Abstract:
This thesis describes the metallurgical and environmental factors that influence
hydrogen embrittlement of super duplex stainless steels and presents a model
to predict the rate at which embrittlement occurs. Super duplex stainless steel
has an austenite and ferrite microstructure with an average fraction of each
phase of approximately 50%. An investigation was carried out on the
metallurgical and environmental factors that influence hydrogen embrittlement
of super duplex stainless steels.
Tensile specimens of super duplex stainless steel were pre-charged with
hydrogen for two weeks in 3.5% NaCl solution at 50º C at a range of applied
potentials to simulate the conditions that exist when subsea oilfield components
are cathodically protected in seawater. The pre-charged specimens were then
tested in a slow strain rate tensile test and their susceptibility to hydrogen
embrittlement was assessed by the failure time, reduction in cross-sectional
area and examination of the fracture surface.
The ferrite and austenite in the duplex microstructures were identified by
analysing their Cr, Ni, Mo and N contents in an electron microscope, as these
elements partition in different concentrations in the two phases. It was shown
that hydrogen embrittlement occurred in the ferrite phase, whereas the
austenite failed in a ductile manner.
An embrittled region existed around the circumference of each fracture surface
and the depth of this embrittlement depended on the hydrogen charging time
and the potential at which the charging had been carried out. The depth of
embrittlement was shown to correlate with the rate of hydrogen diffusion in the
alloy, which was measured electrochemically using hydrogen permeation and
galvanostatic methods. A two-dimensional diffusion model was used to
calculate the hydrogen distribution profiles for each experimental condition and
the model could be employed to provide predictions of expected failure times in
stressed engineering components.
