Search Results (52)

A novel stainless steel with high Mn and Mo content (much higher than traditional stainless steel), designated CH241SS, was developed as a potential replacement for Cr-Mo-V alloy steel in the cold forging applications of precision industry. Through carbon reduction in an environmentally friendly manner and a two-stage heat treatment process, the hardness of as-cast CH241 was tailored from HRC 37 to HRC 29, thereby meeting the industrial specifications of cold-forged steel (≤HRC 30). X-ray diffraction analysis of the as-cast microstructure revealed the presence of a small amount of ferrite, martensite, austenite, and alloy carbides. After heat treatment, CH241 exhibited a dual-phase microstructure consisting of ferrite and martensite with dispersed Cr(Ni-Mo) alloy carbides. The CH241 alloy demonstrated excellent high-temperature stability. No noticeable softening occurred after 72 h for the second-stage heat treatment. Based on the mechanical and room-temperature tensile fatigue properties of CH241-F (forging material) and CH241-ST (soft-tough heat treatment), it was demonstrated that the CH241 stainless steel was superior to the traditional stainless steel 4xx in terms of strength and fatigue life. Therefore, CH241 stainless steel can be introduced into cold forging and can be used in precision fatigue application. The relevant data include composition design and heat treatment properties. This study is an important milestone in assisting the upgrading of the vehicle and aerospace industries. Full article

Laser powder bed fusion (LPBF) is an advanced additive manufacturing technology that is gaining increasing interest for biomedical implants because it can produce dense, patient-specific metallic components with controlled microstructures. This study investigated the LPBF fabrication of 316L stainless steel, which is widely used in orthopedic and dental implants, and examined the effects of laser power and scanning speed on the microstructure and mechanical properties relevant to biomedical applications. The study achieved 99.97% density and refined columnar and cellular austenitic grains, with optimized molten pool morphology. The optimal LPBF parameters, 190 W laser power and 700 mm/s, produced a tensile strength of 762.83 MPa and hardness of 253.07 HV_0.2, which exceeded the values of conventional cast 316L stainless steel. These results demonstrated the potential of optimized LPBF 316L stainless steel for functional biomedical applications that require high mechanical integrity and biocompatibility. Full article

This study focuses on S32654 super austenitic stainless steel (SASS) and systematically characterizes the morphology of the sigma (σ) phase and the segregation behavior of alloying elements in its as-cast microstructure. High-temperature confocal scanning laser microscopy (HT-CSLM) was employed to investigate the effect of the rare earth element yttrium (Y) on the solidification microstructure and σ phase precipitation behavior of SASS. The results show that the microstructure of SASS consists of austenite dendrites and interdendritic eutectoid structures. The eutectoid structures mainly comprise the σ phase and the γ₂ phase, exhibiting lamellar or honeycomb-like morphologies. Regarding elemental distribution, molybdenum displays a “concave” distribution pattern within the dendrites, with lower concentrations at the center and higher concentrations at the sides; when Mo locally exceeds beyond a certain threshold, it easily induces the formation of eutectoid structures. Mo is the most significant segregating element, with a segregation ratio as high as 1.69. The formation mechanism of the σ phase is attributed to the solid-state phase transformation of austenite (γ → γ₂ + σ). In the late stages of solidification, the concentration of chromium and Mo in the residual liquid phase increases, and due to insufficient diffusion, there are significant compositional differences between the interdendritic regions and the matrix. The enriched Cr and Mo cause the interdendritic austenite to become supersaturated, leading to solid-state phase transformation during subsequent cooling, thereby promoting σ phase precipitation. The overall phase transformation process can be summarized as L → L + γ → γ → γ + γ₂ + σ. Y microalloying has a significant influence on the solidification process. The addition of Y increases the nucleation temperature of austenite, raises nucleation density, and refines the solidification microstructure. However, Y addition also leads to an increased amount of eutectoid structures. This is primarily because Y broadens the solidification temperature range of the alloy and prolongs grain growth perio, which aggravates the microsegregation of elements such as Cr and Mo. Moreover, Y raises the initial precipitation temperature of the σ phase and enhances atomic diffusion during solidification, further promoting σ phase precipitation during the subsequent eutectoid transformation. Full article

The microstructure and surface behavior of iron-based 304 stainless steel after temperature exposure was studied by Mössbauer spectroscopy, powder X-ray diffraction, scanning electron microscopy, energy dispersive analysis and positron annihilation. The tested specimens were in the form of cylinders produced by the casting process. The samples were annealed in air in the 600–1000 °C temperature range for 36 h. Under the influence of temperature, cast 304 stainless steel underwent austenitic–ferritic transformation and tended to form an oxide layer on the surface. The oxides were mainly found in the thin surface layer (0.3 μm) and consisted of Fe oxides and oxides of alloying elements (Cr and Mn) in the form of corundum, while, in the bulk region (10 μm), the phase transformation of austenite to ferrite occurred. Surface phase inhomogeneity was studied by Mössbauer spectroscopy. The method of positron annihilation was used to study defects and the effect of annealing on the formation and removal of a defect structure. Full article

The hot deformation behaviors and dynamic softening mechanisms of XM-19 super austenitic stainless steel (SASS) were investigated using the isothermal compression test in the temperature range from 1025 to 1250 °C and a compression rate of 0.01–10 s⁻¹. A hot processing map with a strain of 0.9 was constructed, and the analysis results show that the optimal thermal deformation parameters are a temperature range of 1200–1250 °C and a strain rate range of 0.03–0.2 s⁻¹. The thermal activation energy at 0.7 strain is calculated to be 614.3 kJ/mol by developing constitutive equations under various deformation parameters, which is essentially higher than the range of thermal deformation activation energy of typical austenitic stainless steels. At a high temperature of 1250 °C, the synergistic effect of adiabatic heating and increased dislocation density drives the recrystallization fraction to surge from 20% to 78% as the strain rate rises from 0.01 to 10 s⁻¹, while at a fixed strain rate of 0.1 s⁻¹, the increase in deformation temperature from 1025 °C to 1250 °C promotes dynamic recrystallization (DRX), leading to a parallel rise in recrystallization fraction to 25%. The nucleation mechanism of XM-19 SASS is primarily driven by discontinuous dynamic recrystallization (DDRX), with a supporting role of continuous dynamic recrystallization (CDRX). The contribution of CDRX decreases gradually with increasing deformation temperature. Full article

The objective of this study is to investigate the effect of the solidification microstructure of CJ5L Recycled Stainless Steel in the cast state on its thermoplasticity. Therefore, the residual ferrite, solidification structure, and high-temperature thermoplasticity in both Recycled and Non-Recycled steel ingots are examined. The principal experimental techniques employed include SEM, OM, EPMA, and EDS. It was observed that the solidification microstructure underwent a gradual transformation from a dendritic structure with a skeletal shape to a worm-like dendrite as the thickness increased. This resulted in the formation of large equiaxed grains at the center of the steel ingots. The cooling rate decreased from 3~16 °C/s at the surface to below 0.8 °C/s at the center. The residual ferrite gradually transformed from a skeletal to granular and rod-like form with increasing depth, eventually forming a ferrite network at the center of the casting. In the Recycled steel, the composition segregation resulted in the formation of a network ferrite aggregation at the center of the steel ingots. The analysis of microstructure changes in conjunction with thermodynamic calculations revealed that the solidification mode of CJ5L stainless steel underwent a transition from the ferritic–austenitic (FA) mode to the austenitic–ferritic (AF) mode with increasing casting thickness. This resulted in an increase in the amount of residual ferrite from the surface to the center. The high-temperature thermoplasticity analysis of CJ5L stainless steel showed that at temperatures between 800 °C and 900 °C, the casting displayed optimal properties, minimizing crack formation during subsequent processing. Full article

To meet the requirement of low magnetic permeability, which, in turn, lowers the ferrite content of castings, of special interest is 316 stainless steel, whose low ferrite content renders it suitable also for nuclear power applications. Therefore, the effects of the composition and cooling rate of 316 stainless steel castings on the ferrite content are investigated. Three 316 stainless steel continuous casting samples with different compositions (primarily differing in the Ni content) are studied, i.e., low-alloy type (L-316), medium-alloy type (M-316), and high-alloy type (H-316). The austenite-forming element nickel of three different industrial samples is 10%, 12%, and 14%, respectively. The effect of the cooling rate on the ferrite content and precipitation phases of the high Ni content of the 316 stainless steel casting (H-316) is studied by remelting experiments and different methods of quenching of liquid steel. In both cases, the ferrite content and the precipitate phases in the microstructure are analyzed using SEM and EBSD. The results indicate that compositional changes within the 316 stainless steel range lead to changes in the solidification mode. In the L-316 casting, solidified by the FA mode (ferrite–austenite mode), ferrite precipitates first from the liquid phase, followed by the formation of austenite, and the ferrite content is 11.2%. In contrast, the ferrite content in the M-316 and H-316 castings, solidified by the AF mode (austenite–ferrite mode), is 2.88% and 2.45%, respectively. The effect of the solidification mode on the ferrite content is more obvious than that of the composition. The microstructure of the L-316 casting is mainly composed of the austenitic phase and the ferritic phase. The microstructure of the M-316 casting is composed of austenite, ferrite, and a small amount of sigma phase, with a small amount of ferrite transformed into the sigma phase. The microstructure of the H-316 casting is basically composed of austenite and the sigma phase, with the ferrite has been completely transformed into sigma phase. Changes in composition have a greater influence on the precipitate phases, while the solidification mode has a lesser impact. In the remelting experiments, the ferrite content in the H-316 ingot obtained through furnace cooling and air cooling is 1.49% and 1.94%, respectively, and the cooling rates are 0.1 °C/s and 3.5 °C/s, respectively. Under oil- and water-cooling conditions, with cooling rates of 11.5 °C/s and 25.1 °C/s, respectively, the ferrite content in the ingot is controlled to below 1%. The effect of the cooling rate on the precipitation phase of the H-316L ingot is that the amount of precipitated phase in the ingot decreases with an increase in cooling rate, but, when the cooling rate exceeds a certain value (air cooling 3.5 °C/s), the change in cooling rate has little effect on the amount of the precipitated phase. Full article

This paper reports experimental results concerning the corrosion of 316L austenitic stainless steels produced by ball milling and spark plasma sintering in NaCl electrolyte. Specimens with grain sizes ranging from 0.3 µm to 3 µm, without crystallographic texture, were obtained and compared with a cast that is 110 µm in grain size and an annealed reference. The potentiodynamic experiments showed that the reduction in grain size leads to a degradation of the electrochemical passivation behavior. This detrimental effect can be overcome by appropriate passivation in a HNO₃ concentrated solution before consolidation. The Mott–Schottky measurements showed that the semiconducting properties of the passive layer do not vary significantly on the grain size, especially the donor density, which is responsible for the chemical passivation breakdown by chloride anions. The total electrical resistance of the layer, measured by impedance spectroscopy is always lower than the one of a cast and annealed 316L, but it slightly increases with a reduction in grain size in the ultrafine grain range. This is followed by a slight increase in the thickness of the oxide layer. The effect of chloride ions is very pronounced in terms of passivation breakdown if the powder is not passivated prior to sintering. This leads to the nucleation and growth of subsurface main pits and the formation of secondary satellite pits, especially for the smallest grain sizes. Passivation of the 316L powder before sintering has been found to be an effective way to prevent this phenomenon. Full article

The influence of laser-alloyed stainless steel coatings on the properties of the surfaces of cast iron discs, such as friction-induced vibration and noise, friction coefficient, residual stress, hardness, and corrosion resistance, was investigated in this study. The experimental results show that after laser alloying, the surface hardness of the cast iron discs increased significantly. The residual stresses on the surfaces of the laser-alloyed discs changed from tensile to compressive residual stresses, while any compressive residual stresses increased by more than six times. Most of the laser-alloyed discs demonstrated better performance in friction-induced vibration and noise damping and friction reduction. Metallographic observation and XRD (X-ray diffraction) analysis results show that the laser-alloyed layer is mainly a mixture of acicular martensite and dendritic material, while the phase composition of laser-treated discs is mainly martensitic, [Fe, Ni], Fe₃Si, Cr₂₃C₆, and austenite, which plays a significant role in the improvement of the properties of the laser-alloyed cast iron in physics, tribology and corrosion resistance. This research has significance for the laser surface treatment of various cast irons and steels, which is an increasingly important manufacturing technology in the vehicle friction brake industry. Full article

This paper investigates the casting defects of a stainless steel pump impeller manufactured through the sand casting process. The material characterization of austenitic steel AISI 316L was initially carried out, which examined the chemical composition of the casting and its microstructure. The next step was to determine the cause of the casting defects using numerical simulations. The numerical simulations were performed using ProCAST software (Version 18.0). Initial results of the filling and solidification simulations were conducted using the parameters employed in the actual casting process, revealing casting defects in corresponding locations. The casting process was subsequently modified to achieve improved results. This involved reconstructing the gating system, redesigning the riser, and incorporating a cylindrical chiller. The results show that the modified casting process significantly reduces the occurrence of defects in the final product. The study provides useful insights into the analysis and modification of the casting process for stainless steel pump impellers produced through sand casting. The results can help improve the quality of such products and reduce production costs associated with casting defects. Full article

Carbon steel is subjected to several pretreatments to enable its use in highly corrosive environments, such as marine structures. However, its surface treatment is problematic owing to various processes, and these problems can be solved by replacing it with super duplex stainless steel (SDSS), which exhibits remarkable strength and corrosion resistance owing to its austenite and ferrite phases. EN 1.4410 and EN 1.4501 are the most extensively used SDSS grades in marine structures, as they exhibit exceptional strength and corrosion resistance in seawater. This study subjected EN 1.4410 and EN 1.4501 samples to specific heat treatment after casting and observed their structural alterations through field emission scanning electron microscopy. Their passivation states, with or without the Cu and W layers, were determined by examining their corrosion properties through open-circuit potential measurements, electrostatic polarisation tests, electrochemical impedance spectroscopy (EIS), and critical pitting temperature (CPT) analysis. The inclusion of Cu significantly improved the uniform corrosion resistance within the passivation layers, whereas the addition of W enhanced the pitting resistance (E_pit, CPT). Additionally, the EIS analysis confirmed a double-layer structure in the passivation layer of EN 1.4501. Moreover, Cu did not act as a strengthening element of the passivation layer, whereas W significantly reinforced it. Full article

This investigation aims to analyze the effect of homogenization heat treatment at 1240 °C for 2 and 6 h on the hardness, distribution, morphology, and chemical composition of the δ-ferrite and sigma phases in 316L stainless steel cast billet. A field emission scanning electron microscope, combined with electron back-scattered diffraction, a field emission electron probe microanalyzer with a wavelength dispersive spectrometer, and a Vickers microhardness tester are applied to identify various phase evolutions in the cast billet. The morphology of the δ-ferrite and sigma phases in the austenite matrix of the 316L cast billet are strongly related to the subsequent hot and cold wire drawings. The homogenization heat treatment is expected to provide a driving force to form spheroid interdendritic δ-ferrite and to minimize the amount of the brittle sigma intermetallic compound in the austenite matrix. The homogenization heat treatment at 1240 °C effectively spheroidized all δ-ferrites into blunt ones in the cast billet. The transformation of δ-ferrite into sigma is dominated by temperature and cooling rate. The fast air cooling after homogenization between 1240 and 850 °C retards the precipitation of the sigma in the δ-ferrite. There are two δ-ferrite transformation mechanisms in this experiment. The direct transformation of the δ-ferrite into sigma is observed in the as-cast 316L stainless steel billet. In contrast, the eutectoid transformation of the δ-ferrite into the sigma and austenite dominates the 316L cast billet homogenized at 1240 °C, with a slow furnace cooling rate. Full article

Super austenitic stainless steels are expected to replace expensive alloys in harsh environments due to their superior corrosion resistance and mechanical properties. However, the ultra-high alloy contents drive serious segregation in cast steels, where the σ phase is difficult to eliminate. In this study, the microstructural evolution of 7Mo super austenitic stainless steels under different homogenization methods was investigated. The results showed that after isothermal treatment for 30 h at 1250 °C, the σ phase in steels dissolved, while the remelting morphologies appeared at the phase boundaries. Therefore, the stepped solution heat treatment was further conducted to optimize the homogenized microstructure. The samples were heated up to 1220 °C, 1235 °C and 1250 °C with a slow heating rate, and held at these temperatures for 2 h, respectively. The elemental segregation was greatly reduced without incipient remelting and the σ phase was eventually reduced to less than 0.6%. A prolonged incubation below the dissolution temperature will lead to a spontaneous compositional adjustment of the eutectic σ phase, resulting in uphill diffusion of Cr and Mn, and reducing the homogenization efficiency of ISHT, which is avoided by SSHT. The hardness reduced from 228~236 Hv to 220~232 Hv by adopting the cooling process of “furnace cooling + water quench”. In addition, the study noticed that increasing the Ce content or decreasing the Mn content can both refine the homogenized grain size and accelerate diffusion processes. This study provides a theoretical and experimental basis for the process and composition optimization of super austenitic stainless steels. Full article

Two as-cast low-density steels grades (austenite-based duplex Fe-30.9Mn-4.9Al-4.5Cr-0.4C and austenitic Fe-21.3Mn-7.6Al-4.3Cr-1C) with an initial dendritic microstructure were subjected to hot working conditions to understand the influence of deformation parameters on flow behavior and microstructural evolution. The alloys were produced using electric arc melting, and their phase constituents were determined using optical microscopy and X-ray diffraction analysis. This was then corroborated with the phase predicted from Thermo-Calc simulation. The as-cast alloys were machined to 10 × 10 × 7 mm specimen configurations for rectangular axial testing on the Gleeble 3500 thermomechanical simulator. The samples were deformed to a total strain of 0.5 at different deformation temperatures (800, 900, and 1000 °C) and strain rates (0.1 and 5 s⁻¹). Thereafter, a hardness test was conducted on the deformed samples, and post-deformed microstructures were analyzed using optical and scanning electron microscopes. The results showed that the alloys’ dendritic structures were effectively transformed at temperatures below 1000 °C regardless of the strain rate. At all deformation conditions, the peak flow stress of Fe-21.3Mn-7.6Al-4.3Cr-1C alloy was at least 50% higher than that of Fe-30.9Mn-4.9Al-4.5Cr-0.4C alloy owing to the higher carbon content in the austenitic low-density stainless steel. The hardness of all the deformed samples was superior to that of the as-cast samples, which indicates microstructural reconstitution and grain refinement in the alloys. Dynamic recrystallization, dynamic globularization, and dynamic recovery influenced the softening process and the microstructural changes observed in the alloys under different deformation conditions. Full article

Herein, the vacuum arc-melting process is applied to incorporate various amounts of Ti and C into SUS304 austenitic stainless steel based on the high-entropy alloy concept to obtain wear- and corrosion-resistant alloys with in situ carbide reinforcements. Five compositions containing the equivalent of 5, 10, 15, 20, and 25 volume percentages of TiC in SUS304 stainless steel, named A1, A2, A3, A4, and A5, respectively, were designed, melted, and solidified by the arc-melting method. Microstructural analyses, hardness measurements, immersion tests in four corrosive solutions, electrochemical measurements in a 3.5 wt % NaCl_(aq) solution, and tribological tests were conducted to determine the properties and explain the relevant mechanisms. A1 exhibited a eutectic structure between FCC dendrites, while A2, A3, A4, and A5 possessed proeutectic dendritic TiC, FCC dendrites enveloping the TiC dendrites, and a eutectic structure. A5 represents the optimal composition. Its hardness, wear resistance, and corrosion resistance are 2, 14, and 4 times higher than those of SUS304, respectively. Additionally, its wear resistance is 2.5 times that of high-chromium cast iron. Consequently, A5 could have a 2.5-fold longer lifetime in wear operation. Therefore, A5 could be potentially applied in corrosive and abrasive environments, such as rotary shafts, rotors, bearings, and structural parts in food, chemical, and optoelectronic industries. Full article