Herein, an extensive study is presented on the microstructure–tensile properties relationship in dual‐phase (DP) steels. A series of ferrite‐martensite DP steels with varied martensite volume fractions (V_m) from 0.17 to 0.86, microstructure morphologies (globular and elongated) and structure finenesses (ferrite grain sizes from 1.9 to 10.7 μm) are produced applying appropriate heat treatments. The tensile properties are studied, and the strain hardening behavior is analyzed in terms of Holloman, Crussard–Jaoul (C–J) and modified C–J approaches. The tensile curves reveal up to three strain hardening stages with the highest strain hardening exponent at the beginning of straining. Increasing V_m and refining the structure raises the number of strain hardening stages and improves the strain hardening capacity in the first stage (n₁). For the DP steels with similar morphologies, the mean free path in ferrite (λ_f) is proposed to be the most significant microstructure factor affecting n₁‐value. The n₁ of the elongated morphology shows stronger dependence on λ_f than the globular one. Finally, the DP steels are subjected to aging treatments, which lead to improved yield strength and total elongation, however, the strain hardening exponent decreases significantly.

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.

In this research, different ductile irons and austempered ductile irons were successfully developed using several alloying contents of nickel, copper and microalloying with niobium. Additionally, special nanocarbon powder was added to the molten iron to enhance the nucleation tendency of spheroidal graphite and compensate for the possible negative effect of Nb addition on the nodule morphology. Metallographic analysis showed that increasing the niobium content in the alloy to 0.1 wt % raises the number of graphite eutectic cells and refines the final structure of the graphite. Moreover, the nodule count of graphite slightly increased, but it concurrently decreased the nodularity when the Nb amount reached 0.1 wt %. SEM micrographs illustrated that nano- to microsized niobium carbides (NbC) particles were dispersed in the matrix of the Nb microalloyed ductile irons. Both optical and SEM micrographs clearly showed that alloying of ductile irons with nickel, copper and microalloying with niobium had a significant effect on defining the final pearlite structure. Coarse, fine, broken and spheroidized pearlite structures were simultaneously observed in all investigated alloys. Dilatometry studies demonstrated that the nano NbC particles acted as nucleation sites for graphite and ferrite needles. Therefore, Nb addition accelerated the formation of ausferrite during the austempering stage. Finally, alloying with Cu, Ni and microalloying with Nb led to developing novel grades of ADI with excellent strength/ductility property combination.

This paper shed light on the kinetics of transformation and the developed microstructure during wire arc additive manufacturing (WAAM). Three microalloyed alloys, two of them are high strength low alloyed steel (HSLA) grades and the third is a Ni-Cr-Mo steel, from which the welding wires are being produced, were investigated. Repeated cycles around varied temperatures from a reheating temperature of 1350 °C and down to a temperature 35 °C below the A_e1 are applied using dilatometer on samples from the steels. After applying the cycles, the dilatometric-samples were investigated metallographically and their macro- and microhardness values were measured. It is shown that the WAAM using HSLA steels produce softer structure than the steel of the welding wire. Combined microalloying with Ti and Nb can present a useful strategy for producing finer structure in the WAAM components due to the effect of Ti in inhibiting the prior austenite grain-growth and that of Nb in refining the final structure. Additionally, repeated heating near A_e3 refines the prior austenite grains and produced fine ferrite-pearlite structure in case of HSLA steels and a microstructure predominated by the granular bainite in case of welding wire alloy. The former microstructure was the softest one for the case of HSLA steels, whereas the softest structure in case of the welding wire alloy was the tempered martensite structure developed by reheating below A_e1. Idealized temperature curves were chosen for the heat treatment, which could be characterized in a well-defined manner. In future work such idealized curves together with temperature histories obtained in WAAM-process will be used to set up a database to train an AI-model for predicting structure and material properties.