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03132 TESTING OF SUPERDUPLEX STAINLESS STEEL FOR SOUR SERVICE

Product Number: 51300-03132-SG
ISBN: 03132 2003 CP
Author: Paul Woollin, Manuel Maligas
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Superduplex stainless steels are widely used by the oil and gas industry for their combination of corrosion resistance, chloride stress corrosion cracking resistance, high strength and weldability. Where such materials are approved for sour service by NACE MR0175, maximum hardness limits are stated, together with allowable material condition, limits of H2S partial pressure and in some cases limits on chloride ions, pH, temperature and elemental sulfur. This paper gives results of sour service testing of three different UNS superduplex grades and three batches of one UNS superduplex grade in environments representing conservative worst case conditions. Unstressed pitting corrosion tests were performed on all steels examined. Sulfide stress cracking (SSC) tests were performed on two steels that did not corrode in the unstressed tests. Two test stressing methods, i.e. C-ring testing and slow strain rate tensile (SSRT) testing, were compared. The results indicated that some superduplex steels may be resistant to sulfide stress cracking at higher partial pressures of H2S than currently allowed by NACE MR0175, if other environmental or material conditions are favourable. However, different batches of superduplex steel, either within a UNS composition range or from different UNS compositions, may show quite different resistance to pitting and SSC in sour environments and there is no single maximum partial pressure of H2S to which superduplex steels are resistant. In particular, higher PREN/PRENW values are beneficial but coarse grain size with high ferrite content and nitride precipitates may be detrimental. Levels of primary alloying elements, other details of the service environment (e.g. pH and Cl- level) and the mode of stressing (e.g. load or displacement control) may all affect resistance to SSC of superduplex steels. Constant load, dead weight tensile testing is preferred to constant deflection Cring or bend tests where constant load conditions may exist in service. SSRT testing is very severe forevaluating SSC performance of superduplex steels and a reliable pass/fail criterion is required. Keywords: sour service, superduplex stainless steel, test methods, microstructure, composition.
Superduplex stainless steels are widely used by the oil and gas industry for their combination of corrosion resistance, chloride stress corrosion cracking resistance, high strength and weldability. Where such materials are approved for sour service by NACE MR0175, maximum hardness limits are stated, together with allowable material condition, limits of H2S partial pressure and in some cases limits on chloride ions, pH, temperature and elemental sulfur. This paper gives results of sour service testing of three different UNS superduplex grades and three batches of one UNS superduplex grade in environments representing conservative worst case conditions. Unstressed pitting corrosion tests were performed on all steels examined. Sulfide stress cracking (SSC) tests were performed on two steels that did not corrode in the unstressed tests. Two test stressing methods, i.e. C-ring testing and slow strain rate tensile (SSRT) testing, were compared. The results indicated that some superduplex steels may be resistant to sulfide stress cracking at higher partial pressures of H2S than currently allowed by NACE MR0175, if other environmental or material conditions are favourable. However, different batches of superduplex steel, either within a UNS composition range or from different UNS compositions, may show quite different resistance to pitting and SSC in sour environments and there is no single maximum partial pressure of H2S to which superduplex steels are resistant. In particular, higher PREN/PRENW values are beneficial but coarse grain size with high ferrite content and nitride precipitates may be detrimental. Levels of primary alloying elements, other details of the service environment (e.g. pH and Cl- level) and the mode of stressing (e.g. load or displacement control) may all affect resistance to SSC of superduplex steels. Constant load, dead weight tensile testing is preferred to constant deflection Cring or bend tests where constant load conditions may exist in service. SSRT testing is very severe forevaluating SSC performance of superduplex steels and a reliable pass/fail criterion is required. Keywords: sour service, superduplex stainless steel, test methods, microstructure, composition.
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