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