A Statistical Test for Identifying the Number of Creep Regimes When Using the Wilshire Equations for Creep Property Pred
- PDF / 1,267,124 Bytes
- 15 Pages / 593.972 x 792 pts Page_size
- 14 Downloads / 134 Views
ODUCTION
TO reduce fuel consumption and CO2 emissions from power plants, new high-temperature alloys are required to resist the increase in temperature and pressure needed to raise plant efficiencies. However, at the design stage, information must be available on the stresses to which multiple batches of these new alloys can sustain without creep fracture occurring within 100,000 hours at the service temperatures.[1] Unfortunately, with the traditional parametric, numerical, and computational methods, long-term strengths cannot be predicted by extrapolation of short-term property sets. Consequently, at present, protracted and expensive long-duration test programs are necessary to determine the 100,000-hour creep rupture strengths, with a reduction in the 12- to 15-year ‘‘materials development cycle’’ being defined as the number 1 priority in the 2007 UK Energy Materials-Strategic Research.[2] In response to this problem, over recent years, a new approach—the Wilshire equations—has been devised, which appears to allow accurate long-term strength values to be obtained by extrapolation from accelerated short-term measurements. The last 5 to 6 years has seen MARK EVANS, Associate Professor and Portfolio Director for Materials Engineering, is with the College of Engineering, Swansea University, Swansea, SA1 8EN, Wales, UK. Contact email: [email protected] Manuscript submitted November 3, 2015. Article published online October 6, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
the appearance in the literature of this methodology applied to a wide range of materials used for high-temperature applications in the power generation and aerospace industries in an attempt to verify the validity and accuracy of this approach.[3–8] Specifically, 100,000-hour strength estimates have been produced by analysis of multibatch data lasting up to only 5000 hours for a series of ferritic, bainitic and martensitic steels for power and petrochemical plant and titanium alloys used in aeroengine blades and disc. The Wilshire equation takes the form v ðr=rTS Þ ¼ exp k2 e_ m :exp(Qc =RTÞ ½1a where e_ m is the minimum creep rate, T the absolute temperature, r the stress, rTS the tensile strength, R the universal gas constant, Qc the activation energy for self-diffusion, and k2 and v further model parameters. This equation provides a sigmoidal data presentation such that e_ m fi ¥ as (r/rTS) fi 1 (provided v < 0), whereas e_ m fi 0 as (r/rTS) fi 0. Wilshire and Battenbough[3] proposed a similar expression to Eq. [1] for the stress and temperature dependencies of the time to failure, tf, and time to various different strains. The parameters k2 and v appear to be dependent upon stress (and possibly temperature) for many steel alloys. This approach can be contrasted to the traditional power-law expression for modeling creep properties as a function of stress and temperature, e_ m ¼ Arn expðQc =RTÞ
½1b
VOLUME 47A, DECEMBER 2016—6593
but once again the unknown parameters (Qc and n) change with test conditions. In this approach, the variatio
Data Loading...
