Seeking to reduce CO 2 emissions and improve power generation efficiency, a project to develop a 700°C A-USC (advanced ultra super critical) power plant has been under way in Japan since 2008. HR6W (44Ni-23Cr-7W) is a candidate material for application in the maximum temperature areas of A-USC boilers. In this study, the creep rupture properties of plastic deformed material were investigated in comparison with those of solution treated material, in order to clarify the capability of HR6W as a material for use in A-USC plants. The creep strength of 20% pre-strained HR6W was found to increase substantially as compared with the solution treated material. 20% pre-strained material is in a state where high dislocation density has been introduced by plastic forming strain, with M 23 C 6 and Laves phase precipitating preferentially by dislocation diffusion from the early stages of creep. In particular, since high dislocation density is accumulated in connection with creep deformation near the grain boundaries, precipitation is accelerated and the grain boundaries are covered with M 23 C 6 from the early stages of creep. Then, even though the intragranular precipitate density decreases, given that the fraction of grain boundaries affected by precipitation is maintained in a high state, it is presumed that a high density of dislocation is maintained in the long-term region. This was considered to be the reason why the creep rupture strength of the 20% pre-strained material increased so remarkably in comparison with the solution treated material.

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.

Low thermal expansion precipitation strengthening Ni-base superalloy, Ni-20Cr-10Mo-1.2Al-1.6Ti alloy (USC141TM), was developed for 700°C class A-USC steam turbine material by Hitachi, Ltd and Hitachi Metals, Ltd. USC141 is usually solution treated and then aged to increase high temperature strength for turbine blades and bolts. As the estimated 105h creep rupture strength at 700°C is about 180MPa, USC141 could also be expected to apply for boiler tubes. On the other hand, this alloy seems to be only solution treated to apply for boiler tubes because tubes are usually jointed by welding and bended by cold working and thus tube alloys should have low hardness before welding and bending and should be used as solution treated. In this study, the creep properties of USC141 as solution treated was evaluated, and the results and microstructures after creep tests were compared with those as aged. As a result, USC141 as solution treated exhibited almost as same creep rupture properties as that as aged because precipitation at grain boundaries and in grains gradually increased at testing temperatures around 700°C. Furthermore seamless tubes of USC141 were produced and various properties including creep properties are now being evaluated.

The use of creep strength-enhanced ferritic alloys like Grade 91 has become popular in fossil power plants for applications at temperatures above 566°C (1050°F). Compared to Grades 11 and 22, Grade 91 offers higher stress allowables, better ramp rate tolerance, weight reduction, and lower thermal expansion coefficients at operating temperatures. However, Grade 91's superior elevated temperature strength requires specific microstructure and metallurgical considerations. This paper highlights concerns that warrant further investigation. Initial operating stresses in Grade 91 piping systems may exceed 262 MPa (38 ksi), and lack of creep relaxation below 593°C (1050°F) could lead to weldment failures within years, especially above 159 MPa (23 ksi) after one year. While cold spring can reduce initial stresses for systems below 593°C (1050°F), creep relaxation rates up to 206 MPa (30 ksi) need study. Above 593°C (1050°F) and below 103 MPa (15 ksi), weldments may fail prematurely by Type IV creep mechanism. Long-term creep rupture studies on cross-weld and multiaxially loaded thick-walled specimens should evaluate deteriorated weldment properties, particularly below 103 MPa (15 ksi).

The creep rupture properties of welded joints of advanced 9%Cr-Mo-Co-B steel used for 620°C USC steam turbine have been studied. The welded joints were prepared by means of shielded metal arc welding (SMAW). A lot of creep tests have been conducted and the results indicate that fracture usually occurs in the intercritical heat affected zone (ICHAZ) of the welded joint and is typical of Type IV cracking. The microstructure of the HAZ has been investigated by using optical microscopy, SEM and TEM. The degradation mechanism of welded joint of the 9%Cr-Mo-Co-B steel has been explored by analysing the phases of precipitates. Creep voids were observed in the vicinity of the coarse Laves phase particles, resulting in the degradation of the creep rupture properties.

New Fe-base ferritic alloys based on Fe-30Cr-3Al-Nb-Si (wt.%) were proposed with alloy design concepts and strategies targeted at improved performance of tensile and creep-rupture properties, environmental compatibilities, and weldability, compared to Grade 91/92 type ferritic-martensitic steels. The alloys were designed to incorporate corrosion and oxidation resistance from high Cr and Al additions and precipitate strengthening via second-phase intermetallic precipitates (Fe2Nb Laves phase), with guidance from computational thermodynamics. The effects of alloying additions, such as Nb, Zr, Mo, W, and Ti, on the properties were investigated. The alloys with more than 1 wt.% Nb addition showed improved tensile properties compared to Gr 91/92 steels in a temperature range from 600-800°C, and excellent steam oxidation at 800°C as well. Creep-rupture properties of the 2Nb-containing alloys at 700°C were comparable to Gr 92 steel. The alloy with a combined addition of Al and Nb exhibited improved ash-corrosion resistance at 700°C. Additions of W and Mo were found to refine the Laves phase particles, although they also promoted the coarsening of the particle size during aging. The Ti addition was found to reduce the precipitate denuded zone along the grain boundary and the precipitate coarsening kinetics.

Haynes 282 alloy is a relatively new Ni-based superalloy that is being considered for advanced ultrasupercritical (A-USC) steam turbine casings for steam temperatures up to 760°C. Weld properties are important for the turbine casing application, so block ingots of Haynes 282 alloy were cast for properties studies. Good, sound welds were produced using Haynes 282 weld-wire and a hot gas-tungsten-arc welding method, and tensile and creep-rupture properties were measured on cross-weld specimens. In the fully heat-treated condition (solution annealed + aged), the tensile properties of the welded specimens compare well with as-cast material. In the fully heat-treated condition the creep-rupture life and ductility at 750°C/250MPa and 800°C/200MPa of the cross-weld specimens are similar to the as-cast base metal, and repeat creep tests show even longer rupture life for the welds. However, without heat-treatment or with only the precipitate age-hardening heat-treatment, the welds have only about half the rupture life and much lower creep ductility than the as-cast base metal. These good properties of weldments are positive results for advancing the use of cast Haynes 282 alloy for the A-USC steam turbine casing application.

In order to develop an Fe/Ni dissimilar-weld rotor structure for an Advanced Ultra Super Critical turbine, fundamental studies on the metallurgical properties of Fe/Ni welds are needed. In the work reported in this paper, we studied the microstructure evolution and creep rupture properties of Fe/Ni weld joints with different compositions. Investigation of thermally aged Fe/Ni diffusion couples revealed that Fe-based ferritic steel and Alloy 617 weld joints with a large difference in Cr content showed strong C diffusion at the weld interface. This decreased the creep rupture life of the weld joint, caused by coarsening of a martensitic structure near the interface. Analysis using Fe/Ni diffusion couples and thermodynamic calculations suggested that the driving force of C diffusion is the chemical potential gradient at the interface, and the difference in Cr content between Fe and Ni accelerates the C diffusion.

Continuous and extensive works have been going to develop 700°C A-USC (Advanced Ultra Super Critical) power plants worldwide. Since Japanese national project launched in 2008, Ni based alloy HR6W (45Ni-24Fe-23Cr-7W-Ti, ASME Code Case 2684) was selected as one of the promising candidate materials of A-USC boiler tube and pipe for long-term creep strength evaluation and field exposure test. In the present study, to establish the creep damage and life assessment method for Ni based alloy component, long-term creep rupture properties, microstructural stability, and creep damage morphology of HR6W weldment were experimentally investigated. Creep tests of HR6W weldment were conducted at temperature range of 700 to 800°C for durations up to 70,000 hours. Failure behavior of creep void formation and creep crack growth was identified, and damage mechanism of weldment during creep were discussed and characterized. Furthermore, uniaxial interrupted creep tests were carried out, the creep damage evaluation was conducted and life assessment approach was proposed based on the metallographic quantification evaluation of creep void and microstructure evolution. It demonstrated the possibility and validity to evaluate creep damage of Ni based alloy component with creep void and microstructure parameters.

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.

In order to reduce CO 2 emissions and improve power generation efficiency, a development project involving an advanced USC (A-USC) plant has been carried out in Japan since 2008. Nibased alloys are candidate materials for boiler components with high temperature steam conditions, which are much stronger than conventional heat resistant steel. However, Ni-based alloys have never been applied with respect to the high pressure parts and thick walled components of USC coal-fired power plants. In this study, therefore, fabrication and characteristic properties, such as weldability, the weld joint and bent part properties, and weld cracking susceptibilities of Ni-based alloys such as HR6W, HR35 and two types of Alloy617 (High B and Low B) pipes were evaluated. Additionally, two types of HR6W header mock-ups and a HR6W tube element mock-up were fabricated. With the exception of Alloy617 (High B), the fabrication trials of Ni-based alloy pipes were conducted successfully, and the long-term creep strength of weldments and bends of Ni-based alloy pipes were found to be nearly equivalent to those of base metal. In the case of Alloy617 (High B), hot cracking was observed.

A Japanese national project has been undertaken since Aug. 2008 with the objective of developing an advanced ultra-supercritical power plant (A-USC) with a steam temperature of 700°C. Fe-Ni and Ni-based alloys, namely HR6W, HR35, Alloy617, Alloy740, Alloy263 and Alloy141, were taken as candidate materials for piping and superheater/reheater tubes in an A-USC boiler. Weldments of these alloys were manufactured by GTAW, after which long term creep rupture tests were conducted at 700°C, 750°C and 800°C. Weldments of HR6W, HR35 and Alloy617 showed similar creep strength as compared with these base metals. Weldments of Alloy740 tended to fail in the HAZ, and it is considered that voids and cracks preferentially formed in the small precipitation zone along the grain boundary in the HAZ. The creep strength of Alloy263 in weldments exhibited the highest level among all the alloys, although HAZ failure occurred in the low stress test condition. A weld strength reduction factor will be needed to avoid HAZ failure in Alloy740 and Alloy263. Also, to prevent premature failure in weld metal, optimization of the chemical composition of weld filler materials will be required.

SMST is producing Ni alloy Boiler tubes since more than 10 years with application in several test loops and R&D programs. This paper will give an overview about the experience with the common grades A617 as well as C263 plus some additional information on the new developed austenitic material “Power Austenite MoW”.

Austenitic heat resistant steels are one of the most promising materials to be applied around 650°C, due to its superior creep strength than conventional ferritic steels and lower material cost than Ni based superalloys. The problem of austenitic steels is its high thermal expansion coefficient (CTE), which leads to high deformation and stress when applied in rotors, casings, blades and bolts. To develop low CTE austenitic steels together with high temperature strength, we chose the gamma-prime strengthened austenitic steel, A-286, as the base composition, and decreased the CTE by introducing the invar effect. The developed alloy, Fe-40Ni-6Cr-Mo-V-Ti-Al-C-B, showed low CTE comparable to conventional ferritic steels. This is mainly due to its high Ni and low Cr composition, which the invar effect is prone even at high temperature region. This alloy showed higher yield strength, higher creep rupture strength and better oxidation resistance than conventional high Cr ferritic steels and austenitic steels. The 2 ton ESR ingot was forged or hot rolled without defects, and the blade trial manufacturing was successfully done. This alloy is one of the best candidates for USC and A-USC turbine components.

Recently, a γ’ precipitation strengthened Ni-base superalloy, USC141, was developed for 700°C class A-USC boiler tubes as well as turbine blades. In boiler tube application, the creep rupture strength of USC141 was much higher than that of Alloy617, and the 105 hours’ creep rupture strength of USC141 was estimated to be about 180MPa at 700°C. This is because fine γ’ particles precipitate in austenite grains and some kinds of intermetallic compounds and carbides precipitate along austenite grain boundaries during creep tests. Good coal ash corrosion resistance is also required for tubes at around 700°C. It is known that coal ash corrosion resistance depends on the contents of Cr and Mo in Ni-base superalloys. Therefore the effect of Cr and Mo contents in USC141 on coal ash corrosion resistance, tensile properties, and creep rupture strengths were investigated. As a result, the modified USC141 containing not less than 23% Cr and not more than 7% Mo showed better hot corrosion resistance than the original USC141. This modified alloy also showed almost the same mechanical properties as the original one. Furthermore the trial production of the modified USC141 tubes is now in progress.

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.

Ni-38Cr-3.8Al has high hardness and high corrosion resistance with good hot workability, and therefore, it has been applied on various applications. However, in order to expand further application, it is important to understand the high temperature properties. Then, this study focused on the high temperature properties such as thermal phase stability, hardness, tensile property, creep property and hot corrosion resistance. As the result of studies, we found that the thermal phase stability of (γ/α-Cr) lamellar structure and the high temperature properties were strongly influenced by the temperature. Although the high temperature properties, except for creep property, of Ni-38Cr-3.8Al were superior to those of conventional Ni-based superalloys, the properties were dramatically degraded beyond 973 K. This is because the lamellar structure begins to collapse around 973 K due to the thermal stability of the lamellar structure. The hot corrosion resistance of Ni-38Cr-3.8Al was superior to that of conventional Ni-based superalloys, however, the advantage disappeared around 1073 K. These results indicate that Ni-38Cr-3.8Al is capable as a heat resistant material which is required the hot corrosion resistance rather than a heat resistant material with high strength at high temperature.

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.

A new 9%Cr steel with high boron levels (boron steel) has been developed by optimization studies on steels and alloys that are applicable to advanced ultra-super critical power plants operated at steam conditions of 700°C and 30 MPa and above. The composition and heat treatment condition of boron steel was optimized by the initial hardness, tensile strength, yield strength, and Charpy impact values on the basis of the fundamental investigation with the stability of the long-term creep strength. Creep testing of boron steel was conducted at temperatures between 600 and 700°C. The creep rupture strength at 625°C and 105 h is estimated to be 122 MPa for the present 9% Cr steel with high boron by Larson-Miller parameter method. Furthermore, physical properties as a function of temperature, metallurgical properties, tensile properties, and toughness were examined to evaluate the applicability of the steel for a 625°C USC power plant boiler. It was also confirmed that the steel has good workability for such an application by the flaring and flattening tests with tube specimens having an outer diameter of approximately 55 mm.

Inconel alloy 740H is designated for boiler sueprheater/reheater tubes and main steam/header pipes application of advanced ultra-supercritical (A-USC) power plant at operating temperatures above 750°C. Microstructure evolution and precipitates stability in the samples of alloy 740H after creep-rupture test at 750°C, 800°C and 850°C were characterized in this paper by scanning electron microscopy, transmission electron microscopy and chemical phase analysis in details. The phase compositions of alloy 740H were also calculated by thermodynamic calculation. The research results indicate that the microstructure of this alloy keeps good thermal stability during creep-rupture test at 750°C, 800°C and 850°C. The precipitates are MC, M 23 C 6 and γ′ during creep-rupture test. The temperature of creep test has an important effect on the growth rate of γ′ phase. No harmful and brittle σ phase was found and also no γ′ to η transformation happened during creep. Thermodynamic calculations reveal almost all the major phases and their stable temperatures, fractions and compositions in the alloy. The calculated results of phase compositions are consistent with the results of chemical phase analysis. In brief, except of coarsening of γ′, Inconel alloy 740H maintains the very good structure stability at temperatures between 750°C and 850°C.