Investigation of the Long-term Strength and Deformation Behaviour of Alloy 740 Tubes (Complementary project to COORETEC DE-4)
The implementation of new technologies for high-performance and low-emission power plants with a clear increase in steam parameters (pressure and temperature) needs a material change to nickel-based materials in the highest stress area.
COORETEC is an initiative of the German Federal Ministry of Economics. The abbreviation COORETEC stands for CO2 reduction technologies for fossil-fired power plants. The aim of the COORETEC DE4 project with a total volume of 2.25 million € is to create a scientific data base for the design and operational reliability of steam generators (funding no. 0327705Y). The data base is necessary for the used structural materials of pipes and valves to gain evidence regarding their long-term durability regarding corrosion (oxidation), strength and ductility. The material properties of Alloy 263 (forgings and tubes) and Alloy 617 (forgings) before and after the implementation of the necessary production steps will be determined and characterized to enable the manufacturability of components with stronger wall thicknesses. Design and fracture mechanics' parameters are derived and calculated to establish the corresponding equation of state.
The data flow into the current rules and partly into new design methods. At the end, the development and application of the equation of state allow Design by Analysis (DBA) with numerical methods.
The work program is divided into the following packages:
- Processing and analysis of existing data.
- Production and processing of thin and thick walled components.
- Definition of the welding processes, optimization of the welding consumables and production of welded joints.
- Long-term strength and deformation properties of base materials and welded joints (Alloy 617 and 263).
- Microstructure characterization.
- Evaluation of material and component behaviour.
- Fracture mechanical characterization and brittle fracture validation.
- High-temperature oxidation and corrosion behaviour.
- Quality assurance in particular accuracy of the components.
These investigations shall be expanded by a complimentary program for Alloy 740 financed by more than 70 % by E.ON, EnBW, RWE and Vattenfall Europe.
The main material specific criterion for the selection of eligible alloys is creep strength, which must be at least 100 MPa at 700 to 750 °C. In addition, the material must be processed with hot and cold forming. Like Alloy 263, the new boiler material Alloy 740 has better creep strength values than Alloy 617. In the first instance, this is caused by a hardening heat treatment, which simultaneously can raise processing as well as weldability problems. Therefore an examination of processed Alloy 740 is required. The complimentary program covers this subject and would bring forward the qualification of the new material.
The same examinations are scheduled as for Alloy 263: Manufacture of circumferential welds with TIG, precipitation of base material and welds, determining the impact strength in the origin and precipitation state, creep tests of boiler tube welds with planned experimental periods of up to 30,000 h (target is a life time of 100,000 h)
The project is being carried out from January 2010 until December 2012 by the following Institutes:
- Institute for Materials Testing of the Stuttgart University (Germany)
- Institute for Materials Science of the Darmstadt Technical University (Germany).
The project is supported by the Technical Committee Materials and Quality Supervision.