A study of the metallurgical and mechanical property variability in a dual phase low alloy material

AISI 4161H is a low alloy material that is currently used to manufacture coil springs for the TechnipFMC portfolio of actuated gate valves. The coil springs are designed to operate in a subsea service environment for a minimum of 25 years, which can equate to conditions of a working water depth of 10,000 feet (3048metres) and pressure of 10,000 psi. As the Coil Spring is the main method of valve closure, failure of the respective material can lead to catastrophic consequences. The design has no redundancy, therefore coil spring breakages and loss of load can lead to failure in the closure of the valve gate, which is the main failsafe system, controlling the flow of oil and gas from the seabed. Throughout the initial development of the respective coil spring material, TechnipFMC has discovered that the necessary metallurgical properties requirements have not been consistently met. The initial work, which was conducted between 2012 and 2014, established that the material contained microstructural variability, which produced mechanical properties that did not meet the design intent of the coil spring. These findings were found with material procured from different mills and by two separate OEM's who hot formed the raw bar into final coil spring products. Failure to meet the design requirements, affects the functionality of the coil spring to have enough stored energy to act as failsafe mechanism to close the respective valve. To address this problem, a comprehensive design of experiment programme has been developed as a series of characterisation and validation testing, to determine the fundamental properties of the AISI 4161H material type, using different heat treatment operations and conditions. This is considered paramount, as current industry requirements do not mandate any testing or material characterization, other than a basic metallurgical assessment. These requirements and level of governance are considered inadequate by the author, as the industry controlling standards do not define the correct pass / fail criterion that ensures such a critical product will not fail in-service. The research programme contained within this thesis, addresses the reasons for the variability exhibited by the AISI 4161H material, and determines how the variation can be influenced by exposure to different hot working and heat treatment conditions. The subsequent findings from the programme will enable engineers to determine the limitations of the material, and the effect variability has on the functionality of the coil spring for subsea applications. The characterisation will also allow the industry governing bodies to align their respective test requirements and acceptance standards to that of a heterogeneous material, which contains variability throughout its cross-sectional thickness.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral-pg

Grantor dc:publisher.institution

University of Strathclyde

Year dc:date.issued

2017

Author dc:creator

• Gill, David

Advisors dc:contributor.advisor

• Nash, David
• McLaren, Andrew

DOI

10.48730/8td1-dg86

Identifier

T14752

Author Identifier

201580424

OAI identifier oai:identifier

oai:strathclyde:pc289j07j