Effects of Non-metallic Inclusions on Machinability of Free-Cutting Steels Investigated by Nano-Indentation Measurements
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IT is well-known that the machinability of steel has great effect on cost reduction of products, productivity increase, and realization of up-speed, automation, and high precision of machine work. Control of non-metallic inclusions is one of the most effective methods to improve the machinability of steel.[1] One of the ways to improve machinability of steel is attained by realization of the inclusions mechanical properties.[2] The microvoids and microcracks are nucleated by stress concentration due to different mechanical properties between inclusions and matrix.[3,4] It is considered that soft inclusions in steel can promote ductile fracture of chip formation process.[5] However, hard inclusions which have high melting points in steel can reduce tool life seriously during cutting process because of abrasive wear.[6] Therefore, it is necessary to investigate the mechanical properties of inclusions in steel, as the basis of the investigation of the effects of inclusions on machinability. YU-NAN WANG, Postdoctoral, is with the Steelmaking Research Department, Research Institute, Baosteel Group Corporation, Shanghai 201900, P.R. China, and also with Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, P.R. China. Contact e-mail: [email protected] JIAN YANG, Professor, is with the Steelmaking Research Department, Research Institute, Baosteel Group Corporation. YAN-PING BAO, Professor, is with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China, and also with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. Manuscript submitted March 13, 2014. Article published online October 7, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
In fact, few experimental methods are available to determine the mechanical properties of inclusions because of their small size. In recent years, nanoindentation tests[7–10] have been revealed suitable for measuring hardness, as well as elastic moduli of microscopic volumes of materials such as microstructure, thin-film structures, oxides, and inclusions.[11–32] Nanoindentation is simply an indentation test in which the length scale of the penetration is measured in nanometers (109 m) rather than microns (106 m) or millimeters (103 m), the latter being common in conventional hardness tests. However, in nano-indentation tests, the size of the residual impression is of the order of microns and too small to be conveniently measured directly. Thus, it is customary to determine the contact area by measuring the depth of penetration of the indenter into the specimen surface. This, together with the known geometry of the indenter, provides an indirect measurement of contact area at full load.[10] This paper aims at investigating the effects of inclusions on machinability. In the first part of the paper, the hardness, elastic moduli, and load–displacement curves of BN, MnS, Al2O3, TiN, and matr
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