ABSTRACT

Oil & Gas treatment and petroleum refining involve aggressive environments containing aqueous hydrogen sulfide (H2S). Carbon-manganese steels used in these industrial sectors may be susceptible to various kinds of cracking phenomena namely Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC) and Stress Oriented Hydrogen Induced Cracking (SOHIC).

The HIC risk can be avoided by a strict control of the chemical composition of the steel (sulfur, phosphorus, oxygen) associated to a specially-adapted production route. SSC can be limited by a careful control of the hardness level of both the parent material and the heat affected zone as well as a low impurity level and appropriate PWHT.

SOHIC is a critical problem in the Oil & Gas industry since it has been involved in many in-service failures. This phenomenon can be described as a combination of both SSC and HIC. Previous studies have shown that the measures taken to counter HIC and SSC are not sufficient to protect carbon- manganese steels against SOHIC.

The SOHIC susceptibility of SA516 Grade 70 carbon-manganese steel was studied according to NACE MR0175/ISO15156-2 standard by means of the four-point-bending and constant load test methods. Both base metal and welded specimens were investigated. The results show that the microstructure also plays an important role and that only quenched and tempered steels with appropriate PWHT can offer SOHIC resistance when combined with an optimized chemical composition and steel fabrication route.

INTRODUCTION

Oil & Gas treatment and petroleum refining involve aggressive environments containing aqueous hydrogen sulfide (H2S). Carbon-manganese steels used in these industrial sectors may be susceptible to various kinds of cracking phenomena, namely Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC) and Stress Oriented Hydrogen Induced Cracking (SOHIC).

SOHIC is a critical problem in the Oil & Gas industry since it has been involved in many in-service failures 1-4. This phenomenon can be described as a combination of both SSC and HIC. It has been demonstrated that the measures used to avoid HIC especially the control of sulfur and phosphorus contents will also apply to the control of SOHIC propagation 5-7. However a previous study has shown that these measures are not always sufficient to protect carbon-manganese steels from SOHIC 8.

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