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TENSILE, HIGH STRAIN RATE COMPRESSION AND MICROSTRUCTURAL EVALUATION OF LIGHTWEIGHT AGE HARDENABLE CAST FE-30MN-9AL-XSI-0.9C-0.5MO STEEL


R. Howell and T. Weerasooriya


Army Research Lab, Aberdeen Proving Grounds, MD, USA D. Van Aken


Missouri University of Science and Technology, Rolla, MO, USA Copyright © 2010 American Foundry Society Abstract


Age hardenable, castable, and lightweight Fe-Mn-Al-C steels are currently being developed and evaluated for substitution of high strength low alloy steel and to meet MIL-PRF-32269 criteria. Two nominal Fe-30Mn-9Al-0.9C-0.5Mo steels were cast and modified with 1 and 1.4 wt.% silicon. Ageing, tensile, and high strain rate compression testing were performed on solution treated and aged samples of both chemistries. Each alloy was solution treated at 1050°C for 2 hours. Microstruc- tures of the solution treated and aged alloys show primary austenite with less than 8 volume % ferrite. The solution treat- ed hardness of the low silicon steel was 230 BHN and the high silicon alloy was 225 BHN. Specimens were aged at 530°C


Introduction


Fe-Mn-Al-C alloys were developed as a substitute for austen- itic nickel-chrome stainless steels by the U.S. Navy.1


These alloys Ham and


Cairns designed the alloy with the intent of utilizing high alu- minum content to provide corrosion resistance in an austen- itic steel stabilized by manganese and carbon.2


contain 20-30% Mn, 7-12% Al, and 0.7-1.2% C. All chem- istries are in weight percent. The Fe-Mn-Al-C steels have been proven to show excellent ductility with greater than 80% fracture strain in the solution treated condition.2


It was later


shown that for aluminum content greater than 5% and car- bon content greater than 0.3%, the Fe-Mn-Al-C age hardened by precipitation of the κ-carbide (see Figure 1). Strengths in excess of 2,000 MPa (290 ksi) and Charpy impact toughness greater than 221 J (300 ft-lbs) have also been reported.3,4


Fe-


Mn-Al-C alloys are 12-18% lighter than high strength low alloy steel.5


The high strength properties and low density combine for high specific strength. It is for these reasons that recent studies have been conducted on wrought Fe-Mn-Al-C steels for automotive use as a high energy absorbing material in critical structural components due to their exceptional high strain-rate, work hardening behavior.5


As-cast microstructures consist of primary austenite with 10 to 15 volume % δ-ferrite. Solution treatment can partially or completely dissolve the ferrite. Solution treatment is typi-


International Journal of Metalcasting/Winter 10


for up to 60 hours. Peak ageing occurs after 30 hours with a peak hardness of 371 BHN for the 1% silicon containing al- loy and 377 BHN for the high silicon alloy. Tensile strengths of the 30 hour aged specimens were 1065 MPa (154 ksi) and 1080 MPa (156 ksi) for the low and high silicon alloys. High strain rate compression testing was conducted on solution treated and 10 hour aged 1% Si containing alloy. Compres- sive strength of the 1 wt.% Si alloy exceeded 1,500 MPa (217 ksi) at a strain rate of 3000 s.-1


.


Keywords: tensile, strain rate compression, age hardened steel, shear


cally performed at 1050°C (1922°F) for 2 hours.6,7,8 Wrought


microstructures contain equiaxed austenite grains with an- nealing twins and bands of ferrite parallel to the rolling di- rection. When age hardened, κ-carbide precipitates homo- geneously in the austenite to produce a three phase, or TRI- PLEX microstructure.5


where aluminum atoms occupy the lattice corner positions, iron and manganese at the face positions, and carbon at the octahedral {½, ½, ½}interstitial position. The homogeneous precipitation of κ-carbide is believed to result from an initial spinodal decomposition involving aluminum and carbon.9 Most studies have used a 550°C (1022°F) ageing tempera- ture with an ageing time of 16 hours.10,11


crystal structure with chemical composition (Fe,Mn)3 The κ-carbide is an E21


hours embrittles the material by growth of κ-carbide along grain boundaries and formation of β-manganese.12


Ageing beyond 16 The ad-


dition of silicon has been shown to prevent β-manganese by displacing manganese from the austenite to the κ-carbide.13


In cast form, Howell et al. showed that Fe-Mn-Al-C al- loys with up to 2.24% Si are primarily austenitic with less than 20% ferrite in the solution treated condition.14


Thermal


analysis was used to show that silicon additions lower the liquidus, dendrite coherency point, and solidus temperatures by 30°C (86°F) per weight percent of silicon added, and that silicon additions increased fluidity as shown by the 70% increase in spiral length versus a low alloy steel at 150°C


7


perovskite AlC


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