Influence of peening on the corrosion properties of AISI 304 stainless steel
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.
Section snippets
Experimental procedure
The material used for peening was commercial AISI 304SS, whose chemical composition is 0.05C–0.4Si–1.6Mn–18.2Cr–8.2Ni–0.3Mo–0.3Cu–Fe (wt.%). Steel plates of 70 × 120 × 15 mm3 were machined and ground with 600-grit SiC. Two kinds of peened samples were prepared. Shot-peened specimens were prepared using a shot with a diameter of 0.8 mm and a work flow of 30 kg/min. Ultrasonically peened specimens were treated using ultrasonic equipment, which had a piezoelectric transducer and a tungsten carbide tip.
Microstructure
Fig. 1(a–c) is optical micrographs of the cross-sectional areas of the as-received, shot-peened and ultrasonically peened specimens, respectively. The as-received specimen has a conventional microstructure, which contains γ-austenite and a small fraction of δ-ferrite aligned along the rolling direction (Fig. 1(a)), and a deformed depth less than 30 μm can be seen on the surface. In the shot-peened specimen, the plastically deformed region on the surface layer (the dark region indicated in Fig. 1
Conclusion
After shot-peening and ultrasonic-peening treatments, nano-sized grains, multi-directional mechanical twins and strain-induced martensite formed in the surface layer. The ultrasonically peened specimen had more strain-induced martensite than the shot-peened specimen, because it received more plastic strain energy.
The shot-peened specimen showed the lowest corrosion resistance among the specimens studied, whereas the ultrasonically peened specimen showed equal or even better corrosion
Acknowledgments
This work was supported from the Korea Institute of Energy Technology evaluation and Planning (code #: R-2007-2), Republic of Korea. This work was also supported by a grant (code #: 2009K000424) from ‘Center for Nanostructured Materials Technology’ under ‘21st Century Frontier &D Programs’ of the Ministry of Education, Science and Technology, Korea.
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