Fatigue Crack Growth of a 420 MPa Structural Steel Heat Affected Zone at Low Temperatures

摘要

The development of oil and gas fields in the arctic brings to the tableseveral challenges in the use of structural steels, particularly concerningtheir low-temperature properties. Among others, also fatigue behaviorneeds to be accounted for when using structural steels for arcticapplications. As for static fracture, ferritic steels experience a fatigueductile to brittle transition (FDBT) when temperature is decreasedbelow a certain temperature. This may result in higher crack growthrate and, consequently, unpredicted fatigue-related failure. In order toshed some more light on this phenomenon, fatigue crack growth testshave been performed on a 420 MPa structural steel weld simulatedcoarse grained heat affected zone (CGHAZ) at different temperatures:room temperature, -30, -60, -90 and -120 ˚C, with -60 ˚C considered asa possible design temperature relevant for the most extreme arcticareas. Post-mortem fracture surface investigations have been alsoconducted in order to confirm the expected switch in fatigue crackgrowth mechanisms as temperature is lowered below the FDBTtemperature. Finally, two analytical equations, valid for temperatureranges above the FDBT, were established based on the experimentalresults to relate yield strength and temperature variation of the Parislaw constants . These are used to quantify the temperature impact onthe designed fatigue life, and the results are compared to the actualdesign rules (BS 9710).