Techniques for using oxide thickness measurements for predicting crack formation and growth history in high-temperature metallic components

A method and system to develop the age and history of a crack by exposing a specimen or component to varying predetermined temperature range that covers the designated service temperatures and measuring the thickness of the oxide across the specimen along the thickness direction.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

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BRIEF SUMMARY OF THE INVENTION

The present invention provides unique techniques for predicting the history of both crack formation and crack growth in components. Oxidation kinetics of ASTM Grade P91 Steel were characterized in the temperature range of 600° C. to 650° C. with exposure times ranging from 10 to 1000 hours utilizing weight gain measurements.

Oxide thickness as a function of temperature and exposure time was also measured on the oxidized samples using a scanning electron microscope.

The weight gain and thickness measurement data were used to determine the parabolic rate constant (kp) as a function of temperature for this steel as a part of model development.

The value of kpand oxide thickness measurements from compact type specimens used in creep-fatigue crack growth experiments were used to predict the crack growth rates and were subsequently compared to the measured crack growth rates.

The predicted and measured crack growth rates were found to agree (within a factor of 2). This level of accuracy is considered very satisfactory in these applications.

In yet other embodiments of the present invention, even more accurate predictions can be made. As part of fractographic studies, the measurement of oxide thicknesses in creep and creep-fatigue crack growth specimens with a known crack growth history can be routinely performed and reported with crack growth data.

Other embodiments of the present invention provide a system and method for predicting the remaining life, safe inspection intervals and inspection criteria for high-temperature components. Thus, the embodiments of the present invention may be used to extend the life of expensive power plant systems and components.

The embodiments of the present invention may also be used to extend the life of land-based gas turbines and aircraft engines by reducing components that are prematurely retired.

DETAILED DESCRIPTION OF THE INVENTION

Metal surfaces on various components are known to oxidize in air when exposed to high temperatures. Components subject to oxidation include reheat steam pipes, steam turbine rotors and casings, chemical reactors, land-based as well as aircraft turbine components in the hot-gas path. Oxidation leads to the development of cracks in these components due to creep-fatigue and creep mechanisms. Oxide thickness, x, on the surface of the crack is related to the exposure time, t, by the following equation:

Metal Equation (1) is called the parabolic oxidation rate equation. Further, R in equation 2 is the universal gas constant, Q is the activation energy characteristic of the thermally activated, rate controlling process, and T is the temperature in degrees Kelvin. In one embodiment, the present invention pertains to the use of this relationship for predicting the age of cracks in high-temperature components and the rates at which the cracks have been growing during service.

In a preferred embodiment, the present invention provides a system and method to develop the age and history of cracks. The uniqueness of the invention lies in several of the following unique process and analytical steps used to develop the age and history of cracks:1. Expose several rectangular specimens of well-polished specimens to temperatures in the range that cover the typical service temperatures for varying periods ranging between 10 and 1000 hours.2. Cover the oxidized surface with a conductive resin to protect the oxide layer from spallation. Next, the specimen is sectioned to reveal the thickness of the oxide layer.3. Measure the thickness of the oxide by SEM imaging and chemical mapping across the specimen along the thickness direction.4. Software, such as MATLAB code, may be used, to automate the oxide thickness measurements at several locations and obtain a reliable measure of the average oxide thickness.5. Equation (1) is fitted to the data to obtain the values of kp0and Q.

In other preferred embodiments, the present invention provides a system and method to estimate the age of cracks discovered in service. Plastic replicas of the cracked region may be used to estimate the oxide thickness at the mouth of the crack which is directly related to the age of the crack. If the crack is excavated from the component and the oxide thickness profile measured, the rate at which the crack was growing during service can be estimated. Thus, the full growth history of the cracks can be revealed and then used to validate analytical models used to estimate remaining life and inspection intervals in these components. The various steps in the technique are explained in more depth below.

As shown inFIG. 1, the first step is to excavate cracked region100from component110.

Next the excavated region is fractured to expose the crack surface so that coating120may be applied to protect the crack surface. Fracturing may be accomplished by dipping the excavated region in liquid Nitrogen or some other cold medium that will cause fracturing. Coating may be done by epoxy or Ni may be used by electroplating or vapor deposition.

Next the coat excavated region is sectioned in half in the middle to reveal the oxide thickness profile200as shown inFIG. 2.

Oxide thickness, x, may be measured as a function of crack depth, Δa. x0is the oxide thickness at the mouth of the crack that may also be measured on the surface using of plastic replicas.

More detailed relationships between the oxide thickness measured on surfaces of cracks found in service and their time-history are given below and are shown inFIG. 3.

The age of the crack may be determined using the following equation:

Exposure time, t, may be determined using the following equation:

Crack growth rate at any point on the crack surface may be determined using the below three equations:

Equation 1 is the governing equation for estimating crack growth rate:

referenced to the beginning of crack extension.

d⁡(Δ⁢⁢a)d⁢⁢x
is negative so

d⁢⁢ad⁢⁢t′
is

positive. The estimation of

The below equations may be used in computer algorithms using the equations below to generate the entire crack growth history of the component:

Also, the oxide thickness values measured on the mouth of the crack using plastic replicas may be used to estimate the age of cracks.

In other embodiments, oxide thickness profiles are measured on crack surfaces of components and are used for estimating the rates of crack propagation and the age of cracks. The embodiments of the invention are particularly suitable for high-temperature components fabricated from metals and used in power plants. To implement this technique, it is necessary to fully characterize the kinetics of high-temperature oxidation in the metals used in the fabrication of these components. In this study, the oxidation characteristics of an American Society of Testing and Materials (ASTM) Grade P91 ferritic steel used in steam headers and piping is used as an example.

Two sets of studies were conducted. The first measured oxide thicknesses in polished rectangular specimens after exposure to high temperature for a range of temperatures and times to determine the oxidation kinetics. The oxidized samples were also subjected to SEM examination, and measurements of physical properties such as density were made. The second measured the oxide layer thickness on the fracture surfaces of creep-fatigue crack growth samples tested as part of a study where the crack growth rates were measured. The measured crack growth rates were compared to the predicted crack growth rates from models developed using the controlled oxidation test results. Good agreement was found between the measured crack growth rates and those predicted from oxide thickness measurements.