According to the heat treatment regime stated by the shaft manufacturer, it could be expected that complete austenization
was done, with breaking the dendrite microstructure that had remained from the casting process. However, the obtained
microstructure test results, shown on Fig. 11, demonstrate that some portion of residual dendrite microstructure has remained.
Such non-homogeneous microstructure, together with the observed porosity and microcracks, has contributed to faster
propagation of the fatigue crack, compared with the case of the flange material that completely fulfill requirements [6].
According to [5], fatigue limit at R = 1 in open air is 225 MPa, and 140 MPa in water. However, by testing the flange
material samples, the value of 168 MPa of fatigue limit in open air was obtained, which is 25% less than the reference value
of 225 MPa.