Mechanical Properties of Cerium-Treated Fe-Mn-Al-C Steel Castings

Age-hardening austenitic steels in the Fe-Mn-Al-C system show great promise as a lightweight alternative to quenched and tempered 4130 steel. These Fe-Mn-Al-C steels are sensitive to phosphorus, and phosphorus levels greater than 0.006 wt.% have unacceptable notch toughness. Recent studies have shown that phosphorus mitigation in Fe-Mn-Al-C steels is possible by additions of cerium in the form of misch metal. In this paper, the mechanical properties of two industrially produced Fe-30Mn-9Al-1Si-0.9C-0.5Mo (> 0.03 wt.% P) steel castings with 0.01 and 0.052 wt.% Ce were studied as a function of age hardening at 530C (986F) and 570C (1058F). Age hardening curves show slightly higher hardness values for the 0.01 wt.% Ce alloy than either the 0.052 wt.% Ce alloy or the low phosphorus steels studied previously. On average, there was a 16 Brinell Hardness Number (BHN) increase in hardness as the amount of Ce decreased from 0.052 to 0.01 wt.%, In the solution treated condition, increasing Ce decreased the ultimate tensile strength (UTS) by an average of 100 MPa and decreased the average total elongation from 57 to 23%. Peak-aged alloys exhibited a 60% decrease in elongation with the 0.052 wt.% Ce content. In addition, increasing Ce reduced the room temperature notch toughness from 214 to 90 J in the solution treated condition and from 92 J to 26 J for specimens aged to 302 BHN. The loss in ductility and notch toughness with the 0.052 wt.% Ce content was attributed to the presence of large (>20 mu m), hard and brittle (1170 HV), (Ce, La)Si-2, Laves phase that was found in the interdendritic regions of the 0.052 wt.% Ce alloy.