy steels. Furthermore, the creep strength in Grade 91 can vary by more than a factor of 10 between different heats. This wide variation of creep strength has led to extensive research in understanding the damage mechanisms and progression of damage in this steel. In this study, large cross weld samples were fabricated from thick wall piping in Grade 91 steel using two different heats of material. One weld was fabricated in a ‘damage tolerant’ heat and another weld was fabricated in a ‘damage intolerant’ heat of material. The samples were subjected to a post-weld heat treatment (PWHT) at a temperature of 745°C (1375°F) for 1.50 hours. Hardness maps were collected on the cross-welds in the as-welded and PWHT condition for both weldments. Cross-weld creep test conditions were selected to develop accelerated damage representative of in-service behavior. The test samples were interrupted at multiple stages and nondestructively evaluated (NDE) with advanced phased-array ultrasonic techniques. Samples were developed to variable levels of damage (50% to 100% life fraction) in both weldments. Metallographic sections were extracted at specific locations to validate the NDE findings using light emitting diode, laser and scanning electron microscopy. This research is being used to help validate the level of damage that can be reliably detected using conventional and advanced NDE techniques.

In order to improve thermal efficiency of fossil-fired power plants through increasing steam temperature and pressure high strength martensitic 9-12%Cr steels have extensively been used, and some power plants have experienced creep failure in high temperature welds after several years operations. The creep failure and degradation in welds of longitudinally seam-welded Cr- Mo steel pipes and Cr-Mo steel tubes of dissimilar metal welded joint after long-term service are also well known. The creep degradation in welds initiates as creep cavity formation under the multi-axial stress conditions. For the safety use of high temperature welds in power plant components, the complete understanding of the creep degradation and establishment of creep life assessment for the welds is essential. In this paper creep degradation and initiation mechanism in welds of Cr-Mo steels and high strength martensitic 9-12%Cr steels are reviewed and compared. And also since the non-destructive creep life assessment techniques for the Type IV creep degradation and failure in high strength martensitic 9-12%Cr steel welds are not yet practically established and applied, a candidate way based on the hardness creep life model developed by the authors would be demonstrated as well as the investigation results on the creep cavity formation behavior in the welds. Additionally from the aspect of safety issues on welds design an experimental approach to consider the weld joint influence factors (WJIF) would also be presented based on the creep rupture data of the large size cross-weld specimens and component welds.

Advanced austenitic stainless steels, such as Super 304H, have been used in reheater and superheater tubes in supercritical and ultra-supercritical power plants for many years now. It is important to characterize the microstructure of ex-service reheater and superheater tubes as this will help researchers understand the long-term microstructural evolution and degradation of the material, which can impact the performance and lifetime of the components that are in service. In this research, the microstructure of an ex-service Super 304H reheater tube that has been in service for 99,000 hours at an approximate metal temperature of 873K (600°C) has been characterized. The characterization techniques used were electron microscopy-based and included imaging and chemical analysis techniques. Seven phases were observed as a result of the characterization work. The phases observed were MX carbonitrides rich in niobium, copper-rich particles, M 23 C 6 , sigma phase, Z phase, a cored phase, and a BCC phase.