Effect of Strain Range on High Temperature Creep-Fatigue Behaviour of Fe-25Ni-20Cr (wt.%) Austenitic Stainless Steel (Alloy 709)

ABSTRACT

Since the preliminary data suggest that Fe-25Ni-20Cr austenitic stainless steel (Alloy 709) is an excellent candidate as a structural material for high-temperature applications such as Sodium-cooled Fast Reactor (SFR), the effect of strain range on creep-fatigue interaction of the Alloy 709 is investigated by conducting strain-controlled creep-fatigue tests with tensile hold times of 0, 600, 1,800 and 3,600 s at strain ranges varying from 0.6% to 1.2% at 750°C and 2 × 10⁻³ s⁻¹ strain rate. Strain-controlled fatigue tests were performed at strain ranges from 0.3% to 2.5% at 750°C and 2 × 10⁻³ s⁻¹ strain rate. The predicted fatigue life of Alloy 709 shows a better correlation with the characteristic slopes predictive method. With increasing strain range at a given hold time, the number of failure cycles decreases until saturation. The fractography of the deformed samples exhibited increased number of cracks with strain range along with M₂₃C₆ precipitates and high dislocation density.

KEYWORDS:

• Alloy 709
• creep-fatigue
• strain range
• interaction diagram
• cracks
• dislocations

Acknowledgments

The authors would like to acknowledge the financial support from the Nuclear Energy University Programs (NEUP/Project #15-8582) of the Department of Energy, Office of Nuclear Energy for performing this research, and Dr Sam Sham of Argonne National Laboratory for supplying the experimental material, and Dr Nilesh Kumar, Dr Abdullah Alomari and Dr Boopathy Kombaiah for support and discussion. The TEM work was partially supported by the US. Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office Contract DE-AC07- 051D14517 as part of a Nuclear Science User Facilities project. The authors gratefully acknowledge the use of microscopy facility at Advanced Instrumentation Facility, the use of optical microscopy at the Industrial and Systems Engineering at North Carolina State University, Raleigh, USA and the partial financial support by the US National Science Foundation grant CMMI 1727237.

Data availability statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Nuclear Energy University Program [15-8582].