## 3 Results and Discussion
### Effect of DB and Heat Treatment on Fatigue Behaviour of 304 Ss  
The S-N fatigue curves for the two initial states are shown in Figure 16, and 10\({}^{7}\)-cycle fatigue strength is accepted as the fatigue limit. In general, for all treatments, the as-received state has a higher fatigue strength compared with the initially heat-treated state. The explanation for this is the initial microstructure of the steel (see Section 3.2.1). The as-received state is characterised by a finer-grain structure compared with the initially heat-treated state, as well as by the presence of M\({}_{23}\)C\({}_{6}\) carbides at the grain boundaries, which increase the strength and hardness. In addition, the steel experienced significant hot-mechanical strengthening during the production process of the as-received hot-rolled bars. Conversely, initial heat treatment increases the ductility but lowers the hardness and strength, and the grain size increases. For both initial states, DB significantly increased the fatigue strength, including the fatigue limit. For the as-received state, the fatigue limit for turned and polished specimens, accepted as a reference condition (RC1), was 440 MPa (Figure 16a). The smoothing DB, implemented as a single-pass process (AR2), increased the fatigue limit to 540 MPa, i.e., the increase compared with RC1 was 22.7%. The single-pass hardening DB (AR3) increased the fatigue limit compared with RC1 by 31.8%, from 440 to 580 MPa. The biggest increase in the fatigue limit was achieved by the five-pass hardening DB (AR4), from 440 to 605 MPa, i.e., by 37.5%. The increase in fatigue strength due to DB is greater in the high-cycle fatigue field compared with low-cycle fatigue. The likely reason for this is the higher residual stress relaxation rate in the low-cycle field that is associated with the large amplitude of the bending stresses. For the initially heat-treated state, the fatigue limit of non-DB specimens, accepted as the reference condition (RC2), is 290 MPa. The relative increases in the fatigue limit by smoothing (HT8), single-pass hardening (HT9), and five-pass hardening DB (HT10) are 17.2%, 44.8%, and 27.6%, i.e., from 290 to 340, 420, and 370 MPa, respectively. In contrast to the as-received state, the higher number of tool passes (n = 5) reduces the fatigue limit compared with hardening single-pass DB, regardless of the higher compressive stresses. One of the reasons for this is that the surface and nearby layers reach a stabilised cycle in less than five passes, a consequence of cyclic loading from DB [30], i.e., a softening effect is observed for n = 5. From the point of view of fatigue limit, smoothing DB is significantly more effective for the as-received state compared with the initially heat-treated state. One of the reasons for this is the significantly higher compressive stresses introduced into the material in the as-received state (see Figure 14). The fatigue strength, and especially the fatigue limit, of the specimens subjected to smoothing DB increased (Figure 17a) for both initial states when the specimens were heated at 350 \({}^{\circ}\)C for three hours after SCW, despite the relaxation of the residual stresses introduced by DB (see Figure 15) and the reverse partial \(\alpha^{\prime}-\)martsenti  
Figure 16: S-N fatigue curves: (**a**) as-received initial state and (**b**) heat-treated at 1100 \({}^{\circ}\)C for one hour, then quenched in water. For the as-received state, the fatigue limit increased from 540 to 580 MPa, i.e., by 7.4%, while the increase for the initially heat-treated state was 8.82%, from 340 to 370 MPa. Because of the small slope of the fatigue curves, the fatigue life increase was significant, more than 415 times greater for the as-received state and more than 571 times greater for the initially heat-treated state (see Figure 17a). A possible reason for the increased fatigue strength and fatigue life is the time-dependent diffusion-based strain ageing of the plastically deformed surface and subsurface layers affected by DB (see Section 3.2.2).