aged conditions were selected to measure and bracket tensile properties corresponding to lower and upper MIL-PRF-32269 hardness requirements. Two sets of cylindrical compression specimens were electrical discharge machined from the low silicon steel for Split Hopkinson Bar (SHB) testing (see Fig- ure 2). The Split Hopkinson Bar principle and testing appa- ratus are described in detail in Volume 8 of the ASM Hand- book.20
Age Hardening
The first set of SHB specimens were machined and tested in the solution treated condition. The second set was machined from material solution treated and aged at 530°C (986°F) for 10 hours. SHB specimens were lapped to a thick- ness of 0.3175 cm and measured 0.635 cm in diameter. Test- ing occurred at a strain rate of 3000 s-1
. Specimen load was in-
crementally increased from 270 Pa (40 psi) to 448 Pa (65 psi) in 34 Pa (5 psi) increments until specimen failure to evaluate shear band evolution.
Post compressive and tensile testing analysis was conducted by optical microscopy and scanning electron microscope (SEM) with energy dispersive x-ray spectrometer (EDS). SEM accelerating voltage was 15 keV with 18 mm of work- ing distance at 0° rotation. Metallographic specimens were etched with 2% Nital and images were recorded using a dif- ferential interference contrast technique.
Results Chemical Analysis
The chemical analyses of both silicon steels are reported in Table 1. Chemical analysis showed elevated levels of phos- phorus whereas sulfur was low; phosphorous and sulfur mea- sured 0.06 and 0.005 % in the low silicon alloy and 0.06 and 0.006 % in the 1.4% silicon modified alloy. Manganese, car- bon, and molybdenum contents were equal in both alloys, but aluminum concentrations varied. Aluminum contents were 8.3 and 7.9 in the low and high silicon alloys, respectively.
Figure 3 shows the age hardening curves for the 1 and 1.4% silicon containing alloys. The 1 and 1.4% silicon so- lution treated conditions had hardness 224 BHN and 219 BHN. The Fe-Mn-Al-C alloys increased in hardness to 270 BHN (1% silicon) and 252 BHN (1.4% silicon) after ageing 1 hour. Hardness of both alloys increased steadily from 1 hour to 10 hours ageing time. After 10 hours, the hardness measurements for each alloy were 343 BHN (1% silicon) and 353 BHN (1.4% silicon). From 10 hours to the peak aged condition at 30 hours, the hardness in- creased, but hardening rate decreased. Peak aged hard- ness at 30 hours measured 372 BHN (1% silicon) and 384 BHN (1.4% silicon).
Ferrite volume fraction increased during ageing but the ob- served increase occurred after peak ageing. Figure 4 shows solution treated, 10 hour, and 60 hour aged microstructures of both alloys. The 1% silicon solution treated alloy was primary austenite with less than 1 % ferrite and less than 3% ferrite in the 60 hour aged condition. Dendrite arm spacing measured 53 µm. The 1.4% silicon modified alloy contained 6% ferrite in the solution treated condition and 8% in the 60 hour aged condition. Dendrite arm spacing for the 1.4% silicon alloy measured 48 µm. Figure 4 clearly shows the dendritic micro- structure observed for both silicon containing alloys.
Quantitative inclusion measurement and chemical analysis was conducted using an automated scanning electron micro- scope utilizing energy dispersive chemical analysis. Nitrides, oxides and sulfides are comprised principally of manganese and silicon with a greater number of sulfides and nitrides than oxides as shown by the density of data plotted in Figure 5. Figure 6 shows a histogram of inclusion radii for the com- bined nitride, sulfide and oxide counts. Ninety-five percent of all inclusions were less than 1.3 µm in radius for each steel.
(a)
(b)
(c)
Figure 2. Fe-Mn-Al-C alloys were cast into a risered bar to produce mechanical test bars and metallographic specimens. The casting (a), tensile (b), and compression specimen (c) drawings are shown with dimensions to illustrate the shape and size of the materials utilized for this investigation.
International Journal of Metalcasting/Winter 10 9
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