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.

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).

HR6W (23Cr-44Ni-7W) is a candidate material for application in the maximum temperature locations of A-USC boilers. In this study the creep rupture properties of plastic deformed, notched, and weldment materials were investigated in comparison with those of solution treated material, in order to clarify the capability of HR6W as a material for A-USC plant application. The deterioration of long term creep rupture strength has been reported with respect to metastable authentic stainless steel due to cold working. However the creep strength of the 20% pre-strained HR6W increased. HR6W creep strength showed notch strengthening behavior. The creep ruptured strength of the GTAW joints was nearly the same as that of the solution treated material, and all specimens fractured within the base metal. The creep ductility of the solution treated materials decreased under low stress conditions. The intergranular fracture is considered to be caused of ductility drop. This tendency is the same as for austenitic stainless steel. The potential of HR6W as a material for A-USC was revealed from the standpoint of creep rupture properties.

In this paper, the dissimilar metal welds (DMWs) between 617B nickel-based alloy and 10%Cr martensitic heat-resistant steel filled by 617 filler metal was studied, focused on the high temperature creep rupture properties. The high temperature creep rupture properties of welded joints with different welding processes were tested, and the microstructure of welded joints before and after the creep rupture test was observed by OM and SEM. The results showed that, there were three failure modes: base metal failure, type W failure and interface failure, among which interface failure caused the most serious life reduction. The welded joints using ER NiCr-3 filler metal reduced the strain concentration at the interface, so the fracture location shifted from the interface to HAZ of 10%Cr martensitic heat-resistant steel under high temperature and low stress conditions, and creep rupture life was improved. Similarly, weld cap shifted the creep crack propagation path by changing the groove form, so as to altered the stress state of joint and prolong the creep rupture life.

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.

A new methodology challenges the conventional use of a constant C-value in the Larson-Miller Parameter (LMP) for 9-12% Cr ferritic steels, proposing instead a multi-C region analysis to address creep strength breakdown issues. Using NIMS data and other publications, the study demonstrates that C-values vary both between steel types and across stress regions. The new approach enables prediction of long-term (10 5 hours) creep rupture properties using only short-term (5×10 3 hours) test data, while d[g(σ)]/d[P(t r ,T)] versus P(t r ,T) analysis provides insight into property stability. This methodology offers a more cost-effective and accurate approach to acquiring and assessing long-term creep rupture data for these heat-resistant steels.

Creep deformation and rupture properties of several long-term used Super 304H steel boiler tubes were presented in this paper. The aged superheater tubes that have been in service for about 140,000 hours at the approximate metal temperature ranged from 550°C to 640°C, were investigated. Creep tests were conducted at 650°C and 700°C using standard and miniature specimens taken from the axial and circumferential directions of tubes, and effects of specimen size, sampling direction and position on creep properties were discussed. Creep deformation of long-term used materials with significant microstructural evolution accelerated earlier than that of virgin material, and the time to creep rupture and the fracture ductility were also smaller. The degradation of rupture properties of the long-term used material was discussed in relation with microstructural evolution. In addition, there was little effects of specimen size and sampling direction on creep deformation and rupture time, whereas the time to creep rupture changed significantly due to the sampling position.

The operating temperature of ultrasupercritical (USC) power plants is increasing, with planned temperatures reaching up to 700°C. Austenitic superalloys are promising alternatives to ferritic heat-resistant steels due to their potential for high strength at temperatures around 650-700°C. While austenitic nickel-base superalloys generally exhibit higher creep rupture strength than ferritic heat-resistant steels, they also have drawbacks, including higher thermal expansion, lower creep rupture ductility, and increased costs. Initially, the researchers focused on developing a molybdenum-containing superalloy to achieve low thermal expansion. They systematically investigated the effects of molybdenum and cobalt content, gamma prime phase amount, and aluminum/titanium ratio on thermal expansion, tensile properties, and creep-rupture properties. These investigations were conducted using the conventional molybdenum-containing Alloy 252 as a reference. The developed superalloy, notably free of cobalt and combined with a modified heat treatment, demonstrated significantly improved creep rupture elongation compared to Alloy 252, while maintaining low thermal expansion and high creep rupture strength similar to the reference alloy. Additionally, the research evaluated creep-rupture properties at 700°C for up to approximately 20,000 hours to assess long-term applications. The study also examined the weldability and mechanical properties of weld joints at 750°C, focusing on potential boiler tube applications.

In this study, the role of minor alloying additions in 347H stainless steels (UNS34709, ASTM A240/240M) on creep-rupture properties at 650-750°C and microstructure evolution during isothermal exposure at 750°C has been investigated, aiming to provide the experimental dataset as boundary conditions of physics-based modeling for material/component life prediction. Four different 347H heats containing various amounts of boron and nitrogen additions were prepared and evaluated. The combined additions of B and N are found to stabilize the strengthening secondary M 23 C 6 carbides and retarding the transition from M 23 C 6 to sigma phase precipitates during thermal exposure. The observed kinetics of microstructure evolution reasonably explains the improvement of creep-rupture properties of 347H stainless steels with the B and N additions.

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.

SUPER304H (18Cr-9Ni-3Cu-Nb-N, ASME CC2328) and TP347HFG (18Cr-12Ni-Nb, ASME SA213) are advanced fine-grained microstructure steel tubes developed for high strength and superior steam oxidation resistance. Their exceptional performance is demonstrated by the longest creep rupture tests, with SUPER304H tested at 600°C for 85,426 hours and TP347HFG at 700°C for 55,858 hours, both maintaining stable strength and microstructure with minimal σ phase formation and absence of other brittle phases compared to conventional austenitic stainless steels. HR3C (25Cr-20Ni-Nb-N, ASME CC2115) was specifically developed for high-strength, high-corrosion-resistant steel tubes used in severe corrosion environments of ultra-supercritical (USC) boilers operating at steam temperatures around 600°C. The longest creep test for HR3C, conducted at 700°C and 69 MPa for 88,362 hours, confirmed its high and stable creep strengths and microstructural integrity across the 600-800°C temperature range. These innovative steel tubes have been successfully installed in the Eddystone No. 3 USC power plant as superheater and reheater tubes since 1991, with subsequent microstructural investigations after long-term service exposure revealing their remarkable performance. The paper provides an up-to-date analysis of the long-term creep rupture properties and microstructural changes of these steels following extended creep rupture and aging processes, highlighting their successful application as standard materials for superheater and reheater tubes in newly constructed ultra-supercritical boilers worldwide.

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.

The creep enhanced low alloy steel with 2.25Cr-1.6W-V-Nb (HCM2S; Gr.23, ASME CC2199) has been originally developed by Mitsubishi Heavy Industries, Ltd. and Sumitomo Metal Industries, Ltd. The steel tubes and pipe (T23/P23) are now widely used for fossil fired power plants all over the world. Recently, the chemical composition requirements for ASME Code of the steel have been changed and a new Code Case 2199-4 has been issued with the additional restriction regarding Ti, B, N and Ni, and the Ti/N ratio incorporated. In this study, the effects of additional elements of Ti, N and B on the mechanical properties and microstructure of T23/P23 steels have been evaluated. It is found that N decreases the hardenability of the steel by forming BN type nitride and thus consuming the effective B, which is a key element for hardening of the steel. The addition of Ti, on the other hand, enhances the hardenability of the steel by precipitating TiN and thus increasing the effective B. It is also found that too much addition of Ti degrades the Charpy impact property and creep ductility of the steel to a great extent. This phenomenon might affect the steel's long-term creep rupture properties, although a steel with the original chemical composition has demonstrated high creep strength at temperatures up to 600°C for more than 110,000 h.

The mechanical behavior of a cast form of an advanced austenitic stainless steel, CF8C-Plus, is compared with that of its wrought equivalent in terms of both tensile and creep-rupture properties and estimated allowable stress values for pressurized service at temperatures up to about 850°C. A traditional Larson-Miller parametric model is used to analyze the creep-rupture data and to predict long-term lifetimes for comparison of the two alloy types. The cast CF8C-Plus exhibited lower yield and tensile strengths, but higher creep strength compared to its wrought counterpart. Two welding methods, shielded-metal-arc welding (SMAW) and gas-metal-arc welding, met the weld qualification acceptance criteria in ASME BPVC Section IX for the cast CF8C-Plus. However, for the wrought CF8C-Plus, while SMAW and gas-tungsten-arc welding passed the tensile acceptance criteria, they failed the side bend tests due to lack of fusion or weld metal discontinuities.

Grade 91 creep strength-enhanced ferritic steel is a critical material in power generation, widely used for high-temperature, high-pressure tubing and piping applications. Its superior elevated-temperature strength derives from a distinctive microstructure of tempered martensite with uniformly dispersed secondary phases (carbides and carbo-nitrides). This microstructure, crucial for reliable service performance, is achieved through precise control of the manufacturing process, including steelmaking, hot forming, and final heat treatment. This investigation builds upon earlier research into the relationship between manufacturing parameters and short-term creep-rupture properties in T91 tubes, and a recent update that included test results exceeding 30,000 hours. This study presents a comprehensive metallurgical analysis of ruptured test specimens. The investigation focuses on correlating manufacturing parameters with not only creep strength but also material ductility and microstructural evolution during long-term exposure, providing valuable insights into the material’s behavior under extended service conditions.

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.