Low-cost, fast-transforming carbide-free nano-bainitic steel has been produced with tensile strengths up to 2140 MPa and elongation values of around 10%. An alloying concept of combining low Mn (0.29 wt.%) and moderate Al (~ 1 wt.%) additions enabled full transformation in 10 min to less than 2 h at austempering temperatures ranging from 235 to 310 °C. Dilatometric experiments and thermodynamic simulation via ThermoCalc software were employed to design the heat treatment procedure. The resulting structures were investigated using scanning electron microscopy and synchrotron x-ray diffraction. Mechanical characterization was performed via hardness, tensile, and impact testing at room temperature. The strain-aging response for the alloy system was also investigated.

Additions of 3 and 5 wt.% Al have been investigated as a low-cost method for transformation acceleration in nano-bainitic steels. For both Al contents, two groups of steels with C-content in the range ~0.7 to ~0.95 wt.% were studied. Thermodynamic and physical simulations were used in alloy and heat treatment design. Characterization was performed via dilatometry, scanning and transmission electron microscopy, Synchrotron X-ray diffraction, and tensile and impact testing. Fast bainitic-transformation time-intervals ranging from 750–4600 s were recorded and tensile strengths up to 2000 MPa at a ductility of ~10 elongation percent were attainable for the 3 wt.% Al group at an austempering temperature of 265 °C. Higher Al additions were found to perform better than their lower Al counterparts as the austempering temperature is dropped. However, Al lowered the austenite stability, increased the martensite start temperature, austenitization temperatures and, consequently, the prior austenite grain size, as well as limiting the austempering temperatures to higher ones. Additionally, the lowered austenite stability coupled with higher additions of hardenability elements (here carbon) to maintain the martensite start at around 300 °C, causing the 5 wt.% Al group to have a large amount of low stability retained austenite (and consequently brittle martensite) in their microstructure, leading to a low elongation of around 5%.

The target of this study is to develop routes for processing Al–SiCp (SiC particulate) sheets with improved microstructures; namely, the uniform distribution of SiCp and minimized porosity throughout the whole material, thereby improving its mechanical properties. Al–SiCp composites reinforced with 5, 10, and 15 vol.% SiC and having three average sizes of 29, 17, and 8.5 µm were produced using a semi-automated stir-casting machine; finally, the bars were hot rolled to get the final sheet dimensions. The rolling steps were performed based on the results of thermomechanical simulations performed on the cast blocks. The first rolling steps were applied according to a safe rolling schedule to avoid cracking via encouraging dynamic recrystallization. The last rolling steps were performed to improve the hardness and strength of the metal matrix composites (MMCs). A refinement in grain size from an average of 420 µm in the as-cast condition to an average of about 85 µm after rolling was observed. Some heterogeneity in grain size was observed where the regions with larger, elongated grains were associated with the zones depleted by SiCp reinforcement. Reinforcing with SiCp led to a small increase in tensile strength of 10–20 MPa, and this was further improved to about 60 MPa and 110 MPa for the 5 and 10 vol.% SiCp conditions, respectively, after a T6 heat treatment.

A low‐cost strategy for generating fast transforming nanobainitic (nB) steels in low‐ and medium‐carbon alloys is investigated. Accelerating the bainite transformation relies on adding 0.7–3 wt% Al. Alloys and heat treatments are designed via thermodynamic calculations and dilatometry. nB microstructures are generated via isothermal holding and continuous cooling. The microstructures generated are investigated via microscopy and mechanical characterization. Incubation periods less than 150 s as well as isothermal transformation times ranging from 2000 to 4000 s are recorded for all conditions. Increasing the Al content from 0.7 to 2.8 wt% lowers the incubation and transformation times from 150 to 15 s and 3000 to 2000 s, respectively, at a cost of a reduction in tensile strength and elongation % (EL%) from 1330 to 1270 MPa and from 13.5 to 7.7%, respectively. The introduction of δ‐ferrite to the microstructure of the high Al alloy increased EL% up to 16% and reduced the tensile strength to 1105 MPa. Continuous cooling at a rate of 0.03 K s⁻¹ increased the tensile strength by 100 MPa at similar EL%. Lowering the cooling rate to 0.003 K s⁻¹ yielded similar properties as isothermal treatment because most of the transformation is concluded near the starting temperature.