The influence of surface mechanical attrition treatment (SMAT) on the surface characteristics of AISI 304 stainless steel was studied, with a particular focus on changes in the nanocrystalline structure, corrosion resistance and microhardness profile caused by varying the cumulative time the surface has been subjected to SMAT.
There is a relatively small but steadily growing body of research specifically with respect to the effect SMAT has on the corrosivity of steels. Furthermore, the novel nanocrystalline grain
sizes that can be obtained using SMAT mean treated samples exhibit novel and potentially commercially useful surface mechanical properties.
It was firstly confirmed experimentally that the characteristic martensitic transformation temperature (Ms) of AISI 304 stainless steel is below 1960. SMAT treatment was conducted to
individual samples at various cumulative treatment times [10, 20, 30, 60 and 90 min respectively] using 450g of GCR-15 bearing steel balls. These treated samples were divided into small testing samples (to ensure continuity of microstructure between tests) and a battery of metallographic tests was conducted.
Through potentiodynamic polarisation testing and extended immersion in a corrosive medium respectively, it was found that SMAT increases corrosivity and that there exists a general
correlation between increasing the cumulative time SMAT is conducted and the corrosivity of a sample.
Through Vickers microhardness testing a clear trend was established of maximum surface hardness increasing with increasing cumulative SMAT, while a constant variation in lower
substrate hardness was maintained between samples.
Optical microscopy testing techniques combined with chemical etching and electroetching respectively indicated that no visible martensite was formed in any sample, regardless of the degree of SMA treatment; however significant nanocrystalline austenite grain refinement was clear in all SMATed samples.
Keywords: surface mechanical attrition treatment, SMAT, stainless steel, AISI 304, microhardness, corrosion, polarization