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A Recent Case Of Intermetallic Precipitations In Super Duplex

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Production of oil and gas is well known to cause potential corrosion issues due to the CO2 content in the well stream. Carbon steel is widely used for production facilities as e.g. flowlines and manifolds, however, aging of the reservoir increases the number of corrosive agents, such as e.g. CO2, which are known to cause high corrosion rates in carbon steel. Therefore, carbon steel piping is often being replaced with super duplex stainless steel due to its high strength, excellent toughness and good corrosion resistance. Replacing carbon steel with super duplex has been conducted on several mature offshore oilfields in the European North Sea region.

Product Number: 51322-17627-SG
Author: Henrik Bang, Jakob Mølholm, Kirsten Grønning Sørensen
Publication Date: 2022
Industries: Corrosion Monitoring and Control , Coatings and Linings , Coatings
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This article is based on a failure analysis, where a pressurized 4” super duplex (UNSS32760) elbow from a hydrocarbon system suddenly leaked during service, due to a 90 mm long crack.
The crack originated from a corrosion pit and was caused by intermetallic precipitations in the microstructure in a small area of the elbow. The intermetallic precipitations were the result of inadequate heat treatment of the elbow during production. A screening procedure based on ferrite measurements applying handheld ferrite measurement devices was developed. If the screening revealed ferrite values outside nominal values, in-situ metallography was performed to either confirm or deny the presence of intermetallic precipitations. On-site metallography was performed by grinding, polishing and electrolytical etching with NaOH. Images were recorded, applying a portable USB microscope at 450x magnification.
Intermetallic precipitations were found in other SSDS (Super Duplex Stainless Steel) items from other suppliers as well. In total, 5681 items were screened, 96 (1.7 %) had a ferrite content lower than 35%. Of these 96 items, 25 were examined with in-situ metallography and all 25 had significant amounts of intermetallic precipitations. Impact toughness testing (Charpy V-notch) of SSDS items with intermetallic precipitations at different temperatures ranging from -50°C to +20°C revealed very poor toughness without any indications of a transition temperature.

This article is based on a failure analysis, where a pressurized 4” super duplex (UNSS32760) elbow from a hydrocarbon system suddenly leaked during service, due to a 90 mm long crack.
The crack originated from a corrosion pit and was caused by intermetallic precipitations in the microstructure in a small area of the elbow. The intermetallic precipitations were the result of inadequate heat treatment of the elbow during production. A screening procedure based on ferrite measurements applying handheld ferrite measurement devices was developed. If the screening revealed ferrite values outside nominal values, in-situ metallography was performed to either confirm or deny the presence of intermetallic precipitations. On-site metallography was performed by grinding, polishing and electrolytical etching with NaOH. Images were recorded, applying a portable USB microscope at 450x magnification.
Intermetallic precipitations were found in other SSDS (Super Duplex Stainless Steel) items from other suppliers as well. In total, 5681 items were screened, 96 (1.7 %) had a ferrite content lower than 35%. Of these 96 items, 25 were examined with in-situ metallography and all 25 had significant amounts of intermetallic precipitations. Impact toughness testing (Charpy V-notch) of SSDS items with intermetallic precipitations at different temperatures ranging from -50°C to +20°C revealed very poor toughness without any indications of a transition temperature.

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Duplex stainless steels (DSSs) are based on the Fe-Cr-Ni system and are constituted of 30 to 70 % ferrite and austenite. They combine high tensile strength, good toughness, weldability, and excellent corrosion resistance including stress-corrosion cracking and resistance to localized corrosion.1-3 DSSs can be classified according to the Pitting Resistance Equivalent Number (PREN = Cr + 3.3 Mo + 16 N) in lean duplex (PREN= 22-27), standard (PREN = 28-38), super duplex (PREN = 38-45) and hyperduplex (PREN > 45).
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Metallurgical investigations on a welded sample of 25% Cr super duplex stainless steel (UNS S32750) from a vessel that had operated above 300-350°C (570-660°F) for six months, resulting in brittle fracture.
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