Qualification of 12CrCoMo as a New Material for Steam Generators
Project Number 221
The aim of the project is to qualify and specify the new structural steel for boilers, 12CrCoMo, for use above 620 °C. The critical parameters for the economic feasibility of the material are:
- adequate creep strength
- good processing capabilities - particularly welding - without a permanent effect on the strength of the base material
- oxidation resistance.
Based on the experience with the development of comparable steels, because of its chemical composition, this steel has a creep strength potential of 100 MPa at 620 °C. Because of the Cr content, an adequate oxidation resistance can be assumed even in this temperature range.
The aim is to create a reliable and adequate data and knowledge base for designing and operating boiler components made from the new material. To do this, investigations will be carried out into the mechanical and technical properties of typical welded connections (TIG; TIG-E; TIG-E/UP. TIG = tungsten inert gas, E = electric, UP = submerged arc) in the initial state and under service conditions, the creep behaviour, taking into account the long-term behaviour in particular and also intensive investigations into the structure state. The latter will mainly describe the initial state of the base material as a function of the heat treatment parameters (austenitising temperature, annealing temperature, dwell times) with reference to particle size, structure constituents, homogeneity of the structure, hardness and characteristic precipitations.
Light, scanning electron and transmission electron microscopy will be used to guarantee adequate investigation detail. This kind of "structure atlas" can be used to make assessments of the initial structure with regard to the complete heat treatment. By simulating the structure of the HAZ structure in a Gleeble installation, conclusions can be derived concerning stress relief sensitivity when welding, damage development in the form of creep pore formation (together with investigations of welded connections), the substance laws necessary for the digital calculation of welded connections and also the lower bound limit of the creep rupture behaviour in the long-term range.
The investigations form the basis for the design and operation of components in modern steam generator engineering. They are used for quality assurance purposes when ordering new components and may also find their way into the rules and regulations. The results of the investigations are also suitable for assessing the damage process in components and can therefore also be used in future monitoring processes.
The project will be conducted by MPA Stuttgart under the management of Dr. K. Maile and Dr. A. Klenk. The VGB Technical Committee "Materials and Quality Assurance" will provide technical support for the project.