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Effect of the Superduplex Microstructure on the Hydrogen Embrittlement Susceptibility

Superduplex stainless steel UNS S32750 / EN 1.4410 presents very high corrosion resistance together with elevated mechanical strength making it especially suited for subsea systems. Despite the material excellent properties a number of in-service failures have been recorded especially when it is subjected to cathodic protection. This grade is also known to be sensitive to intermetallic phase precipitation e.g. σ-phase whose deleterious effect is well documented. Therefore manufacturers have optimized metallurgical routes to deliver superduplex products free from intermetallic phases. However very few residual particles may be present under the form of centerline intermetallic stringers and their effect on hydrogen embrittlement is not well documented.The present study aims at evaluating the influence of the intermetallic phases in centerline stringers on the hydrogen embrittlement susceptibility of superduplex stainless steel. Heat treatment have been performed on superduplex sheets in order to obtain different configurations of intermetallic phase – free from σ-phase presence of centerline intermetallic stringers presence of σ-phase over the full thickness. Specimens have been then tested by means of slow strain rate tensile test in simulated deep sea water (0.5 M NaCl 4 °C) under cathodic polarization at -1100 mV/SCE. All duplex microstructures and fracture surfaces were investigated using low and high magnification microscopy. The strain to fracture is estimated by the section reduction at the fracture surface and the hydrogen embrittlement susceptibility is defined for each microstructure as the reduction of the strain to fracture compared to a reference test in deionized water at 4 °C (RAR).With the presence of hydrogen fracture mode changes from ductile to transgranular quasi-cleavage. No effect of centerline intermetallic stringers (up to 0.08% of sigma σ-phase) on hydrogen embrittlement susceptibility has been measured.Considering the sample with σ-phase over the full thickness the resistance to hydrogen embrittlement is artificially high due to the low initial ductility measured in the reference test. Therefore the influence of hydrogen was only insignificant as observed on the fracture surface which remains ductile.

Product Number: 51319-13062-SG
Author: Pauline Huguenin
Publication Date: 2019
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