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51316-7025-The Detection Of Alpha Prime In Duplex Stainless Steels

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 The alloys (used in chem/process industries) are usually in the solution annealed condition & must be fast cooled from the annealing temperature to prevent the precipitation of third phases such as sigma chi nitrides & alpha prime. This paper describes a reactivation test to detect alpha prime - used with 22%Cr and 25% Cr alloys. Case studies.

Product Number: 51316-7025-SG
ISBN: 7025 2016 CP
Author: Roger Francis
Publication Date: 2016
Industries: Process Industries , Corrosion Monitoring and Control
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Duplex stainless steels are widely used by the oil and gas and chemical and process industries because of their combination of high strength and corrosion resistance. The alloys are usually used in the solution annealed condition and must be fast cooled from the annealing temperature to prevent the precipitation of third phases such as sigma chi nitrides and alpha prime. Alpha prime precipitates in the temperature range 550 to 300°C. It forms less readily than sigma phase and other intermetallic precipitates and so is not normally found in commercially produced duplex alloys. However poor cooling of duplex steels through this temperature range or repeated excursions in to this temperature range can result in its formation. Alpha prime dramatically reduces impact toughness and increases susceptibility to hydrogen induced stress corrosion cracking due to cathodic protection. However corrosion resistance in chloride environments appears to be unaffected by this phase. Alpha prime cannot be seen under optical microscopes because of its very small size. This means that the combination of corrosion testing in ferric chloride solution microstructural examination and impact testing can be ineffective in detecting alpha prime precipitation principally because of the low toughness acceptance level of 45 Joules at -46°C that is commonly specified. This paper describes a simple electrochemical reactivation test to detect alpha prime that can be used with both 22%Cr and 25% Cr alloys. The paper also presents some case studies where low toughness occurred but no third phases were visible in optical microsections. The test was used to confirm the presence of alpha prime and determine what should be done to correct the problem.

Key words: downloadable, Duplex Stainless Steel, Third Phases, Impact Toughness, Hydrogen Embrittlement

Duplex stainless steels are widely used by the oil and gas and chemical and process industries because of their combination of high strength and corrosion resistance. The alloys are usually used in the solution annealed condition and must be fast cooled from the annealing temperature to prevent the precipitation of third phases such as sigma chi nitrides and alpha prime. Alpha prime precipitates in the temperature range 550 to 300°C. It forms less readily than sigma phase and other intermetallic precipitates and so is not normally found in commercially produced duplex alloys. However poor cooling of duplex steels through this temperature range or repeated excursions in to this temperature range can result in its formation. Alpha prime dramatically reduces impact toughness and increases susceptibility to hydrogen induced stress corrosion cracking due to cathodic protection. However corrosion resistance in chloride environments appears to be unaffected by this phase. Alpha prime cannot be seen under optical microscopes because of its very small size. This means that the combination of corrosion testing in ferric chloride solution microstructural examination and impact testing can be ineffective in detecting alpha prime precipitation principally because of the low toughness acceptance level of 45 Joules at -46°C that is commonly specified. This paper describes a simple electrochemical reactivation test to detect alpha prime that can be used with both 22%Cr and 25% Cr alloys. The paper also presents some case studies where low toughness occurred but no third phases were visible in optical microsections. The test was used to confirm the presence of alpha prime and determine what should be done to correct the problem.

Key words: downloadable, Duplex Stainless Steel, Third Phases, Impact Toughness, Hydrogen Embrittlement

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