Austenitization of steel during chip formation
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quick-stop device in which the cutting action is arrested by withdrawing the tool by means of an explosive charge was used. 3 During subzero machining the workpiece temperature increased to between -140 ~ to - 1 0 0 ~ after 30 seconds continuous cutting. On quick-stopping, the chip (0.5 mm thick x 1.3 mm wide) left attached to the much larger workpiece (76 mm diameter x 400 mm long) was cooled rapidly as soon as the cutting action was arrested. It was this self-quenching action by the cooler workpiece that enabled the pre-existing chip structure to be retained, any pearlite that had transformed to austenite transforming again to martensite on quenching. High resolution metallography was carried out using a scanning electron microscope in the secondary electron emission mode on specimens polished and etched in 2 pct nital as for conventional optical metallography. Figures 2(a) and 2(b) are optical micrographs taken of the area outlined by the rectangle in Figure 1, the chip in Figure 2(b) being formed during room-temperature cutting. Figures 3(a) and 3(b) show the areas outlined by rectangles
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Fig. 1--Diagrammatic representation of the cutting process.
J.L. HAU-BRACAMONTE, formerly Research Student, Department of Metallurgy and Materials, University of Birmingham, is now with INTEVEP, S. A., Apdo. 76343, Caracas 107, Venezuela. M. L. H. WISE is Lecturer, Department of Metallurgy and Materials, University of Birmingham, P.O. Box 363, Birmingham, BI5 2TT, United Kingdom. Manuscript submitted October II, 1982. METALLURGICALTRANSACTIONS A
Fig. 2--Optical micrographs showing microstructure of chips at about 3 to 4 mm from the original position of the cutting edge as indicated in Fig. 1; etched 2 pct nital. (a) Subzero machining, (b) room-temperature machining. The number indicated on the indentations is the Vickers Pyramid Number at a load of 50 g, i.e., HV 0.05. VOLUME 14A, AUGUST 1983-- 1743
Fig. 3 - - Scanning electron micrographs showing details in areas outlined by rectangles in Fig. 2(a). (a) Equiaxed grains of 0.2 to 0.8 /xm in size have formed in the ferrite. (b) Martensite appears featureless; a thin ferritic band is indicated by an arrow. The number in (a) is the microhardness value, HV 0.05.
in Figure 2(a) as seen under the scanning electron microscope. Because of its etching characteristics, featureless appearance and high hardness, it is postulated that the white layer in Figure 2(a) is martensitic (compare with Figure 2(b)). During room-temperature cutting the rate of cooling experienced by the chip as soon as cutting was arrested was not sufficiently high to allow the austenitemartensite transformation to take place in Figure 2(b). The average temperature of the chip body was estimated to be about 400 ~ during RT-cutting and about 200 ~ during subzero cutting (for estimation of temperature see Reference 4). The maximum temperature developed at the tool-chip interface was determined by the metallographic method developed by Wright and Tr
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