Hydrogen embrittlement stress corrosion cracking of superduplex stainless steel

摘要

Two superduplex stainless steel hubs on a subsea manifold subject to cathodic protection failed as a result of hydrogen embrittlement stress corrosion cracking. A series of tests was performed to establish the threshold condition for cracking. The tests included: (i) constant-load smooth bend tests, (ii) constant-deflection smooth bend tests, (iii) constant-load pre-cracked bend tests, (iv) interrupted slow strain rate tensile tests, (v) constant-load tensile tests, (vi) approximately constant-strain tensile tests and (vii) full-scale hub tests. The testing identified a very marked difference in material response under load-control and displacement-control. Under constant-load conditions, tensile testing indicated a threshold stress for crack initiation and propagation in the hub material in 50 days of 545MPa, equivalent to an initial strain of 0.5 percent. Strain continued to develop over the test duration, due to low temperature creep, to 0.9 percent after 50 days. Full-scale hub tests confirmed that this threshold level was appropriate to the hubs and that residual stress in the hubs contributed to cracking. In displacement-controlled bend tests, with deflection prior to exposure, threshold strains of 2.1 percent and 8 percent were identified for crack initiation and propagation respectively. Comparison with previously published work and powder metallurgy pipe indicated that the hub material was particularly sensitive to hydrogen embrittlement stress corrosion as a consequence of its microstructure, which had coarse aligned grains and nitrides/carbonitrides. Ferrite volume fraction and hardness were apparently of secondary importance.