This study evaluates the role of thermomechanical processing and heat treatment on the microstructure and mechanical properties of a hot rolled, annealed, and aged Fe-18Mn-10Al-0.9C-5Ni (wt%) steel. The steel exhibited rapid age hardening kinetics when aged in the temperature range of 500–600 °C for up to 50 h, which has been shown in other work to be the result of B2 ordering in the ferrite and κ-carbide precipitation within the austenite matrix. The ultimate tensile strength increased from 1120 MPa in the annealed condition to 1230 MPa after 2 h of aging at 570 °C. Charpy V-notch toughness was evaluated at −40 °C in sub-sized specimens with a maximum in the annealed and quenched condition of 28.5 J in the L-T orientation.

In this study, a novel lightweight Fe-Mn-Al-C steel composition and thermomechanical processing route was developed to produce a fully austenitic microstructure with a uniform intragranular dispersion of B2-NiAl precipitation in order to overcome the significant challenge of strengthening hot-rolled Fe-Mn-Al-C steels while retaining toughness. The new composition and processing methods allow for the processing of ultrahigh-strength Fe-Mn-Al-C steel containing nickel as thicker gauge plate for a multitude of new automotive and structural applications where lightweighting is critical. The composition investigated in this study was a fully austenitic Fe-21Mn-9Al-1C-8Ni wt% steel. Two hot rolling methods were investigated: the first procedure involved lower temperature rolling cycles to precipitate B2-NiAl during hot rolling and reheating. The second method involved higher temperature rolling to precipitate B2-NiAl after thermomechanical processing during a short isothermal treatment. The lower temperature rolling produced plate with an ultimate tensile strength of 1120 MPa and a Charpy V-Notch (CVN) toughness of 24 J at −40 °C. After the high temperature rolling procedure, precipitation of B2-NiAl through a subsequent precipitation hardening step resulted in reduced B2-NiAl size and improved the ultimate tensile strength above 1300 MPa. The two novel processing routes of a single composition can be performed with current manufacturing capabilities to produce hot rolled plate strengthened by B2-NiAl precipitation to various hardness (ranging from 33 to 41 HRC) and strength levels (ranging from 1100 to 1320 MPa ultimate tensile strength) while retaining 22–27% elongation.

Gating systems play an important role in determining the quality and mechanical properties of castings. To understand the efficiency of gating systems, four systems, namely pressurized system, non-pressurized system, naturally pressurized system with a side riser and a naturally pressurized system with a top riser, were studied. The naturally pressurized systems were provided with overflows which collected the incoming metal swirl. Parameters like velocity of metal flow, air entrapment, microporosity and Niyama criterion were considered, and a design was developed with a common pouring basin. 8630 alloy was poured into two molds using a teapot ladle. The inclusion analysis revealed the presence of alumina and an alumina + manganese sulfide complex, and the naturally pressurized top riser system provided the cleanest casting with the best toughness. The formation of eutectic type II MnS affected the toughness properties. Naturally pressurized side riser and the non-pressurized system performed on par while the pressurized system had the highest inclusion area fraction.

To understand the efficiency of gating systems, four systems, namely AFS styled pressurized system, AFS styled non-pressurized system, naturally pressurized system with a side riser and a naturally pressurized system with a top riser, were studied. The naturally pressurized systems were provided with overflows which collected the incoming metal swirl while the pressurized and non-pressurized systems had runner wells. Parameters like velocity of metal flow, air entrapment, microporosity and Niyama criterion were considered, and a design was developed with a common pouring basin using solidification software. Two molds were 3D printed using carbonitride sand, and steel of composition Fe-29Mn-8.2Al-0.91C-0.88Si-0.49Mo was poured into both the molds. Optical microscopy revealed extensive oxide-film-like features on samples obtained from the top section of castings. Across the limited number of molds and a single heat, the pressurized and non-pressurized systems had Charpy toughness values comparable to the naturally pressurized top riser system. The observed fracture surfaces commonly contained Al-oxide-rich film features and presence of AlN, AlN-MnO and AlN-MnS inclusions.