A creep resistant martensitic steel, CPJ7, was developed with an operating temperature approaching 650°C. The design originated from computational modeling for phase stability and precipitate strengthening using fifteen constituent elements. Approximately twenty heats of CPJ7, each weighing ~7 kg, were vacuum induction melted. A computationally optimized heat treatment schedule was developed to homogenize the ingots prior to hot forging and rolling. Overall, wrought and cast versions of CPJ7 present superior creep properties when compared to wrought and cast versions of COST alloys for turbines and wrought and cast versions of P91/92 for boiler applications. For instance, the Larson Miller Parameter curve for CPJ7 at 650°C almost coincides with that of COST E at 620°C. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M 23 C 6 precipitates at 650°C. The cast version of CPJ7 also revealed superior mechanical performance, well above commercially available cast 9% Cr martensitic steel or derivatives. The casting process employed slow cooling to simulate the conditions of a thick wall full-size steam turbine casing but utilized a separate homogenization step prior to final normalization and tempering. To advance the development of CPJ7 for commercial applications, a process was used to scale up the production of the alloy using vacuum induction melting (VIM) and electroslag remelting (ESR), and underlined the importance of melt processing control of minor and trace elements in these advanced alloys.

Modified 9Cr-1Mo steel (ASTM Gr.91) is widely used in components of fossil fueled power plants around the world today. This grade of steel has however been shown to exhibit significant variations in creep life and creep ductility, which has led to premature in-service failures. The aim of this work is to define potential metallurgical risk factors that lead to this variation in performance. To achieve this, a set of creep test samples that represent a wide range in this variation of creep behavior in this steel grade have been studied in detail. As a first stage in this characterization the macro-scale chemical homogeneity of the materials were mapped using micro-XRF. Understanding the segregation behavior also allows quantification of microstructural parameters in both segregated and non-segregated areas enabling the variations to be determined. For example this showed a significant increase in the number per unit area of Laves phase particles in high compared with low Mo content areas. To study the effect of MX particles on segregation a methodology combining SEM and TEM was employed. This involved chemically mapping the larger V containing particles using EDS in the SEM in segregated and unsegregated areas and then comparing the results to site-specific TEM analysis. This analysis showed that although the average size of the V containing samples is in the expected 0-50 nm size range, these particles in some samples had a wide size distribution range, which significantly overlaps with the M 23 C 6 size distribution range. This together with the segregation characteristics has important implications for determining meaningful quantitative microstructural data from these microstructurally complex materials.

The effect of boron nitrides (BN) and aluminum nitrides (AlN) on long-term creep life and rupture ductility has been investigated for martensitic 9 to 12Cr steels at 550 to 700 °C. The BN particles form in 9 to 12Cr steels during normalizing heat treatment at 1050 to 1150 °C, suggesting no change in the amount of BN particles during creep. On the other hand, the AlN particles gradually form during creep at 550 to 700 °C, decreasing the concentration of nitrogen free from the AlN particles. The degradation in creep life takes place more significantly with test duration in the steels containing high Al but not in those containing the BN particles. The rupture ductility is evaluated by using a semi-logarithmic diagram of the RA and total elongation, showing the necking dominant and void swelling dominant regions. The BN and AlN particles are responsible for the degradation in RA at low stresses and long times by accelerating the formation of creep voids at interfaces between the BN and AlN particles and alloy matrix.

A new method of creep life assessment was developed to consider heat-to-heat variations of welded joints of materials used in power plants. This paper explains a scheme of the assessment method and also describes an actual implementation of the method for Grade 91 steel. In the method, creep properties of the welded joints are related to those of each base metal because the heat-to-heat variations of welded joints strongly depend on the creep properties of the corresponding base metals. To estimate the creep properties of each base metal of the target pipe, microstructure analyses and small punch creep tests were conducted using small samples cut from the base metals in service, and evaluations were done on the basis of material data base obtained using standard test samples of long-term service exposed pipes. It is expected that the precision of the life assessment of pipes will be significantly improved using the developed method because it can consider the heat-to-heat variations of their materials, which are not considered in existing life assessment methods.

700°C advanced ultra-supercritical system and supercritical CO 2 turbine system are developed for high efficiency turbine systems for next generation. This study covered the feasibility of creep life assessment of γ’-Ni 3 (Al,Ti) precipitation strengthened Ni-based superalloy rotor material, TOS1X-2, a modified alloy of UNS N06617 for these systems, based on hardness measurement method. It was found that the hardness of TOS1X-2 was governed by the change in precipitation strengthening and strain hardening during creep. The clear relationship between hardness increase in crept portion and macroscopic creep strain was observed, suggesting that it might be possible to estimate the creep strain or initiation of acceleration from hardness measurement. Microstructure inhomogeneity and microstructure evolutions during creep especially focused on dispersion of creep strain were characterized by EBSD quantitative analysis. It was found that creep strain was accumulated along the grain boundary, while it was relatively absent in coarse grains with low Schmid factor of {111} <110> slip system in fcc structure. The upper limit of hardness scatter band is thought to be important, since it represents the local and critical creep damage of the alloy.

The effect of taking miniature sample scoops on the creep life of ASME Grade 91 steel pipes was experimentally and analytically assessed in this work. Internal pressure tests were conducted on tubular specimens having defects on their outer surface, which simulate sampling scoops. The creep life did not decrease until the depth ratio of the defect to the wall thickness of the specimens was about 5%, and the creep life decreased with increasing defect depth when the depth ratio exceeded about 5%. When the depth ratio was about 11%, the creep life decreased to four-fifths of that of a specimen with no defects. In addition, as a result of investigating the stress concentration around a defect with a depth ratio of about 5% by the finite element method, stress concentration was clearly observed around the defect. These results suggest that taking a miniature sample up to a depth of 5% of the thickness of a Grade 91 steel pipe in service has a negligible effect on the creep life of the pipe.

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.

ASME Grade 91 steel seam-welded elbow pipe, which has been used in a USC plant (A-Plant) for about 6 × 10 4 h, was investigated to clarify the microstructure and remaining creep life of the material at long-term region. SEM and TEM observations were conducted on specimens cut from the welded portions of the intrados and extrados of the elbow, and the number density of creep voids in fine-grained HAZ was measured in the wall-thickness direction. Then, creep rupture tests were performed to examine the remaining life of each portion of the base metal and welded joint. On the basis of the results, it was suggested that the microstructural changes were small and that the cumulative creep damage was also small for the elbow pipe during its use at the USC plant for about 6 × 10 4 h. The present result was compared with the result of an investigation on Grade 91 steel elbow used in another USC plant (B-Plant) for about 5 × 10 4 h. The A-Plant material had a creep life about ten times longer than that of the B-Plant material for not only the base metals but also the welded joint. Through the comparison of the investigation results, it was suggested that the difference in the creep deformation property between the base metals of the elbows was the main reason for the difference in their creep lives.

A methodology is developed for evaluating its creep rupture life from analysis of an on-going creep curve with the aid of an Ω creep curve equation. The method is applied to on-going creep curves of grade 91 steel for evaluating their rupture lives. Quick decrease in creep rupture strength has been reported recently in long-term creep of grade 91 steel. The quick decrease of the steel is discussed by using the rupture lives evaluated. The quick decrease is confirmed in the present study in the time range longer than 3 x 10 4 h at 600°C.

Through inner wall oxidation scale thickness measurement, sampling tests and installation of wall temperature measuring device in the boiler, the equivalent wall temperature and its distribution of secondary high temperature reheater tube were estimated and verified, and the temperature field distribution of tube platen which is of single peak distribution in the direction vertical to tube platen and an apparent lower temperature distribution covered by the smoke shield at the side of boiler wall were both obtained. For the middlemost 10CrMo910, the wall temperature of individual tube was getting close to 600°C. Afterwards material state and residual creep life of tube platen were estimated and calculated. The results of estimate and calculation show that the tube platen in the middle is not suitable for further service due to its degraded material states and lower antioxidant ability. Thus with consideration of distribution characteristics of temperature field, parts of tube platens in the middle are proposed to be replaced with T91 tubes. Furthermore, to avoid onsite heat treatment, 10CrMo910 tube covered by the smoke shield in the boiler was reserved, and a small piece of 10CrMo910 tube was welded at the inlet and outlet ends respectively in the manufactory.

Creep properties of 9Cr heat resistant steels can be improved by the addition of boron and nitrogen to produce martensitic boron-nitrogen strengthened steels (MarBN). The joining of this material is a crucial consideration in the material design since welds can introduce relatively weak points in the structural material. In the present study, creep tests of a number of MarBN weld filler metals have been carried out to determine the effect of chemistry on the creep life of weld metal. The creep life of the weld metals was analysed, and the evolution of creep damage was investigated. Significant differences in the rupture life during creep have been observed as a function of boron, nitrogen and molybdenum concentrations in the weld consumable composition. Although the creep lives differed, the particle size and number in the failed creep tested specimens were similar, which indicates that there is a possible critical point for MarBN weld filler metal creep failure.

High chromium HiperFer (High performance ferritic) materials present a promising concept for the development of high temperature creep and corrosion resistant steels. The institute for Microstructure and Properties of Materials (IEK-2) at Forschungszentrum Jülich GmbH, Germany develops high strength, Laves phase forming, fully ferritic steels which feature excellent resistance to steam oxidation and better creep life than state of the art 9-12 Cr steels. Mechanical strength properties of these steels depend not only on chemical composition, but can be adapted to various applications by specialized thermo(mechanical) treatment. The paper will outline the sensitivity of tensile, creep, stress relaxation and impact properties on processing and heat treatment. Furthermore an outlook on future development potentials will be derived.

The effects of Cr and W on the creep rupture life of 8.5-11.5Cr steels at 650°C were evaluated. Throughout this paper the specimen composition is expressed in mass percent. The creep rupture life of 8.5Cr steel is the longest in 8.5-11.5Cr steels at 650°C under the stress of 78MPa. The creep rupture life of 9Cr steel at 650°C was extended with increasing W content. The creep strength of the modified steel, 9Cr-4W-3Co-0.2V-NbBN steel, at 650°C did not decrease sharply up to 32000h. The 105h creep rupture temperature of this steel under the stress of 100MPa was estimated to be approximately 635°C using Larson-Miller parameter. M 23 C 6 type carbides and VX type carbonitrides were observed on the lath boundary of the modified steel. The stability of these precipitates in the modified steel is likely to suppress the degradation of the long term creep strength at 650°C.

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 change in hydrogen desorption characteristic due to creep was investigated to examine the possibility of hydrogen as tracer for detecting and evaluating the creep damage accumulated in high Cr ferritic boiler steel, Gr.91. Hydrogen charging into the creep specimen was conducted by means of cathodic electrolysis. Next, the thermal desorption analyses (TDA) were carried out at temperature range from room temperature to 270°C for measuring the hydrogen evolution curve. The experimental results revealed that the amount of hydrogen desorbed during analysis, C H , increased with increasing creep life fraction, although the trend of increase in C H was strongly dependent on the stress level. Moreover, there was an almost linear correlation between the logarithm of C H measured on the creep ruptured specimen and the Larson-Miller parameter (LMP), which was approximated by “log C H = 0.39 LMP – 13.4”. This can be a criterion for creep rupture and means that as far as the C H does not reach the line, the rupture never occurs.

This paper investigates the cause of premature failures in certain Grade 91 steel components used in UK power plants. The failures were linked to both low material hardness and specific chemical compositions that fell within ASTM specifications but had a low nitrogen-to-aluminum ratio (N:Al). The investigators examined eight material batches, including those involved in failures, new stock, and in-service components with similar properties. Testing confirmed these materials had lower creep resistance compared to standard Grade 91 steel. Microscopic analysis revealed the presence of large aluminum nitride precipitates, which limited the formation of beneficial vanadium nitride precipitates, leading to reduced creep strength. These findings suggest that even within the ASTM specification limits, a low N:Al ratio can negatively impact the performance of Grade 91 steel.