The resistance to hydrogen assisted cracking of welded supermartensitic stainless steel. (SMSS) under cathodic protection conditions has been evaluated by ...
Testing of supermartensitic stainless steel welds under cathodic protection at very low strain rates G Hinds and A Turnbull Materials Centre National Physical Laboratory Teddington, Middlesex U.K., TW11 0LW
ABSTRACT The resistance to hydrogen assisted cracking of welded supermartensitic stainless steel (SMSS) under cathodic protection conditions has been evaluated by the slow strain rate technique. Slow strain rate testing at very low strain rates (nominally 10-8 s-1) generated failure close to the 0.2 proof stress of the parent material, whereas failure in conventional testing at 10-6 s-1 occurred at much higher plastic strains (6% - 11%). In comparison to the effect of strain rate, the influence of temperature and surface condition was relatively small. These results demonstrate that adopting very low strain rates is critical when evaluating the resistance of SMSS welds to hydrogen embrittlement. KEY WORDS: slow strain rate testing, supermartensitic stainless steel, cathodic protection, hydrogen embrittlement, welds. INTRODUCTION Failure of welded supermartensitic stainless steel (SMSS) pipeline in seawater under cathodic protection is a major concern for the oil and gas industry but as yet no consensus has been reached on the most appropriate laboratory qualification tests1-5. Recent work at NPL has assessed the suitability of both cyclic and monotonic slow strain rate testing for evaluating resistance of supermartensitic stainless steel welds to hydrogen embrittlement in 3.5% NaCl under cathodic protection conditions5. Tests were carried out on ground cylindrical specimens at –1.100 VSCE at a nominal strain rate of 10-6 s-1 at 23 C. It was demonstrated that there is no benefit in cyclic slow strain rate testing as most of the cyclic plastic strain is generated in the first cycle with negligible cyclic plastic strain in subsequent cycles due to work hardening. In the monotonic slow strain rate tests, failure occurred at plastic strain values in the range 6-11%, which is well above the levels encountered in service. The purpose of this work was to evaluate whether a more representative test could be established by investigating the effect of: strain rate (10-6 s-1 and 10-8 s-1); surface condition (ground and as-welded specimens); temperature (5 C and 23 C).
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EXPERIMENTAL PROCEDURES The material tested was a 12 Cr-5 Ni-2 Mo steel supplied in the form of a pipe with an external diameter of 203 mm and a wall thickness of 11.1 mm. The chemical composition is shown in Table 1.
Table 1. Chemical composition of parent material (mass %, balance Fe). C
