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Creep resistant steels and welds for ultra supercritical power plants

Periodic Reporting for period 1 - CRESWUP (Creep resistant steels and welds for ultra supercritical power plants)

Reporting period: 2015-08-01 to 2017-07-31

The primary method of generating electricity worldwide is by fossil-fuel fired power plants, which have high CO2 emissions. In developing countries like India, fossil-fired power plants based on domestic coal reserves will remain a main source of electricity for many years to come. An immediate goal is to generate power from fossil-fired stations that are more efficient and therefore less polluting.

Conventional coal fired power plants operate at steam temperatures between 538 and 566C and at a sub-critical pressure below 240 bars. Such power stations have high CO2 emissions due to low thermal efficiency (35-40%). To reduce emissions, new power stations need to operate at higher temperatures and pressures, which will improve the thermal efficiency. However, this leads to intensified loads on the materials in the hottest components of the steam cycle (boiler, steam lines and steam turbine), and new construction materials with improved resistance against creep, corrosion and thermal fatigue are needed for these components.

State-of-the-art coal fired power plants operate at 600-620C and ultra-supercritical pressure of 300 bar, leading to thermal efficiencies of 43-47%, corresponding to 10-20% less specific emissions of CO2 compared with conventional plants. Construction materials for the most critical hot components in these USC plants are creep strength enhanced martensitic 9%Cr steels, such as Grade 91 and 92.

Current R&D of materials for enhanced efficiency of coal fired power plants follow two directions. 1) the development of improved martensitic steels, which could enable USC plants with steam parameters up to 650C and 325 bar, and efficiencies of 48-50%. 2) Advanced Ultra Supercritical (A-USC) power plants operating at 700C and 350 bar pressure with more than 50 % efficiency. Nickel base alloys are the chosen construction materials for the hottest components in A-USC plants. However, the cost of these materials are 5-10 times higher than the cost of steels, and a number of technical issues related to the long-term performance of base materials and welded joints have been identified.

Research in the present project focused on the characterization of microstructure and properties of new martensitic 11–12% Cr steels and welded joints currently under development . Improved understanding of relations between microstructure and properties of 11-12% Cr steels will improve the chances of a successful materials development of such steels, which could enable the construction of USC power plants with steam parameters of 325 bar and 650C. Furthermore, such steels would cut back the necessary amount of expensive Nickel base alloys in the more efficient A-USC power plants, and increase the overall economy of such plants.

A final objective of the project was to establish contacts between research communities in Europe (Technical University of Denmark (DTU)) and India (Indira Ghandi Centre for Atomic Research (IGCAR)), working on materials development for USC power plants in order to further the research in both areas.
At the Technical University of Denmark (DTU) a new concept of Z–phase strengthened 12% Cr steels has been evolved. However, test steels based on the Z-phase strengthening concept has been found to possess poor impact toughness and they show creep instabilities in the form of accelerated creep strain accumulation in restricted time periods. The research in the project focused on investigations of modified heat treatments, which could improve the impact toughness of the steels, and on detailed microstructure investigations aimed on understanding reasons for the poor toughness and creep instability.

A number of low C 12%CrWCoCu test steels were studied by light microscopy, in-situ synchrotron XRD, dilatometric, transmission electron microscopy, and atom probe tomography investigations, and by ThermoCalc modelling. The candidate C.R. Das from IGCAR learned these techniques during his stay at DTU (electron microscopy) and during visits to the BESSY facilities in Berlin, Germany (Synchroton XRD), TU Rostock, Germany (Dilatometry) and TU Aachen, Germany (Atom Probe Tomography).

The normal heat treatment of such steels consists of austenitization at 1050-1150C with cooling in air. This results in martensitic transformation leading to high hardness and brittleness. Thus, the steels are tempered at 600-750C in order to restore ductility and toughness and to activate precipitation hardening. Transmission electron microscopy observations revealed segregation of W during tempering at the prior austenite grain boundaries (PAGB) and other boundaries followed by formation of intermetallic Laves phase and growth of austenite. This leads to reduced toughness of the steels.

The gathered information was used to design a new 12%Cr Z-phase strengthened allow with balanced additions of carbon and W in order to achieve good creep properties high toughness and weldability without post weld heat treatment. The proposed steel was produced in the RFCS CRESTA II project, and improved toughness was confirmed.

The Z7 steel is now being upscaled and the weldability without PWHT is being demonstrated inb the CRESTA II. The steel is a candidate alloy for waterwall application in the boiler of A-USC power plants, where it could replace large quantity of expensive Nickel base alloy.

Results from the work was presented at the14th International Conference on Creep and Fracture of Engineering Materials and Structures, Creep 2017, Saint Petersburg, Russia, 19-21 June 2017.

Further presentation of the results will be made in high impact scientific journals. However, the understanding of the results was developed too late for this to happen inside the project period.
New fundamental understanding of tempering processes in martensitic 12%r steels was developed in the project. The segregation of W to PAGB leading to local formation of austenite at temperatures at below the Ac1 temperature, where austenite is not thermodynamically stable based on evaluation of the overall steel composition, has not been observed before. It offers an explanation for poor toughness and creep instability in 12%Cr steels based on the Z-phase strengthening concept.

The new understanding was used to design a new 12%Cr2W steel with improved toughness and creep stability. Due to low carbon content the steel can be welded without post weld heat treatment. This means that it has a good combination of properties for application in waterwalls of future A-USC power plants operating at steam temperatures up to 700C. Here it can replace expensive nickel base alloys, and reduce the investment costs for A-USC power plants. This will significantly improve the overall economy of A-USC projects and thereby its chances for commercial and technical success.

India has initiated a national programme to develop A-USC power plants for future [1]. Upon his return to India, C.R. Das has joined this project and is implementing the results of the project into the national A-USC project in India.

Realization of A-USC power plants in the future development of electricity supply in India has potentials to save Gigatonnes of CO2 emissions as compared to state of the art power plants, and to accelerate the electrification of India at affordable cost.

Figure 1: (a) Creep curve at 650⁰C/80 MPa/2,200h, (b) Dark field and (c) Bright field HAADF imaging
Figure 2: HAADF images of the material (a) as-normalized, (b) aged at 650C/24h and (c) aged at 650
Figure 3: (a) HAADF images of the material aged at 675C for 44 h and (b) composition profile across