Strain-Induced Phase Transformation and Nanocrystallization of 301 Metastable Stainless Steel Upon Ultrasonic Shot Peeni
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ustenitic 301 stainless steels exhibit good ductility and are used in applications that require good formability, such as the formation of sheets, strips, and wires. Following annealing, the soft austenite structure
KEJIAN LI is with the School of Metallurgy and Materials Engineering, Chongqing University of Science & Technology, Chongqing 401331, China and also with The Center of Material Analysis and Testing, Chongqing University of Science & Technology, Chongqing 401331, China. Contact e-mail: [email protected] KEESAM SHIN is with the School of Metallurgy and Materials Engineering, Chongqing University of Science & Technology and also with the School of Nano & Advanced Materials Engineering, Changwon National University, Changwon 51140, Korea. PENGJUN CAO is with the School of Metallurgy and Materials Engineering, Chongqing University of Science & Technology. Manuscript submitted February 21, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
could be obtained that could undergo the martensite transformation during subsequent operations, such as cold working or cryogenic treatments. These steels are not as formable as other conventional stable austenitic stainless steels, although they exhibit a good combination of formability, strength, ductility, and corrosion resistance, making them competitive with other highstrength steels and alloys.[1,2] The coarse grain refinement of metals and alloys that have dimensions as low as 100 nm could improve the properties of the resulting materials.[3,4] There have been efforts made to refine coarse grains into nanocrystals via strain-induced grain refinement using various techniques, including shot peening.[5,6] Ultrasonic shot peening (USP) was used to enhance the surface properties of the metallic parts by inducing phase transformation/microstructural refinement or nanocrystallization that originates from the high strain rate introduced by high energy ultrasonic vibration.[7–10] The mechanical properties of steels with low-stacking fault energy, such as metastable austenite 301 stainless steel, is affected by the strain-induced grain refinement and the phase transformation. The 301 stainless steel was chemically composed of C 0.05 to 0.15, Si< 2.00, Mn< 2.00, P< 0.045, S< 0.015, N < 0.11, Cr 16.00 to 19.00, Mo < 0.80, and Ni 6.00 to 9.50 (wt. pct). The description of the USP equipment was as previously reported.[9,10] Prior to the treatment, the plate surface was ground to 2000-grit with SiC metallographic paper. The USP treatments were performed at room temperature with an amplitude of 70 lm, frequency of 20 kHz, ball size of 1.5 mm, and a peening area of 2800 mm2 in the center of a plate (110 9 30 9 5 mm3). Following the USP treatment, an analysis of the surface and a cross-sectional region of the specimens were performed via X-ray diffraction (XRD), Vickers hardness, electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). XRD analyses of the untreated specimen and the treated surface layer were performed on a Philips X’pert MPD X-ray (Philips, Nethe
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