## 4 Conclusions  
This article aimed to quantify the effects of different DB processes and heat treatments on the microstructure, SI, rotating bending fatigue strength, and electrochemical performance of 304 austenitic SS in two initial states: as-received and initially heat-treated. Based on the experimental results, it is possible to choose an optimal DB process and heat treatment depending on the operational purpose of the steel component. The major findings can be expressed as follows: \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & \multicolumn{5}{c}{**Ranking of the Different Treatments of 304 SS**} \\ \cline{3-6}
**X\(\mathbf{\hat{n}}\)** & **Specimen** & \multicolumn{5}{c}{**Criteria (Requirements)**} & \multirow{2}{*}{
\begin{tabular}{c} **Integral** \\ **Efficiency** \\ \end{tabular} } \\ \cline{3-6}  & & & **Roughness** & & **Micro-** & **Fatigue** & **Corrosion** & **Efficiency** \\ \hline
1 & AR1 & 5 & 9 & 3 & 5 & 22 \\
2 & AR2 & 2 & 8 & 2 & 3 & 15 \\
3 & AR3 & 4 & 4 & **1** & 6 & 15 \\
4 & AR5 & 2 & 6 & **1** & 2 & **11** \\
5 & AR6 & 4 & 2 & **1** & 10 & 17 \\
6 & HT7 & 6 & 10 & 7 & 8 & 31 \\
7 & HT8 & **1** & 7 & 6 & 9 & 23 \\
8 & HT9 & 3 & 3 & 4 & 4 & 14 \\
9 & HT11 & **1** & 5 & 5 & **1** & 12 \\
10 & HT12 & 3 & **1** & 4 & 7 & 15 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of the effectiveness of the types of treatments. Figure 21: Impact of DB and heat treatment on the (**a**) fatigue limit and (**b**) CR. * For the as-received and initially heat-treated states, smoothing DB provided the lowest roughness: R\({}_{\rm a}\) = 0.1 \(\upmu\)m and R\({}_{\rm a}\) = 0.09 \(\upmu\)m, respectively. Both DB processes, smoothing and hardening, led to smaller roughness parameters for initially heat-treated samples, a consequence of the increased plasticity of the steel after quenching. The maximum surface micro-hardness was obtained after hardening DB and subsequent heating at 350 \({}^{\circ}\)C for three hours for both initial states 677 and 691 HV, respectively, and the micro-hardness improvement, compared with turning, was 75.4% and 82.8%, respectively. * For all treatments, the as-received state showed a higher fatigue strength compared with the initially heat-treated state due to the finer-grained structure; the presence of M\({}_{23}\)C\({}_{6}\) carbides at the grain boundaries, which increase the strength and hardness; and the significant hot-mechanical strengthening during the production process of the as-received hot-rolled bar. The largest increase in the fatigue limit compared with turning was achieved by five-pass hardening DB: from 440 to 605 MPa, i.e., by 37.5%. * The fatigue strength of the specimens subjected to smoothing DB significantly increased for both initial states when, after SCW, the specimens were heated at 350 \({}^{\circ}\)C for three hours, although relaxation of the residual stresses introduced by DB and reverse partial \(\alpha^{\prime}-\)martensitic transformation were observed. For the as-received state, the fatigue limit increased from 540 to 580 MPa, i.e., by 7.4%, while the increase for the initial heat-treated state was 8.82%, from 340 to 370 MPa. Furthermore, the slope of the fatigue curves was small, and the fatigue life increase was significant: more than 415 times for the as-received state and more than 571 times for the initially heat-treated state A possible reason for the increased fatigue strength is the time-dependent diffusion-based strain ageing of the plastically deformed surface and subsurface layers affected by DB. Another possible reason, for the as-received initial state, is the dispersion of carbides along the grain boundary, thereby increasing the resistance to inter-crystalline deformation. * Clearly expressed correlation of the volume fraction of the \(\alpha^{\prime}-\)martensite phase, surface roughness, and residual stresses with the corrosion behaviour of DB 304 SS specimens in 0.5 M H\({}_{2}\)SO\({}_{4}\) was not observed.