The flow equation and its necking criterion in austenitic cryogenic Fe-Mn-Al-X steels
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YOUNG G. KIM and CHA Y. LIM The behavior of increasing elongation with decreasing temperature in austenitic Fe-30Mn-5A1-0.3C-0.1Nb steel was previously reported, m The steel exhibited a 29 pct total elongation at room temperature (RT), compared to a 57 pct at 77 K (LNT). We also reported that the inverse ductility behavior with decreasing temperature in austenitic Fe-30Mn-A1-0.3C steels was dependent on the amount of aluminum content, m The inverse ductility phenomenon from RT to 77 K occurred when the A1 content exceeded 3 wt pct. The inverse ductility behavior was found to be very beneficial in enhancing the strain-controlled low cycle fatigue resistance at 77 K of the cryogenic Fe-30Mn-5A10.1Nb-0.3C steel, p] This work is concemed with low temperature tensile behavior of an austenitic Fe-25Mn-5Ni-5A1-0.3C (wt pct) steel. The alloy used in this study was prepared by air induction melting. The ingot, about 20 kg, was homogenized at 1523 K for 2 hours. The ingot was subjected to a hot controlled rolling process. The final rolling temperature and rolling reduction ratio were 1123 K and 25 pct, respectively. Plate tensile specimens with 6.3 mm by 3.0 mm gage cross-section and 30 m m gage length were prepared from the controlled rolled plates. Tensile tests were performed at RT, 233 K, 173 K, and 77 K, respectively. The true stress-true strain curves at the different temperatures are plotted in Figure 1. The maximum elongation occurred at 77 K and necking occurred at lower strains as the test temperature increased from 77 K to RT. This alloy also exhibited the inverse ductility behavior. The total elongation at RT was 25 pct, compared to 50 pct at 77 K. The plots of logarithmic true stress v s logarithmic true strain for this alloy are shown in Figure 2. In obtaining strain hardening exponents (n), the relationship between (In o-) and (In e) was assumed to be quadratic at 233 K, 173 K, and 77 K: In G = a(ln e) 2 + b(ln e) + c
[1]
In the above equation, the values of a, b, c were obtained by means of least square method. At 77 K, the values of a, b, and c were found to be 0.103, 0.641, and 8.069, respectively. The strain hardening exponents could be obtained from the Eq. [1] by differentiating (ln o9 with respect to (ln e). The variation of strain hardening exponents with true strain at various temperatures for this alloy is shown in Figure 3. The strain hardening exponent did not change with the strain at RT, whereas the strain hardening exponents increased with increased true strains with the decreasing temperatures.
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f
I'-" 400L/ 0
~233K
~arro..... k UTS.ineng. / 0.1 0.2 0.5 True Strain
METALLURGICAL TRANSACTIONS A
0.4
Fig. 1 - - T r u e stress-true strain curves at RT, 233 K, 173 K, and 77 K.
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7.2 z 7~
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~ R T / /
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-3.o -2'.5 -s
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vs
logarithmic true strain at RT, 233 K,
Assuming a linear relationship between strain hardening exponent and true strains at 233 K, 173 K, and 77 K in Figure 3, the
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