Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel
- PDF / 628,455 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 94 Downloads / 225 Views
INTRODUCTION
INCREASING understanding of nanostructure (d < 100 nm) and ultrafine grain (d 100 nm–1 lm) polycrystalline materials resulted in a surge of activity in the property enhancement through the formation of nanostructures and ultrafine grain. These materials have shown an increase in strength 5 to 10 times higher than their coarse grain counter parts. Severe plastic deformation (SPD) techniques are commonly used laboratory methods to produce bulk nanocrystalline materials. These techniques include high-pressure torsion, equal channel angular pressing,[1–7] accumulative roll bonding,[8–10] and bidirectional compression.[11] The SPD methods are based on metal forming processes involving both strain accumulation and intense grain refinement. These techniques involve grain subdivision leading to subgrain formation and finally transformation of subgrains into nano- or ultrafine grains. In addition to these emerging techniques, advanced thermomechanical methods, which, unlike SPD techniques, depend on conventional cold rolling and annealing for grain refinement, are also being investigated. Using the advanced thermomechanical processing route, it was demonstrated that by reverse transformation of straininduced a¢-martensite (SIM) to austenite, it was possible to produce nanocrystalline or ultrafine-grained stainless steel.[12–23] This technique involves transformation of B. RAVI KUMAR, Scientist, B. MAHATO, Senior Technical Assistant, SAILAJA SHARMA, Project Assistant, and J.K. SAHU, Scientist, are with the Materials Science and Technology Division, National Metallurgical Laboratory, Jamshedpur, India. Contact e-mail: [email protected] Manuscript submitted March 5, 2009. Article published online October 27, 2009 3226—VOLUME 40A, DECEMBER 2009
metastable austenite to SIM by heavy cold rolling and SIM then reverts to recrystallized austenite grains during subsequent annealing. Recently, an alternative technique of the repetitive thermomechanical process was employed to successfully produce nanostructured stainless steel.[24] This technique consists of repetitive cold rolling and annealing to reduce grain size. Nanostructured or ultrafine-grained stainless steel, thus produced, demonstrated excellent strength and elongation of about 30 pct.[18,24,25] This was contrary to the general understanding of the dramatic decrease in ductility in the nanostructured materials.[26,27] A good combination of strength and ductility attained by the nanostructured or ultrafine-grained stainless steel was attributed to its ability to work harden through SIM transformation during deformation.[18,24,25] The types of stainless steels investigated in the preceding studies were highly metastable and readily transform to SIM during cold deformation. During annealing, nearly 90 pct of the material consists of SIM, which was reported to promote high recrystallization and nucleation rates of new nano- or ultrafine austenite grain.[12,13] The grain refining process depends on the type of reversion mechanism, martensitic shear reversion, or diffusional re
Data Loading...
