Novel Insights and Advances in Steels and Cast Irons

1. Introduction and Scope

The manufacturing sector relies extensively on ferrous alloys, with steels and cast irons serving as essential materials for structural and functional components across a wide range of industries, including automotive, energy generation, and civil engineering.

Cast irons continue to play a significant industrial role due to their excellent castability, cost-effectiveness, and intrinsic properties, such as vibration damping and wear resistance, provided by their graphite microstructure. These features make them ideal for complex, near-net-shape components. However, the rapid technological evolution, especially in fields such as Concentrated Solar Power (CSP) plants, high-speed motors, and advanced automotive bodywork, places ever-increasing demands on material performance. This ongoing progress calls for the development of innovative alloys with improved high-temperature corrosion resistance, enhanced processing efficiency, and mechanical stability under severe deformation.

Alongside cast irons, steels continue to represent the cornerstone of numerous industrial and structural applications. Continuous research efforts are crucial for developing high-performance materials in both families, aimed at improving in-service performance under increasingly stringent conditions. In particular, steels constituting the core focus of this Special Issue are subject to intense research directed towards (i) optimizing mechanical properties through controlled microstructural evolution, (ii) mitigating degradation mechanisms in harsh environments, including hot corrosion by molten salts at elevated temperatures and addressing complex paint failures like filiform corrosion on coated substrates and (iii) advancing manufacturing precision and structural integrity, through techniques including additive manufacturing or novel welding methods designed to minimize structural defects.

Within this framework, the present Special Issue collects ten high-quality research papers, addressing these areas of investigation, providing valuable theoretical and experimental insights that advance the understanding of ferrous alloy behavior and promote sustainable progress in material performance and manufacturing technologies.

2. Contributions

The contributions encompass a wide range of topics, including material behavior under severe plastic deformation, optimization of inclusions in free-cutting steels, microstructural stability in heat-resistant alloys, and innovative approaches to corrosion prevention.

The first group of papers explores the interaction between ferrous alloys and their service environments, focusing on corrosion and oxidation under aggressive or functional conditions. The studies address key degradation mechanisms—such as hot corrosion in molten salts and filiform corrosion in coated steels—and propose mitigation strategies including eco-friendly inhibitors and combined thermomechanical-surface treatments.

Abu-warda et al. (Contribution 1) investigated the corrosion behavior of laser powder bed fusion (L-PBF) 316L stainless steel exposed to various molten salts intended for thermal energy storage in concentrating solar power (CSP) plants. Corrosion tests conducted at 650 °C and 700 °C revealed that salt composition had a greater influence than temperature. Chloride-based salts produced the most severe degradation, followed by carbonate- and nitrate/nitrite-based mixtures. Surface and cross-sectional analyses by X-ray diffractometry (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) identified LiFeO₂ and LiCrO₂ as key corrosion products in carbonate media, while chloride salts induced active oxidation through a chlorine cycle. The results highlight that L-PBF 316L is unsuitable for CSP systems using chloride salts above 650 °C without protective measures.

Cristoforetti et al. (Contribution 2) examined the effect of disodium sebacate (SB), an environmentally friendly corrosion inhibitor, in mitigating filiform corrosion on acrylic-coated carbon steel substrates. Electrochemical impedance spectroscopy, polarization tests, and a dedicated filiform corrosion-simulating electrochemical setup revealed that SB acts primarily as an anodic inhibitor in near-neutral conditions, achieving an inhibition efficiency of about 98%. When incorporated into the coating primer (1 wt. %), SB reduced corrosion-creep propagation by about 35% during accelerated weathering. The molecule mitigated cathodic activity under alkaline, aerated environments typical of filiform corrosion and formed a hydrophobic interfacial layer that slows water ingress. These findings confirm sebacate’s promise as an environmentally benign additive for enhancing the durability of organic-coated steels against localized delamination phenomena.

Vontorová et al. (Contribution 3) explored the enhancement of the mechanical properties of low-carbon DC03 steel through severe plastic deformation via the dual-rolling equal-channel extrusion (DRECE) method, followed by hot-dip zinc galvanizing. The effects on microstructure, hardness, and corrosion behavior were evaluated. The DRECE process enhanced hardness by up to 20%, without significantly affecting the uniformity or thickness of the zinc coating (of about 70–85 µm), while microstructural analyses confirmed the presence of a typical Fe–Zn intermetallic layer. Electrochemical testing, based on Tafel extrapolation, revealed that DRECE treatment had a negligible impact on corrosion potential and current, while galvanizing drastically improved corrosion resistance. Hence, the combined DRECE and galvanizing route provides mechanically strengthened, corrosion-protected steels suitable for structural and automotive applications.

Another line of investigation explores how thermal treatments and phase transformation control can be leveraged to optimize the performance of steels. By linking heat treatment parameters to wear resistance, hardness, and microstructural evolution, the works collectively demonstrate the value of precise phase control in improving tribological performance, mechanical balance, and thermal stability.

Li et al. (Contribution 4) investigated the hot deformation behavior and optimized processing parameters of a Cu-alloyed 4Cr₁₆MoCu martensitic stainless steel through Gleeble-3500 compression tests conducted between 900 °C and 1150 °C and at strain rates of 0.001–1 s⁻¹. The addition of 1 wt. % Cu enhanced precipitation hardening and corrosion resistance but significantly affected hot workability. Flow-stress data were modeled using a strain-compensated Arrhenius constitutive equation, yielding an activation energy of ≈470 kJ mol⁻¹ and excellent agreement with experiments. Based on microstructural analysis and the hot processing map, the authors determined the optimal forging window at 1125–980 °C and 0.1 s⁻¹, which was able to promote dynamic recrystallization and fine grains, while strain rates > 0.18 s⁻¹ caused flow instability. The results provide quantitative guidance for forging Cu-bearing martensitic stainless steels.

Wang et al. (Contribution 5) studied the influence of B2 phase precipitation on the hot ductility of Fe₂₂Mn₉Al₀.₆C low-density steel, a candidate material for lightweight automotive components. High-temperature tensile tests conducted between 800 °C and 950 °C revealed a marked drop in ductility and strength within 850–900 °C, associated with the dissolution of κ-carbide and the emergence of the ordered B2 phase, which occurred as temperature increased. The B2 phase is believed to cause crack generation during the hot-forging process. XRD, SEM, and transmission electron microscope (TEM) analyses confirmed the sequential phase transformations, revealing that the precipitate promotes intergranular cracking and embrittlement during hot deformation. The findings demonstrate that avoiding the B2 precipitation temperature range is critical to maintain hot workability and prevent cracking in FeMnAlC low-density steels.

In the study of Qiao et al. (Contribution 6), the effects of solution treatment temperature on the microstructure, mechanical properties, and wear resistance of Fe–25Mn–0.37C–3.69Cr high-manganese steel were examined. Samples treated between 900 °C and 1050 °C exhibited grain coarsening after initial refinement, with optimal recrystallization and uniform austenitic grains at 950 °C. The steel solution-treated at 950 °C achieved the best balance of tensile strength (574 MPa), yield strength (268 MPa), elongation (88%), and wear resistance, associated with enhanced work-hardening during friction. The friction coefficient reached a minimum of 0.273 at this temperature due to oxidation-assisted wear. The dominant wear mechanism transitioned from abrasive and fatigue wear to adhesive wear with increasing solution temperature. These findings provide insights for optimizing heat treatment of alloyed high-manganese steels.

Finally, Timotijević et al. (Contribution 7) provided an insight into the short-term microstructural evolution of HP40Nb austenitic heat-resistant steel, a centrifugally cast alloy used in reformer furnaces, exposed to 950 °C, 1050 °C, and 1150 °C for 2 h and 8 h, simulating overheating conditions in petrochemical reformer tubes. Optical, SEM/EDS, and XRD analyses revealed an as-cast austenitic dendritic matrix containing primary eutectic-like M₂₃C₆ and MC carbides. Increasing temperature and holding time promoted carbide coarsening, spheroidization of primary carbides, and progressive dissolution of secondary Cr₂₃C₆ and NbC precipitates into the matrix. The highest hardness occurred after 8 h at 1150 °C, attributed to partial carbide dissolution and carbon diffusion within austenite. However, exposure at 1150 °C for 8 h resulted in a reduction in secondary carbides due to dissolution, leading to the formation of carbide-free zones. The findings clarify carbide transformation mechanisms under transient overheating and underscore the importance of microstructural monitoring for extending the service life of reformer tubes in extreme environments.

The final group of articles delves into the broader processing-structure-property relationships that govern the mechanical and functional performance of steels in industrial contexts. Topics include microalloying strategies (e.g., Cu, Bi, Te), solidification behavior, inclusion modification, and hot working behavior. Studies in this section explore inverse segregation, dynamic recrystallisation, B2 intermetallic suppression, and machinability enhancement through inclusion engineering. Special emphasis is placed on translating laboratory insights to real-world applications, highlighting how alloy design, casting control, and thermomechanical routes interact to define the final performance of advanced steel components.

The study by Liu et al. (Contribution 8) proposed a novel indentation-free resistance spot welding method for joining SUS301L stainless steel sheets in paint-free rail vehicle applications, utilizing a 1.5 mm steel ball as an intermediate filler. The latter was intended to redistribute current density and confine heat generation within the nugget, thereby minimizing surface indentation. Systematic variation of welding current (7.0–8.5 kA), time (120–210 ms), and pressure (0.35 MPa) revealed optimal joint strength (6414 N) at 8.0 kA and 150 ms, without surface ablation or spatter. The nugget exhibited columnar austenite and a narrow heat-affected zone, similar to conventional RSW. Under optimized conditions, indentation depth was <1% of plate thickness, confirming that the proposed method enables mechanically robust, visually defect-free welds suitable for lightweight stainless-steel body structures.

Wang et al. (Contribution 9) examined the effects of environmentally friendly alloying elements bismuth (Bi) and tellurium (Te) on the inclusion characteristics of sulfur-containing free-cutting steels. Three compositions, S-only, S and Bi, and S, Bi and T, were prepared and analyzed through metallography, SEM/EDS, and 3D electrolytic etching. The results revealed that Bi and Te significantly modify MnS inclusions, transforming elongated class II morphologies into fine flake- or spindle-shaped class III inclusions with lower aspect ratios (<4), thereby improving machinability and reducing anisotropy. The Te addition promoted MnS-MnTe composite inclusions of diverse morphologies (heart-, droplet-, or butterfly-shaped). Both elements decreased the overall inclusion count and homogenized their distribution. The combined Bi-Te modification offers a sustainable route to lead-free free-cutting steels with enhanced cutting performance and isotropy.

Finally, Gao et al. (Contribution 10) investigated the microstructural evolution, texture development, and strengthening mechanisms of Fe–3.3 wt. % Si non-oriented electrical steel subjected to double cold rolling and final annealing. Microstructure and texture were characterized using optical microscopy (OM,) XRD, and quasi-in situ electron backscattered diffraction (EBSD) techniques to reveal recrystallization behavior. The double cold-rolling route effectively suppressed the detrimental γ-fiber and promoted η-fiber texture formation, optimizing the texture factor and magnetic response. At reduction ratios of 50–65%, the steel exhibited enhanced magnetic induction (B50 = 1.698 T), reduced iron loss (P10/400 = 21.84 W/kg), and increased yield strength (Rp0.2 = 578 MPa) compared with single-stage rolling. The combined dislocation-strengthening and texture-control strategy offers a pathway for high-strength, low-loss silicon steels suitable for next-generation electrical applications.

3. Conclusions and Outlook

The research compiled in this Special Issue effectively demonstrates the dynamic and multifaceted nature of current advancements in ferrous metallurgy. The collected contributions underscore the continued industrial and scientific relevance of advanced steels and cast irons, particularly in terms of their mechanical, tribological, and corrosion-related performance.

The findings demonstrate that significant property enhancements can be achieved through careful control of processing conditions and in-depth analyses of microstructural evolution. Steels and cast irons maintain a high degree of industrial and scientific relevance, with potential for enhanced performance. Altogether, the contributions gathered in this Special Issue offer valuable insight and theoretical support for the design of robust, cost-effective, and high-performance components, serving as a reference for both academic research and industrial innovation.