Feasibility of Using Oxide Thickness Measurements for Predicting Crack Growth Rates in P91 Steel Components
There are only few methods available for predicting the age of cracks that are found in high temperature structural components during service; among the promising ones is the oxide thickness measurement technique. Oxide thickness profiles are taken from crack surfaces of components and used for predicting the rates of crack propagation. This technique is particularly suitable for high temperature components fabricated from ferritic steels commonly used in power plants that run on fossil fuels. To implement this technique, it is necessary to fully understand the kinetics of high temperature oxidation in these steels. In this study, the oxidation characteristics of an American Society of Testing and Materials (ASTM) Grade P91 ferritic steel used in high temperature piping is characterized. The literature shows that there are four primary mechanisms that influence the oxide thickness during high temperature exposure. Initially, the oxide thickness increases in a linear fashion with time and then as steady-state conditions are established, the parabolic relationship takes over. Multiple types of oxides with different rate characteristics can also form. Oxide degradation can occur by spallation due to porosity and formation of cracks. Evaporation or volatility can also occur and result in loss of oxide thickness. These factors must be considered in oxide thickness analysis to determine crack growth history. Two sets of laboratory experiments were conducted. The first consisted of measurement of oxide thicknesses after exposure to high temperature for various periods to determine the oxidation kinetics. The oxidized samples were subjected to SEM examination and measurements of physical properties such as density and porosity levels. The second set of experiments consisted of measuring the oxide layer thickness on the fracture surfaces of creep-fatigue crack growth samples tested as part of a previous study where the crack growth rates were measured. These reported measurements are used to compare with the predicted crack growth rates from the analytical models that are developed as part of this study. The success of the technique is measured by finding the correlation coefficient, which is within a factor of 2.58.
Fracture Mechanics, Oxidation, Oxidation Kinetics, P91 Steel, Predicting Crack Growth, Engineering Mechanics, Mechanical Engineering
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