Search Results for segregation

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.

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 new nickel-base superalloy GH750 has been developed as boiler tube of advanced ultrasupercritical (A-USC) power plants at temperatures about and above 750°C in China. This paper researched the weld solidification of GH750 filler metal, microstructure development and property of GH750 welded joint by gas tungsten arc weld. Liquid fraction and liquid composition variation under non-equilibrium state were calculated by thermo-dynamic calculation. The weld microstructure and the composition in the dendrite core and interdendritic region were analyzed by SEM(EDX) in detail. The investigated results show that there is an obvious segregation of precipitation-strengthening elements during the weld solidification. Titanium and Niobium are the major segregation elements and segregates in the interdendritic region. It was found that the changing tendency of the elements’ segregation distribution during the solidification of GH750 deposit metal is agree with the thermodynamic calculation results. Till to 3,000hrs’ long exposure at 750°C and 800°C, in comparison with the region of dendrite core of solidification microstructure, not only the coarsening and the accumulation of γʹ particles are remarkable in the interdendritic region, but also the small quantity of the blocky and needle like η phases from. The preliminary experimental results indicate that the weakening effect of creep-rupture property of the welded joint is not serious compared with GH750 itself.

The microstructural evolution of the Ni-based superalloy CMSX-4 including the change in gamma prime size and distribution and the degree of rafting has been examined in detail using field emission gun scanning electron microscopy (FEGSEM) and transmission electron microscopy (TEM) after high temperature degradation and rejuvenation heat treatments. The relationship between the microstructure, mechanical properties and the applied heat treatment procedures has been investigated. It is shown that there are significant differences in the rafting behaviour, the size of the ‘channels’ between the gamma prime particles, the degree of rafting and the size of the tertiary gamma prime particles in each of the different microstructural conditions studied. Chemical segregation investigations were carried out to establish the cause of reduced mechanical properties of the rejuvenated sample after high temperature degradation compared to an as-received sample after the same degradation procedure. The results indicate that although the microstructure of as-received and rejuvenated samples were similar, the chemical segregation was more pronounced in the rejuvenated samples, suggesting that chemical segregation from partitioning of the elements during rejuvenation was not completely eliminated. The aim of this research is to provide greater understanding of the suitability of rejuvenation heat treatments and their role in the extension of component life in power plant applications.

To develop 10-ton class forgings with adequate long-term strength and without segregation defects for A-USC steam turbine rotors, researchers modified the chemical composition of Alloy 706 to improve its microstructure stability and segregation properties. The modified Alloy, named FENIX-700, is a γ' phase strengthened alloy without a γ" phase, and its microstructure stability is superior to Alloy 706 at 700°C, as demonstrated by short-term aging tests and phase stability calculations using the CALPHAD method. A trial disk 1-ton class forging of FENIX-700 was manufactured from a double-melted ingot, with tensile and creep strength of the forging equivalent to that of 10-kg class forgings, indicating a successful trial. Long-duration creep tests were performed using 10-kg class forgings, revealing an approximate 105-hour creep strength at 700°C higher than 100 MPa. Manufacturability tests showed that FENIX-700 performs better than Alloy 706, as evidenced by segregation tests using a horizontal directional solidification furnace and hot workability tests. Microstructure observation and tensile tests on 10,000-hour aged specimens (at temperatures of 650, 700, and 750°C) revealed degradation of tensile strength and yield stress due to coarsening of the γ' phase, but also showed enhanced ductility through aging. The microstructure stability of FENIX-700 at 700°C was confirmed as excellent through microstructure observation of the 10,000-hour aged sample and supporting thermodynamic considerations.

To maximize the mechanical properties of Ni-base superalloys, solution heat treatment is essential to sufficiently homogenize the dendritic segregations formed during solidification. To investigate the homogenization behavior during solution heat treatment, a Ni-base single crystal superalloy, TMS-238, was heat treated under various conditions; temperatures ranging from 1573 to 1613 K for times ranging from 2 to 100 h. After solution heat treatment, the average concentrations of Re, an element that exhibits the highest degree of segregation, in dendrite core and inter-dendritic regions were analyzed. From these results, apparent diffusion constants, D app , were determined based on a proposed homogenization model. Obtained D app values were significantly smaller than the diffusion constant of Re in Ni, strongly suggesting that the apparent diffusion coefficients should be obtained experimentally when using the target alloy.

The US Advanced Ultra-Supercritical (A-USC) Consortium conducted an extensive program to evaluate available superalloys for use in rotors for steam turbines operating at a nominal temperature of 760 °C (1400 °F). Alloys such as 282, Waspaloy, 740H, 720Li, and 105 were tested in the form of bar supplied from the alloy producers. Ultimately, alloy 282 was down-selected for the turbine rotor based on its combination of creep strength, phase stability, ductility, and fatigue resistance. The next step in development was to produce a full-size rotor forging for testing. A team was established consisting of GE Power (project management and testing), Wyman-Gordon (forging and testing) and Special Metals (melting and billetizing) to pursue the work. A research license to melt the alloy was obtained from Haynes International. The first step of the development was to devise a triple melt (VIM-ESR-VAR) practice to produce 610 mm (24 inch) diameter ingot. Two ingots were made, the first to define the VAR remelting parameters and the second to make the test ingot utilizing optimum conditions. Careful attention was paid to ingot structure to ensure that no solidification segregation occurred. A unique homogenization practice for the alloy was developed by the US Department of Energy (DOE) and National Energy Technology Laboratory (NETL). Billetization was performed on an open die press with three upset and draw stages. This procedure produced an average grain size of ASTM 3. A closed die forging practice was developed based on compressive flow stress data developed by Wyman Gordon Houston for the consortium project. Multiple 18 kg forgings were produced to define the forging parameters that yielded the desired microstructure. The project culminated with a 2.19 metric ton (4830 lb), 1.22 m (48 inch) diameter crack-free pancake forging produced on Wyman Gordon’s 50,000 ton press in Grafton, MA. The forging process produced a disk with an average grain size of ASTM 8 or finer. Forging cut-up, microstructural characterization, and mechanical property testing was performed by GE Power. Fatigue and fracture toughness values of the disk forging exceeded those previously reported for commercially available rolled bar.

In this study, a possibility of application of advanced 9%Cr steel containing 130 ppm boron for boiler components utilized at around 650 °C to higher temperature steam turbine rotor materials has been investigated by means of reduction in silicon promoting macro-segregation in the case of large size ingots, using laboratory heats. Tempered martensitic microstructure without proeutectoid ferrite in all steels studied is obtained even at the center position of a turbine rotor having a barrel diameter of 1.2 m despite lower amounts of nitrogen and silicon. The strength at room temperature is almost the same level of practical high Cr steels such as X13CrMoCoVNbNB 9-2-1 for ultrasuper critical steam turbine rotors. The toughness is sufficient for high temperature rotors in comparison with CrMoV steels utilized as sub-critical high pressure steam turbine components. The creep rupture strength of the steels is higher than that of the conventional 9-12Cr steels used at about 630 °C. The creep rupture strength of 9%Cr steel containing 130 ppm B, 95 ppm N, 0.07 % Si and 0.05 % Mn is the highest in the steels examined, and it is therefore a candidate steel for high temperature turbine rotors utilized at more than 630 °C. Co-precipitation of M 23 C 6 carbides and Laves phase is observed around the prior austenite grain boundaries after the heat treatments and the restraint of the carbide growth is also observed during creep exposure. An improvement in creep strength of the steels is presumed to have the relevance to the stabilization of the martensitic lath microstructure in the vicinity of those boundaries by such precipitates.

In 9~12% Cr containing martensitic stainless steels, Laves phase usually occurs after long term high temperature exposure, while in the present work, some sparse relatively large particles of (Fe,Cr)2Mo type Laves phase were observed in virgin FB2 steel. It is speculated that the large Laves phase particles formed in casting process due to dendritic segregation. Then the evolutionary behavior of Laves phase during welding thermal cycle was studied and constitutional liquation of Laves phase was found, suggesting a liquation crack tendency in FB2 steel. At last, the hot ductility tests showed that the area where constitutional liquation occurred would act as crack initiation site, and the tested specimen fractured without any obvious plastic deformation. This work provided some guidance for the practical production of welded turbine rotors made of FB2 steel.

The EU NextGenPower-project aims at demonstrating Ni-alloys and coatings for application in high-efficiency power plants. Fireside corrosion lab and plants trials show that A263 and A617 perform similar while A740H outperforms them. Lab tests showed promising results for NiCr, Diamalloy3006 and SHS9172 coatings. Probe trials in six plants are ongoing. A617, A740H and A263 performed equally in steamside oxidation lab test ≤750°C while A617 and A740H outperformed A263 at 800°C; high pressure tests are planned. Slow strain rate testing confirmed relaxation cracking of A263. A creep-fatigue interaction test program for A263 includes LCF tests. Negative creep of A263 is researched with gleeble tests. A263 Ø80 - 500mm trial rotors are forged with optimized composition. Studies for designing and optimizing the forging process were done. Segregation free Ø300 and 1,000mm rotors have been forged. A263 – A263 and A293 – COST F rotor welding show promising results (A263 in precipitation hardened condition). Cast step blocks of A282, A263 and A740H showed volumetric cracking after heat treatment. New ‘as cast’ blocks of optimized composition are without cracks. A 750°C steam cycle has been designed with integrated CO 2 capture at 45% efficiency (LHV). Superheater life at ≤750°C and co-firing is modeled.

Large scale components of the conventional 600°C class steam turbine were made of the ferritic steel, but the steam turbine plants with main steam temperatures of 700°C or above (A-USC) using the Ni-base superalloys are now being developed in order to further improve the thermal efficiency. The weight of the turbine rotor for the A-USC exceeds 10ton. A lot of high strength superalloys for aircraft engines or industrial gas turbines have been developed up to now. But it is difficult to manufacture the large-scale parts for the steam turbine plants using these conventional high strength superalloys because of their poor manufacturability. To improve high temperature strength without losing manufacturability of the large scale components for the A-USC steam turbine plants, we developed Ni-base superalloy USC800(Ni-23Co-18Cr-8W-4Al-0.1C [mass %]) which has temperature capability of 800°C with high manufacturability achieved by controlling microstructure stability and segregation property. The 700°C class A-USC materials are the mainstream of current development, and trial production of 10 ton-class forged parts has been reported. However, there have been no reports on the development and trial manufacturing of the A-USC materials with temperature capability of 800°C. In this report, results of trial manufacturing and its microstructure of the developed superalloy which has both temperature capability 800°C and good manufacturability are presented. The trial manufacturing of the large forging, boiler tubes and turbine blades using developed material were successfully achieved. According to short term creep tests of the large forging and the tube approximate 100,000h creep strength of developed material was estimated to be 270MPa at 700 °C and 100MPa at 800°C.

Monoblock low-pressure (LP) turbine rotor shaft forgings for nuclear power plants have been produced from up to 600 ton ingots. However, ingots greater than 600 tons are necessary to increase the generator capacity. Segregation, non-metallic inclusions, and micro porosities inevitably increase with the increase in ingot size. Manufacturing such massive ingots with high soundness is quite difficult. Thus, the development of 650 ton ingot production was carried out in 2010. The 650 ton ingot was dissected and investigated to verify its internal quality. The internal quality of the 650 ton ingot was found to be equal to that of 600 ton ingots. Subsequently, in 2011, we produced a 670 ton ingot, the world’s largest, to produce a trial LP rotor shaft forging with a diameter of 3,200 mm. Results show that the internal quality, mechanical properties, and heat stability are the same as LP rotor shaft forgings made from 600 ton ingots.

Advanced 700°C-class steam turbines demand austenitic alloys for superior creep strength and oxidation resistance beyond 650°C, exceeding the capabilities of conventional ferritic 12Cr steels. However, austenitic alloys come with a higher coefficient of thermal expansion (CTE) compared to 12Cr steels. To ensure reliability, operability, and performance, these advanced turbine alloys require low CTE properties. Additionally, for welded components, minimizing CTE mismatch between the new material and the welded 12Cr steel is crucial to manage residual stress. This research investigates the impact of alloying elements on CTE, high-temperature strength, phase stability, and manufacturability. As a result, a new material, “LTES700R,” was developed specifically for steam turbine rotors. LTES700R boasts a lower CTE than both 2.25Cr steel and conventional superalloys. Additionally, its room-temperature proof strength approaches that of advanced 12Cr steel rotor materials, while its creep rupture strength around 700°C significantly surpasses that of 12Cr steel due to the strengthening effect of gamma-prime phase precipitates. To assess the manufacturability and properties of LTES700R, a medium-sized forging was produced as a trial run for a turbine rotor. The vacuum arc remelting process was employed to minimize segregation risk, and a forging procedure was meticulously designed using finite element method simulations. This trial production resulted in a successfully manufactured rotor with satisfactory quality confirmed through destructive evaluation.

The addition of boron without the formation of any boron nitrides during normalizing heat treatment at high temperature minimizes the degradation in creep strength of both base metal and welded joints of 9Cr steel at 650 °C and long times. The enrichment of soluble boron near prior austenite grain boundaries (PAGBs) by the segregation is essential for the reduction of coarsening rate of M 23 C 6 carbides in the vicinity of PAGBs, enhancing boundary and subboundary hardening, and also for the production of same microstructure between the base metal and heat-affected-zone (HAZ) in welded joints, indicating no Type IV fracture in HAZ. Excess addition of boron and nitrogen promotes the formation of boron nitrides during normalizing, which reduces the soluble boron concentration and accelerates the degradation in creep rupture ductility at long times. 9Cr- 3W-3Co-VNb steel with 120 - 150 ppm boron and 60 - 90 ppm nitrogen (MARBN) exhibits not only much higher creep strength of base metal than Gr.92 but also substantially no degradation in creep strength due to Type IV fracture at 650 °C. The pre-oxidation treatment in Ar gas promotes the formation of protective Cr 2 O 3 scale on the surface of 9Cr steel, which significantly improves the oxidation resistance in steam at 650 °C.

To improve microstructure stability at temperature up to 700°C and avoid segregation of Nb during melting processes, we modified the chemical composition of conventional Ni-Fe base super alloy(Ni-36Fe-16Cr-3Nb-1.7Ti-0.3Al:Alloy706). It is known that Alloy706 is strengthened by γ'(Ni 3 Al) phase and γ”(Ni 3 Nb) phase. But these phases are unstable at high temperature and transform into Nb rich δ or η) phase after long-term exposure to elevated temperature. Therefore modified alloy contains lower Nb and higher Al than those of Alloy706, and it is mainly strengthened by γ’(Ni 3 Al) phase. In fact we could not find δ or η phase in the modified alloy after creep and aging at 700 °C. Tensile strengths of the modified alloy at temperature from room temperature to 700 °C were almost same as those of Alloy706. Yield strength of modified alloy at room temperature was slightly lower than that of Alloy706, but equivalent to that of Alloy706 at higher temperatures. Tensile and yield strengths of the modified alloy at temperature from room temperature to 700 °C were higher than those of Alloy706 after aging at 700 °C. In this paper, we discuss effects of Nb and Al on mechanical properties and microstructure at elevated temperature up to 700 °C, using mechanical testing, TEM observations and thermodynamics calculation results. And we show advantages of the microstructure stabilized Ni-Fe base superalloy(FENIX-700), which is a candidate material for 700 °C class USC steam turbine rotor.

Hitachi and Hitachi Metals have developed low thermal expansion Ni-base superalloy, Ni-20Cr-10Mo-1.2Al-1.6Ti alloy (USC141) for use as A-USC steam turbine material. The approximate 10 5 h creep rupture strength at 740° C is 100MPa, so USC141 can be expected to apply for blades and bolts. Now we have been studying to get better creep properties by microstructure controlling such as grain size or grain boundary morphology. In addition, the segregation test of USC141 shows good Freckle tendencies, it means that it would be easy to make a large ingot which could be used as rotors or pipes. From these calculation results, we have been tried to make an 850mmϕ ESR ingot of USC141.

A modified version of Alloy 706, designated FENIX-700, was developed using the CALPHAD method to improve high-temperature stability above 700°C. The new alloy features reduced Nb and increased Al content, relying on γ' (Ni 3 Al) strengthening while eliminating γ'' (Ni 3 Nb), δ, and η phases. This modification improved both creep temperature capability (from 650°C to 700°C) and segregation properties. Successful manufacturing trials included a 760 mm² forging shaft using triple melt processing and a 1050 mm ESR ingot, demonstrating industrial viability. The study also explores compatible Ni-base welding materials for joining FENIX-700 to 12% Cr ferritic steel in 700°C class steam turbine applications.

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.

Microstructure change during creep at 650°C has been examined for a high-B 9%Cr steel by FIB-SEM serial sectioning 3D observation, Nano-SIMS, SEM, EBSD and TEM. The precipitates formed in the steel were M 23 C 6 , Laves phase, and a quite small amount of MX. For as-tempered steel, precipitation of M 23 C 6 on the prior austenite grain boundaries was clearly found, while precipitation of the Laves phase was not confirmed during tempering. The volume fraction of the Laves phase gradually increased with elapsed time, while M 23 C 6 appeared to increase once and decrease afterward, based on the comparison between the 2,754 h ruptured sample and the 15,426 h ruptured sample. Nano-SIMS measurements have revealed that B segregates on the prior austenite grain boundaries during normalizing, and it dissolves into M 23 C 6 .

In order to understand the microstructural evolution during service that 9Cr steels experience it is important to be able to quantify key microstructural parameters that define the characteristics of the secondary phases (e.g. precipitated phases and inclusions) and the steel matrix. The average size of M 23 C 6 , Laves phase and MX particles in these materials have been reported in many studies, however comparability between these studies is compromised by variations in technique and different/incomplete reporting of procedure. This paper provides guidelines on what is required to accurately measure these parameters in a reproducible way, taking into account macro-scale chemical heterogeneities and the statistical number of particles required to make meaningful measurements. Although international standards do exist for inclusion analysis, these standards were not developed to measure the number per unit area of hard particles that can act as creep cavity nucleation sites. In this work a standardized approach for measuring inclusions from this perspective is proposed. In addition the associated need to understand the segregation characteristics of the material are described, which in addition to defining the area that needs to be analysed to measure the average number of inclusions per unit area, also allows the maximum number of inclusions per unit area to be determined, a parameter which is more likely to define the damage tolerance of the material.