# Study of the Rotary Bending Fatigue Resistance of 30 MnB5, 41 CrS4 and 30 MnVS6 Steels
## 3 Results and Discussion
### Fatigue Resistance  
In Figure 4 the experimental S-N curves of the tested steels and the fitted curve are shown, and in Table 6 the values obtained for the fitting coefficients of the Basquin equation and the regression coefficient, \(\text{R}^{2}\).  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Steel** & **Key** & **A (MPa)** & **b** & \(\text{R}^{2}\) \\ \hline
30 MnB5 & B1 & 2048 & \(-\)0.123 & 0.62 \\
30 MnB5 & B2 & 3701 & \(-\)0.188 & 0.98 \\
41 Cr54 & EC & 4407 & \(-\)0.198 & 0.95 \\
30 MnV56 & MV & 1755 & \(-\)0.126 & 0.42 \\ \hline \end{tabular}
\end{table}
Table 6: Fitting coefficients, A and b, obtained with the Basquin method and regression coefficient of the fitting coefficients.  
In Figure 4a it can be observed that in the case of the 30 MnB5 reference steel (B1), only the data of three stress levels are represented, since for the lowest level tested (50% of R\({}_{\text{e}}\)) none of the specimens broke, and at the next level, 60% R\({}_{\text{e}}\), only two of them did. For B1 and MV steels a high dispersion is observed, in particular at the highest load levels; this was due to the fact that the specimens supplied by the manufacturer had a higher surface roughness than the other two materials. In all cases, a surface roughness, Ra, lower than the value of 0.2 microns established was ensured, so it is guaranteed that the four steels have an adequate surface finish to obtain results in valid fatigue tests.  
As already indicated, despite the great dispersion in the data, the Wohler curves can be analyzed by means of a statistical treatment that allows defining a calculation S-N curve for any probability of fatigue. In this way, this curve will be used as a reference to compare the fatigue response of the four types of steel studied. With the data from the fatigue tests in the finite life range, the arithmetic mean of the failure cycles was calculated, \(\overline{\text{N}}\), he standard deviation, D, and the standard error of the mean value of the number of cycles, \(\text{D}_{\overline{\text{N}}}\). Table 7 shows the values obtained for these parameters.  
From the data in Table 7, the compared fatigue resistance was represented for the four steels evaluated (Figure 5).  
From Figures 4 and 5 it can be observed that the fatigue resistance of the reference 30 MnB5 (B1) steel (blue line) is slightly higher than that observed for the same modified steel (B2), despite the fact that the yield strength and the tensile strength of the latter is greater. Microalloyed steel with vanadium (30 MnVS6, orange line) has a resistance to fatigue very similar to that of reference steel for low load values. Since its elastic limit is significantly lower, its application with high loads is ruled out. Furthermore, with load values lower than 500 MPa, the fatigue behavior is clearly worse than that of the reference steel, which at that load level seems to have reached the fatigue limit. The quenched and tempered chrome steel (41 CrS4-EC) has a higher fatigue resistance than the other steels in the range of stresses studied. However, in this stress range it was not possible to establish  
Figure 4: S-N curves obtained with the experimental data of the rotary-bending fatigue tests for the reference quenched and tempered steel (**a**) modified (**b**), alternative chromium quenched and tempered steel (**c**) and microalloyed steel (**d**).  
a fatigue limit, as was the case for both the reference steel and the microalloyed steel. To determine this limit, further tests will be necessary, which will be addressed in later studies.