Mechanical Properties of INCONEL 740H at 725°C

Poputa, Mircea Cristian (2023). Mechanical Properties of INCONEL 740H at 725°C. MPhil thesis The Open University.


Worldwide, demand for electricity usage is increasing. The temperature at which conventional fossil fuel Power Plants currently operate is insufficient to increase their efficiency and reduce their CO2 emissions. The typical operational temperature of fossil fuel power plants in Europe is around 560°C. Increased efficiency and decreased CO2 emissions can only be achieved by increasing steam temperature above 700°C and pressures in the region of 35MPa. New alloys, including INCONEL 740H were developed to meet the new requirements, in the late 1990’s under the European Thermie project.

INCONEL 740H, is an age-hardened nickel-based superalloy with a gamma prime strengthening phase and a melting range of 1288-1362°C, similar to Nimonic 263, but with optimised chemical composition for creep strength, microstructure stability, hot corrosion, and heavy section weldability in order to withstand harsher environment conditions.

Allowable stresses for pressure containing components made of this new alloy UNS (N07740) have been established in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessels Code Case 2702. At present INCONEL 740H is mostly used for thin-wall elements of superheaters and thick-walled chambers and pipelines of Ultra-supercritical steam boilers.

This thesis focuses on characterising and understanding the interactions between chemistry, microstructure stability, creep, and fatigue behaviour of parent material (as manufactured) and after stress-free long-term ageing at 725°C for up to 20,000 hours. The temperature of 725°C was selected based on the literature review, and discussion with the material provider. At the time of the study there was very sporadic date points at 725°C for the material. Also another reason was that in Europe the aim of the material used was to be operated around 700°C, so testing the material at 25°C higher would bring confidence on the current data for creep and fatigue.

The observed behaviours are quantified and assessed to establish how INCONEL 740H can be utilized in high temperature power plant within and beyond a design service life of 100,000 hours (11,4 years). The effect of long-term stress-free ageing at 725°C on the mechanical properties is of particular interest as few studies have been conducted at this temperature. The reported experimental work will contribute to the definition of a creep-fatigue interaction diagram for this material condition of use for future design life and integrity assessment purposes. The research has demonstrated the microstructure of the alloy is stable after long term stress-free aging of the alloy at 725°C, with a main strengthening phase ɣ’ (Ni3Ti) homogenously distributed within the matrix. The creep-fatigue model analysis for 1h and 16h dwell times shown that the main dominant failure mode is creep rather than fatigue.

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