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Location/Division: Cambridge

PhD Studentship - Quantitative Assessment of Resistance to Hydrogen-Induced Stress Cracking (HISC)  of Duplex Stainless Steels: Development of a Standardised Environmental-Mechanical Test Methodology

Institution National Structural Integrity Research Centre (NSIRC)
PhD Supervisor Kasra Sotoudeh
Application Deadline: Open All Year Round
Funding Availability Funded PhD Project (Students Worldwide)

Background
Duplex stainless steels (DSSs) are widely used in oil and gas subsea applications, such as pipelines, and manifolds and risers in production systems, due to their superior properties. In these environments, DSS components are often connected to ferritic steel components, which necessitate the application of cathodic protection (CP) as a means of corrosion prevention. Whilst successful in preventing degradation of the ferritic parts, CP can generate hydrogen at the bare surface of the subsea structures, whereupon, it can be absorbed into the alloy and cause embrittlement. Cracking of this embrittled material is known as hydrogen-induced stress cracking (HISC) and is recognised as a major cause of catastrophic failures in service, which are of significant fatal and financial losses. However, there are currently no established/standardised guidelines for evaluating resistance to cracking of DSSs in such environments, in a quantitative manner and against any established acceptance criterion.

The difficulties with evaluating resistance to cracking, and determining crack initiation quantitatively, are inherent to test methodologies which had been originally developed for testing ductile materials, in air, and mainly to provide inputs for Engineering Critical Assessments (ECAs).  Whilst it is highly desirable to use such standardised fracture toughness assessments for determining maximum tolerable flaw sizes in air, experience from failure investigations and testing, shows difficulties with obtaining meaningful data using existing test methods, for evaluating HISC of   corrosion resistant alloys (CRAs), including DSSs, in hydrogen charging environments. Thus, the suitability of testing which is employed for conventional fracture assessments, using specimens with a pre-existing crack, i.e. a fatigue pre-crack, is called into question, and requires further development.

Reference:
1- NACE corrosion C2018-11439 - Environmental Testing of Rolled and Hot Isostatically-Pressed Duplex Stainless Steels.

Project Outline
This PhD is a follow-on to a number of research programmes and projects pursued in the past 20 years at TWI. The main objectives of this programme of work are to develop an in-depth insight to the cracking behaviour of DSSs once exposed to hydrogen, and provide new, quantitative methods for assessing and designing against HISC, as well as providing data to enable production of more reliable assists. This also helps risks of HISC to be managed, and therefore increase safety and reduce the operation costs and issues associated with, potentially fatal, catastrophic failures. A series of experimental and modelling tasks (but not limited to the following items), will be undertaken:

• Advanced microstructural characterisation of a few DDS product forms (and weldments)
• Modelling  of test specimen geometries, and numerical calculation of strain/stress distributions along the stress raiser to be incorporated in those specimens
• Mechanical and environmental  testing of a number of specimens with different geometries (size, notch acuity)
• Post characterisation of tested specimens using metallography and fractography 
• Data analysis and interpretation of the findings obtained.

Key words
Duplex stainless steel (DSS), hydrogen-induced stress cracking  (HISC), environmental-mechanical testing, quantitative assessment

About University/Department

Brunel University London is a home of technological education, championing innovation and advancement, and giving the UK the knowledge base it needed to compete on the international stage. Their ambition is to equip students for the world of work and to address society’s challenges on a global scale.

About NSIRC

NSIRC is a state-of-the-art postgraduate engineering facility established and managed by structural integrity specialist TWI, working closely with a number of academic partners and universities and a number of leading industrial partners. NSIRC aims to deliver cutting edge research and highly qualified personnel to its key industrial partners. 

Candidate Requirements

Candidates should have a relevant degree at 2.1 minimum, or an equivalent overseas degree, in a Materials or Mechanical Engineering field. Overseas applicants should also submit IELTS results (minimum 6.5) if applicable.

https://www.brunel.ac.uk/research/Research-degrees/Research-degree-entry-requirements

Funding Notes

This project is funded by Lloyds Foundation and University. The studentship will provide successful Home/EU students with a minimum stipend of £16k/year and will cover the cost of tuition fees. Overseas applicants are welcome to apply, with total funding capped at £24k per year.