Effect of laser shock peening on high cycle fatigue properties of aluminized AISI 321 stainless steel

Highlights

• Laser shock peening (LSP) was applied to aluminized 321 stainless steel.

• High fraction twinning and compressive residual stress were induced by LSP.

• More obvious de-twinning occurred in LSP sample during cyclic deformation.

• Dynamic recovery is fully developed in LSP sample during cyclic deformation.

• Fatigue lives are increased by 204.4–230.8% though LSP treatment.

Abstract

In this work, the effect of laser shock peening (LSP) treatment on fatigue property of aluminized AISI 321 stainless steel was studied. The high fraction twinning and compressive residual stress (CRS) induced by LSP at the severe plastic deformation (SPD) layer were found to be responsible for the enhanced fatigue property. De-twinning was conspicuously noticed in LSP sample. Moreover, the dynamic recovery (DRV) was only noticed at SPD layer during cyclic deformation related to CRS. In sum, the aluminized stainless steel achieved an enhanced fatigue property by LSP treatment.

Introduction

AISI 321 is a Ti-stabilized austenitic stainless steel generally characterized by excellent overall corrosion resistance and outstanding ductility. Therefore, AISI 321 is widely applied as preferential material for manufacturing heat exchangers and pressure vessels for solar thermal generation [1]. In general, steel could operate under extremely rigorous conditions, such as corrosive media and severe cyclic stress at elevated temperatures [2], [3]. However, in the absence of protective coatings, such conditions may cause fatigue failure, oxidation and hot corrosion, seriously damaging the components of solar thermal systems and leading to their failure [4], [5], [6]. To prevent oxidation and corrosion, Fe-Al coatings are often used as protective components to segregate metallic substrate from detrimental environments [7], [8], [9], [10], [11], [12], [13].

Pack cementation is often utilized as an aluminizing technique to form desired coatings on larger sized components with complicated geometries [14]. Nevertheless, the brittle nature of Al-rich coatings leads to deterioration of mechanical properties of substrate materials, especially the fatigue performance. For instance, Serre et al. [15] attributed the degradation in the mechanical behavior of aluminized steel to the large plastic deformation in coatings, which promoted the formation of voids in the bulk of specimens. In our previous work [16], similar conclusions were drawn, in which the fatigue-ductility coefficient of aluminized steel (3.528) was found lower than that of AISI 321 as-received steel (4.347). Thus, more attention should be paid to improving the fatigue properties of aluminized steel.

Laser shock peening (LSP) is a superior laser-based surface processing technique used to treat metallic materials to yield enhanced surface strengths and extended fatigue life services [17]. Using LSP, the severe plastic deformation (SPD) and compressive residual stress (CRS) can be introduced in subsurface layer of workpieces when the induced shock pressure exceeds the Hugoniot Elastic Limit of the material [18]. Zhang et al. [19] employed CRS parallel to the specimen surface to inhibit the fatigue crack growth by confining the cracks to the center, leading to fatigue life increasing by 125%. Apart from the ability to restore the pre-fatigued steel, LSP may also prolong the fatigue life of specimens by more than 27-fold upon the introduction of twin lamellae [20].

During hot deformation, the restoration phenomena, such as dynamic recovery (DRV) and dynamic recrystallization (DRX) significantly affect the deformation behavior [21]. In general, the kinetics of DRV are slow due to the low stacking fault energy (SFE) of austenite. Thus, DRX process initiated at the critical strain would preferentially occur during the hot formation of steels. Ghazani et al. [22] investigated the impacts of temperature on hot compression behavior of AISI 321 stainless steel and confirmed the occurrence of DRV as the main restoration process at temperatures ranging from 800 to 950 °C, while complete recrystallization took place at 1000 °C. Sajadifar et al. [23] noticed the importance of DRV in the softening process during cyclic deformation at 600 °C.

Additionally, de-twinning is also an inevitable phenomenon strongly affecting the fatigue performance of materials since high-level fraction mechanical twins would be induced by LSP at SPD region. Wu et al. [24] noticed alternate disappearance of twins or narrowing followed by reappearance during subsequent unloading and reloading. However, the mechanisms associated with the cyclic deformation on aluminized steel pre-deformed after LSP treatment have not yet been deeply explored.

In this work, high-cycle fatigue life of characteristics of aluminized steel samples with and without LSP treatment were investigated at 620 °C, imitating the potential service temperature. To gain a better understanding of the deformation mechanisms, the microstructures of aluminized steel samples before and after cyclic loadings were observed by an electron back-scattered diffraction detector (EBSD) and transmission electron microscope (TEM). A reasonable understanding of the factors governing the fatigue behavior of aluminized steel subjected to LSP process was also provided.