10309 Validation of the Constant Extension Rate Test for High Strength OCTG Steels as a Ranking Tool for SSC Resistance

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Product Number: 51300-10309-SG

ISBN: 10309 2010 CP

Author: Michaela Hoerstemeier, Christoph Bosch, Laurent Delattre and Bertine Orlans-Joliet

Publication Date: 2010

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Susceptibility of steels for OCTG to brittle cracking (sulfide stress cracking, SSC) is most frequently evaluated by NACE TM0177 Method A, which covers the testing of metals subjected to tensile stresses for resistance to cracking failure in low-pH aqueous environments containing H₂S. As the NACE tensile test is a long term test which successfully qualifies a material not before 720 hours test duration, this test is difficult to apply as a quick ranking tool, although this test is mandatory for the final qualification of a material for certain test conditions. On the other hand, the CERT (Constant Extension Rate Test), which is also known as SSRT (Slow Strain Rate Test) when used for corrosion resistant alloys under elevated pressure and temperature, has been used in the past as a standardized method for screening CRA materials for use in oilfield production environments.

In the present investigation different CERT test series have been performed on 110, 125 and 140 ksi grade materials for OCTG to validate the capability of the CERT for ranking materials regarding their susceptibility to SSC. Test results were evaluated using different ductility parameters and time-to-failure ratios which provide the comparison to NACE A tensile test data. Suitable test conditions (mildly sour environment with 3 % H₂S in CO₂ at pH 4.5) have been identified for successful ranking HSLA steels with regard to SSC performance. The correlation of NACE A test results with the plastic elongation ratio EPR from CERT was clearly shown for different chemical composition having similar yield strength as well as for same chemistry subjected to different heat treatments.

Keywords: sulfide stress cracking, NACE Method A, constant extension rate test, OCTG, high strength materials