and are manganese rich. Howell et al.24
has previously shown
that calcium treated alloys have a greatly reduced sulfide con- tent, and has reported an increase in Charpy toughness for a reduction in phosphorous and sulfur content in a similar Fe- Mn-Al-C alloy. The high phosphorous content in the alloys investigated here (0.06 wt.%P) appears to be due to phos- phorous in the ferromanganese. However, at 100% recovery, phosphorous contribution from ferromanganese would only account for 0.018%P. Other potential sources include furnace and ladle refractory materials, and residual metal from prior heats. Phosphorous reduction is possible by changing to low phosphorous electrolytic manganese. Improved foundry prac- tice investigations to further reduce phosphorous, sulfur, and inclusion content are ongoing.
Tensile strength, ductility, and work hardening followed classic trends for age hardening materials. Except for the so- lution treated condition, the 1.4% silicon alloy had a higher strength and hardness, but lower ductility for equivalent ageing treatment. The lower ductility may result from an increase in the volume fraction of ferrite and the number density of non-metallic inclusions. Pickering25
has reported
a decrease in ductility for steel for an increase in volume fraction of second phase particles and the same trend is ob- served for the alloys studied here.
Microstructure of aged and deformed materials were simi- lar between the two silicon levels. Ferrite content followed a trend previously reported14
where a minimum of ferrite occurs
at approximately 1% Si and increases with greater silicon con- tent. It should be noted that the high phosphorous content in the alloys studied here may promote higher volume fractions of ferrite in both the solution treated and aged microstructures. In the previous study, a fully austenitic microstructure was ob- tained for a 0.82% Si alloy after solution treatment. The tensile deformed microstructure shows evidence of planar slip in both alloys (see Figure 8). Planar slip was also observed in the high strain rate compression samples (see Figure 12). Both solution treated alloys failed by transgranular microvoid coalescence during tensile testing as seen in Figure 9. Ageing resulted in a transition to a transgranular cleavage failure that appears to follow the crystallography of the dendrites as shown in Figure 9(d).
High strain rate compression samples failed by adiabatic shear band formation, which preceded crack nucleation and growth. Three specific examples show this phenomenon. First, the through crack in Figure 11 that occurred at 45° to the loading direction is associated with adiabatic shear band formation. Ex- amples of shear bands that lead to through cracking are shown in Figures 14, 15, and 16, and were observed along a macro-
Table 3. Comparison of Specific Compressive Strengths of Two Heat Treated Conditions Between the 1% Silicon Containing Fe-Mn-Al-C Alloy and RHA Tested at 3000 s-1
Strain Rate.
Figure 16. Multiple shear bands are shown in a 1% sili- con modified alloy in the solution treated condition after high strain rate testing. Planar slip is visible between shear bands. The loading direction is indicated in the upper right hand corner.
16 International Journal of Metalcasting/Winter 10
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