Microstructure and FCG path in seawater under sinewave

A metallographic study was carried out to assess the cause of the disparity in the FCGR s between NR and TMCP as observed in Fig. 16. The fractographs for the NR and TMCPsteels in SW are shown in Fig. 17. Fig. 17(a) is the fractograph obtained in the present study for J2N and Fig. 17(b) is for the same steel obtained by Adedipe [63]. Fig. 17(c) is the corrosion-fatigued surface of the G10 obtained at about 20MPa√m and Fig. 17(d) is the common feature of the corrosion-fatigued surface of the same material above about 23 MPa√m. Generally, cleavage cracking was not observed and the failure mode remains by ductile striation which is typical of the failure mechanism in the Paris Region for low carbon steel.The striations in SW simply appeared to be washed out by the corrosion process and the surfaces were covered by the corrosion products. This washing out progressed with time as can be seen from (c) to (d) in Fig. 17. The secondary cracks in the TMCP steel were larger than that of NR and the corrosion products appear to collect into these secondary cracks over time as seen in Fig. 17(d).
The collection of the corrosion products into the cracks has the potential of reducing the crack driving force, hence, the rate at which the crack is moving. However, the flow of test environment and pumping of theSW in and out of the crack tip as a result of the cyclic action – especially under sinewave, has the capacity of removing corrosion products at the crack tip. Hence, oxide-induced retardation effect may be minimal. Moreover, crack closure phenomenon is little or negligible in the Paris Region. Since there was no cleavage cracking in the J2N, retardation due to the corrosion product or the oxide-induced crack closure is not sufficient to account for the large difference inCFCGR s in Fig. 16. An altervative approach again is to examine the crack path and the microstructural features influencing crack growth in the medium. Fig. 18(a) shows crack path in the G10 material inSW . Fig. 18(b) is a schematic of Fig. 18(a). The direction of all the crack growth is from left to right and the direction of applied fatigue force is as given by the yellow double-edge arrow. The crack path was observed for the G10 under sinewave, 9kN and 0.2Hz between the ∆K of 19.34 and 30.19 MPa√m. The same phenomena of high angle crack path diversion, multiple crack branching and metal crumbs phenomena characteristic of this TMCP steel in air were also observed in SW (as shown in Fig. 18(b)). But, the number of the branched crack fronts is somewhat less and the extension or length of the branched arm is short compared to that of air. It then appears that the corrosion process had limited the length of the arm of the branched crack through a blunting process. Metal crumbs were also observed, but the extent is small compared to the air test.
Another feature that can be seen is the corrosion attack of the separated surfaces which resulted to widening of the fatigued surfaces as compared with the test in air (see Fig. 11). This widening process may be due to elimination of the small metal crumbs, plastically deformed wake and the ‘microplastic -zone by the corrosion process. Themicroplastic zone as used, refers to the closest area(s) to the crack tip that experienced the greatest lattice distortion, stress intensity or plastic deformation as a result of the amplified cyclic force at the crack tip as it opens and closes. These are areas of high lattice energy and are expected to be the points of greatest corrosion rate. Another observation of high importance is that the crack tips of both the main and branched crack fronts from number 1 to 5 are largely sharp as shown by the magnified micrographs of Fig. 18(c). However, the areas immediately behind the crack tips are widened by the corrosion process as compared with that of the air. It was observed also that the sharpness of the crack tip decreased gradually with increase in the ∆K . It may be that at high ∆K , the crack-tip was sufficiently open to allow some degree of blunting by the corrosion process. This generally suggests that the crack tip under 9kN, 0.2Hz, sinewave was always sharp up to a particular ∆K , before it started experiencing some significant blunting effect of the corrosion process. Note that the sharpness of crack tip ensured that the crack growth was not retarded. The blunting phenomenon may not have much retarding effect at very high ∆K because the crack driving force is enough to continually nucleate sharp crack fronts. In other words, the mechanical fatigue component leads the corrosion process in this domain. This suggests then that at the Paris Region, two domains appear to exist – corrosion dominated and mechanical-fatigue dominated domains. The corrosion-controlled domain appears to be strongly influenced by the K max and time of crack tip exposure to the corrosive environment [67].
Fig. 19(a) is a typical nature of the crack path in the J2N steel inSW . Fig. 19(b) is a schematic of Fig. 19(a). The crack propagated with predominance of low angle crack-tip diversion, very limited low angle crack branching with the shorter arm in comparison with theTMCP steels. The crack moved on a horizontal plane with minimal zig-zag motion. The crack growth pattern is generally the same with that of air (see Fig. 12(a)) except that the branched arms are shorter and there is widening of the crack gap as a result of the corrosion process. The branched crack tips are relatively blunted, but the main crack tip appeared to maintain its sharpness. No crumb formation was seen, and crack diversion and bifurcation are less than that in air. Hence, one would expect a very high CFCGR for this crack growth pattern if blunting of the main active crack tip did not occur.
In Fig. 19, it appears that there is no predominant features of the phases influencing the crack growth, except that the crack diverted in a zig-zag manner which is a consequence of the crack propagation mechanism. A closer observation shows some influencing microstructural features. Some of the distinct micrographs showing phases and morphologies influencing the CFCGR for the J2N are presented in Fig. 20. However, we could see that the branched crack tips appeared to be following the α/α andα/P interfaces as shown by the arrows. In other words, the preferred crack paths appeared to be the α/α and α/P boundaries. The black arrow is the α/α while the blue arrow is the α/P. In fact, these boundaries are decorated by the ferrite phase ribbon of high alloy content and the crack followed the αHA phase. We can also note the widening activity near and immediately behind the crack tip as it propagated. This is likely to be as a result of intense corrosion activity around the microplastic zone.
Fig. 21 shows the comparison of the FCGR curves for the air andSW tests under sinewave and the experimental conditions are shown in Table 8. Fig. 21(a) shows that CFCGR of the 0.2Hz is higher than that of air by an average factor of 2.5 for the J2N steel. Above about 24 MPa√m, the rate increased up to an average factor of 4.0. These values are typical of normalisedα-P steels of the same microstructure. The sudden jump in the CFCGR to a factor of about 4 can be explained by examining the crack path in air and SW .
Table 8: Specimen dimensions and loading conditions in air and SW