**On the Influence of Ultrasonic Surface Mechanical Attrition Treatment (SMAT) on the Fatigue Behavior of the 304L Austenitic Stainless Steel**

**Clément Dureau 1,2,3,\*, Marc Novelli 1,2,4, Mandana Arzaghi 3, Roxane Massion 1,2, Philippe Bocher 4, Yves Nadot <sup>3</sup> and Thierry Grosdidier 1,2**


Received: 25 November 2019; Accepted: 1 January 2020; Published: 8 January 2020

**Abstract:** The potential of ultrasonic surface mechanical attrition treatment (SMAT) at different temperatures (including cryogenic) for improving the fatigue performance of 304L austenitic stainless steel is evaluated along with the effect of the fatigue loading conditions. Processing parameters such as the vibration amplitude, the size, and the material of the shot medias were fixed. Treatments of 20 min at room temperature and cryogenic temperature were compared to the untreated material by performing rotating–bending fatigue tests at 10 Hz. The fatigue limit was increased by approximately 30% for both peening temperatures. Meanwhile, samples treated for 60 min at room temperature were compared to the initial state in uniaxial fatigue tests performed at R = −1 (fully reversed tension–compression) at 10 Hz, and the fatigue limit enhancement was approximately 20%. In addition, the temperature measurements done during the tests revealed a negligible self-heating (Δt < 50 ◦C) of the run-out specimens, whereas, at high stress amplitudes, temperature changes as high as 300 ◦C were measured. SMAT was able to increase the stress range for which no significant local self-heating was reported on the surface.

**Keywords:** surface mechanical attrition treatment (SMAT); ultrasonic shot peening (USP); cryogenic temperature; 304L austenitic stainless steel; rotating–bending fatigue; tension–compression fatigue
