**1. Introduction**

In the high cycle fatigue regime, cracks mostly nucleate at the surface of the loaded workpiece. Therefore, the surface or near-surface features such as surface roughness and residual stresses affect the fatigue life of a component. The surface roughness has an influence essentially at the crack initiation stage [1]. Indeed, the presence of surface irregularities leads to high local stresses, which may create a high amount of plastic deformation locally, eventually contributing to crack-type defect creation depending on the intrinsic behavior of the studied material. Furthermore, the presence of residual stresses may affect the crack propagation behavior [2]. While a tensile stress loading promotes crack opening and faster propagation, the introduction of compressive residual stresses can delay or stop crack propagation with the consequence of increasing the total fatigue life of mechanical components.

In this context, plastic deformation treatments such as burnishing [3], laser shock-peening [4,5], or hammering [6,7] were used on stainless steels to delay early fatigue crack initiation and growth and increase part performance.

In addition, surface mechanical attrition treatment (SMAT) and ultrasonic shot peening (USP) were used to impart severe plastic deformation (SPD) at the surface of components. The effects of SMAT and similar kind of surface severe plastic deformation techniques on the surface and near-surface microstructure modifications and associated mechanical properties were reviewed in several papers [8–12]. It is noticeable that positive effects on the fatigue behavior were observed on different materials such as Fe- [13–15], Ti- [16], or Mg-based [17] alloys. In some cases, the use of these treatments pushed the initiation site underneath the peened surface, thus enhancing the fatigue limit significantly [18,19]. At the same time, the peening energy also strongly influences the treated surface roughness [20] and integrity, which can decrease the fatigue resistance of the components in spite of the nanostructured surface layer and higher compressive residual stresses [16,21,22].

The fatigue resistance of 316L austenitic stainless steels was enhanced by nearly 30% (from 300 to 380 MPa) due to the presence of a superficial refined microstructure delaying crack initiations and a high compressive residual stress impeding crack propagation [13].

The effect of SMAT treatments on the fatigue behavior of the 304L was investigated at room temperature (RT), as well as elevated temperature, by Kakiuchi et al. [23]; the shot-peened specimens exhibited higher fatigue strengths than the untreated ones at both RT and 300 ◦C. By decreasing or removing the surface topology induced by severe shot peening through a repeening or grinding process, the fatigue life of a shot peened low-alloyed steel can be further improved as shown in the work of Bagherifard et al. [20].

The idea of practicing cryogenic SMAT on 304L comes from the fact that some Fe–Ni–Cr austenitic stainless steels, such as 304L, are metastable and undergo stress- or strain-induced martensitic transformations through severe plastic deformation. Accordingly, the volume fraction of martensite in the SMAT samples increases at lower temperatures [24–26]. Although no definitive explanation was given for the influence of martensite (α') prior to the cyclic loadings [27], its presence is usually reported to be beneficial regarding tensile strength [28]. Also, for a given SMAT processing condition, the use of cryogenic temperature was shown to decrease the surface roughness [25,26], a modification that may result in improved fatigue performance.

In this context, the present paper has two major goals: (i) to investigate the effect of lowering the SMAT temperature on the fatigue behavior of a 304L stainless steel, and (ii) to analyze differences that may exist between rotating–bending and fully reversed uniaxial tension–compression fatigue tests on a 304L stainless steel.
