PSI - Issue 13

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

2074

4

Fatigue properties of sulfur-rich batches (FT5, 6) are not quite comparable with the others from sulfur-lean steel, however, they are useful in terms of NMI analysis and fatigue limit range representability. Knowing the batch hardness (Table 1) and crack initiating defect area estimated by SEM the fatigue limit σ w was calculated according to well known empirical formula of Murakami [3, p. 91]: ( ) 1 6 1.56(HV + 120) w area  − = (1) Here area is estimated by enclosing the crack-initiating NMI (including FGA, if present) using circle of minimum radius, as shown on Fig. 2e and Fig. 2f, respectively. Calculations were done for all the failures (circles and squares on Fig. 1a) and grouped dependin g on the FT batch. Resulting σ w range (Fig. 1b, dot lines) is almost two times smaller than the experimental σ f (Fig. 1b, triangles). However, it is well known that nitriding introduces compressive residual stresses, which can enhance fatigue limit significantly. Assuming that residual stress is equivalent to a local mean stress σ m eq. (1) was modified by Matsumoto et al. [3, p. 108] in the following way: ( ) 1 6 ' 0.5 1.56(HV + 120) 0.5 w w m m area     − = − = − (2) Although the real stress state at the crack initiating NMI is unknown, this equation was used as a first approximation in order to describe the enhancement of fatigue limit by compressive residual stresses. We used σ m = – 350 MPa, which corresponds to compressive residual stresses on NHD of about 0.27 mm (Table 1) according to Weidner et al. [15]. Eq. (2) results then in corrected σ ’ w values (solid lines on Fig. 1b) which show reasonable agreement with σ f . More detailed analysis of the influence of residual stresses on the fatigue limit has to be provided by future work.

600 MPa 277 152

(a)

525 MPa 1 865 289

525 MPa 4 209 332

(c)

(b)

Al

Mn

S

1 mm

O

475 MPa 40 619 345

560 MPa 17 633 344

515 MPa 4 403 857

(d)

(f)

(e)

NMI area

FGA area

Fig. 2. Investigation of fracture surfaces of 42CrMo4 specimens after USFT failure with SEM using BSE detector (a – c), SE detector (d – f) and EDX mapping (examples are shown on the insets of b and c). Fatigue cracks show typical behavior, forming fisheye zone (a) and FGA around crack initiator (d-f). Crack-initiating NMIs were MnS dendrites (b) and Al 2 O 3 plates (c-f) of polygonal (d), rectangular (e) and hexagonal (f) shapes, which act solitary (d-f) or form a cluster (c). Scale bar length is 1 mm (a) and 50 μm (b -f) respectively. Stress amplitude and cycles to failure are given in each image. Example of minimum enclosing circle area estimation is shown with dashed circles for NMI (e) and for FGA (f).