## 3 Results and Discussion
### Effect of DB and LTGN on Fatigue Behavior of 304 CNASS  
#### 3.3.1 S-N Curves  
Figure 11 shows the S-N curves of specimens treated with different types of DB. The reference condition is the S-N curve obtained for samples processed only by turning and polishing, which leads to a fatigue limit of 440 MPa. Smoothing DB increases the fatigue limit to 540 MPa, i.e., an improvement of 22.7% compared to the reference samples. If the samples after smoothing DB are heated to 350 \({}^{\circ}\)C for 3 h, the fatigue limit increases to 580 MPa, and the improvement compared to the base level is 31.8%. The reason for this improvement is the time-dependent diffusion-based strain-aging [3]. The same improvement of 31.8% is achieved by single-pass hardening DB. The highest fatigue limit of 605 MPa is achieved by five-pass hardening DB, an improvement of 37.5% compared to the reference samples. However, the hardening DB creates a significant plastic deformation of the surface layer, which is the cause of the \(\gamma\rightarrow\alpha\) transformation. The strain-induced \(\alpha^{\prime}\)-matrensite increases the surface microhardness but decreases resistance to electrochemical corrosion [3].  
Figure 12 shows the S-N curves of the three groups of specimens treated via LTGN after different pretreatments. The pretreatment for Group I samples increases the fatigue limit from 440 MPa (reference point) to 580 MPa, i.e., an improvement of 31.8%, which was achieved by smoothing DB followed by heating up to 350 \({}^{\circ}\)C for 3 h as well as by single-pass hardening DB. Groups II and III specimens achieve the same fatigue limit of 600 MPa, i.e., an improvement over the reference value by 36.3%. The improvement compared to Group I is 3.45%. However, the pretreatment of the Group II and Group III specimens increases the fatigue life more than 178 times compared to the turning and LTGN processes. The main reason for this improvement is the smaller residual stress distribution gradients and the deeper compressive zones obtained by the two combined DB and LTGN processes. The factors determining the comparable fatigue limit of 600 MPa for the two combined processes (smoothing DB and LTGN and single-pass hardening DB and LTGN) are of interest. In the literature, the improvement of the fatigue behavior of austenitic steels after LTN is primarily associated with the quality of the formed S-phase in the surface layer [36]. The obtained equal fatigue limit for the two combined processes based on DB and LTGN facilitates the explanation of the following conclusion. The fatigue behavior depends on the combination of SI characteristics obtained, including the types of phases and their ratio in the AL on the one hand and the profile of residual stresses on the other. The resulting combination of SI characteristics is the result of the superimposition of strain hardening followed by transformation hardening (resulting in the formation of new hard  
Figure 11: S-N curves of specimens treated via different types of DB.  
phases with an increased volume of crystal lattice). The presence of a higher-intensity S-phase in the AL and the creation of larger surface compressive residual stresses are not, by themselves, dominant factors in raising the fatigue limit. Evidence of this is provided by the fact that the fatigue limit obtained for the combined process based on hardening DB and LTGN results in an S-phase of lower intensity and smaller compressive axial and hoop surface stresses. At the same time, however, this process provides the hard phase \(\varepsilon\) and stabilized nitrogen-bearing martensite with greater intensity (Figure 2), as well as a greater depth of the compressive zone (Figure 9). Therefore, the combined processes based on an appropriate combination of static surface cold work and LTN parameters can provide a synergistic effect to improve the fatigue behavior of CMASS.