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A Constitutive Creep Equation for 9Cr‐1Mo‐0.2V (P91‐type) Steel under Constant Load and Constant Stress
Author(s) -
Nagode A.,
Ule B.,
Jenko M.,
Kosec L.
Publication year - 2007
Publication title -
steel research international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.603
H-Index - 49
eISSN - 1869-344X
pISSN - 1611-3683
DOI - 10.1002/srin.200706259
Subject(s) - creep , materials science , arrhenius equation , activation energy , constant (computer programming) , stress (linguistics) , constitutive equation , diffusion creep , thermodynamics , composite material , physics , finite element method , microstructure , chemistry , linguistics , philosophy , organic chemistry , computer science , grain boundary , programming language
The creep behaviour of 9Cr‐1Mo‐0.2V (P91‐type) steel was studied at elevated temperatures of 625 to 675 °C. These temperatures considerably exceed the maximum service temperature of this steel, which is approximately 600 °C. Uniaxial creep tests were performed under constant load and constant stress at initial stresses of 120 to 240 MPa. These tests are considered as short‐term tests and they can even be applied for creep lifetime assessment, albeit with certain limitations. However, the main purpose of this work was to find a constitutive creep law that would satisfactorily describe the creep behaviour of P91‐type steel and enable a comparison of the results obtained with both kinds of creep tests, i.e., under constant load and constant stress. It is well known that the minimum creep strain rate of P91‐type steel cannot be satisfactorily described by the simple Arrhenius‐type power‐law constitutive model. Therefore, an improved stress‐dependent energy‐barrier model for the description of the creep behaviour of P91 steel was used. The model showed that there is a small difference in the apparent activation energy between constant‐load and constant‐stress creep tests. The obtained values of the stress‐dependent activation energies are, in both types of test, considerably higher than the activation energy for lattice diffusion. This is in good agreement with the literature data, although the obtained activation energies are not so strongly stress dependent. The Monkman‐Grant relation showed that the minimum creep strain rate is not quite inversely proportional to the time‐to‐rupture for both constant‐load and constant‐stress creep tests.