# Study of the Rotary Bending Fatigue Resistance of 30 MnB5, 41 CrS4 and 30 MnVS6 Steels
## 3 Results and Discussion
### Fractography  
In order to complement the analysis of the fatigue behavior, the study of the fracture surface was carried out by means of scanning electron microscopy (SEM). Representative samples of each steel were chosen. In the most interesting cases, photos were taken at higher magnifications to study the cracks or micro holes that could be associated with the fracture process, due to the inclusion of second-phase particles.  
The images in Figure 6 correspond to the reference tempering and tempering steel specimen (30 MnB5-B1) tested at a stress level of 70% of its yield strength, equivalent to the stress of 596 MPa. The fracture occurred at 29,987 cycles. Image (a) shows the global fracture surface. The area marked with a white circle indicates the separation between the fatigue failure zone and the final ductile fracture. Image (b) shows the density of holes found in the ductile fracture zone.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Steel** & **Key** & \(\rm\overline{S_{i}}\)**(MPa)** & **D (MPa)** \\ \hline
30 MnB5 & B1 & 326.16 & 7.34 \\
30 MnVS6 & MV & 233.72 & 153.49 \\ \hline \end{tabular}
\end{table}
Table 8: Fatigue limit values and standard deviation obtained for the 30 MnB5-B1 and 30 MnVS6-MV steels with the maximum likelihood method.  
Figure 7a shows the complete fracture surface observed for the 30 MnB5 (B2) steel subjected to a stress of 559 MPa (60% R\({}_{\text{e}}\)). The fracture occurred at 35,259 cycles. The final fracture occurs in the marked area. Figure 7b shows an enlargement of this fracture zone, where very little ductility is observed.  
Figure 8 shows the SEM images obtained for the 30 MnVS6 microalloyed steel tested with a stress of 581 MPa, which corresponds to 90% of its elastic limit. The fracture was reached at 32,754 cycles. Figure 8a shows the complete fracture of the specimen. On this image, the line that marks the difference between the fatigue fracture and the final ductile fracture was represented. Figure 8b shows the ductile fracture zone in detail, where the high density of cracks associated with ductile fracture can be seen, with an average size between 30 and 75 \(\upmu\)m. In its interior are some second phase particles, which are possibly the triggers of the crack.  
Figure 9 shows the images of the failure surface obtained for the quenched and tempered steel alloyed with chromium (41 Cr54-EC) when it is tested with a stress equivalent to 60% of its elastic limit (586 MPa). The number of cycles to failure was 36,062. Image (a) shows the general appearance of the fracture. The area where the final fracture occurs was marked. Image (b) shows this area with higher magnifications. A brittle fracture with tear can be observed, as well as micro voids generated by the cohesion of particles.  
Figure 6: Fracture surface of the reference quenched and tempered steel (30 MnB5-B1) subjected to the rotary bending fatigue test with a stress of 70% R\({}_{\text{e}}\). (Complete fracture surface (**a**) and detail (**b**)).  
Figure 7: Fracture surface of modified quenched and tempered steel (30 MnB5-B2) subjected to rotary bending fatigue test with a stress of 552 MPa. (Complete fracture surface (**a**) and detail (**b**)).