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.