Abstract
In the oil and gas industry, low alloyed quenched and tempered martensitic steels are commonly used as OCTG (Oil Country Tubular Goods) materials for well construction and completion. Environmental assisted failure in the form of sulfide stress cracking (SSC) is a major risk related to the hydrogen absorption promoted by corrosive acidic well environments containing hydrogen sulfide (H2S). Therefore, an accurate determination of the SSC threshold stress intensity factor (KISSC) is essential for a safe and cost-effective well design. The Double Cantilever Beam (DCB) is the standardized crack arrest test (method D of NACE TM0177) currently used for the evaluation of KISSC although suffers from limitations: KISSC as obtained by DCB is not a material property for a given hydrogenating environment. Klimit has been introduced in the latest revision of the standard for suppressing the influence of the wedge thickness on KISSC, but it is dependent on initial crack length. In recent years, there has been increasing interest in using the Single Edge Notch Tension (SENT) test as an alternative approach for obtaining threshold KISSC values. The proposal to use SENT specimens is strengthened by its potential to offer a mechanical response closer to cracked tubular structures providing more consistent KI values. The present work aims to further compare these two testing methodologies by performing DCB and SENT tests on a low-alloyed, quenched and tempered martensitic steel with a yield strength of 820 MPa. Tests were performed in two environments presenting different ability to charge the material with hydrogen. Thereafter the provided threshold KISSC values from SENT and the Klimit from DCB are compared and discussed. The results show that KISSC-SENT tends to be more conservative than Klimit, especially in mild sour environment, which can be explained by crack arrest versus crack initiation configurations and time-dependent hydrogen absorption inside specimens.