PSI - Issue 13

M. Seleznev et al. / Procedia Structural Integrity 13 (2018) 2071–2076 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

2075

5

Fractographic SEM analysis reveals typical internal fatigue fracture morphology, consisting of the NMI, surrounded by FGA in the center of a fisheye (Fig. 1a). All FT batches except FT5 had crack initiators as single (Fig. 2d-f) or clustered (Fig. 2c) alumina plates of polygonal, rectangular or hexagonal (Fig. 2f) shape. Alumina plates, identified on fracture surfaces are 50 – 100 μm wide and only 0.5 – 1 μm thick, which result in semi -transparency on images, obtained with back-scattered electrons (BSE) detector (Fig. 2c). MnS dendrites initiate internal cracks in FT5 batch giving no noticeable FGA and low fatigue strength due to their large size of 150 – 250 μm (Fig. 2b). All the observed plate-like NMIs (dendrites or plates) are opened in their local plane, even being tilted to the main crack plane on angle α . Only NMIs with α ≈ 90° are cross-sectioned. This means that plate-like NMIs concentrate stress with their entire planar area. Thus, cross-section area of plate NMI is not acceptable for fatigue limit calculation. For the given NMI volume the cross-section area and the stress-equivalent area can be equal in case of a spherical shape (Fig. 3a) but can differ greatly in case of plate shape, because disk sectioning reveals only a thin bar (Fig. 3b). With all else being equal, NMI form-factor can change the fatigue limit of steel sufficiently and should be taken into account along with the total NMI amount, distribution and volume. Qualitative analysis on NMIs morphology can be made already from metallographic sectioning. As in most cases plate-like NMI section is a thin bar, the difference between its maximum and minimum sizes (Feret max. and Feret min. diameters, respectively) is significant (Fig. 3b). In contrary, regular- shaped NMI doesn’t show any big difference between F max and F min (Fig. 3a). This can be used to divide the size distribution of NMIs in two groups, depending on F max / F min relation, where F max / F min < 5 denotes regular-shaped NMIs (Fig. 3d, e, blue bars) and F max / F min > 5 denotes plate-shaped NMIs (Fig. 3d, e, red bars). In case of steel with regular-shaped NMIs the influence of plate-like inclusions is avoidable, as their size (F max ) distribution is completely overlapped by the regular-shaped NMI sizes (Fig. 3d). In contrary, NMI size distribution of FT batch shows unavoidable influence of plate-like NMIs: the biggest F max is almost doubled from 40 to 70 μm (Fig. 3e). As area 0.5 ~ biggest F max , fatigue limit is reduced by plate-like NMIs, which can be qualitatively detected by presented size distribution analysis. Quantitative estimation of stress-concentrating NMI area is to be done by fatigue testing. Confocal laser scanning microscopy (CLSM) analysis (Fig. 3c) shows that crack-initiating plate-like NMI is tilted to the main crack. However, for observed NMIs tilt angle α does not exceed 30° (Fig. 3f). The observed fatigue limits σ * w (Fig. 1b) were calculated from SEM imaging of fractured surfaces, which means that projected NMI area was taken for eq. (1) and (2) . As σ ’ w ranges consist with experimental fatigue limits, one can conclude that projection area usage is valid for small tilt angles (α<30°). Estimation of equivalent area of highly-tilte d (α>30°) plate -like NMIs requires further investigations.

(b)

(c)

(a)

F max ≈ F min

25 μm

50 μm

(d)

(e)

Spheroids

(f)

Plates

α = 27°

Feret max (μm)

Fig. 3. Comparison of alumina inclusions from industrial casting (a, d) [10] and FT batches (b, c, e, f) of 42CrMo4 steel. Typical metallographic sections show regular-shape (a) and plate-like (b) NMIs. Size distribution diagrams (d, e) consist of two inclusion types: NMIs with F max / F min < 5 are considered as regular-shaped (blue bars) and F max / F min > 5 – as plate-shaped (red bars). 3D scanning of fatigue fracture origin (c) shows that plate-like NMI initiates crack with its entire area even being tilted to the crack plane, as presented on surface section (f).