Thermal stability of a nanostructured layer on the surface of 316L stainless steel
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Kangning Sun Key Laboratory for Liquid-Solid Structure Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China (Received 13 October 2013; accepted 13 January 2014)
To obtain a nanocrystalline surface layer, 316L stainless steel was treated by fast multiple rotation rolling (FMRR). The microstructure, after FMRR treatment and annealing treatment, was characterized by transmission electron microscopy and x-ray diffraction. Equiaxed nanocrystalline with the average grain size about 12 nm is obtained on the surface layer of FMRR sample. The investigation of thermal stability of the nanocrystalline layer indicates that the grains are still nanocrystalline and the average grain size is about 60 nm for annealing at 500 °C. In addition, the amount of a-martensite increases markedly as the annealing temperature increases from 300 to 500 °C. However, it begins to reduce at 600 °C due to the reversion transformation from martensite to austenite. After annealing at 400 °C, the microhardness of the annealed FMRR sample reaches a maximum value of about 660 HV, and it is four times higher than that of the original sample.
I. INTRODUCTION
In recent years, 316L stainless steel is a widely used engineering material in environments such as petrochemical, chemical, nuclear, and food industries due to its excellent corrosion resistance and its biocompatibility. However, structural application of this material with a coarse grained state is limited by its relatively low mechanical strength and poor abrasion resistance properties.1 It is well known that nanocrystalline materials have attracted considerable scientific interest due to their superior mechanical properties (hardness, strength, and wear resistance) different from conventional coarsegrained polycrystalline counterparts. This is consistent with the classical Hall–Petch relationship2,3 that the refinement of the grain size can improve the mechanical strength. Therefore, a reasonable approach to enhance the mechanical strength of the 316L stainless steel is refinement of its grain size by the recently developed severe plastic deformation (SPD)4–6 technique. To date, many methods based on SPD have been developed, such as high pressure torsion (HPT),7–11 equal channel angular pressing,12–19 surface mechanical attrition treatment (SMAT),20–23 and fast multiple rotation rolling (FMRR),24 to refine the grain to nanometer size to improve the properties of materials. According to the study of Chen et al.,25 a nanocrystalline 316L stainless a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.16 556
J. Mater. Res., Vol. 29, No. 4, Feb 28, 2014
http://journals.cambridge.org
Downloaded: 04 Mar 2014
steel was prepared by SMAT, and the nanocrystalline 316L sample exhibits a yield strength as high as 1450 MPa. But we should notice that the outstanding property of the nanocrystalline sample depends on its thermal stability. In addition, the thermal stability of the nanostructured materials has been s
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