Performance Characterisation of Additively Manufactured Grade 2209 Duplex Stainless Steel for Oil & Gas Industry Applications

Abstract

The work summarised in this PhD thesis aims to investigate and correlate the microstructure of wire and arc additively manufactured (WAAM) duplex stainless steel (DSS) type 2209 to its corrosion performance in a chloride-containing environment for the oil and gas industry application. DSS is utilised for its excellent corrosion resistance and mechanical performance. The adaption of WAAM offers advantages, such as freedom of design and reduced supply chain cost. However, not much information is available in the open literature about the feasibility of producing DSS via WAAM, and there is a lack of an in-depth discussion of corrosion performance of WAAM produced microstructures when exposed to a chloride-containing environment. The work reported here focuses on the effect of deposition parameters on the produced microstructure and its localised corrosion performance. A complex banded microstructure consisting of ferrite and multiple austenite types was formed. The austenite content varies across the building direction, with the lowest austenite fraction observed at the top pass and close to the build plate. Some oxide inclusions and nitride particles were observed with no evidence of the presence sigma phase was found. In a neutral pH chloride environment, the formation of acicular austenite affected the localised corrosion performance due to its lower Cr, Mo, and N content compared to ferrite, reducing the pitting resistance equivalent number (PREN). The utilisation of high heat input and interpass temperature resulted in higher pitting susceptibility due to the formation of a higher fraction of acicular austenite. Therefore, it is essential to minimise the amount of acicular austenite by utilising lower heat input and interpass temperature. In a low pH chloride environment, depassivation and general corrosion can occur in WAAM-DSS. The austenite phase exhibited a higher electrochemical corrosion potential due to the higher Ni content compared to the ferrite. The lower corrosion potential of ferrite leads to a higher dissolution rate. Moreover, some discrete regions, such as oxide inclusions, Cr-nitride precipitates, and phase boundaries, displayed lower potentials, leading to higher dissolution rates. Overall, WAAM-DSS components displayed a promising resistance against general and localised pitting, which was higher than those reported for conventional welding, with a sufficient level of austenite content for engineering application which requires strength, ductility, with an acceptable hardness level.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Dirk Engelberg (Supervisor) & Matthew Roy (Supervisor)