Fraunhofer Institute for Mechanics of Materials IWMToday, power plant components are conservatively designed according to standards using the worst tolerable material quality, even though the steel actually used is often significantly better. For the first time, this project provides a practical method for determining the individual material quality of a component during ongoing operation and using this to derive a more accurate prediction of its remaining service life. At its core is a creep deformation model further developed at Fraunhofer IWM, which is combined with in-service strain measurements and finite element simulations. The method was successfully demonstrated on a real intermediate superheater header with approximately 142,000 operating hours and is directly applicable for plant operators, condition assessors, and SMEs.
Improved residual life assessment through determination of the actual creep dispersion band behavior of power plant components during operation
Completed research project
Project description
Power plant components made of high-temperature-resistant steels are subjected to creep stresses during operation that limit their service life. According to current standards, the conservative approach to assessing remaining service life requires using the lower range of material properties—even though a material of above-average quality can have a service life up to ten times longer than one of below-average quality. This regularly leads to the premature replacement of components that are still operational, resulting in significant costs for operators.
There is therefore a market need for methods that determine the individual material quality of a component already in service, thereby enabling a significantly more precise, component-specific assessment of remaining service life.
IGF Project 22615 N developed, validated, and demonstrated such a method for 9% chromium steel Grade 91, which is widely used in coal-fired and combined-cycle gas and steam power plants. Based on in-service strain measurements on power plant components, the batch-specific variation range of the installed material was determined by comparing the data with a finite-element simulation and a specially adapted creep deformation model. This information enables a well-founded assessment of remaining service life, which yields service life gains of up to a factor of 2 compared to the conservative standard method.
Socially and industrially, the project thus contributes to the safe extension of the operating life of existing thermal power plants, which remain indispensable as flexible backup capacity in the context of the energy transition.
Fraunhofer IWM’s work packages in the project:
Data set and creep modeling: The IWM collected and analyzed extensive creep test data for Grade 91 steel from 13 different batches (including 67 tests with durations exceeding 10,000 hours) and determined test- and batch-specific variation factors. Based on this, Holmström’s “Logistic Creep Strain Prediction” (LCSP) model was extended at the IWM to include a diffusion creep term and a variation band concept. The resulting model thus describes both dislocation and diffusion creep using a unified set of parameters—an essential prerequisite for the correct evaluation of components in the operationally relevant low-stress range (< 100 MPa). The implementation of the model as a CREEP user subroutine for the Abaqus FE program enabled its direct use in component simulations.
Component Demonstration and Procedure: The IWM demonstrated the method using a real intermediate superheater header (Grade 911) from the RWE power plant in Niederaußem with approximately 142,000 operating hours. It created a detailed FE model, simulated the complete load history, and compared the calculated creep strains with the measured ones—with good agreement. As part of this work, the creep-life model for Grade 911 was also revised, as the existing ECCC model produced unrealistic predictions at low stresses. All procedures were documented in a practice-oriented procedure manual and Python scripts as methodological tools.
Based on the project results, Fraunhofer IWM can offer industrial companies the following research and development services:
Assessment of hydrogen sensitivity for high-strength steels
Fraunhofer IWM offers comprehensive characterization, including defined hydrogen loading, microstructure analysis using EBSD and nanoindentation, as well as quantitative hydrogen measurement via TDS and TGH. In addition, hydrogen permeation measurements and mechanical characterization of materials after hydrogen loading or under in situ loading can be performed.
Benefits for you:
- Reliable conclusions regarding whether—and after what dwell time—your own process chain poses a risk of hydrogen-induced edge cracks
- Avoidance of costly batch failures through early risk detection
- Reliable and well-founded process planning based on material-specific measurement data
- Determination of materials’ susceptibility to hydrogen embrittlement
Coupled microstructure and hydrogen diffusion simulation for individual materials and processes
Fraunhofer IWM develops material- and process-specific simulation models that quantitatively calculate the time-dependent redistribution of hydrogen in different microstructural states.
Your benefits:
- Virtual determination of critical dwell times, optimal process sequences, and safe forming windows
- Significant reduction in time-consuming and expensive experimental test series
- A digital tool for process design and proactive damage prevention
