times longer than 30 hours at 500°C (932°F). Si7 Al5 and aged condition5
(302°F) superheat. In previous studies using silicon addi- tions, Prodham and Chakrabarti reported rapid age harden- ing in a Fe-30Mn-XAl-YC-1.36Si with aluminum content between 7.5 and 10% and carbon less than 1%. Precipitation of a Mn12
intermetallic was also observed at ageing
High strain rate compression testing has not been reported on cast Fe-Mn-Al-C alloys. Wrought alloys in solution treated condition15
strengths greater than 1,500 MPa (217 ksi) and true fracture strains exceeding 40% at strain rates between 103 s-1
. Fracture in this loading regime occurs by highly localized deformation bands or shear bands.5
added followed by the addition of the required ferroman- ganese. A solid electrolyte sensor measured active oxygen contents. Both heats contained 2 ppm active oxygen. No ad- ditional deoxidation practice was conducted. Melt stirring was limited to the induction heating, natural convection, and pouring from the furnace to the ladle and casting. Deslag- ging was conducted with a low-density granular coagulant.
have shown compressive true s-1
The high deformation rate
Low thermal conducting materials cannot dissipate the heat causing lower stress within the band. This phenomenon has therefore been designated as adiabatic shear band (ASB) formation. Solution treated Fe-Mn-Al-C alloys have demonstrated a resistance to fracture by adiabatic shear bands as work hardening occurs prior to shear localization.15
causes momentary localized high temperatures to occur with- in these bands.16
The current investigation of silicon additions to Fe-Mn-Al-C alloys is part of an evaluation of these age hardenable, high strength, lightweight steel alloys for cast, perforated armor per MIL-PRF-32269.17
A nominal Fe-30-Mn-9Al-0.9C was cho-
and a wrought mechanical property optimization study conducted by Kalashnikov et al.18
sen based on historic Fe-Mn-Al-C steel research showing high strength3
An ageing curve was
constructed for both alloys at 530°C (986°F), a temperature that constrains heterogeneous grain boundary precipitation of κ-carbide from occurring.19
From the 530°C (986°F) ageing
study, times were selected to produce a hardness within the prescribed MIL-PRF-32269 acceptance range. The dynamic properties of the cast Fe-Mn-Al-C-Si alloys were compared to those of rolled homogeneous armor on a specific strength basis. The resultant tensile and dynamic properties are also a useful contribution to determining Johnson-Cook parameters for finite element analysis models.
Experimental Procedure
Two heats with differing silicon contents were produced. Specimen bars were cast into phenolic no-bake silica sand molds. The molds were coated with a zircon wash to prevent reaction between the manganese and the silica sand. Cast bars measured 3 cm in diameter and 20 cm in length (see Figure 2). A large center riser with 15 cm of head height over the mid section of the horizontal bar was also utilized as the down sprue. No filter was used. High purity induction iron, aluminum, carbon, ferromanganese, ferrosilicon, and ferromolybdenum were melted in a 45 kg (100 lb) induc- tion furnace under argon cover. A recovery rate of 95% was utilized for the manganese and aluminum. The furnace was charged with aluminum, carbon, ferrosilicon, ferromolyb- denum, and 30% by weight of the required induction iron. After the charge liquefied, the remaining induction iron was
8 and 104
Castings were solution treated in atmosphere at 1050°C (1922°F) for 2 hours, air-cooled, and then the gating was re- moved. Round bars were machined in preparation for section- ing hardness specimens, coupons for chemical analysis, ma- chining of tensile bars, and compression specimens. Chemical analysis was conducted by ion coupled plasma spectrometry after sample dissolution in perchloric acid. Aluminum and sil- icon contents were verified with x-ray fluorescence by single element wavelength dispersive spectrometry. Chemical cou- pons were taken from end cuts; one from each heat. Nitrogen content was measured by inert gas fusion or thermal evolution method per ASTM E1019. Seven round specimens measur- ing 2.5 cm in diameter by 1.25 cm thick were machined and ground parallel per ASTM E18. Ageing curves for 530°C (986°F) were constructed from hardness measurements for each alloy. One specimen was retained after initial machining and sectioning to record solution treated hardness. Specimens were aged at 530°C (986°F) for 1, 3, 6, 10, 30, and 60 hours in atmosphere and subsequently air-cooled to room temper- ature. The reported hardness is an average of ten measure- ments and the uncertainty is based upon a sample standard deviation. Tensile specimens were machined per ASTM E8. The gage section measured 2.54 cm in length with a 0.635 cm gage diameter (see Figure 2). One tensile specimen from each heat was retained in the solution treated condition. One was aged at 530°C (986°F) for 10 hours, and one at 30 hours. The
Figure 1. Age hardened strength in Fe-Mn-Al-C alloys
results from homogenous precipitation of the κ-carbide. The к-carbide crystal structure is E21
and consists of
aluminum atoms occupying corners of the unit cell, iron and manganese occupying the face centered positions, and carbon at the body center, i.e. {½, ½, ½}.
International Journal of Metalcasting/Winter 10
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