Knowledge to avoid coarse grain structure in austenitic materials to improve the flexibility of power plant components
Project Number 365
In the past, boiler components such as superheater tubes or life steam pipes made from austenitic steels in fossil-fired steam power plants have basically been designed with regard to long-term creep load and oxidative stress.
On the one hand, the increasing requirements on the cyclic operating mode of thermal power plants lead to increased start-up and shutdown processes, as well as power changes. On the other hand, in the process, higher temperature gradients within the components lead to an increased fatigue load with an impressed alternating strain. These thermally caused secondary loads superimpose a relevant primary load caused by the inter-nal pressure of the tube. Therefore, with the current operation of boiler components, fatigue loads must always be taken into account.
Due to their good creep strength and fatigue limits, austenitic steels with a coarse grain structure are used in the boiler construction of superheater tubes. However, so far it is not sufficiently known if the low deformability of this grain structure - as known from other coarse-grained materials such as casting alloys on a nickel basis - has an adverse effect on fatigue properties.
The study comprises:
- Detailed literature research on five selected austenitic power plant steels regarding the effect of coarse grain on fatigue
- Analysis of the literature and evaluation of the coarse grain effect
- Data on materials and examinations
- A comparison of the fatigue behaviour considering the grain structure
- A summary of the results and their evaluation.
The result is a clarification of coarse grain effect on the fatigue properties of austenitic power plant steels or an identification of knowledge gaps so that a need for further examinations can be determined.
The project is supported by the VGB-Technical Committee “Materials and Quality Supervision".