Search Results (14)

This study investigates the mechanical, tribological, and electrochemical behaviour of a novel precipitation-hardenable martensitic alloy produced by selective laser melting (SLM). The alloy was specifically engineered with an optimised composition, free from cobalt and molybdenum, and featuring reduced nickel content (7 wt.%) and 8 wt.% chromium. It has been developed as a cost-effective and sustainable alternative to conventional maraging steels, while maintaining high mechanical strength and a refined microstructure tailored to the steep thermal gradients inherent to the SLM process. Several ageing heat treatments were assessed to evaluate their influence on microstructure, hardness, tensile strength, retained austenite content, dislocation density, as well as wear behaviour (pin-on-disc test) and corrosion resistance (polarisation curves in 3.5%NaCl). The results indicate that ageing at 540 °C for 2 h offers an optimal combination of hardness (550–560 HV), tensile strength (~1700 MPa), microstructural stability, and wear resistance, with a 90% improvement compared to the as-built condition. In contrast, ageing at 600 °C for 1 h enhances ductility and corrosion resistance (Rp = 462.2 kΩ; Ecorr = –111.8 mV), at the expense of a higher fraction of reverted austenite (~34%) and reduced hardness (450 HV). This study demonstrates that the mechanical, surface, and electrochemical performance of this novel SLM-produced alloy can be effectively tailored through controlled thermal treatments, offering promising opportunities for demanding applications requiring a customised balance of strength, durability, and corrosion behaviour. Full article

Powder metallurgy is widely used to fabricate high-nitrogen, nickel-free austenitic stainless steel. However, after sintering and nitriding, additional solution treatment is typically required to achieve uniform nitrogen distribution and a homogeneous austenite phase. This work proposes a novel method to eliminate the need for lengthy and high-temperature solution treatment by switching the nitrogen atmosphere to argon during the cooling process. The effects of different N₂-Ar atmosphere-switching temperatures (750–1320 °C) on the phase composition, element distribution, microstructure, mechanical properties, and corrosion resistance of the studied steels were systematically investigated. Results show that cooling in the N₂ atmosphere initially transforms the matrix to a fully austenitic structure enriched with nitrogen. Excessive nitrogen infiltration leads to Cr₂N precipitation, inducing partial austenite decomposition and forming a multiphase structure comprising austenite, α-Fe, and Cr₂N. Strategic switching from N₂ to Ar reverses this reaction, yielding a high-nitrogen, chemically uniform austenitic structure. Specifically, switching at 1150 °C, the steel exhibits excellent mechanical properties and corrosion resistance, with a yield strength of 749 MPa, an ultimate tensile strength of 1030 MPa, an elongation of 38.7%, and a corrosion current of 0.06 mA/cm², outperforming the steels cooled solely in N₂ and subsequently solution-treated. This novel method offers advantages in cost reduction, energy saving, and operational effectiveness, highlighting its potential for broad industrial application. Full article

This research examines the behavior of electrochemical passivation and the chemistry of surface films on 654SMO super austenitic stainless steel and C276 nickel-based alloy in simulated condensates from flue gas desulfurization in power plant chimneys. The findings indicate that the resistance to polarization of the protective film on both materials initially rises and then falls with either time spent in the solution or the potential of anodic polarization. Comparatively, 654SMO exhibits greater polarization resistance than C276, indicating its potential suitability as a chimney lining material. Mott–Schottky analysis demonstrates that the density of donors in the passive film formed on 654SMO exceeds that on C276, potentially due to the abundance of Fe oxide in the passive film, which exhibits the characteristics of an n-type semiconductor. The primary components of the passive films on both materials are Fe oxides and Cr oxides. The formation of a thin passive film on C276 in the simulated condensates is a result of the low Gibbs free energy of nickel oxide and low Cr content. The slower diffusion coefficient of point defects leads to the development of a thicker and more compact passive film on the surface of 654SMO. Full article

In the present study, the bioactivity, cytotoxicity, and tribological properties of a nickel-free austenitic stainless steel produced via the mechanical alloying of elemental iron, chromium, and manganese nitride powders following by hot isostatic pressing was investigated. Powders after 90 h of mechanical alloying were consolidated via hot isostatic pressing at 1150 °C (1425 K) and heat treated at 1175 °C (1448 K) for 1 h in a vacuum with furnace cooling. Tribological tests were performed to determine the resistance of the as-received nickel-free steel. It was noticed that applying heat treatment after hot isostatic pressing decreases the average friction coefficient and wear rate of the austenitic steel. An immersion test in a simulated body fluid for 28 days at 37 ± 1 °C has been used to determine the biocompatibility of the tested material. The SEM-EDS analysis allowed us to characterise the morphology of the films and the elements of the steel on the thin-film layer. Elements typical of apatite (calcium and phosphorus) were detected on the surface of the sample. Cellular toxicity tests showed no significant toxic side effects for Saos-2 human osteosarcoma cells and the number of Saos-2 human osteosarcoma cells on the nickel-free steel was greater than on the 316LV grade steel. Full article

Today, laws protecting the population at the global level aim to minimize the induction risk of allergies to type IV contact dermatitis. In the European population, the prevalence of nickel allergy is at 10%–15% of adult females and 1%–3% of adult males. A total of 30% of nickel-sensitive people in the general population develop hand eczema. This study concerns the possibility of assembling a bottom of nickel-free austenitic steel (Panacea^®) in a watch case middle made of a grade of austenitic steels, steel 316L (DIN 1.44359), to avoid the risks of Ni release and to develop a galvanic pile between these two dissimilar materials. Two types of methods were used: direct measurements and prediction techniques (mixed potentials theory). For the degradation of thbottom-middle watch assembly, Nielsen–Tuccillo tests were performed, and Ni release measurements according to EN 1811 completed the study. All direct electrochemical investigations and galvanic current prediction measurements show low current values of 40–400 nA. Measurements of nickel release of Panacea^® and 316L reveal small quantities of nickel, much lower than the 0.5 µg/cm² per week that the European legislation enforces. The nickel-free steel Panacea^® in the work hardening conditions 280, 427, and 510 HV0.1s were also studied. The cation extractions reveal the large quantities released from Cr, Mo, Mn, and Fe, so there is a risk of toxicity in contact with the skin. Full article

The uniaxial tensile behaviors of Fe-19Cr-16Mn-2Mo-0.49N and Fe-18Cr-16Mn-2Mo-0.85N high-nitrogen and nickel-free austenitic stainless steels at two strain rates of 10⁻² s⁻¹ and 10⁻⁴ s⁻¹ were comparatively investigated. The related deformation microstructure was characterized and fracture mechanism was analyzed. The results show that the nitrogen content and strain rate both have significant effects on the tensile properties of the tested steels. As the strain rate is the same, the tested steel containing a higher nitrogen content has higher R_m and R_p0.2. However, R_m is higher at a lower strain rate and R_p0.2 is higher at a higher strain rate in the case of the same nitrogen content. The tested steel with a lower nitrogen content (0.49 wt.%N) tensioned at a lower strain rate of 10⁻⁴ s⁻¹ obtains the highest elongation, while the tested steel with a higher nitrogen content (0.85 wt.%N) tensioned at a higher strain rate of 10⁻² s⁻¹ has the lowest elongation. The tensile plastic deformation is mainly governed by slip and twinning, affected jointly by stacking fault energy and short-range order. Dislocation slip featured by planar slip bands and twin-like bands is the main deformation structure in the tested steel containing a higher nitrogen content (0.85 wt.%N) tensioned at a lower strain rate of 10⁻⁴ s⁻¹, whereas twinning deformation becomes more prominent with decreasing nitrogen content and increasing strain rate. Full article

In this study, microstructural evolution and phase transition of nickel-free Fe-18Cr-18Mn (wt. %) austenitic steel powders, induced by mechanical alloying, were investigated. X-ray diffraction, scanning electron microscopy, and microhardness testing techniques were used to observe the changes in the phase composition and particle size as functions of milling time. The first 30 h of mechanical alloying was performed in an argon atmosphere followed by nitrogen for up to 150 h. X-ray diffraction results revealed that the Fe-fcc phase started to form after 30 h of milling, and its fraction continued to increase with alloying time. However, even after 150 h of milling, weak Fe-bcc phase reflections were still detectable (~3.5 wt. %). Basic microstructure features of the multi-phase alloy were determined by X-ray profile analyses, using the whole powder pattern modeling approach to model anisotropic broadening of line profiles. It was demonstrated that the WPPM algorithm can be regarded as a powerful tool for characterizing microstructures even in more complicated multi-phase cases with overlapping reflections. Prolonging alloying time up to 150 h caused the evolution of the microstructure towards the nanocrystalline state with a mean domain size of 6 nm, accompanied by high densities of dislocations exceeding 10¹⁶/m². Deformation-induced hardening was manifested macroscopically by a corresponding increase in microhardness to 1068 HV_0.2. Additionally, diffraction data were processed by the modified Williamson–Hall method, which revealed similar trends of domain size evolutions, but yielded sizes twice as high compared to the WPPM method. Full article

Production of high-quality maraging steel is dependent not only on the production technology but also on the alloying design and heat treatment. In this work, cobalt-free, low nickel, molybdenum-containing maraging steel was produced by melting the raw materials in a vacuum induction melting furnace and then refining with a shielding gas electroslag remelting unit. The critical transformation temperatures of the investigated steel samples were determined experimentally by differential scanning calorimetry (DSC) analysis and theoretically aiding Thermo-Calc software. Types and chemical composition plus volume fraction and starting precipitation temperature of suggested constituents calculated with the aid of Thermo-Calc software. The microstructures of forged steel specimens that were heat-treated under several conditions were evaluated by X-ray diffraction (XRD), optical microscopy (OP), scanning electron microscopy (SEM), and electron backscattering (EBSD), in addition to transmission electron microscopy (TEM). The mechanical properties of the investigated steel specimens were evaluated by measuring the tensile strength properties and micro-hardness, furthermore, estimating their fracture surface using scanning electron microscopy at lower magnification. The metallographic results show that the microstructure of steel in aged conditions includes high-alloyed martensite and nickel-rich phase, in addition to the low-alloyed-retained-austenite, intermetallic compounds, and lavas-phase (MoCr). Furthermore, TEM and EBSD studies emphasized that the produced steel has high dislocation density with nano-sized precipitate with an average size of ~19 ± 1 nm. Moreover, the metallographic results show that the mentioned microstructure enhances the tensile properties by precipitation strengthening and the TRIP phenomenon. The tensile strength results show that the n-value of investigated steel passes two stages and is comparable with the n-value of TRIP-steel. Steel characterized by 2100 MPa ultimate tensile strength and uniform elongation of more than 7% can be produced by the investigated production routine and optimum heat treatment conditions. Full article

Multiaxial stress states frequently occur in technical components and, due to the multitude of possible load situations and variations in behaviour of different materials, are to date not fully predictable. This is particularly the case when loads lie in the plastic range, when strain accumulation, hardening and softening play a decisive role for the material reaction. This study therefore aims at adding to the understanding of material behaviour under complex load conditions. Fatigue tests conducted under cyclic torsional angles (5°, 7.5°, 10° and 15°), with superimposed axial static compression loads (250 MPa and 350 MPa), were carried out using smooth specimens at room temperature. A high nitrogen alloyed austenitic stainless steel (nickel free), was employed to determine not only the number of cycles to failure but particularly to aid in the understanding of the mechanical material reaction to the multiaxial stresses as well as modes of crack formation and growth. Experimental test results indicate that strain hardening occurs under the compressive strain, while at the same time cyclic softening is observable in the torsional shear stresses. Furthermore, the cracks’ nature is unusual with multiple branching and presence of cracks perpendicular in direction to the surface cracks, indicative of the varying multiaxial stress states across the samples’ cross section as cross slip is activated in different directions. In addition, it is believed that the static compressive stress facilitated the Stage I (mode II) crack to change direction from the axial direction to a plane perpendicular to the specimen’s axis. Full article

Low-temperature nitriding allows to improve surface hardening of austenitic stainless steels, maintaining or even increasing their corrosion resistance. The treatment conditions to be used in order to avoid the precipitation of large amounts of nitrides are strictly related to alloy composition. When nickel is substituted by manganese as an austenite forming element, the production of nitride-free modified surface layers becomes a challenge, since manganese is a nitride forming element while nickel is not. In this study, the effects of nitriding conditions on the characteristics of the modified surface layers obtained on an austenitic stainless steel having a high manganese content and a negligible nickel one, a so-called nickel-free austenitic stainless steel, were investigated. Microstructure, phase composition, surface microhardness, and corrosion behavior in 5% NaCl were evaluated. The obtained results suggest that the precipitation of a large volume fraction of nitrides can be avoided using treatment temperatures lower than those usually employed for nickel-containing austenitic stainless steels. Nitriding at 360 and 380 °C for duration up to 5 h allows to produce modified surface layers, consisting mainly of the so-called expanded austenite or γ_N, which increase surface hardness in comparison with the untreated steel. Using selected conditions, corrosion resistance can also be significantly improved. Full article

This investigation attempts to explore the weld characteristics of a laser welded dissimilar joint of ferritic/martensitic 9Cr-1Mo-V-Nb (P91) steel and Incoloy 800HT austenitic nickel alloy. This dissimilar joint is essential in power generating nuclear and thermal plants operating at 600–650 °C. In such critical operating conditions, it is essential for a dissimilar joint to preserve its characteristics and be free from any kind of defect. The difference between the physical properties of P91 and Incoloy 800HT makes their weldability challenging. Thus, the need for detailed characterization of this dissimilar weld arises. The present work intends to explore the usage of an unconventional welding process (i.e., laser beam welding) and its effect on the joint’s characteristics. The single-pass laser welding technique was employed to obtain maximum penetration through the keyhole mode. The welded joint morphology and mechanical properties were studied in as-welded (AW) and post-weld heat treatment (PWHT) conditions. The macro-optical examination shows the complete penetrations with no inclusion and porosities in the weld. The microstructural study was done in order to observe the precipitation and segregation of elements in dendritic and interface regions. Solidification cracks were observed in the weld fusion zone, confirming the susceptibility of Incoloy 800HT to such cracks due to a mismatch between the melting point and thermal conductivity of the base metals. Failure from base metal was observed in tensile test results of standard AW specimen with a yield stress of 265 MPa, and after PWHT, the value increased to 297 MPa. The peak hardness of 391 HV was observed in the P91 coarse grain heat-affected zone (CGHAZ), and PWHT confirmed the reduction in hardness. The impact toughness results that were obtained were inadequate, as the maximum value of impact toughness was obtained for AW P91 heat-affected zone (HAZ) 108 J and the minimum for PWHT Incoloy 800HT HAZ 45 J. Thus, difficulty in obtaining a dissimilar joint with Incoloy 800HT using the laser beam welding technique was observed due to its susceptibility to solidification cracking. Full article

This work aims to show the impact of the allowed chemical composition range of AISI 316L stainless steel on its processability in additive manufacturing and on the resulting part properties. ASTM A276 allows the chromium and nickel contents in 316L stainless steel to be set between 16 and 18 mass%, respectively, 10 and 14 mass%. Nevertheless, the allowed compositional range impacts the microstructure formation in additive manufacturing and thus the properties of the manufactured components. Therefore, this influence is analyzed using three different starting powders. Two starting powders are laboratory alloys, one containing the maximum allowed chromium content and the other one containing the maximum nickel content. The third material is a commercial powder with the chemical composition set in the middle ground of the allowed compositional range. The materials were processed by laser-based powder bed fusion (PBF-LB/M). The powder characteristics, the microstructure and defect formation, the corrosion resistance, and the mechanical properties were investigated as a function of the chemical composition of the powders used. As a main result, solid-state cracking could be observed in samples additively manufactured from the starting powder containing the maximum nickel content. This is related to a fully austenitic solidification, which occurs because of the low chromium to nickel equivalent ratio. These cracks reduce the corrosion resistance as well as the elongation at fracture of the additively manufactured material that possesses a low chromium to nickel equivalent ratio of 1.0. A limitation of the nickel equivalent of the 316L type steel is suggested for PBF-LB/M production. Based on the knowledge obtained, a more detailed specification of the chemical composition of the type 316L stainless steel is recommended so that this steel can be PBF-LB/M processed to defect-free components with the desired mechanical and chemical properties. Full article

Duplex stainless steels (DSSs) exhibit excellent corrosion resistance and are being used in a variety of industrial applications. Reducing/eliminating the amount of nickel in such alloys will contribute significantly to its economic viability. Moreover, a well-established wear behavior for these alloys is also an essential development in most of their applications. Hence, in this work, the Taguchi technique was effectively implemented to investigate the effect of operating factors such as sliding speed and applied load on the wear behavior of different compositions of nickel-free DSSs. It was observed that the composition had a higher contribution of 33.66% to the wear rate (WR) and the contribution of the sliding speed to the coefficient of friction (COF) was found to be 68.17%. With a good agreement, a regression model was also developed to predict the WR and COF within a certain range of factors. Wear tests have also shown that the developed nickel-free DSS is a promising candidate in terms of wear resistance as compared to austenitic stainless steels (ASS). Full article

An influence of the powder metallurgy route on the phase structure, mechanical properties, and corrosion resistance of Fe–18%Cr–12%Mn–N nickel-free austenitic stainless steel as a potential material for medical applications were studied. The powder was mechanically alloyed in a high purity nitrogen atmosphere for 90 h followed by Hot Isostatic Pressing at 1150 °C (1423 K) and heat treatment at 1175 °C (1423 K) for 1 h in a vacuum with furnace cooling and water quenching. More than 96% of theoretical density was obtained for the samples after Hot Isostatic Pressing that had a direct influence on the tensile strength of the tested samples (Ultimate Tensile Strength is 935 MPa) with the total elongation of 0.5%. Heat treatment did not affect the tensile strength of the tested material, however, an elongation was improved by up to 3.5%. Corrosion properties of the tested austenitic stainless steel in various stages of the manufacturing process were evaluated applying the anodic polarization measurements and compared with the austenitic 316LV stainless steel. In general, the heat treatment applied after Hot Isostatic Pressing improved the corrosion resistance. The Hot Isostatic Pressing sample shows dissolution, while heat treatment causes a passivity range, the noblest corrosion potential, and lower current density of this sample. Full article