Sour Testing of 316L Stainless Steel: Comparison of Four-Point Bend and Uniaxial Tensile Test Methods

In order to resolve reported discrepancies between the results of four-point bend tests and uniaxial constant load tests on UNS S31603 (316L SS) in sour environments, a series of tests has been conducted to compare the two test methodologies under carefully controlled conditions. It was demonstrated conclusively that tests performed in uniaxial tension at constant load are inherently more severe than tests performed in constant displacement four-point bending due primarily to the presence of low-temperature creep in the former. In the presence of H₂S, the combination of dynamic plastic strain, accumulated plastic strain, and applied stress in the uniaxial tension specimen is considered to more readily activate pitting and cracking. In contrast, for the four-point bend specimen, plastic deformation is limited because creep is constrained and the work-hardened, nanocrystalline layer formed during surface preparation deforms primarily elastically. In selecting a test method for qualifying alloys for sour service, consideration needs to be given to the extent to which creep is significant, while recognizing that this may be constrained by cold work, e.g., from hydrostatic testing. For four-point bend testing, the specified 0.2% plastic strain level may not be achieved when adopting the current NACE TM0316 standard due to work-hardening during surface preparation. Similarly, in constant load testing at 90% σ_0.2 in the absence of creep, the plastic strain at the surface would be less than expected for the same reason.