Microstructural Refinement in Steels by Machining
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P9.5.1
Microstructural Refinement in Steels by Machining S. Swaminathan, C. Swanson, T. L. Brown, R. F. Kezar, S. Chandrasekar, W. D. Compton and K. P. Trumble Center for Materials Processing and Tribology, Schools of Engineering, 315 N. Grant Street, Purdue University, West Lafayette, IN, 47907-2023 ABSTRACT A study has been made of microstructure and hardness of machining chips created from commercially pure iron and carbon steels. Large shear strains imposed during chip formation in machining are found to produce significant microstructure refinement in the chips, resulting in higher hardness compared to the bulk. Transmission electron and scanning electron microscopy have shown the chips to consist entirely of ultra-fine grain structures with ferrite grain sizes in the range of 100-800 nm. With high carbon steels, the microstructure of the bulk material prior to machining is also seen to have a significant influence on the characteristics of the chip. INTRODUCTION It has been known for some time now that significant microstructure refinement can be realized in metals and alloys by the application of large strain deformation [1,2]. Examples of this approach can be found in the use of high-energy ball milling to create nano-particles [3]; and the use of severe plastic deformation (SPD) processes such as equal channel angular extrusion (ECAE) [4] and high pressure torsion (HPT) [5] to prepare bulk and thin samples, respectively, with nanocrystalline microstructure. It has been shown, recently, that chip formation by machining provides a simple configuration in which large shear strains of known magnitude can be imposed in metals and alloys in a single stage of deformation [6,7]. Furthermore, the method is equally applicable to materials of both low and high initial strength. The creation of nanocrystalline microstructures in a variety of non-ferrous materials and commercially pure iron was demonstrated [6,8]. The present study was undertaken with the objective of studying the effects of large strain deformation, imposed in a single stage of deformation, on the microstructure of steels of varying carbon content. Preliminary experiments have also been carried out to explore the effect of initial microstructure of steel on microstructural changes arising from the large deformation. BACKGROUND Machining is a process in which a hard, wedge-shaped tool removes material (chip) from the surface of a solid (bulk) by very large strain deformation (Fig. 1a)). Chip formation occurs by concentrated shear along a narrow deformation zone called the shear plane (Figs. 1a) and 1b)). The effective plastic strain imposed at the shear plane can be varied by changing the tool rake angle and the friction between tool and chip. Shear strains in the range of 2-10 and strain rates of up to 1×106 s-1 are common features of machining [9]. The temperature in the deformation zone may be systematically varied by changing the cutting velocity.
P9.5.2
EXPERIMENTAL PROCEDURE Chips from commercially pure iron (99.8%), plain carbon steels, 52100
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