CREEP-RUPTURE DATABASE

Creep-rupture resistance is the primary property critical for the design of high temperature equipment. It may be specified in terms of a stress for a limiting creep strain in a specific time or the stress for rupture in a specific time. Because designs for gas turbines and steam turbines require design lives between 50,000 and 100,000 hours, this usually requires significant data extrapolation. The most extensive generally available data has been published by the National Institute for Materials Science (NIMS) in Japan. This database includes about fifty alloys tested for times up to 100,000 hours. Since there is normally a significant scatter in data, stemming from both alloy and test variability, it is appropriate to rate the data source quality to allow incorporation of data from different sources as we build a global alloy database. Thus, because the NIMS data covers very long times on a number of heats for each alloy, it merits an A rating. Similarly, a B rating might be assigned to100,000 hour data on one heat, a C rating for data to 10,000 hours on one heat, and a D rating for a short time parametric study or estimate based on stress relaxation testing. There have been a few programs to study variability among different test laboratories, among different heating methods and even different humidity levels. However, these have generally been of short test duration so all that can really be concluded is that the statistical variations and calculated sigma values in the NIMS data are likely to be optimistic since they were limited to one test laboratory using standardized procedures. It should also be noted that the factors contributing to data scatter in the creep-rupture test may not be directly related to the factors contributing to service failure probabilities. Because of these uncertainties, design codes usually require a general safety factor of two-thirds on stress. Since data scatterbands are alloy and process sensitive, rupture data may also be presented with three sigma limits on life.

The creep-rupture database is being setup using the Cambridge Engineering Software (CES), available from Granta Design. The software currently contains several thousand records of materials in which each record contains a systematic and generally complete set of attributes used in materials selection for room temperature applications. There are currently no high temperature creep-rupture data but the structure of the software and the power of the comparative materials selection tools make this an excellent foundation for the creep-rupture database. The NIMS data are fitted with an optimized Larson-Miller, Manson-Haferd, Manson-Succop or Orr-Sherby-Dorn parameter. Second order polynomial equations could then be set up for rupture life in terms of stress and temperature for the range covered, with the standard error of estimate in terms of logarithmic rupture times (SEE or sigma) and the coefficient of determination (COD). Minimum creep rates were calculated from linear regression between rupture time and minimum creep rate (Monkman-Grant relation). Provision was also made to estimate times to specific creep strains using linear regression between rupture stress and creep stress in the same time (Goldhoff-Gill correlation). To maximize functionality of the data the parametric equations were converted to high resolution gridded functional data which could then be used to show different relationships among the variables within the CES software.

Rupture curves for IN700 at 700, 750, 800 and 900C showing 3sigma  and 6sigma  ranges

Design curves for IN700 at 700, 750, 800, and 900C showing two-thirds stress values

Minimum creep rate curves for IN700 showing 3 sigma and 6 sigma ranges

Rupture life vs. temperature curves for IN700 at different stresses

A hierarchical descriptive structure was incorporated in each alloy record. This included alloy group, processing route, basic alloy chemistry, heat treatment, product form, specification, name, and application. The final template will include much of the data that already is covered in the CES software but will additionally allow alloy selection based on a combination of creep-rupture strength with chemistry limits, processing requirements, and limits on other mechanical or physical properties. Statistical comparisons for each alloy can be made by relating the design stresses (two-thirds of the nominal stress) to the actual sigma performance. For example, for a Cr-Mo-V steel the mean rupture stress for 100,000 hour life at 550C is 130.333MPa. The design code would allow 86.89MPa. With a standard deviation of 10.05 the design code gives a sigma value of -4.32 or approximately 8 failures due to creep-rupture per million opportunities. For a 12% Cr steel a similar calculation yields approximately 74 failures due to creep-rupture per thousand opportunities. For the NIMS data, scatterbands may be plotted up to six sigma limits.