## 2 Materials and Methods  
AISI 304 CNASS, obtained as cylindrical hot-rolled bars, was from the same batch used in our previous studies [2; 3]. All subsequent treatments were conducted on the material as received. The chemical composition (in terms of wt%), established by optical emission spectrometry, was as follows: Fe--71.5, C--0.036, Si--0.193, Mn--1.52, P--0.03, S--0.026, Cr--17.7, Ni--8.3, Mo--0.182, Cu--0.25, Nb--0.042, Ti--0.003, V--0.07, W--0.05 and others--balance. The basic mechanical characteristics, established at room temperature using a Zwick/Roell Vibrophore 100 testing machine to conduct tensile tests, were as follows: Young's modulus--198 GPa; yield limit--432 MPa; tensile strength--734 MPa and elongation--41%. A Bruker D8 Advance X-ray diffractometer, in conjunction with the Crystallography Open Database to determine the peak positions, was used to conduct the phase analysis. The microstructure and surface fracture were observed via scanning electron microscopy (SEM, LYRA I XMU Tescan, Brno, Czechia).  
Specimens with dimensions of \(\phi 20\times 30\) mm were used to measure the physical-mechanical characteristics of SI. The surface microhardness measurements were made using a ZHV\(\mu\) Zwick/Roell microhardness tester using a 0.015 kgf load and a holding time of 10 s. Twenty measurements were made for each specimen. The final value of the surface microhardness corresponded to the grouping center. The residual stresses were measured according to the methodology previously described [3].  
DB implementation was described in our previous studies [2; 3; 4]. In this study, smoothing and single-pass hardening DB processes were implemented. Low-temperature gas nitriding (LTGN) was carried out using the experimental setup shown schematically in Figure 1. The LTGN installation was created at the Technical University of Varna by [PERSON]. The laboratory installation consists of a vacuum furnace, a gas supply with gas flow regulation and a separate secured area for the storage of cylinders. The process is cyclical, with one working cycle lasting one hour at a working temperature of 420 \({}^{\circ}\)and under vacuum (0.7 \(\upmu\)bar). Twenty working cycles were used for the LTGN of the specimens. To depassivate the surface layer of the samples, ammonium chloride was used as a depassivating agent in the furnace.  
Rotating bending fatigue tests were conducted on a UBM testing machine. The cycle asymmetry factor is R = \(-1\). The loading frequency was 50 Hz in the air. The accuracy of counting the number of cycles to fatigue failure was 100 cycles. The experimental setup, methodology and fatigue specimen shape and size have been described [2; 3]. The tests were conducted in two stages. In the first stage, samples treated using only different DB processes were tested. The fatigue outcomes previously obtained [2; 3] were supplemented and significantly expanded. In the second stage, the fatigue tests were conducted with three groups of samples: turned, polished and gas nitrided (Group I); turned, smoothing diamond burnished and gas nitrided (Group II); and turned, hardening diamond burnished with one pass and gas nitrided (Group III). The fatigue specimens from Group I were polished before LTGN in order for the three groups of specimens to have approximately the same roughness and height parameters.  
Figure 1: Scheme of the LTGN experimental setup: (**a**) cyclogram; (**b**) LTGN scheme in a shaft furnace and (**c**) air cooling.