Search Results for austenitic heat-resisting steel

For raising thermal efficiency and decreasing CO 2 emission, China had constructed the first 600°C ultra-supercritical(USC) fossil power plant in 2006. Now more than a hundred 600°C, 1000MW USC electric power units have been put in service. Recently, China has also developed 620°C USC power units and some of them have been put in service already. Meanwhile, more than fifty 620°C USC boilers will be produced by various China boiler companies. The austenitic steels TP347H, Super304H and HR3C are routinely used for 600°C USC boilers. Among these steels, a big amount of Super304H has been used for boiler superheater/reheater components application. However, Super304H is characterized by good stress-rupture strength but poor corrosion/oxidation resistance. On the other side, HR3C is characterized by very good corrosion/oxidation resistance but lower stress-rupture strength than Super304H. Now, the China 620°C USC project needs a new austenitic heat resisting steel with high stress-rupture strength and good corrosion/oxidation resistance to fulfill the superheater/reheater tube components application requirement. A new austenitic heat resisting steel SP2215 is based on 22Cr-15Ni with certain amount of Cu and also Nb and N for multiphase precipitation (MX, Cu-rich phase, NbCrN) strengthening in Fe-Cr-Ni austenitic matrix and M 23 C 6 carbide precipitation at grain boundaries. This SP2215 new austenitic steel is characterized by high stress-rupture strength (650°C, 105h>130MPa) and good corrosion/oxidation resistance. SP2215 austenitic steel has been commercially produced in tube product form. This SP2215 new austenitic heat-resisting steel is recommended to be used as superheater/reheater components for 620°C USC boiler application.

The precipitation behavior of various phases in austenitic heat-resistant model steels, including the Fe 2 Nb Laves phase (C14 structure) on grain boundaries (GB) and grain interiors (GI), and the Ni 3 Nb metastable γ“ phase and stable δ phase on GI, was investigated through experimental study at different temperatures and thermokinetic calculation. The steel samples were prepared by arc melting followed by 65% cold rolling. Subsequently, the samples were solution treated within the γ single-phase region to control the grain size to approximately 150 μm. Aging of the solution-treated samples was carried out at temperatures ranging from 973 K to 1473 K for up to 3600 hours. Microstructural observations were conducted using FE-SEM, and the chemical compositions of the γ matrix and precipitates of Laves and δ phases were analyzed using EPMA. The precipitation modeling was performed using MatCalc software, utilizing a thermodynamic database constructed by our research group to calculate the chemical potential of each phase. Classical nucleation theory was applied for nucleation, while the SFFK model was employed for the growth and coarsening stages. Distinct phases were defined for grain boundary and grain interior Laves phase, with all precipitates assumed to have spherical morphology in the calculations. The precipitation start time was defined as the time when the precipitate fraction reached 1%. Experimental results indicated that above 973 K, Laves phase nucleation primarily occurred on grain boundaries before extending into the grain interior, with the nose temperature located around 1273 K. To replicate the experimentally determined Time-Temperature-Precipitation (TTP) diagram, interaction parameters among elements were adjusted. Additionally, by introducing lower interfacial energy between the γ matrix and Laves phase, the TTP diagram was successfully reproduced via calculation, suggesting relative stability at the interface.

In the present study, the precipitation kinetics of topologically close-packed (TCP) Fe 2 Nb Laves and geometrically close-packed (GCP) Ni 3 Nb phases is studied quantitatively in experimental alloys with different Ta / Nb+Ta ratio, to clarify the mec4hanism of the Ta effect. The microstructure of alloys is changed from Widmanstätten structure to lamellar structure due to discontinuous precipitation, with increasing Ta / Nb+Ta. It is confirmed that Ta partitions into both Fe 2 Nb Laves and Ni 3 Nb phases. However, two phases stability is changed by added Ta content. Ta accelerates the formation kinetics of the precipitates at grain boundaries, as well as γ“-GCP phase within grain interiors, due to increased supersaturation by Ta addition. Besides, Ta retards the transformation kinetics of metastable γ“-Ni 3 Nb to stable the δ-Ni 3 Nb phase. The results indicate that Ta decreases the driving force for the transformation of the δ-GCP phase.

The delivery state of austenitic heat resistant steel boiler tubes is paramagnetic, such as TP304H, TP347H and S30432, the material state, however, appears obviously magnetic after long-time high-temperature service. Vibrating Sample Magnetometer (VSM) has been employed to test the magnetism difference after high-temperature service, and XRD, SEM, TEM, SAED and EDS has been adopted to observe and analyze their microstructure, phase structure and composition. The research results show that compared with the delivery state, the lath α´-Martensite and sometimes the lamellar ε-Martensite will occur in areas adjacent to grain boundaries due to martensite transformation in the microstructure of austenitic heat resistant steel boiler tube after high temperature service. There are high density dislocations tangled together in the substructure of α´-Martensite, and lamellar stacking faults arrayed orderly by a large number of dislocations in the substructure of ε-Martensite. The magnetism of α´-Martensite, its internal stress and carbides is the reason why the austenitic heat resistant steel boiler tubes appear obviously magnetic after high temperature service, and the α´-Martensite plays a major role.

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.

This overview paper contains a part of structure stability study on advanced austenitic heat-resisting steels (TP347H, Super304H and HR3C) and Ni-base superalloys (Nimonic80A, Waspaloy and Inconel740/740H) for 600-700°C A-USC fossil power plant application from a long-term joint project among companies, research institutes and university in China. The long time structure stability of these advanced austenitic steel TP347H, Super304H, HR3C in the temperature range of 650-700 °C and Ni-base superalloys Nimonic80A, Waspaloy and Inconel740/740H in the temperature range of 600-800°C till 10,000h have been detailed studied in this paper.

In this study, we have examined the creep of a novel austenitic heat resistant steel of Fe-20Cr- 30Ni-2Nb (at.%) steel at 1073K in steam and air atmospheres. Our studied steels were Fe-20Cr- 30Ni-2Nb (base steel) and that with 0.03 at. %B (B-doped steel) . The addition of boron is to intentionally increase the area fraction of Laves phase on grain boundaries (ρ). The specimen with ρ = 43% (base steel pre-aged at 1073 K/240 h) exhibits the rupture life of 262 h, whereas the rupture life of the specimen with higher ρ of 80% (B-doped steel pre-aged at 1073 K/240 h) is 833h, which is about three times longer than that of the specimen with ρ = 43%. The specimen with ρ = 80% exhibits smaller creep rate than those with lower ρ than 43% in the entire creep stage. In addition, all specimens show the creep rupture strain of about 60%. The creep rupture life is almost same to that tested under air, whereas the creep rupture strain is slightly smaller (a few percent) than that under air. In the surface of the creep ruptured specimen in steam, the intergranular oxides associated with voids or cavities are often present and grow along grain boundaries to over 100 μm in depth. The intergranular oxidation occurs more extensively in steam rather than air. These results demonstrate that stable Fe 2 Nb Laves phase on grain boundary could increase the creep resistance of the present steel at 1073K without ductility loss in steam as well as air, resulting in the pronounced extension of rupture life. The intergranular oxidation accelerated by steam would not give a serious effect on the creep properties of the present steel below 103 hours in rupture life.

Energy requirements and environmental concerns have promoted a development in higher-efficiency coal fired power technologies. Advanced ultra-super critical power plant with an efficiency of higher than 50% is the target in the near future. The materials to be used due to the tougher environments become therefore critical issues. This paper provides a review on a newly developed advanced high strength heat resistant austenitic stainless steel, Sandvik Sanicro 25, for this purpose. The material shows good resistance to steam oxidation and flue gas corrosion, and has higher creep rupture strength than any other austenitic stainless steels available today, and has recently obtained two AMSE code cases. This makes it an interesting option in higher pressures/temperature applications. In this paper, the material development, structure stability, creep strength, steam oxidation and hot corrosion behaviors, fabricability and weldability of this alloy have been discussed. The conclusion is that the Sanicro 25 is a potential candidate for superheaters and reheaters in higher-efficiency coal fired boilers i.e. for applications seeing up to 700°C material temperature.

This study examines the influence of carbon and austenite stabilizing elements (Ni, Mn, Co, Cu) on Laves phase precipitation, Fe 2 W formation, and creep rupture strength (CRS) in 9-12% Cr steels at 600-700°C. Nickel and manganese had minimal impact on Laves phase and coarse carbide formation up to 1% content. While cobalt increased Laves phase fraction at 650°C, it did not improve long-term CRS and even caused a rapid decrease in short-term CRS. Copper, on the other hand, promoted the precipitation of fine Cu-rich particles that acted as nucleation sites for Laves phase and M 23 C 6 carbide. This resulted in a different needle-like Laves phase morphology compared to the globular type observed in nickel and cobalt alloys, leading to improved CRS in the copper alloy. Increasing carbon content from 0.1% to 0.2% effectively suppressed Laves phase formation, as confirmed by Thermo-Calc calculations. Notably, for cobalt alloys with higher tungsten content, higher carbon content (0.09% to 0.19%) improved CRS at 650°C, whereas the opposite effect was observed in nickel and nickel-manganese alloys. Copper alloys maintained improving CRS trends even with increased carbon, leading to the overall best CRS performance among the tested alloys with 0.2% carbon.

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.

This overview paper summarizes part of structure stability study results in China on advanced heat-resistant steels, nickel-iron and nickel base superalloys such as 12Cr2MoWVTiB(GY102) ferritic steel, Super 304H austenitic steel, GH2984, Nimonic 80A and INCONEL 740 superalloys for fossil power plant application. China had established first USC power plant with steam parameters of 650°C and 25 MPa in the year of 2006. Austenitic heat-resistant steel Super 304H is mainly used as boiler superheater and reheater material. Ni-Cr-Fe base superalloy GH2984 was used as tube material for marine power application. Ni-Cr-Co type INCONEL 740 has been studied in a joint project with Special Metals Corp., USA for European USC model power plant with the steam temperature of 700°C. Nimonic 80A has been used as several stage USC steam turbine bucket material at 600°C in China. Structure stability study of Nimonic 80A shows its possibility of 700°C application for USC steam turbine buckets.

This paper briefly introduces the state-of-the-art of the research and development of candidate heat resistant materials used for the manufacturing of 700°C advanced ultra-super-critical (AUSC) fossil fuel power plants (PP) in China, especially, focus on the impressive progress in the past three years. The detailed advancements (technical exploration and industrial investigation) of candidate materials spectra for the boiler system of A-USC PP will be presented in the current paper, including novel ferritic heat resistant steels, advanced austenitic heat resistant steels, Fe- Ni-based alloys and Ni-based alloys, which serve and cover the steam temperature scope from 600°C to 720°C. Some newly available data associated with above materials will be released.

The long-term performance of superheater super 304h tube during the normal service of an ultra-supercritical 1000mw thermal power unit was tracked and analyzed, and the metallographic structure and performance of the original tube sample and tubes after 23,400h, 56,000h, 64,000 h, 70,000 h and 80,000 h service were tested. The results show that the tensile strength, yield strength and post-break elongation meet the requirements of ASME SA213 S30432 after long-term service, but the impact toughness decreases significantly. The metallographic organization is composed of the original complete austenite structure and gradually changes to the austenite + twin + second phase precipitates. With the extension of time, the number of second phases of coarseness in the crystal and the crystal boundary increases, and the degree of chain distribution increases. The precipitation phase on the grain boundary is dominated by M 23 C 6 , and there are several mx phases dominated by NbC and densely distributed copper phases in the crystal. The service environment produces a high magnetic equivalent and magnetic induction of the material, the reason is that there are strips of martensite on both sides of the grain boundary, and the number of martensite increases with the length of service.

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.

Most effective method to increase the boiler efficiency and decrease emissions is to increase the steam temperature of modern coal-fired power plants. The increase in the steam temperature of the AUSC power plants will require higher grade heat-resistant materials to support the long-term safety and service reliability of power plants. The corrosion resistance of alloys is one of the most important factors for the application in AUSC power plants.

Sanicro 25 material is approved for use in pressure vessels and boilers according ASME code case 2752, 2753 and VdTUV blatt 555. It shows higher creep rupture strength than any other austenitic stainless steels available today. It is a material for superheater and reheaters, enabling higher steam parameters of up to about 650 °C steam (ie about max 700 °C metal) without the need for expensive nickel based alloys. The aim of the present study is the investigation of the steam oxidation resistance of the Sanicro 25. The long term test was conducted in the temperature range 600 -750 °C up to 20 000 hours. The morphology of the oxide scale and the microstructure of the bulk material were investigated. In addition, the effect of surface finish and pressure on the steam oxidation were also studied.

To save fossil fuel resources and to reduce CO 2 emissions, considerable effort has been directed toward researching and developing heat-resistant materials that can help in improving the energy efficiency of thermal power plants by increasing their operational temperature and pressure conditions. Instead of conventional 9-12Cr ferritic heat-resistant steels with a tempered martensitic microstructure, we developed “Precipitation Strengthened 15Cr Ferritic Steel” based on a new material design concept: a solid-solution treated ferrite matrix strengthened by precipitates. Creep tests for 15Cr-1Mo-6W-3Co-V-Nb steels with ferrite matrix strengthened by a mainly Laves phase (Fe 2 W) showed that the creep strengths of 15Cr ferritic steel at temperatures ranging from 923 K to 1023 K were twice as high as those of conventional 9Cr ferric heat-resistant steel. 15Cr steels have higher steam oxidation resistance than that of conventional steel in the same temperature range as the creep tests. Thus, the new material design concept of heat-resistant steel pro- vides improved creep strength and steam oxidation resistance. We are attempting to determine the optimum compositions, especially that of carbon, in order to improve the high-temperature creep strength.

This work concerns a study into the design of creep resistant precipitation hardened austenitic steels for fossil fuel power plants using an integrated thermodynamics based model in combination with a genetic algorithm optimization routine. The key optimization parameter is the secondary stage creep strain at the intended service temperature and time, taking into account the coarsening rate of MX carbonitrides and its effect on the threshold stress for secondary creep. The creep stress to reach a maximal allowed creep strain (taken as 1%) at a given combination of service temperature and time is formulated and maximized. The model was found to predict the behavior of commercial austenitic creep resistant steels rather accurately. Using the alloy optimization scheme three new steel compositions are presented yielding optimal creep strength for various intended service times up to 105 hours. According to the evaluation parameter employed, the newly defined compositions will outperform existing precipitate strengthened austenitic creep resistant steels.

Steam oxidation of a novel austenitic steel, of which composition is Fe-20Cr-30Ni-2Nb (at.%), has been conducted at 973 K to evaluate steam oxidation resistance based on detail analyses of scale morphology and scale growth. Two types of scale morphologies were observed in the solution treated sample, depending on the grain of the steel. Although thin duplex scale with the Cr-rich layer was formed in the early stage, most of the surface was covered with thick duplex scale which consists of magnetite as the outer scale and the mixture of Fe-Cr spinel and metallic Ni as the inner scale. On the other hand, surface morphology of the oxide scale was independent of grain of the steel and thick duplex scale as seen on the solution treated sample was formed on the pre-aged sample. Steam oxidation resistance of the steel is almost the same as that of commercial austenitic steels and it can be improved by the surface treatment such as shot peening. Based on the results, this steel has both enough creep rupture strength and good steam oxidation resistance for A-USC power plants.

Recently advanced ultra-super critical (A-USC) pressure power plants with 700°C class steam parameters have been under development worldwide. Japanese material R&D program for A- USC beside the plant R&D program started in 2008, launched in 2007 under the METI/NEDO foundation includes not only alloy design explores and novel ideas for developing new steels and alloys that can fill critical needs in building 700°C class advanced power plants, but also fundamental studies on creep strength and degradation assessment, which are absolutely needed to assure the long-term safe use of newly developed steels and alloys at critical temperature conditions, for instance, 650°C for ferritic steels, 700°C for austenitic steels and 750°C for Ni- based alloys. This program concept has been based on the lessons from materials issues recently experienced in the creep strength enhanced ferritic steels used for 600°C class ultra-super critical power plants. Particular outputs from the program up to now are recognized as the ferritic steel having the creep strength of 100MPa at 650°C beyond 30,000h without any Type IV degradation and as the austenitic steel developed by means of inter-metallic compounds precipitation strengthening of grain boundary which should be strongest in creep ever found. Concurrently great progresses have been seen in the research works with positron annihilation life monitoring method applicable to various kinds of defects, structural free energy values, small punch creep test data for very limited interest area, crystallographic analyses, optimum time-temperature parameter regional creep rupture curve fitting method, hardness model, etc. which would highly contribute to find out and establish the structural parameters affecting to creep strength and degradation resulting in accurately estimating the 100,000h creep strength.