High Creep Damage Research Articles

Revealing the influences of strain amplitudes on hybrid stress–strain controlled creep-fatigue interaction responses for 9 %Cr steel

The hybrid stress–strain controlled creep-fatigue interaction (HCFI) tests were performed on 9 %Cr steel at strain amplitudes ranging from ± 0.25% to ± 0.6%. The strain amplitude increase brings more possibilities of cracking but fewer crack deflections, along with some differences in LABs and grain size. But for HCFI tests with high creep damage, strain amplitude plays a minor role since considerable creep strain during stress holding is converted into fatigue strain even at low strain amplitudes. Additionally, a dwell indicator (DI) benchmarked against macroscopic LCF responses is proposed to evaluate strain amplitude-induced microscopic HCFI damage. When DI is approximately equal to (or greater than) 0.9, the differences in strain amplitudes can bring significant changes in internal degradation and fracture behaviors. When DI is close to (or lower than) 0.5, HCFI damage exhibits low sensitivity to strain amplitude.

Study of dual grain microstructure heat treatment in steel specimen representing gas turbine rotor disc

Gas turbine discs which support the rotor blades operate at relatively high speeds and high rim temperatures and are therefore subjected high creep damage at the rim area and to very high low-cycle fatigue damage at the bore. Any heat treatment which results in homogenous grain structure and one small band of grain size will limit the life of the disc either by creep at the rim or by the fatigue at the bore. If the heat treatment parameters are optimized to increase the grain size at the rim area closer to the ASTM 6.5 and to finer grain size at the bore area closer to ASTM 11.5, the heat treated disc is likely to have much superior operational life as compared to that of heat treated which results in homogenous grain structure and size throughout the disc. There is therefore the need for exploring possibilities of obtaining variable grain structure during heat treatment operation.This study is aimed at evaluating two possible heat treatment processes that are likely to result in distinctly different grain size in a single test specimen such as a circular rod. These test rods represent rotating gas turbine discs and the objective of achieving differential grain size is to optimize and balance the creep life at one half portion of the test rod (representing the disc rim or Outer Diameter) and fatigue life at another half portion of the test rod(representing the bore or Inner Diameter).

Estimation of high energy steam piping degradation using hybrid recurrent neural networks

The degradation of high energy piping systems is very complex to simulate due to the many variables that influence the useful lives of such systems. Estimation of the extent of degradation is however important in the maintenance planning process. In this research the use of data driven machine learning techniques to deal with this complex problem is investigated. A hybrid recurrent neural network is created that consists of a combined recurrent neural network and a feed forward neural network. The hybrid model is trained on historical data that has been captured over a six-year time period. The following variables related to piping system components are used as inputs to the machine learning model: operating temperature and pressure time history, distance to the closest anchor point, distances to neighbouring supports, elevation survey readings, as well as the last known creep damage measurements on the component. The model is created in Python using the Tensorflow library. A recurrent neural networks (RNN) is employed, namely the gated recurrent unit (GRU). The adaptive movement estimation optimization algorithm, called Adam, is used to optimize the machine learning model. The trained model is able to predict the degradation classification of a component with an accuracy of up to 92% on the training dataset and up to 55% on the validation data set. When using this model to predict components with high creep damage, more than 400 voids per mm2 a hit rate of 25% is achieved. The current system employed at operating power stations shows a historic hit rate of 14%. This is a significant increase in performance and could be used to compile more efficient inspection plans. The model is successful in recognising patterns within the data and offers an automated way to parse large data sets that consist of a temporal and static data mixture simultaneously. Conventional data driven models are only able to look at either temporal data or static data. This suggests a generic approach to make objective decisions on similar complex data driven problems and its application is not limited to this particular problem. The methods applied in this research are expected to perform even better on problems where the frequency of data collection is higher than what is used in this research.

Evolution of creep damage in a modified ferritic heat resistant steel with excellent short-term creep performance and its oxide layer characteristic

By performing short-term creep tests at 923K, the evolution of creep damage in a modified heat resistant steel and its oxide layer characteristic were investigated. Compared with conventional ferritic steels, the modified steel exhibited excellent short-term creep resistance. And the fracture mode remained transgranular under all test conditions. By analyzing creep data, high creep damage tolerance factors were obtained, indicating that precipitation coarsening should be the pronounced factor causing creep strain accumulation and the onset of tertiary creep stage. Combined with microstructural examination, it is found that the coarse precipitates at the prior austenite boundaries, mainly M23C6, contribute to the void formation and eventual transgranular failure. The oxide layers formed on the specimen surface consisted of Fe-rich [Fe,Cr]2O3 outer layer and Cr-rich [Fe,Cr]3O4 inner layer.

Creep behaviour of modified 9Cr–1Mo ferritic steel

Creep deformation and fracture behaviour of indigenously developed modified 9Cr–1Mo steel for steam generator (SG) tube application has been examined at 823, 848 and 873 K. Creep tests were performed on flat creep specimens machined from normalised and tempered SG tubes at stresses ranging from 125 to 275 MPa. The stress dependence of minimum creep rate obeyed Norton’s power law. Similarly, the rupture life dependence on stress obeyed a power law. The fracture mode remained transgranular at all test conditions examined. The analysis of creep data indicated that the steel obey Monkman–Grant and modified Monkman–Grant relationships and display high creep damage tolerance factor. The tertiary creep was examined in terms of the variations of time to onset of tertiary creep with rupture life, and a recently proposed concept of time to reach Monkman–Grant ductility, and its relationship with rupture life that depends only on damage tolerance factor. SG tube steel exhibited creep-rupture strength comparable to those reported in literature and specified in the nuclear design code RCC-MR.