The NACE TM0177 Method A Uniaxial Tensile test using a proof-ring device is extensively employed in order to assess the Sulfide Stress Cracking (SSC) susceptibility which can occur on Oil Country Tubular Goods (OCTG) and Line Pipes (LP). It is also well understood that different test methods may not necessarily provide the same ranking. This is the reason why replicate testing is advisable to tighten as much as possible the scattering of the SSC threshold. Nevertheless, SSC test results can show statistical variability for the same material. The aim of this paper is to check whether the applied stresses are uniformly distributed within the tensile specimen. Shared data regarding the presence of bending phenomenon are compared when stressing test specimen instrumented with strain gages. Order of magnitude is also quantified on different test equipment. Discussion on the influence of other parameters such as machining, polishing, storage of the specimens and test procedure is presented as well in order to share the experience on the NACE Method A test.
The NACE TM0177 Method A1 uniaxial tensile test is extensively used to assess the Sulfide Stress Cracking (SSC) susceptibility of Oil Country Tubular Goods (OCTG) and Line Pipes (LP). SSC phenomenon may occur in materials subjected to stress (applied or residual) in a hydrogen sulfide (H2S) containing environment. Method A tensile test consists of uniaxial stressing of specimen up to a specified stress level (below the Yield Strength). Several types of stressing frames may be used to perform the test, but this paper will only focus on tests carried out with proof ring as stressing frame. This test gives a failure/not-failure result after one month testing in sour service environment. The sour service environment consists of chloride containing solution saturated with H2S gas. A testing device is presented in Figure 1. When multiple specimens are tested at various stress levels, an apparent threshold stress level for the occurrence of SSC can be obtained. NACE Method A tests can be carried out in environments with various combinations of H2S partial pressure and pH for Fit-for-Purpose investigations. A key issue of the SSC NACE Method A test is the statistical variability of results for a specific material tested in the same conditions. Some factors were identified as influencing Method A test results and are presented in the first part of this paper. The second and third parts of this study are devoted to bending assessment in tensile test specimen stressed using proof rings.
Method A test result dispersion may be linked to a wide number of factors. An inventory of every factor which is likely to affect the Method A test results is presented on Figure 2. Some of them were spotted as critical factors towards SSC test result dispersion. These factors can be divided into three groups, the first one referring to specimen preparation, the second one referring to specimen loading, and the third one referring to test set up and control of test conditions.
