Search Results (113)

Hot stamping dies fabricated from Mo–W hot-work steels are exposed to severe thermo-mechanical fatigue (TMF), high-temperature oxidation, and complex tribological loading, which collectively accelerate die degradation and reduce production stability. Although individual failure modes have been reported, an integrated understanding linking microstructural evolution, interfacial reactions, and wear mechanisms remains limited. A failed Mo–W hot-work steel die removed from an industrial B-pillar hot stamping line was examined using Rockwell hardness mapping, optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) with Williamson–Hall (W–H) microstrain analysis. Surface (0–2 mm) and subsurface (~8 mm) regions of 10 × 10 × 10 mm samples were compared. Pits, cracks, reaction layers, and debris were quantified from calibrated SEM images. A 17% hardness reduction from surface (46.2 HRC) to subsurface (37.6 HRC) revealed pronounced TMF-induced softening. W–H analysis indicated microstrain of ~0.0021 and crystallite sizes of 50–80 nm in the surface region, reflecting high dislocation density. SEM/EDS showed pit diameters of 150–600 μm, reaction-layer thicknesses of 15–40 μm, and crack lengths of 40–150 μm. Fe–O oxides, Fe–Al intermetallics, and FeSiAl4 reaction phases were identified as major constituents of brittle surface layers and debris. Wear morphology confirmed a mixed mode of adhesive galling and oxide-assisted abrasive plowing. Full article

In this study, sheets of experimental high-carbon low-density steels (LDSs) with a thickness of 1.7 mm were processed in a combined tool designed for press-hardening. Press hardening, also known as hot stamping or hot press forming, is a manufacturing process used to create car body parts with exceptional mechanical properties and safety standards. These components often require tailored properties, meaning different mechanical characteristics in various parts of the component. LDSs have a lower specific density than conventional steels, so their use would be particularly suitable in automotive applications. Combined tools achieve distinct mechanical properties within a single part through thermomechanical processing. Simultaneous forming and heat treatment create tailored zones of high strength and ductility within the sheet metal. The hardened zone provides crashworthiness, while the more ductile zone absorbs kinetic energy and converts it into deformation energy. Hot stamping enables forming complex geometries from high-strength sheets with limited cold formability, a capability that can also be exploited for the aluminium-alloyed LDS under investigation in this work. Three different high-carbon LDSs with differences in chemical composition were subjected to this experiment, and the hardness, microstructure, and mechanical properties of the two areas of each sheet were evaluated. The aim is to determine their suitability for processing by press hardening and to try to achieve tailored properties (i.e., differences in ductility and strength across one part) as in a typical representative of 22MnB5 boron steel, where a strength limit of 1500 MPa at 5% ductility is achieved in the cooled part and 600 MPa at 15% in the heated part. Tailored properties were also achieved in the investigated LDS, but with only relatively small differences between the two tool areas. The omega profiles were produced by press hardening without visible defects, and it was possible to process the steels without any difficulties. Full article

AlSi and nitrogen-doped AlSi (AlSiN) coatings were deposited onto 22MB5 steel, while h-BN coatings were applied to H13 steel using the magnetron-sputtering method. The thermal stability and tribological properties of the AlSi and AlSiN coatings were systematically investigated from room temperature to 800 °C in ambient air. The results indicate that the AlSiN coatings with an FeAl transition layer exhibited outstanding wear resistance and high thermal stability behaviors at elevated temperature because the FeAl layer can inhibit the diffusion of Al and absorb Fe, forming iron-rich intermetallic compounds with a high bonding strength. The FeAl layer plays a critical role in enhancing the coating’s performance. Analysis of the wear mechanisms revealed that the AlSiN coating primarily underwent adhesive wear, while the AlSi coating suffered from abrasive and oxidative wear. These findings offer valuable insights for developing protective coatings for the hot-stamping-forming process. Full article

This study investigated the microstructure and corrosion behavior of 30MnB5 hot-stamped steel after applying a zinc coating using the cold-spraying method followed by heat treatment (HT). Al-10 wt%Si coating is essential for improving the high-temperature corrosion resistance of 30MnB5 steel during the hot-stamping process. Before HT, the coating layer primarily consisted of Al, whereas after HT, Fe–Al-based intermetallic compounds were formed throughout the layer. The Zn in the coating layer applied using the cold-spraying method was not uniformly distributed before HT. However, during HT, the low-melting-point Zn melted and re-solidified, allowing it to combine with Fe diffusing from the substrate. Consequently, Zn–Al–Fe-based intermetallic compounds were formed on the surface of the coating layer. In the Zn-coated specimens, the current density near the corrosion potential tends to be lower than that of the Al–Si-coated specimens because Zn corrodes preferentially owing to its sacrificial anode effect, thereby protecting the underlying Al–Si-coated layer and steel. Full article

In the industrial practice of metal forming, the consistent and reasonable characterization of the material behavior under the coupling effect of strain, strain rate, and temperature on the material flow stress is very important for the design and optimization of process parameters. The purpose of this work was to establish an appropriate constitutive model to characterize the rheological behavior of a hot-formed steel plate (22MnB5 steel) produced through the TSCR (Thin Slab Casting and Rolling) process under practical deformation temperatures (150–250 °C) and strain rates (0.001–3000 s⁻¹). Subsequently, the material flow behavior was modeled and predicted using the Johnson–Cook flow stress constitutive model. In this study, uniaxial tensile tests were conducted on 22MnB5 steel at room temperature under varying strain rates, along with elevated-temperature tensile tests at different strain rates, to obtain the engineering stress–strain curves and analyze the mechanical properties under various conditions. The results show that during room-temperature tensile testing within the strain rate range of 10⁻³ to 300 s⁻¹, the 22MnB5 steel exhibited overall yield strength and tensile strength of approximately 1500 MPa, and uniform elongation and fracture elongation of about 7% and 12%, respectively. When the strain rate reached 1000–3000 s⁻¹, the yield strength and tensile strength were approximately 2000 MPa, while the uniform elongation and fracture elongation were about 6% and 10%, respectively. Based on the experimental results, a modified Johnson–Cook constitutive model was developed and calibrated. Compared with the original model, the modified Johnson–Cook model exhibited a higher coefficient of determination (R²), indicating improved fitting accuracy. In addition, to predict the material’s damage behavior, three distinct specimen geometries were designed for quasi-static strain rate uniaxial tensile testing at ambient temperature. The Johnson–Cook failure criterion was implemented, with its constitutive parameters calibrated through integrated finite element analysis to establish the damage model. The determined damage parameters from this investigation can be effectively implemented in metal forming simulations, providing valuable predictive capabilities regarding workpiece material performance. Full article

In order to determine the failure reason for the non-working area of a cracked A-pillar hot-stamping die insert, various instruments were used to detect the properties and microstructures of the cracks and matrix. The results show that the cracks are located in the area where the oxidative corrosion is more serious, and the cracks do not appear in the pitting area, verifying that crack initiation is related to the stress concentration on the upper half of the inner wall of the cooling channel. Meanwhile, pores and cracks exist in the grain boundary and crystal, making the impact energy of the die steel poor. Therefore, crack initiation and propagation easily occur along the brittle oxide layer. In summary, the die insert is damaged by stress-induced corrosion. In engineering applications of hot-stamping dies, we should pay more attention to the cracking of the cooling channel caused by stress and corrosion. Full article

This study presents the development of a deep drawing process under an indirect hot stamping method for manufacturing an automotive internal combustion engine oil pan from ultra-high-strength steel (UHSS) sheets, specifically 22MnB5. The forming process involves two stages—cold stamping followed by hot stamping—and is finalized with rapid quenching to achieve a martensitic microstructure. Finite element simulation using AutoForm R8 was conducted to determine optimal forming conditions. The simulation results guided the design of the forming tools and were validated through experimental trials. The final oil pan component exhibited no cracks or wrinkles, with maximum thinning below 18%, a hardness of 550.63 HV, and a fully martensitic phase. This research demonstrates a novel and effective solution for producing deep-drawn, high-strength components using indirect hot stamping, contributing to the advancement of automotive forming processes in Thailand. Full article

Tensile tests are a common method for characterizing plastic behavior for sheet metal forming applications. During tensile testing at the beginning of the deformation, the stress state is uniaxial; however, as the deformation proceeds, the state changes to triaxial, making the post-processing of experimental data challenging using analytical methods. In contrast, inverse approaches in which the behavior is represented by constitutive equations and the parameters are fitted using an iterative procedure are extremely dependent on the empirical equation chosen at the outset and can be computationally expensive. The inverse piecewise flow curve determination method, previously developed for compression tests, is extended in this paper to tensile testing. A stepwise approach is proposed to calculate constant strain rate flow curves accounting for the unique characteristics of tensile deformation. To capture the effects of localized strain rate variations during necking, a parallel flow curve determination strategy is introduced. Tensile test flow curves for manganese-boron steel 22MnB5, a material commonly used in hot stamping applications, are determined, and the approach is demonstrated for virtual force–displacement curves. It has been shown that these curves can replicate the virtual experimental flow curves data with a maximum deviation of 1%. Full article

Hot stamping is a promising method to manufacture ultra-high-strength automotive steel components with high dimension accuracy. In this work, two actual industrial production flows (with and without Al-Si hot dipping) were investigated to reveal their microstructural evolution and hydrogen content at different production steps. Meanwhile, the variations in composition and phase structures of the Al-Si coating layer were studied in terms of energy-dispersive spectrometry and electron backscattering diffraction techniques. The results showed that the microstructure at the steel substrate changed from the pancake-shaped pearlite and ferrite, degenerated pearlite and annealed ferrite, lath martensite, and then tempered martensite with the progress of the production steps, which was not affected by the Al-Si hot dipping. The final coating layer exhibited a multi-sublayer structure with the alternative distribution of FeAl and Fe₂Al₅, which contained many microcracks on the brittle phase Fe₂Al₅. The Al-Si-coated specimens always had higher hydrogen content than the bare steel specimens because of the hydrogen generation at the hot stamping stage and hydrogen absorption during the hot-dip aluminizing stage. Full article

A unified viscoplastic constitutive model based on internal physical variables was proposed to predict the viscoplastic mechanical behavior and microstructure evolution of metals during hot forging. Based on the phase transformation theory of materials under the effect of temperature, the evolution mechanism of residual stress during the cooling process after hot forging and stamping was explored. The determined unified viscoplastic constitutive equation was written in the VUMAT subroutine and employed for the explicit FE analysis of the hot forging and stamping process of thin-walled spherical shells. In the data transfer process, the stress field, temperature field, and deformation characteristics calculated during the high-temperature transient of the hot forging and stamping process were inherited. Meanwhile, the thermoplastic constitutive equation considering the influence of phase transformation was written in the UMAT subroutine and utilized for the implicit FE analysis of the cooling process of thin-walled spherical shells. Through comparison with the measured stress results of the spherical shells after actual forging, it was shown that the proposed constitutive model can effectively predict the microstructural evolution and the final residual stress distribution pattern of medium-carbon steel during the hot forging process. Full article

This investigation aimed at evaluating the weldability of a 2.1 GPa-grade hot stamping steel (HSS) containing 0.40 wt.% carbon using laser butt welding. It is shown that the subject HSS can be properly joined by laser welding without welding defects, such as voids and micro-cracks. The mechanical properties of joints before and after hot stamping were examined using cross-weld uniaxial tension and Vickers hardness, while microstructure was systematically characterized using optical microscopy and electron backscatter diffraction. The experimental results demonstrate that fresh martensite was formed in the weld nugget after welding, leading to a hardness much higher than that of the base metal. Nevertheless, such cross-weld microstructural heterogeneity was erased after hot stamping and low-temperature baking heat treatments, resulting in a uniform microstructure of lath martensite across the weld. As a result, the joint after hot stamping and baking exhibited an ultimate tensile strength of 2140 MPa and a total elongation of 12.03%, with the fracture occurring in the base metal. Such excellent mechanical properties of the joint demonstrate the great weldability of the present 2.1 GPa-grade HSS during laser welding. Full article

This study involved the investigation of the mechanical and tribological behaviors of DIN 1.2344 and WP7V tool steels, quenched in a salt bath after austenitization at 1050 °C, followed by triple tempering for 2 h. The selection of tempering temperatures produced two hardness levels under four metallurgical conditions, with the hardest level found only for WP7V steel (54 and 57 HRC). The mechanical properties were evaluated using Rockwell C, Vickers, and nanoindentation methods, along with unnotched impact tests, according to the SEP 1314 guidelines. Wear tests were conducted in a tribometer configured for a reciprocating setup, with a frequency of 5 Hz, a load of 25 N, and a time of 60 min, at room temperature and at 200 °C. As counterbodies, alumina balls of 5 mm in diameter were used. Wear tracks were evaluated through scanning electron microscopy, EDS, interferometry, and Raman spectroscopy. Friction and wear behaviors were affected by the variation in temperature for softer steels (DIN 1.2344 and WP7V of 48.5 HRC): the higher the temperature, the better the tribological performance. The harder steels were not sensitive to temperature testing. These effects depend on maintaining iron oxide (hematite) at the point of contact. The wear rates determined for the hardest material (57 HRC), considering its impact resistance, make it unsuitable for severe conditions such as hot stamping. Full article

This study investigates the effects of hot stamping on boron steel surface properties, comparing uncoated steel to Al–Si-coated steel, with a focus on developing atmosphere-controlled hot stamping technology. Experiments using a hat-shaped specimen revealed that uncoated steel formed a thick oxide layer due to exposure to atmospheric oxygen at high temperatures, negatively impacting surface quality and weldability. In contrast, the Al–Si-coated steel showed no oxide formation. Although uncoated steel exhibited higher average Vickers hardness, the detrimental effects of the oxide layer on weld quality necessitate advancements in process technology. A lab-scale hot stamping simulator was developed to control atmospheric oxygen levels, utilizing a donut-shaped induction heating coil to heat the material above 1000 °C, followed by rapid cooling in a forming die. Results demonstrated that maintaining oxygen concentrations below 6% significantly reduced oxide layer thickness, with near-vacuum conditions eliminating oxide formation altogether. These findings emphasize the critical role of oxygen control in enhancing the surface quality and weldability of uncoated boron steel for ultra-high-strength automotive applications, potentially reducing manufacturing costs while ensuring part performance. Full article

Hot stamping is a forming process widely used in the manufacturing of structural components in automobiles. It is a versatile process that enables the fabrication of complex-shaped components with high strength. It also facilitates the manufacturing of components that incorporate high-strength sections and high-ductility sections, by controlling the cooling rate. The process is versatile in terms of the microstructures and mechanical properties that can be obtained. This versatility, however, puts high demands on the materials pertaining their stability, wear resistance, costs, etc. This study has focused on understanding the effect of temperature on the tribological response of different tool materials when these are exposed to high temperatures. The results show that friction significantly stabilises with increased temperature for most tool steels. One tool steel behaves more unstably at high temperature, and this is attributed to the presence of Cr₇C₃, MoO₃, and VO and severe wear on the workpiece material. The most severe wear on the workpiece is caused by a partially melted interdiffusion layer, which facilitates the detachment of the Al-Si coating and subsequent transfer onto the tool; this effect is maximised at the highest temperatures of the workpiece. An important finding is that friction and material transfer severity decrease as the workpiece temperature decreases, and friction is stabilised as tool temperature increases without minimising wear or the average friction coefficient. Full article

This study investigates enhancing the high-temperature oxidation resistance of hot-stamped steels by adding the Cr/Mn/Si elements to form an extremely thin oxide layer. Under low oxygen partial pressure conditions and high Cr content in the matrix, the oxide layer of a 38Cr3MnNbVMo hot-rolled plate containing the Mo element and high Si content was further thinned to 0.6 μm after cooling at 900 °C for 5 min. The structure of the ultra-thin oxide layer consists of Fe₃O₄, Mn oxides, FeCr₂O₄, Cr₂O₃, and Fe₂SiO₄ oxides. Compared to other antioxidant elements, under low oxygen partial pressure conditions, Si is more prone to oxidation, forming ultra-thin (22 nm) Fe₂SiO₄ oxides at the matrix interface. Combined with Cr₂O₃, FeCr₂O₄, and Mn oxides, it collectively inhibits the mutual diffusion of external O ions and matrix Fe ions. Furthermore, the addition of the Mo element improves the oxidation resistance. The synergistic effect of multiple powerful oxidation-resistant elements and oxide products effectively inhibits the growth of the iron oxide scale, enhancing the oxidation resistance of hot-rolled, hot-stamped steel. Full article