Fatigue crack propagation in high strength steels for use offshore
- Cranfield Univ., Bedford (United Kingdom)
The influence of applied cathodic protection levels on the corrosion fatigue crack propagation behavior of a number of welded high strength microalloyed steels has been determined. The types of steel and the yield strengths selected, which ranged from 450 to 650MPa, were intended to cover both existing and likely developments in offshore structures over the next decade or so. The steels were welded using the submerged arc process at a heat input of 3kJ/mm and the corrosion fatigue properties of the coarse grained heat affected zones (CGHAZ) were examined using S.E.N. specimens subjected to a loading frequency of 0.5Hz and a load ratio of 0.6 in a synthetic seawater solution. At high over potentials ({minus}1100mV) all steels showed significantly enhanced corrosion fatigue crack propagation rates compared to similar samples protected at {minus}800mV. Growth rates of up to an order of magnitude faster were measured in some cases. Crack path deviation in general decreased as the potential decreased. Much flatter, cleavage type modes of fracture propagation were sometimes observed, indicating that the faster growth rates were associated with hydrogen embrittlement problems caused by the excess hydrogen generated by the cathodic protection process. However, the resulting fatigue performance was found to be no worse than for conventional lower strength 350MPa structural steels which are presently being used offshore. The work examined the influence of alloy composition and microstructure on fatigue crack growth behavior and emphasizes the difficulties associated with assessing and comparing fatigue behavior of welded samples because of the different microstructures which are sampled by the growing fatigue crack tip in conventional test procedures. Finally, the work makes recommendation on suitable high strength alloy types for further development for offshore applications.
- OSTI ID:
- 27926
- Report Number(s):
- CONF-940230--; ISBN 0-7918-1266-9
- Country of Publication:
- United States
- Language:
- English
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