Microstructures of Cutting Chips of SUS430 and SUS304 Steels, and NCF 750 and 6061-T6 Alloys

Abstract

Microstructures of chip specimens of high-strength materials produced by machining were examined by FE-SEM/EBSP method (an orientation imaging microscopy, OIM). In the ferritic SUS430 (16Cr) steel, the chip specimen with shear strain (γ) of ~7.5 was principally composed of equiaxed submicron grains which were for the most part surrounded by large angle grain boundaries (misorientation, θ≥15°). A similar microstructure was observed in the chip specimen with γ ≈ 22 of the Inconel X-750 (NCF 750) nickel-base alloy. However, the chip specimen of the austenitic SUS304 (18Cr–8Ni) steel (γ ≈ 14) and that of the 6061-T6 (aluminum) alloy (γ ≈ 10) exhibited principally a deformed microstructure with elongated grains and sub grains separated by small angle (2°≤θ<5°) or medium angle grain boundaries (5°≤θ<15°), although equiaxed submicron grains were partly observed. The chip specimens exhibited very high hardness compared to the original materials except 6061-T6 alloy. The maximum hardness value (609 Hv) was observed in the chip specimen with γ ≈ 22 of the Inconel X-750 alloy. Strong particles with equiaxed submicron grain structure, which can be easily produced by milling of cutting chips of commercial alloys, will be potential strengthener for metal matrix composites.