Ductility coaxing during creep and tensile deformation of high temperature alloys

ABSTRACT

Ductilities generated after four types of creep test are compared: (i) constant load, (ii) constant stress (calculated); (iii) constant stress (from direct observation of the minimum section) and (iv) constant strain rate. Supplementary information is obtained from a regularly interrupted plastic deformation test. The materials considered are 2¼Cr1Mo and 9Cr1Mo steels at 550°C, type 304 and 316 alloys at 650°C and 700°C, respectively, and 20Cr25Ni/Nb steel at 750°C. The principal parameter of interest is the ‘uniform ductility’ in the gauge section, that is, that part which has not severely necked. It is shown that uniform ductility is effectively described by the Monkman-Grant relation and this in turn can be used to sum creep damage via the ductility exhaustion method. It is further demonstrated that, assuming constant volume conditions, gauge length behaviour can be modelled by considering a truncated cone rather than a traditional cylindrical shape.

KEYWORDS:

• Uniform elongation
• uniaxial tests
• necking
• Monkman-Grant relation
• constant load-stress- and strain-rate-testing
• ductility-strain-rate relation
• stress relaxation
• ductility exhaustion
• damage summation

Nomenclature

Table

Acknowledgments

The author would like to thank Mr M. W. Spindler for helpful discussions during the preparation of this paper. The author’s experiments on the 20Cr25Ni/Nb alloy were performed at the former Central Electricity Research Laboratories of the CEGB and are published by permission of successor company EDF Energy. The additional data on P92 steel arise from tests undertaken by the author at the Mechanical Engineering Department, Imperial College, London.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.