Solid State Phenomena Vol. 383

Abstract: Austenitic stainless steels are strong candidates for cryogenic applications such as liquid hydrogen storage (20 K) and nuclear fusion technology (4 K) but suffer from low yield strength. In this study, austenitic stainless steels with varying nitrogen contents were evaluated. TMCP-processed and solution-annealed plates were manufactured using a pilot-scale rolling mill, and their microstructures were characterised. Tensile tests were performed from room temperature down to-180°C to assess the cryogenic yield strength of the plates. At all temperatures TMCP significantly increased the yield strength e.g. by a factor of 2 at room temperature, with the effect being mainly due to substantial dislocation hardening. The thermally activated component of yield strength depended mainly on nitrogen content via dislocation-nitrogen interactions, which was found to be much weaker in TMCP plates. Solution annealed plates therefore presented remarkable yield strength at cryogenic temperatures for the highest nitrogen level investigated.

Abstract: This study investigates the microstructural evolution and mechanical response of 51CrV4 spring steel subjected to flash quenching and tempering using a continuous high-speed induction heating line. The steel, supplied as 8 mm thick sheets with a composition of Fe–0.5C–0.9Mn–1Cr–0.16V (wt.%), was processed through rapid austenitisation at 900 °C (~200 °C/s), followed by water quenching and tempering at 300 °C. Rapid induction heat treatment was utilized to produce a hardened surface layer with refined microstructure and balanced mechanical properties. Optical microscopy revealed a uniform, crack-free martensitic layer extending to approximately ~1.2 mm from the surface, while hardness profiling showed a gradient from 590 ± 20 HV at the surface to 240–330 HV in the core. Electron Backscatter Diffraction (EBSD) analysis confirmed a fully martensitic surface structure with refined prior austenite grains (~3.2 µm), and FESEM imaging indicated minimal carbide coarsening, supporting the effectiveness of short-time tempering. These results demonstrate that flash induction processing can produce a hardened shell with retained core ductility. The consistency between EBSD, FESEM, and hardness data validates the process as an energy-efficient, scalable alternative to conventional furnace-based treatments.

Abstract: The effects of common electric arc furnace (EAF) impurities, including copper (Cu), nickel (Ni), molybdenum (Mo), and chromium (Cr), were investigated in low-carbon steels. These steel scrap originating tramp elements can influence the microstructures and mechanical properties of steel products. Tramp elements containing test materials were thermo-mechanically rolled to achieve yield strengths between 400–450 MPa with different cooling routes. Various methods of microstructure characterization and mechanical testing were utilized to study the resulting steels. Additionally, thermo-mechanical simulations were conducted using Gleeble 3800 equipment to gather information about flow stress properties. The results indicate that with a lower cooling rate, the microstructure is not significantly affected by tramp elements, however strength levels can be increased and elongation properties decreased, mostly due to the solid solution strengthening effect of impurities. In water-quenched steels, the addition of tramp elements can alter the final microstructure morphology, increasing the ultimate tensile strength but simultaneously improving the ductility. Flow stress is not significantly affected by tramp elements in the temperature range of 950–1050 °C.

Abstract: Characterization, classification, and size distribution of non-metallic inclusions (NMIs) in a cast high-Mn TWIP steel (Fe-20Mn-0.6C-1.5Al-0.3V, in wt.%), were studied to explore their interplay with the fracture mode during tensile deformation. NMIs were separated by electrochemical extraction, and subsequent X-ray dispersive analysis was performed to characterize their compositions. Subsequently, 70 % cold rolled TWIP sheets were processed and undergone fast heating annealing (FHA) at a heating rate of 200°C/s to anneal at temperatures 750 - 850 °C for 30 s. The grain structures achieved by FHA were evaluated by EBSD. The mechanical properties were determined by tensile testing. Distinct categories of NMIs, including Al2O3 and Mn (S,Se), and (Ti,V)N nitrides, and intricate combinations of inclusions, were identified. FHA process at low temperatures, 750-800 °C, promoted partially recrystallized microstructures. Fully recrystallized structures were obtained at 850 °C characterized by an average grain size of 2 µm at 850 °C. The structure promoted at FA 850 °C displayed noteworthy elongation of 60% with a yield strength (YS) and tensile strength (TS) of 410 and 830 MPa, respectively. A minimal effect of NMIs in TWIP steel was observed due to activating mechanical twinning mechanism, which overcomes the detrimental impact of NMIs and retard the necking induced by void formation related to NMIs.

Abstract: High-throughput computational screening (HTCS) based on CALPHAD (Calculation of Phase Diagram) was employed to investigate potential chemical compositions within the medium manganese steel family for achieving desired austenite stability and stacking fault energy (SFE). The primary objective was to identify optimal alloy compositions that balance the complex effects of various alloying elements on retained austenite fraction and related mechanical properties. Utilising TC-Python Thermo-Calc software coupled with a custom-developed algorithm, two optimised compositions were determined: 0.35C, 9Mn, 1Mo, 3Al, 1Si, 0.05Nb, 0.3V (alloy 353), and 0.35C, 9Mn, 1Mo, 3Al, 1Si, 0.1Nb (alloy 310) in wt.% to be the best fited composition to our selected criteria. The alloys were subsequently produced via open-air induction furnaces, and the microstructure was analysed after the hot forging condition. The initial multiphase as-cast structure, primarily composed of lath martensite, δ-ferrite (34 vol.%), and retained austenite (RA, 5–7 vol.%), experienced notable grain refinement. Forging reduced δ-ferrite grain sizes from 39 µm to 12 µm (alloy 310) and from 46 µm to 9 µm (alloy 353), accompanied by increased RA content (28 vol.% for alloy 310 and 46 vol.% for alloy 353) and reduced RA grain sizes (1.2 µm and 1.9 µm, respectively). Non-metallic inclusions (NMIs) were analysed using field emission scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, classifying inclusions primarily as AlN, MnS, (Mo,Nb)C, or their combinations. No significant differences in inclusion types were observed, but forged samples displayed reduced inclusion sizes. In summary, the results showed that HTSC effectively identified optimal compositions with a high fraction of retained austenite.

Abstract: The transition to hydrogen-based energy systems presents a critical need for materials capable of withstanding the harsh conditions of hydrogen storage. Our project addresses this challenge by developing a multilayer steel designed specifically for hydrogen environments. This material combines austenitic steel, known for its resistance to hydrogen embrittlement, with carbon steel, which provides strength and cost efficiency. Hydrogen embrittlement poses a well-known issue in the storage and transport of hydrogen, often degrading various metals. Although many stainless steels provide superior resistance, its high-cost limits widespread application. Our solution involves a multilayer approach, where austenitic layer serves as the primary barrier against hydrogen-induced degradation, and the carbon steel layer ensures the material’s structural strength under high pressure. The manufacturing process involves hot roll bonding, where the surfaces of the two materials are cleaned of oxides, welded together, heated up to 1200 °C, and then hot rolled to form a strong bond. This method not only strengthens the material but also makes it a potential solution for large-scale hydrogen storage applications.

Abstract: The global production of niobium-microalloyed steels is now a well-established industrial practice. Initially driven by experimental insights into niobium's ability to refine steel microstructures during thermomechanical processing, this technology has become especially prevalent in low-alloy steels. An important aspect of niobium's production is its natural association with tantalum, which often leads to the co-extraction of both elements. This paper investigates the impact of tantalum traces, present as a contaminant in FeNb, on the microstructure and mechanical properties of niobium-microalloyed steels. The study reveals that tantalum's presence leads to further refinement of austenitic grains without negatively affecting the alloys' yield strength. Additionally, this tantalum contamination enhances the steel samples' toughness. By exploring these subtle effects, this study provides new insights into tantalum's influence on microalloyed steels, particularly regarding microstructural refinement and mechanical performance in two specific Nb-microalloyed steel compositions.

Abstract: The impact of Si on Zn-induced liquid metal embrittlement (LME) in 3^rd generation advanced high strength steels (AHSS) during resistance spot welding has been widely studied, but the effect of Al is rather unknown. This study investigates the substitution of Al for Si by analyzing two steels with the fixed C and Mn-contents of 0.2 and 3 wt.-% respectively. Si and Al-contents are both set to 1.4 wt.-%. To minimize microstructural effects, all steels were quenched and tempered before electro-galvanizing. The effects of Si and Al were examined using hot tensile testing (600 – 900 °C, in 50 K steps) on a Gleeble 3800, resistance spot welding with prolonged welding times, thermodynamic calculations with Thermo-calc® and dilatometry. Results indicate that the use of either Si or Al increases the LME-susceptibility but substituting Al for Si significantly reduces Zn-induced LME-cracking. In hot tensile testing, higher testing temperatures generally increase the steel’s vulnerability to LME. But comparing both alloying elements to one another, Si causes a higher LME-susceptibility.

Abstract: The automotive industry’s push for lightweight, high-strength materials has led to the advancement of third-generation advanced high-strength steels (AHSS). Known for their blend of ultra-high tensile strength and ductility, these steels are ideal for structural applications. However, their adoption has faced obstacles, notably due to Zinc (Zn)-assisted liquid metal embrittlement (LME), particularly in cases where Zn-based corrosion-resistant coatings are used during resistance spot welding (RSW). This study explores testing methodologies for evaluating LME susceptibility and examines the impact of silicon (Si) concentration, specifically between 0.5 and 1.4 wt.-%, on Zn-LME susceptibility in AHSS. This research introduces two primary testing approaches: spot welding and hot tensile tests, each designed to quantify LME behavior. Both methods show that higher Si levels correlate with increased LME sensibility, evidenced by greater ductility loss and the formation of longer critical cracks during welding. Thermodynamic modeling further demonstrates that Si affects phase stability in the Fe-Zn system, broadening the stability of liquid Zn. The findings highlight Si’s significant role in Zn-LME susceptibility and underscore the importance of robust testing methods to facilitate the safer application of AHSS in automotive manufacturing.