This study aims to elucidate the chemical compositions and microstructural factors that affect longterm creep rupture strength and creep rupture ductility using multiple heats of Gr.92 steel. Evaluating the reduction behavior in long-term creep rupture strength, we propose a relative creep rupture strength value, which is expressed as the logarithmic ratio of the estimated creep strength for each rupture time exceeding 10,000 hours, with 10,000 hours as the reference. Higher initial hardness correlates with greater pronounced strength reduction in the long-term regime. While smaller prior austenite grain sizes lead to greater reductions in creep rupture strength, this effect diminishes above 30 μm. However, no clear correlation was observed between Cr content and creep strength reduction in this study. Brittle creep ruptures with smooth test specimens were observed just below the extensometer ridge in the parallel section of test specimen, indicating notch weakening. Even in heats with excellent creep ductility, the amount of inclusions tended to be higher than in heats with lower creep ductility. Factors other than inclusions also seem to influence long-term creep ductility.

There is a critical lack of data on the mechanical behavior of candidate structural materials for advanced nuclear reactors under molten halide salt environments. Limited legacy data from the molten salt reactor experiment (MSRE) program showed a significant reduction in creep rupture strength of a Ni-base alloy in molten fluoride salt. With ongoing efforts to commercialize different molten salt reactor concepts, the industry can considerably benefit from quantitative information on the impact of molten halide salts on the engineering properties such as creep and fatigue strength of materials of interest. The present work aims to assess the role of molten salt corrosion on the creep behavior of three alloys 316H, 617 and 282 at 650-816 °C. Creep tests were conducted in fluoride (FLiNaK) and chloride (NaCl-MgCl 2 ) salts. Initial results from the ongoing testing will be presented which suggest that the molten salt environment caused a 25-50% reduction in creep rupture lifetime compared to air exposures. Physics-based corrosion and creep models were employed to gain some insights into the potential degradation mechanisms.

This study investigates the role of grain-boundary precipitates in enhancing creep rupture strength of Ni-based alloys through analysis of Ni-15Cr-15Mo and Ni-15Cr-17Mo (at.%) model alloys. The investigation focused on the “Grain-boundary Precipitation Strengthening (GBPS)” effect from the thermally stable TCP phase, a phenomenon previously observed in Fe-Cr-Ni-Nb austenitic heat-resistant steels. Through multi-step heat treatments, specimens were prepared with varying grain boundary coverage ratios (ρ) of TCP P phase (oP56) and consistent grain-interior hardness from GCP Ni2(Cr, Mo) phase (oP6). In the 15 at.% Mo alloy, specimens with a higher coverage ratio (~80%) demonstrated significantly improved creep performance, achieving nearly four times longer rupture time (3793 h vs. 1090 h) at 300 MPa and 973 K compared to specimens with lower coverage (~35%). However, the 17 at.% Mo alloy showed unexpectedly lower performance despite high coverage ratios, attributed to preferential cavity formation at bare grain boundaries. These findings confirm that GBPS via thermally stable TCP phase effectively enhances creep properties in Ni-based alloys, with grain boundary coverage ratio being more crucial than intragranular precipitation density.

Creep strength of Grade 91 steels has been reviewed and allowable stress of the steels has been revised several times. Allowable stress regulated in ASME Boiler and Pressure Vessel Code of the steels with thickness of 3 inches and above was reduced in 1993, based on the re-evaluation with long-term creep rupture data collected from around the world. After steam leakage from long seam weld of hot reheat pipe made from Grade 122 steel in 2004, creep rupture strength of the creep strength enhanced ferritic (CSEF) steels has been reviewed by means of region splitting method in consideration of 50% of 0.2% offset yield stress (half yield) at the temperature, in the committee sponsored by the Ministry of Economy, Trade and Industry (METI) of Japanese Government. Allowable stresses in the Japanese technical standard of Grade 91 steels have been reduced in 2007 according to the above review. In 2010, additional long-term creep rupture data of the CSEF steels has been collected and the re-evaluation of creep rupture strength of the steels has been conducted by the committee supported by the Federation of Electric Power Companies of Japan, and reduction of allowable stress has been repeated in 2014. Regardless of the previous revision, additional reduction of the allowable stress of Grade 91 steels has been proposed by the review conducted in 2015 by the same committee as 2010. Further reduction of creep rupture strength of Grade 91 steels has been caused mainly by the additional creep rupture data of the low strength materials. A remaining of segregation of alloying elements has been revealed as one of the causes of lowered creep rupture strength. Improvement in creep strength may be expected by reducing segregation, since diffusional phenomena at the elevated temperatures is promoted by concentration gradient due to segregation which increases driving force of diffusion. It has been expected, consequently, that the creep strength and allowable stress of Grade 91 steels can be increased by proper process of fabrication to obtain a homogenized material free from undue segregation.

A newly developed ferritic heat-resistant steel; 9Cr-3W-3Co-Nd-B steel has higher creep rupture strength both in the base metal and welded joints than the conventional high-Cr ferritic heat-resistant steels. The creep rupture strengths of 9Cr-3W-3Co-Nd-B steel welded joints were below the lower limit of the base metal in long-term creep stage more than 20,000 hours. The creep rupture position was heat-affected zone (HAZ) from 1.0 to 1.5 mm apart from the fusion line on the welded joint specimen ruptured at 34,966 hours. The equiaxed subgrains and coarsened precipitates were observed in HAZ of the ruptured specimen. In order to clarify the creep fracture mechanism of the welded joints, the microstructures of HAZ were simulated by heat cycle of weld, then observed by EBSD analysis. Fine austenite grains formed along the prior austenite grain boundaries in the material heated just above A C3 transformation temperature, however there were no fine grains such as conventional steel welded joints. The prior austenite grain boundaries were unclear in the material heated at 1050 °C. The creep rupture life of the material heated at just above A C3 transformation temperature exceeded the lower limit of base metal and there was no remarkable degradation, although it was shorter than the other simulated materials. It is, therefore, concluded that the creep fracture of 9Cr-3W-3Co-Nd-B steel welded joint in long-term stage occurred at HAZ heated at from just above A C3 transformation temperature to 1050 °C. It is speculated that the fine austenite grains formed along the prior austenite grain boundaries and inhomogeneous microstructures cause the coarsening precipitates and recovery of lath structure during long-term creep deformation.

Long-term creep rupture tests up to 10 5 hours at 600℃ and 650℃ were carried out on mod.9Cr- 1Mo steel base metal and weldments from five different materials, consisting of various chemical compositions and heat treatments as well as welding conditions. As a result, positive correlations of creep rupture strength were clarified between the base metal and weldments from the same materials. Microstructural observations and thermokinetic calculations revealed that the strength correlations were attributed to the precipitation strengthening behavior of finely dispersed M 23 C 6 carbides and V-type MX carbonitrides, where their precipitation distribution characteristic in the fine-grained HAZ microstructures partially or almost entirely took over those in base metal. This finding implies that the long-term creep rupture strength of mod.9Cr-1Mo steel weldment might be able to be evaluated as long as the corresponding base metal strength is obtained.

Creep strength enhanced ferritic steels like T/P 91 and T/P 92 are widely used for the fabrication of pressure vessel components in the petro-chemical and thermal power industry. Today, a new generation of 9-12% Cr CSEF steels like MARBN, Save12AD, G115 and Super VM12 are entering into the market. All CSEF steels require an accurate post-weld heat treatment after welding. This paper discusses the impact of chemical composition on Ac1 as well as the transformation behavior during post-weld heat treatment in a temperature range below and above Ac1. The Ac1 temperature of weld metals with variations in chemical composition has been determined and thermodynamic calculations has been carried out. Simulations of heat treatment cycles with variations in temperature have been carried out in a quenching dilatometer. The dilatation curves have been analyzed in order to detect any phase transformation during heating or holding at post weld heat treatment. Creep rupture tests have been carried out on P91 and Super VM12 type weld metals in order to investigate the effect of sub- and intercritical post weld heat treatment on creep rupture strength.

Creep strength degradation behavior of normalized and tempered 2.25Cr-1.6W-V-Nb(Gr.23) steel was investigated by conducting extra long-term creep rupture tests. Creep strength drop was observed in long-term creep range at 600°C and above, while signs of a creep strength drop were not identified at 550°C and 575°C. Creep strength drop of around 110 MPa at 600°C was caused not by the effect of oxidation but rather by a change of the deformation mechanism or the weakening of deformation resistance by the microstructural change during creep. The effect of oxidation was significant for causing a further creep strength drop in the range which exceeded 20,000 h in rupture time at 600°C. As a result, the creep strength at 60 MPa and 600°C was almost the same regardless of long tempered or aged steel.

The newly developed 12%Cr steel Super VM12 is characterized by excellent creep rupture strength properties (better than Grade 92) and enhanced steam oxidation resistance of 12%Cr steels such as VM12-SHC. Balanced properties profile of the new steel development in comparison to the existing well-established steels such as Grade 91 and Grade 92, opens opportunities for its application as construction material for components in existing or future high-efficiency power plants. In this study the oxidation behavior of typical 9%Cr steels was compared with the new steel development. The oxide scale morphologies and compositions of different oxide layers as function of temperature and exposure time in steam-containing atmospheres were characterized using light optical metallography, Scanning Electron Microscopy (SEM). Creep testing has been carried out in the temperature range between 525°C and 700°C. Selected creep specimens were investigated using the Transmission Electron Microscopy and the Atom Probe Tomography techniques.

In this study, creep rupture behaviors and rupture mechanisms of dissimilar welded joint between Inconel 617B and COST E martensitic steel were investigated. Creep tests were conducted at 600 ℃ in the stress range 140-240 MPa. Scanning electron microscopy (SEM) and micro-hardness were used to examine the creep rupture behaviors and microstructure characteristics of the joint. The results indicated that the rupture positions of crept joints shifted as stress changed. At higher stress level, the rupture position was located in the base metal (BM) of COST E martensitic steel with much plastic deformation and necking. At relatively lower stress level, the rupture positions were located in the fine-grained heat affected zone (FGHAZ) of COST E or at the interface between COST E and WM both identified to be brittle fracture. Rupture in the FGHAZ was caused by type Ⅳ crack due to matrix softening and lack of sufficient precipitates pinning at the grain boundaries (GBs). Rupture at the interface was related to oxide notch forming at the interface.

Inconel 740H is one of the most promising candidate Ni-base superalloys for the main steam pipe of 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. After processing and welding in manufacturing plant in solution-annealed state, large components was commonly suggested to have an extra aging treatment at 800 °C for 16 h, in order to obtain homogeneous γ′ precipitates. In this present work, creep tests and microstructure analyses were conducted on Inconel 740H pipe specimens under two different heat treatments to verify the necessity of aging process. Here we show that aging treatment has limited effect on the creep rupture life of Inconel 740H pipe. Both in grain interiors and along grain boundaries, crept specimens under two different heat treatments have the same precipitates. But the shape and distribution of γ′ in solution annealed sample is not as regular as the aged ones. Our results provide the underlying insight that aging treatment is not so necessary for the straight pipes if the on-site condition was hard to control. But for both groups of specimens, a small amount of h particles and some banded like M 23 C 6 were emerged during creep, which would be harmful to mechanical properties for the long run.

This paper investigates creep rupture and damage behaviors of HR6W weldment using full thickness specimen cut from the circumferentially welded pipe. Creep tests were conducted at 750°C for durations up to 8,000 hours, and damage morphology of weldment during creep was characterized. The applicability of several nondestructive detection methods to the creep damage evaluation was discussed. It was found that full thickness specimen was broken at the base metal and main crack was inclined approximately at 45 degrees to the axial direction of the specimen. Times to creep rupture of full thickness specimen were comparable with those of the standard specimen. In addition, a small crack in base metal on the outer surface was first observed at life fraction of 35% by replication. PT can detect the crack in about half of the life. The crack whose length is longer than 3mm can be detected by UT in latter half of the life.

18Cr-9Ni-3Cu-Nb-N steel is widely used for heat exchanger tubes such as super-heaters and reheaters of ultra-super critical power generation boilers. In this study, long-term creep rupture tests were carried out on 18Cr-9Ni-3Cu-Nb-N seamless steel tubes of 7 heat materials, and the specimens of 2 heat materials with different creep rupture strengths were observed by ultra-low voltage scanning electron microscope after creep rupture tests. The results of the investigation of the creep rupture specimens and the coverage ratios of M 23 C 6 on grain boundary were different. The cause of this was estimated to be the difference in B content between the 2 heat materials. Creep rupture tests with different final ST temperatures were also carried out using the same heat material, and it was revealed that the higher final ST temperature, the higher the creep rupture strength. As the final ST temperature is higher, the amount of Nb(C, N) solid solution in the matrix increases, and the amount of precipitation of NbCrN and M 23 C 6 increases during creep, therefore it is assumed that the creep rupture strength increases.

Creep properties and microstructural changes of 25Cr-20Ni-Nb-N steel (KA-SUS310J1TB) were investigated. Creep tests were performed under 20MPa to 380MPa at 600°C to 800°C. Time to rupture was from 53.5h to 23950h. At 650°C or higher, creep strength degraded in the long-term. Rupture elongation and reduction of area decreased with increasing time to rupture at 600°C to 800°C. The reduction of area was lower than 12% after creep rupture for more than 10000h. Creep voids and cracks were observed on grain boundaries in creep ruptured samples. The hardness of head portion of creep ruptured samples increased with increasing time to rupture at 600°C to 800°C. The hardness of gauge portion of creep ruptured samples was higher than that of as received sample. However, the hardness of gauge portion does not strongly depend on time to rupture. No precipitates were observed in as received sample. On the other hand, a large number of precipitates were confirmed after creep rupture at 600°C to 800°C. M 23 C 6 , sigma phase, eta nitride and Z phase were detected in creep ruptured samples. The precipitation was confirmed on grain boundaries after short-term creep. The precipitates were also formed inside grains after long-term creep. It was confirmed by optical microscope that the grain boundary seemed to have band-like structure after short-term creep exposure. The Cr depletion zone was detected around grain boundary after short-term creep exposure. The Cr depletion zone can be visible when Cr rich precipitates such as M 23 C 6 and sigma phase are formed on grain boundaries. However, the bandlike structure was not observed after long-term creep exposure because the Cr depletion zone became unclear after long-term creep exposure. Creep voids were formed on grain boundaries and at the interface between precipitates such as M 23 C 6 and sigma phase and matrix.

Advanced martensitic 9% chromium steels have been identified as the most favored group of materials for high temperature applications in thermal power plants. To extend the temperature range of martensitic steels up to 650°C large effort was put on the development of new alloy concepts. The so-called MARBN concept (Martensitic steel with defined Boron/Nitrogen relation) provides increased creep rupture strength due to higher solid solution strengthening and improved microstructural stability. The major improvement is the reduction of type IV cracking in welded joints, which shifts the focus to the creep rupture strength of the weld metal. This paper illustrates the process experience of the steel foundry for production of heavy cast components in latest state of the art 9-12%Cr-MoCoVNbNB-alloyed cast steel grades and the newest state of development and prototype components in MARBN cast steel grades. Metallurgy, solidification, heat treatment and welding are main items to be considered for development of new, complex steel grades for foundry processing with the help of empiric processing in test programs and thermo-physical simulation. As welding is an essential processing step in the production of heavy steel cast components a good out-of-position weldability is required. Moreover a stress-relieve heat-treatment takes place subsequently after welding for several hours. This contribution also deals with the development of matching welding consumables for the production of heavy cast components and discusses the challenges of defining appropriate welding and heat treatment parameters to meet the requirements of sufficient strength and toughness at ambient temperature. Additionally, first results of creep rupture tests are presented.

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.

This paper presents results and analyses from long-term creep-rupture testing of alloy CCA617 (also known as alloy 617B) in wrought and welded forms at temperatures and stresses relevant to power generation under advanced steam conditions. The refined controlled chemical composition of CCA617 resulted in increased creep-rupture strength compared to the conventional alloy 617 chemistry at applied stress levels of ~150 MPa and above. Long-term creep rupture testing of weldments (in one case, over 100,000 h) showed that their creep-rupture lives were dependent on the welding process. Gas-tungsten-arc and shielded metal-arc weldments of CCA617 performed nearly equivalent to standard alloy 617 base metals in creep, but there was some debit in creep-rupture resistance when compared to CCA617 base metal. Submerged arc welding produced weldments that were notably weaker than both versions of alloy 617 base metal under creep conditions, possibly due to lack of optimization of filler wire composition and flux.

Approximately 75% of the worldwide energy supply is based on fossil energy but the discussions on CO 2 emission require improvements of the conventional power technologies and also an increase of renewable energy resources. Over the past 40 years, enormous efforts, especially in the development of new materials, were made to establish the technology for the ultra-supercritical power plants, which are the standard of today’s power generation. For decades voestalpine Boehler Special Steel has been a full package supplier of customized high quality special steels and forgings with close relationships to plant manufacturers to provide products ahead of their time. This paper reports on improvements and research activities of the currently best available martensitic 9% Cr steel FB2 and the latest generation, the so-called MarBN steels, raising the operating temperatures of the 9% Cr steel class from 620 °C to 650 °C. Increasing the operating temperature requires adaptations in processes and manufacturing methods to adjust optimized microstructures with improved toughness properties and increased creep rupture strength at the same time. The microstructure of two Boron containing 9% Cr steels, FB2-2 and NPM1, developed within the framework of COST / KMM-VIN, have been investigated comparatively after different heat treatments and discussed after creep rupture tests at 650°C. The results show a dependency of the creep rupture strength on the stability of precipitates and the creep rupture time of both steels was increased by more than 30 % without negatively affecting the creep rupture strain and impact values.

The reasonable procedures for estimation of 100,000 h creep rupture strength have been investigated for Alloy 617 and Alloy 740 for A-USC power plants by Larson Miller method. The creep rupture data of longer duration than 500 h in the temperature range between 593 and 816 °C and between 600 and 850 °C were used for the analysis on Alloy 617 and Alloy 740, respectively. The data were obtained by Special Metals. In these temperature ranges, Ni3Al-γ’ can precipitate in Alloy 617 and Alloy 740 during creep. The maximum time to rupture was 40,126.7 and 24,066 h for Alloy 617 and Alloy 740, respectively. The rupture data for Alloy 617 exhibit large scattering, especially at 760 °C, showing a split into two groups. After eliminating the shorter time to rupture data at 760 °C, the regression analysis using the second order equation of Larson-Miller parameter gives us the Larson-Miller constant C of 12.70 and the 100,000 h creep rupture strength of 100 MPa at 700 °C. The regression analysis underestimates the constant C and corresponding 100,000 h creep rupture strength of Alloy 617, as shown by the regression curves locating below the rupture data at long times, while those locating above the rupture data at short times. The underestimation of constant C is caused by large data scattering. The linear extrapolation of log tr versus reciprocal temperature 1/T plot to 1/T = 0 at constant stresses gives us the constant C of 18.5, which is much larger than that by the regression analysis. Using an appropriate constant C of 18.45, the 100,000 h creep rupture strength of Alloy 617 is estimated to be 123 MPa at 700 °C. On the other hand, the rupture data for Alloy 740 exhibit only a little bit scattering. The regression analysis gives us C = 18.45, which agrees very well with that by the linear extrapolation of log tr versus 1/T plot to 1/T = 0. The 100,000 h creep rupture strength of Alloy 740 is estimated to be 214 and 109 MPa at 700 and 760 °C, respectively.

A new nickel-based superalloy, designated as GH750, was developed to meet the requirements of high temperature creep strength and corrosion resistance for superheater/reheater tube application of A-USC power plants at temperatures above 750°C. This paper introduces the design of chemical composition, the process performance of tube fabrication, microstructure and the properties of alloy GH750, including thermodynamic calculation, room temperature and high temperature tensile properties, stress rupture strength and thermal stability. The manufacturing performance of alloy GH750 is excellent and it is easy to forge, hot extrusion and cold rolling. The results of the property evaluation show that alloy GH750 exhibits high tensile strength and tensile ductility at room and high temperatures. The 760°C/100,000h creep rupture strength of this alloy is larger than 100MPa clearly. Microstructure observation indicates that the precipitates of GH750 consist of the precipitation strengthening phase γ’, carbides MC and M 23 C 6 and no harmful and brittle TCP phases were found in the specimens of GH750 after long term exposure at 700~850°C. It can be expected for this new nickel-based superalloy GH750 to be used as the candidate boiler tube materials of A-USC power plants in the future.