Assessment of the hydrogen induced stress cracking resistance of precipitation hardened nickel-based alloys using the slow strain rate tensile test method : Influence of microstructure

• Within the framework of a Joint Industrial Project sponsored by several petroleum companies, the behavior of several Precipitation Hardened (PH) Ni-based alloys with respect to Hydrogen Induced Stress Cracking (HISC) resistance was studied, using the Slow Strain Rate Tensile (SSRT) test method conducted under hydrogen charging conditions (applying a constant cathodic current density throughout the test). Among the test materials, several industrial Heats of UNS N07718, UNS N09945/945X, UNS N09925, UNS N09935, UNS N07725 and UNS N07716 were evaluated. A detailed microstructural analysis was performed on each heat, at different levels, involving SEM and TEM examinations at high magnification on etched samples, to reveal the phases present at grain boundaries. Methodologies were developed to quantify the grain boundary coverage (length of precipitates at grain boundary divided by the total grain boundary length), and to study the distribution in size (equivalent diameter, surface, shape factor) of the grain boundary precipitates, and applied to the studied materials. Elemental chemical analysis of the precipitates was also conducted by Energy Dispersive X-ray spectrometry. The performance of the different alloys with respect to HISC resistance is discussed on the basis of the plastic elongation obtained in the SSRT tests under cathodic polarization, the microstructural features and the microstructural criteria given in the API 6A CRA standard. 

Nyckelord

Grain boundary precipitates

Hydrogen Embrittlement

Hydrogen Induced Stress Cracking Resistance

Linear coverage

Slow Strain Rate Tensile test

Cathodic polarization

Chemical analysis

Grain boundaries

Hardening

Hydrogen

Metal testing

Nickel alloys

Petroleum industry

Precipitation (chemical)

Tensile testing

Cathodic current density

Elemental chemical analysis

Energy dispersive X-ray spectrometry

Grain boundary precipitate

Induced stress

Microstructural analysis

Strain rate

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