R. A. Howell, D. C. Van Aken, V. L. Richards, Missouri University of Science and Technology, Rolla, MO
FeMnAlC alloys with chemistries in the range of 18-28wt%Mn, 9-12wt%Al, and 0.7-1.2wt%C are age hardenable austenitic steels. The high aluminum content gives the steel corrosion resistance, reduces the density (10-15% ) to a range of 6.5 to 7.2 g/cm3, and produces age hardening in combination with carbon. Wrought materials are typically solution treated above 1000°C, quenched and aged at 550°C for 16 hours to produce precipitation of ?-carbide in the austenite phase. FeMnAlC alloys possess tensile strengths from 627 to 1200 MPa, and an elongation to failure as great as 35% in the peak-aged condition. The combination of strength and low density makes the FeMnAlC alloys attractive as a low cost alternative to titanium alloys. Furthermore, FeMnAlC alloys have casting characteristics similar to cast irons, e.g. high fluidity and low melting temperatures. In this paper, the quench sensitivity and age hardening behavior of a cast Fe-30Mn-9Al-1Si-0.9C-0.5Mo alloy is reported.
Quench sensitivity is an important consideration in the heat treatment of these alloys, since distortion and adverse residual stresses increase with quenching rate. At slower rates of cooling the age hardening response can be adversely affected by precipitation of brittle intermetallic phases along austenite grain boundaries. In this study, cast Jominy end-quench bars were used to examine cooling rate effects upon the age hardening potential. Four Jominy bars were solution treated at 1050°C and quenched using a standard end-quench apparatus. End quenching produced a steady increase in hardness with increasing distance from the quenched end. The Jominy bars were subsequently aged at 500°C for 3, 6, 10, and 30 hours and compared to the same cast material that was water quenched and aged. The peak-aged hardness was obtained at shorter aging times. A minimum cooling rate was determined based upon microstructure and aging response.
Summary: FeMnAlC alloys with chemistries in the range of 18-28wt%Mn, 9-12wt%Al, and 0.7-1.2wt%C are age hardenable austenitic steels. The high aluminum content gives the steel corrosion resistance, reduces the density (10-15% ) to a range of 6.5 to 7.2 g/cm3, and produces age hardening in combination with carbon. Wrought materials are typically solution treated above 1000°C, quenched and aged at 550°C for 16 hours to produce precipitation of ?-carbide in the austenite phase. FeMnAlC alloys possess tensile strengths from 627 to 1200 MPa, and an elongation to failure as great as 35% in the peak-aged condition. The combination of strength and low density makes the FeMnAlC alloys attractive as a low cost alternative to titanium alloys. Furthermore, FeMnAlC alloys have casting characteristics similar to cast irons, e.g. high fluidity and low melting temperatures. In work reported elsewhere, the addition of Si is shown to lower solidus and liquidus temperatures approximately 30°C /wt.% Si. In this paper, the quench sensitivity and age hardening behavior of a cast Fe-30Mn-9Al-1Si-0.9C-0.5Mo alloy is reported.
Quench sensitivity is an important consideration in the heat treatment of these alloys, since distortion and adverse residual stresses increase with quenching rate. At slower rates of cooling the age hardening response can be adversely affected by precipitation of brittle intermetallic phases along austenite grain boundaries. In this study, cast Jominy end-quench bars were used to examine cooling rate effects upon the age hardening potential. Four Jominy bars were solution treated at 1050°C and quenched using a standard end-quench apparatus. End quenching produced a steady increase in hardness with increasing distance from the quenched end. The Jominy bars were subsequently aged at 500°C for 3, 6, 10, and 30 hours and compared to the same cast material that was water quenched and aged. The quench sensitivity of these alloys was related to spinodal decomposition (first stage in aging process) that occurred at slower cooling rates. The peak-aged hardness was obtained at shorter aging times. A minimum cooling rate was determined based upon microstructure and aging response.
