Fig. 11 Relationship of FCG rate and the stress intensity factor range
of BM, AS-HAZ and WNZ.
3.4. Morphological features of fracture surface
Fig. 12 shows the typical SEM fractographs of fatigue fracture features
at stable propagation (ΔK = 15 MPa m1/2) region. From
the fracture surfaces of BM and AS-HAZ, it can be seen that obvious
tearing ridges distributed along the propagation direction,
tearing dimples and small-scale
facets formed during crack propagation. Some coarse intermediate
Fe-containing particles which usually formed during casting were found
in tearing dimples (Fig. 12a). At higher magnification, fatigue
striations on the facets can be observed clearly. The characteristic
fatigue fracture features of BM and AS-HAZ were substantially similar
but with two notable differences. Firstly, the amount of tearing ridge
on the fracture surface of AS-HAZ was fewer than BM while the tearing
dimples were more numerous. Secondly, the average spacing of fatigue
striations in AS-HAZ was smaller than BM, the statistics average values
were 1.5 μm and 1 μm, respectively. By contrast, the fatigue fracture
features of WNZ were quite different. The tearing ridges turned to be
smaller and not distributed strictly parallel to the direction of
propagation, and the amount of shallow tearing dimples increased
significantly. Large size intermediate particles were probably broken
down by pin tool during welding, therefore coarse Fe-containing phases
were scarcely observed in WNZ (Fig. 12e). It is noted that many second
cracks and facets with smaller scale
formed on the fracture surface of WNZ, the spacing of fatigue striation
became indistinguishable attributed to the existence of second cracks.
These fracture features revealed that the crack path of WNZ would be
more tortuous and in favor to slower FCG rate as compared with BM and
AS-HAZ.
Fig. 12 SEM micrograph of fatigue fracture surfaces at the stage of
stable expanding region for BM, AS-HAZ and WNZ specimens.