Influence of peening on the corrosion properties of AISI 304 stainless steel

Abstract

The effects of peening treatment on the microstructure and corrosion behavior of AISI 304SS were investigated. Shot and ultrasonic peening were performed on the austenitic stainless steel, and peened specimens were compared in terms of microstructure, surface roughness and corrosion resistance. Nano-sized grains, multi-directional mechanical twins and strain-induced martensite were formed on the surfaces, and the volume fraction of strain-induced martensite in the ultrasonically peened specimen was higher than that of the shot-peened specimen. The ultrasonically peened specimen which had smoother surface and contained more strain-induced martensite showed superior general and localized corrosion resistance to the as-received and shot-peened specimens.

Introduction

Peening is mostly used as a mechanical surface treatment method in the automotive and aerospace industries. When a peening ball impacts a metal surface, its kinetic energy induces severe plastic deformation and residual compressive stress that remains on the surface, which enhances wear and fatigue properties. In addition to the traditional shot-peening, recent technologies, such as ultrasonic peening [1], [2], laser shock peening [3], [4], [5] and water jet peening [6], [7], [8], have been developed and applied widely. In particular, the ultrasonic peening technique is based on the combined effect of the high frequency impacts of special strikers and ultrasonic oscillations in the treated material [9]. Suh et al. applied ultrasonic cold forging technology (ultrasonic peening) to tool steel, SKD61, and observed extended fatigue life, higher surface hardness and deep residual compressive stresses [10].

In some austenitic stainless steels, including AISI 304SS, strain-induced martensite (α′-martensite) can be formed by the peening treatment at room temperature. Several studies have been published indicating the significance of the martensitic formation and grain refinement in austenitic stainless steels [11], [12], [13], [14], [15], [16]. However, there is still insufficient information on corrosion properties after peening treatment. Reports on corrosion resistance after peening treatment are conflicting without showing a clear trend [17], [18], [19], [20]. Mordyuk et al. [21] applied ultrasonic peening on AISI 321SS for 1–4 min, and obtained optimal corrosion behavior with ultrasonic peening for 1 min. The corrosion resistance is reported to decrease as the peening time increased, due to the increase of strain-induced martensite which gave rise to a galvanic effect between the austenite and martensite. Wang and Lee [22] reported decrease of corrosion resistance on the surface of AISI 304SS upon sandblasting. However, corrosion resistance increased after annealing at 350 °C for 1 h, compared to the as-received specimen. The annealing treatment induces to a dense, Cr-rich passive film, which has better interfacial bonding to the substrate [23], [24].

In the present study, a traditional shot-peened specimen and an ultrasonically peened specimen using ultrasonic cold forging technology were prepared. After peening treatment, the corrosion behavior of each specimen of AISI 304SS was investigated in addition to analysis of the microstructure, surface roughness and surface hardness.