Study on the machinability of resulfurized composite free-cutting steels
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International
Study on the Machinability of Resulfurized Composite Free-Cutting Steels D. Lou, K. Cui, and Y. Jia The machinability of S, S-Ca, S-RE, and S-RE-Ca system resulfurized composite free-cutting steels were investigated, where RE is rare earths, mostly cerium. The experimental results showed that in the low cutting speed range (_ 120 m/min).'f0 = - 1 0 ~ ~0 = 8~ ~'0 = 6 ~ ~.s = - 6 ~ K~=75 ~ K'.t= 14 ~ Low cutting speed (v < 35 m/min): depth of cut d = 1.5 mm, feed f = 0.1 mm/rev, total cutting time t = 240 min. High cutting speed (v > 120 m/min): d = 1'.0 mm, f = 0.1 mm/rev, t = 20 min. Dry cutting
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3.1 Analyses of the Inclusion Compositions, Shape, Size
and Area Fraction in Experimental Steels Electro-probe microanalysis (EPMA) and image analyses were employed to determine the composition, shape, size, and area fraction of the inclusions. The results are given in Table 2. 3.2 The Flank Wear of T1 HSS Tools Machining tests of the four steels were carried out at the following cutting speeds: at 5 m/rain for 80 rain, then at 10 m/rain for 40 min and 15 rn/min for 40 rain, and then at 20, 25, 30, and 35 m/min for 20 min, respectively. The total cutting time was 240 min. The total flank wear curves for T 1 tools are shown in Fig. 2. In the cutting speed range, wear on the flank of the tool was the dominant failure mechanism, not rake face wear due to the formation of built-up edges on the rake face. Figure 2 shows that the highest degree of wear is for steel C. This may be attributed to the effect of inclusion shape on the maximum stress concentration (Ref 5):
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Gmax =
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where B and L are the width and length of the inclusion, respectively. The expression shows that, the higher the ratio B/L, the higher the stress concentration, which leads to improved chip breakage, and thus reduced tool wear. Therefore it is not difficult to explain why steel A exhibits better machinability than steel C, and so do steels B and D, for their differences lie in the B/L ratio. Fig. 1 Geometry terms of cutting tools 3.3 The Wear of Cemented Carbide Tool (P30)
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C U T T I N G TIME ( m / n ) Fig. 2 Flank wear curves for type TI HSS cutting tools
216--Volume 6(2) April 1997
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Four steels were machined at 120 and 160 m/rain. Catastrophic failure of the tools took place in several seconds when machining steel C, so the machining process for steel C was stopped. Cutting of the other steels was carried out smoothly. In the high cutting speed range, however, rake wear was the main factor affecting the tool life, for the flank wear was relatively smaller (as seen in Fig. 3), which agrees with an earlier study (Ref 6). SEM images of rake wear morphologies are presented in Fig. 3. As can be seen from Fig. 3, the tools exhibited the maximum abrasive wear when machining steel D (Fig. 3d), followed by steel A (Fig. 3
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