Oxide Thickness and Estimation of Remaining Creep Life

Something that I have to keep pushing myself more and more, so I didn't give up with the situation, so I keep trying to post this Piping & Fabrication and now we try to talk about Oxide Thickness and Estimation of Remaining Creep Life. 

Another method for estimating remaining creep life of certain high-temperature tubing and piping components considers the amount of metal oxide scale that has formed on the metals surface. Understandably, this method only applies when the tubular items contain relatively benign substances under oxidizing conditions. It has found its use in steam-carrying piping and components. This method is based on the knowledge that a given thickness of oxide scale on the tube or pipe surface represents growth for a certain time at some temperature. Since oxide growth kinetics of many alloys are well characterized, the effective temperature at which the tube was operating for a known time (service life) can be estimated. The combination of effective temperature and time can then be compared to the typical creep life of the alloy at an applied stress or stresses that are known to have acted on the component during its service life.

FIGURE A3.21 Variation of Larson-Miller rupture parameter with stress for wrought 1¹⁄₄ Cr ¹⁄₂ Mo-Si steel.

FIGURE A3.22 Variation of Larson-Miller parameter with stress for rupture of annealed 2¹⁄₄ Cr-1 Mo steel

As noted, the two principal tools needed by the metallurgist to estimate life using the oxide measurement technique are (1) steam oxidation data for the alloy in question, and (2) uniaxial creep-rupture data for that alloy across the temperature range of interest. This latter information can be found for many of the most widely used ferrous alloy piping materials in ASTM references. The specific steps followed in this approach are as follows:
1. Oxide thickness is measured either metallographically on a sample or using specialized ultrasonic techniques. Operating time is known.
2. The effective operating temperature is determined from the oxidation data. The effective temperature is defined as the constant temperature that the particular tube metal would have had to have operated at for the known service time to have resulted in the measured oxide thickness. (This is an approximation, since the tube or pipe would have operated at various temperatures, perhaps even in
upset conditions well above the design temperature limit.)
3. The hoop stress is calculated using an appropriate formula, knowing the tube or pipe size and operating pressure.
4. The Larsen-Miller Parameter (LMP) is calculated for the service time and effective temperature of the subject tube. The LMP is defined as:

Where T is temperature in degrees Rankine and t is time in hours. This is a simple factor representing the actual condition of the operating component.
5. Uniaxial creep-rupture data is obtained for the alloy in question. Examples of data for 11⁄₄Cr-1⁄₂Mo-Si and 21⁄₄Cr-1Mo, taken from creep data sources ASTM DS50 and DS652 are shown as Fig. A3.21 and Fig. A3.22. This rupture data is normally represented by curves of minimum and average behavior, and lists
applied stress versus LMP.
6. The ASTM rupture curve is entered on the stress axis at the level of appropriate calculated operating stress (from step 3). In this manner, the LMP representing the expected minimum and average total creep life at that stress is determined.
7. The operating LMP calculated in step 4 is compared to the LMPs derived in step 6. The differential in time represented by these parameters can be easily calculated from the Larsen-Miller formula, and the percentage of expended life versus minimum and average expected life can be determined by taking a ratio of these values.

This method for estimating remaining creep life has found its greatest use in the fossil power boiler industry, particularly for ferritic alloy steam piping and superheater tubing. Since a great majority of the operating power boilers in the United States are approaching their originally intended lifetime, the method is critical for establishing when major repair or replacement is necessary to restore the unit to safer and more reliable operation.