Corrosion Fatigue Crack Propagation Behaviour of a High Strength Low Alloy Steel in a Synthetic Sea Water Environment.

摘要

The corrosion fatigue crack propagation behaviour of a highstrength low alloy steel, N-A-XTRA 70, in a synthetic seawater solution was tested using S. E. N. specimens subjectedto a loading frequency of 0.1 Hz and a load ratio of 0.6.In order to simulate the conditions encountered by a thumbnailtype crack several specimens from each of themicrostructural types tested, namely parent plate, heataffected zone and heat treated material, had their cracksides covered by transparent plastic covers. Severe overprotectionand slight underprotection conditions wereproduced using cathodic protection potentials of -1400,-1300, -1200 and -700 mV (S. C. E. ).The Paris relationship da/dN = CLKm was found to be a usefultool in describing the crack propagation rate data. Resultsobtained, presented in the form of plots of log da/dN againstlog AK, show that for parent plate, H. A. Z. and heat treatedmaterial, covering the crack sides of specimens producesenhanced corrosion fatigue crack propagation rates, atcathodic protection potentials of -1400 and -1300 mV (S. C. E. ),when compared to non covered specimens. This trend was alsotrue for H. A. Z. specimens at a potential of -700 mV (S. C. E. ).For parent plate specimens, however, covering the crack sidesat a potential of -700 mV (S. C. E. ) produced reduced crackpropagation rates over non covered specimens. It is believedrestriced oxygen access may account for these results.Plots of the Paris exponent m and constant C for the threemicrostructures tested produced three lines of the formm= alnC +b where a and b were found to be dependent uponmaterial parameters.Comparison of results with BS 4360: 50D revealed that N-A-XTRA70 exhibited superior fatigue performance when tested in airbut behaved worse under conditions of free corrosion.