Hydrogen Assisted Cold Cracking is a phenomena that manifests itself in weldments when a
critical hydrogen concentration is trapped within a susceptible microstructure and subjected to
a threshold level of stress and poses a significant threat to pipeline girth weld integrity, in
particular the root pass of girth welds.
This thesis explores the thermomechanical factors influencing the formation of Weld Metal
Hydrogen Assisted Cold Cracks (WMHACC) in High Strength Low Alloy line pipe steel
when welded with cellulosic electrodes using the Manual Metal Arc Welding process. The
overarching objective of this body of work is to delineate a safe boundary across which the
characteristics of weld metal samples can be defined to enhance the understanding of the
factors which influence the formation of weld metal hydrogen cracks. The significance of
achieving this objective is the improved ability to predict the onset of the phenomenon,
consequently facilitating the development of strategies, which can be assimilated by industry
to minimise the presence of cold cracks increasing the safety and reliably of pipeline girth
welds. To address the overarching objective and address the limitations identified though the
literature review, in particular the limited number of studies focusing on welding in the
Australian context, a three-part experimental and analytical program was developed and
undertaken.
Development of an Enhanced Weldability Test. To facilitate deposition of tests welds
under high restraint in a range of heat inputs which reflected the desired test window, an
enhanced weldability test, the MWIC, was designed and commissioned. The test’s geometry
and characteristics were based on the well-established Welding Institute of Canada
Weldability test, but was enhanced to allow for deposition of welds at very low heat inputs, of
which no published cracking data exits. Additionally, the test was modified to allow for the
extraction of critical welding data, facilitating its use as a research tool.
Delineation of Safe Welding Envelope. Using the MWIC test, deposition envelopes were
created for thick (20mm) and thin (10mm) sections of API 5L X70 line pipe steel welded with
E6010 electrodes under high and low restraint. Welds were deposited at low heat inputs
(<1kJ/mm) over a range of preheats and examined under magnification to establish the
critical/ threshold preheats above which no cracking was observed.
Characterisation of weld metal samples across the derived cracking boundary. The
weldability test samples generated from weldability testing were characterised using a range of macroscopic, microstructural, and micromechanical techniques to establish the
interrelationship between cracking and thermomechanical parameters. The influence of heat
input on bead eccentricity was established and its consequent effect on the welds susceptibly
to hydrogen cracking was proposed. The potential interrelationship between solidification and
hydrogen cracks was discussed in relation to restraint levels. The inefficiency of the currently
accepted hardness threshold of 350HV used to predict the onset of HAZ-HACC as a proxy for
Weld Metal Hydrogen Assisted Cold Cracking susceptibly was established for all test cases.
Additionally, the use of micromechanical properties as a proxy, in particular the use of the
ratio of E/H was introduced as a means of quantifying susceptibly to WMHACC.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2017
