Objectives
Identify a suitable test method to assess the susceptibility to hydrogen embrittlement of precipitation hardened nickel alloys.
Establish the relationship between the microstructural factors, such as the grain size, the quantity of secondary phases and the strength, and the susceptibility to hydrogen embrittlement of a precipitation hardening nickel alloy.
Identify a test method to allow quantification of the long term hydrogen embrittlement behaviour of cathodically protected duplex stainless steel containing buried flaws.
Project Outline
Firstly, a review of the available published data and TWI studies will be carried out, in order to identify a suitable test method, aimed at assessing the resistance to embrittlement of a precipitation hardening nickel alloy when hydrogen charged. Nickel Alloy 718 specimens will be obtained and heat treated accordingly, in order to produce different microstructures, eg different levels of delta phase, and strength levels.
Testing of the specimens with and without hydrogen charging will be undertaken, to establish the degree of embrittlement of each metallurgical condition. Characterisation of the tested specimens will be performed, to quantify the microstructure as far as possible, identify fractographic features associated with hydrogen embrittlement, and include EBSD analysis to assess the strain development during test. The results will relate the microstructure and strength level of nickel Alloy 718 with the susceptibility to hydrogen embrittlement.
Secondly, duplex stainless steel will be obtained. Trials will be undertaken to hydrogen-charge rectangular bars in a range of high temperature environments. The hydrogen profile obtained by each method will be determined. Hydrogen embrittlement testing will be undertaken on hydrogen pre-charged samples that have been pre-cracked, such that the hydrogen concentration at the crack tip can be identified. A correlation between material resistance to crack extension and the crack-tip hydrogen concentration will be derived. The work will be supported by metallographic characterisation of the materials to define austenite spacing, phase balance and anisotropy of the microstructure, which are likely to influence resistance to HISC.
Relevant Industry Sectors
Oil and Gas
Technical and Economic Benefits
An improved fundamental understanding of the effect of microstructure on the susceptibility to hydrogen embrittlement, together with a suitable test method to assess the acceptability of the material for application in marine environments should ultimately reduce the incidence of failure in precipitation hardening nickel alloys, saving millions of pounds in lost revenue and improving the safe operation of subsea systems.
Failures of duplex stainless steel subsea equipment due to hydrogen embrittlement have proved extremely expensive. Consequently, in the absence of reliable information on the behaviour of material with buried flaws, very conservative approaches have been adopted to date. If this challenging project is successful, the data developed will assist in removal of unnecessary conservatism.
Industrial Member Reports
Access the Industrial Member Reports/Papers resulting from this programme:
Effect of hydrogen and strain rate on superduplex stainless steel weld metal fracture toughness and fracture morphology.