Microstructure and Mechanical Properties of AISI 8620 Steel Processed by ECAP
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Microstructure and Mechanical Properties of AISI 8620 Steel Processed by ECAP Diana M. Marulanda1, Jair G. Cortés1, Marco A. Pérez1 and Gabriel García1 1 Research Group in energy and materials (REM), Universidad Antonio Nariño, 22th Street 12D81 South, Bogotá, Colombia.
ABSTRACT The aim of this work is to process by equal channel angular pressing (ECAP) a low carbon – triple-alloyed steel containing 0.2% C, 0.5% Cr, 0.6% Ni, 0.2% Mo and 0.8 Mo. The process is performed at room temperature for up to four passes using route Bc with an equivalent strain of a0.6 after a single pass. Structure evolution before and after deformation is studied using scanning electron microscopy (SEM) and x-ray diffraction (XRD) and mechanical properties are assessed by microhardness and tensile testing. A significant improvement of the mechanical properties is found with increasing number of ECAP passes. Micro-hardness increases from 216 HV for the initial sample to 302 HV after four passes and tensile strength increases to 1200 MPa compared with 430 MPa prior to ECAP. X-ray diffraction and SEM analysis show changes in the original ferritic-perlitic structure through ferrite grain refinement and the deformation of perlite. This nickel-chromium-molybdenum alloy is used in manufacturing as gear material, and when it is hardened and formed through carburizing or boronizing it can be used to make hard-wearing machine parts. However, the ECAP process has not been used to harden this steel and to change its structure to obtain better mechanical performance. INTRODUCTION In engineering applications, most of the components are subjected to friction, wear, fatigue and corrosive environments. Guarantying the efficient use of these components includes selecting the adequate material for the specific purpose. Among these materials, the AISI 8620 steel, low carbon – triple-alloyed steel, is used in manufacturing as gear material, and when it is hardened and formed through carburizing or boronizing [1], it can be used to make hard-wearing machine parts. Through these techniques, a surface layer is formed that improves mechanical properties of the steel. However, only the surface is treated. Improving the bulk properties of the material requires the use of techniques that modify its internal structure. Among these techniques, the equal channel angular pressing (ECAP) has attracted attention because of the capacity to produce full dense bulk materials with sub-micron or even nano-scale grains [2]. The grain refinement obtained using this technique improves mechanical properties owing to the large volume fraction of grain boundaries and in some cases it is possible to obtain high angle grain boundaries which gives an optimization in strength without loss of ductility. The ECAP process has not been used to harden AISI 8620 steel and to change its structure to obtain better mechanical performance. In that sense, the microstructure and mechanical properties of this steel are presented before and after the ECAP processing using route Bc.
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