Ferrite Effects in Fe-Mn-Al-C Triplex Steels

In order to determine the ferrite effect on tensile properties of Fe-28.8Mn-9.4Al-C Triplex steels, four Triplex steel sheets with different carbon contents were solution treated under various conditions leading to different ferrite fractions ranging from 0 to 39 pct. This approach was used as it was very difficult to prepare samples with a wide range of ferrite contents from only one grade. However, in order to isolate the ferrite effect from this approach, it was necessary to precisely know the effect of the austenite carbon content and that of the austenite grain size on tensile properties of fully austenitic samples. This is why, a preliminary study was performed to assess these two effects, which required a thorough analysis of the microstructures of the investigated samples combining electron backscattered diffraction experiments and transmission electron microscopy observations. The study of fully austenitic samples led us to the conclusion that a decrease in the austenite grain size leads to higher strength and lower elongation and that the austenite carbon content has a greatly significant influence on tensile properties. However, this influence was difficult to estimate with a very good accuracy. Finally, the study of samples with different ferrite contents showed that ferrite had a positive effect on strength when its content was lower than 5 pct. In contrast, a higher content of ferrite (more than 30 pct) has a deleterious effect on mechanical properties.