Abstract

The Super duplex stainless steels (SDSS) are characterized by a remarkable combination of corrosion resistance and mechanical strength, mainly in chloride containing environments. Because of this, its demand is growing every year in offshore applications, becoming an alternative to the ferritic and austenitic stainless steels. Nevertheless, due to the lack of oil, the offshore companies are moving toward deeper waters and harsher environments, where it calls for the need of stronger materials. Facing this scenario, the Hyper duplex stainless steel (HDSS) was developed to achieve better properties than SDSS.

Despite the pronounced properties, the duplex grades can suffer environmentally induced cracking, as recorded in some failure cases of subsea structures. The driven force for these occurrences was the combination of three main factors: stress/strain, inappropriate microstructure and presence of hydrogen due cathodic protection (CP). In general, this set of events is called Hydrogen Induced Stress Cracking (HISC) within the offshore industry.

The SDSS and HDSS tubes in as-received condition were divided in two groups. One group was strain-hardened in order to simulate fabrication and laying operation. The second group was tested in annealed condition. For each, the tubes were cathodically charged for 160 hours in artificial seawater, under or without elastic stress, at cathodic potential equivalent of -1350 mVSCE. Then, the charged tubes were submitted to tensile testing and their HISC susceptibility was assessed. The fracture surfaces were analysed in order to identify the active micro mechanisms of failure.

The results showed that for the stressed/strain-hardened samples the embrittlement was more pronounced than the unstressed/annealed ones.

Introduction

Super duplex stainless steels (SDSS) are dual-phase steels containing austenite (γ) and ferrite (δ) as close to 50/50% distribution as possible. The synergic effect between both phases promotes a good combination of high mechanical strength and excellent resistance to stress corrosion cracking in chloride solutions, as well as in the presence of H2S and CO2. Due to the excellent agreement of properties, these steel grades are widely employed in the petroleum industry, highlighted in offshore applications such as subsea equipment, umbilical tubes and liner pipes [1–4].

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