Despite numerous studies to understand the hydrogen stress cracking behavior of precipitation hardened nickel-based alloys there is still a need for improving industry understanding. A number of data were published in a study at AMPP 2021 (paper C2021-16709) to quantify the hydrogen stress cracking resistance of high strength alloys used in subsea fastener applications. The cracking threshold of various high strength corrosion-resistant alloys was determined through successive step loading and constant load tests using cathodically polarized notched tensile specimens to simulate subsea fastener service exposure.

This paper presents work in follow-up to the previous study. It is focused on UNS1 N07718, UNS N09925, UNS N07725 and UNS N09946. A series of incremental step load tests of compact tensile specimens were conducted to measure the fracture toughness during testing and cracking was monitored by the Electric Crack Growth Monitoring technique. A new engineering technique, referred to as statistical fractography, was used to investigate the fracture surface morphology and extract from it the fracture properties of the alloys. The local toughness values measured by this technique was found to be consistent with the ones measured during the tests carried on CT specimens. Interestingly, the toughness was also measured from the analysis of the fracture surface of the tensile loaded specimens, again showing consistent results. Overall, the combination of mechanical tests, acoustic emission monitoring and statistical fractography provides further insights on the resistance to HSC of the studied alloys.

In addition, hardness mapping was undertaken of specimens with rolled, machined and notched specimens to elucidate if an increased hardness at the surface had an effect on the hydrogen stress cracking resistance or behavior. This was shown not to be the case.

Subject
Test methods, Crack growth, Materials, Fracture toughness, Cathodic protection, Crack initiation, Stress cracking, Mechanical failure, Hardness, Alloys, Fractography, Toughness, Hydrogen stress cracking
Keywords:
nickel alloy, hydrogen stress cracking, statistical fractography, hardness, threads
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2023
Association for Materials Protection and Performance (AMPP)
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