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.

Microstructures of Machined Chips in Pure Titanium, Pure Iron and 0.83%C Steel

“…As shown in the image quality map ( Fig. 2(a)), the chip specimen is principally composed of deformed microstructure with large elongated grains (~4 to ~40 μm in length and 2 to ~15 μm in width) surrounded by large angle grain boundaries, which is similar to the one observed in the chip specimens of the SUS304 steel (γ ≈ 14) and 6061-T6 alloy (γ ≈ 10), 14) although equiaxed SMGs of ~0.5 μm are partly observed between large grains in the inverse pole figure map ( Fig. 2(b)).…”

“…11,12) In SUS430 steel, 14) ~90% in volume of the chip specimen with γ ≈ 7.5 had equiaxed SMG structure, while very small grains were observed near the surface contacting with cutting tool and a little larger grains were partly involved in the region near free surface. Similar result was Fractions of these grain boundaries were examined in chip specimens.…”

“…3), 0.83%C steel (Fig. 4) and SUS430 steel 14) were principally composed of equiaxed SMGs mostly separated by large angle grain boundaries (θ ≥ 15°). Similar microstructure was observed in the chip specimen of Inconel X-750 (NCF 750) alloy at much larger strain of ~22.…”

“…14) McQueen 21) reported that the degree of dynamic recovery observed in metals at a certain value of strain rate and homologous temperature depends on the relative abilities of dislocations to leave the slip planes. This has been found to be limited in metals of low SFE because the dislocations are extended on the slip plane and require thermal activation to constrict in order to cross-slip or climb.…”

“…8) Ni and his co-workers reported similar results on 1 100 aluminum 9) and pure Cu. 10) According to the microstructural analysis by the FE-SEM/EBSP method, 11,12) Tanaka et al 13,14) recently found that the chip specimen with γ ≈ 7.5 of the ferritic SUS430 (16Cr) steel with body-centered cubic (bcc) structure was principally composed of equiaxed submicron grains (SMGs). However, the chip specimens with shear strain (γ) up to ~10 of the 6061-T6 alloy as well as the chip specimen with γ ≈ 14 of the austenitic SUS304 (18Cr-8Ni) steel (fcc) had principally a deformed microstructure with elongated grains.…”