Papers by Keyword: Resistance Heating

Abstract: Hot stamping of manganese–boron steels is widely used in automotive manufacturing to produce ultra-high-strength components with tensile strengths exceeding 1500 MPa . Conventional industrial heating relies on gas-fired roller hearth furnaces, which require 5 – 10 min to reach austenitization and exhibit low energy efficiency . Resistance heating offers a compact and energy-efficient alternative, enabling heating rates above 100 K/s and full austenitization within seconds. However, rapid heating of uncoated steels leads to severe oxidation, and established coating systems such as AlSi are not designed for diffusion-controlled bonding within such short times . This study demonstrates that resistance heating in an XHV-adequate atmosphere – consisting of nitrogen and monosilane – suppresses oxidation while simultaneously enabling adhesion of a pre-laminated aluminum foil to the steel substrate. For coating preparation, 22MnB5 sheets were roughened by corundum blasting, cleaned, and laminated with an aluminum foil using a flat-die pressing tool. The pre-coated blanks were heated in a self-developed resistance-heating chamber, in which the oxygen concentration was reduced to an XHV-adequate level. Several heating profiles were investigated to determine suitable process windows for coating formation. The results show that resistance heating achieves austenitization within a few seconds, reducing heating times by more than an order of magnitude compared to furnace heating. The XHV-adequate atmosphere reliably prevents scale formation, enabling completely oxidation-free surfaces during rapid heating. Under these conditions, the laminated aluminum foil bonds uniformly to the substrate, forming a continuous coating layer. Metallographic cross-sections and SEM analyses confirm the formation of Al–Fe intermetallic phases at the interface, demonstrating robust metallurgical bonding suitable for subsequent hot stamping operations. Overall, the combination of resistance rapid heating and an XHV-adequate atmosphere provides a highly energy-efficient process route for hot stamping while offering an opportunity to integrate aluminum-based protective coatings directly into the heating step. This approach addresses the limitations of current furnace-based heating and coating technologies and opens a promising pathway toward more flexible, sustainable, and functionally integrated hot-stamping process chains.

Abstract: The main reason for appearance and development of corrosion damage of the nuclear power plant (NPP) steam generator heat exchanger pipes is the process of stress corrosion cracking of metal under the influence of the residual tensile stresses. The method for heat treatment and straightening of the heat exchanger pipes using distension is proposed. The approbation of the process in a production environment is carried out. The construction and operation of the experimental installation are described. The experimental results show that the proposed method provides mechanical properties, grain size and resistance to intergranular corrosion of metal pipes in accordance with the requirements of Standard Specification 14-3P-197-2001 (Seamless pipe of corrosion-resistant steels with improved surface condition). In pipes, heat-treated by the proposed method, the level of residual tensile stresses and the curvature are one order less than in the tubes, heat treated by the technology which is usually used.

Abstract: Strain gradient is known as an important factor that influences springback of bent components in microscale. Compared with thicker foils, thinner foils usually indicate more strain gradient due to non-uniform material deformation. A resistance heating (RH) method is an effective approach to obtain homogenous material flow by heating foils within only several minutes. To predict springback of foils bent at elevated temperatures, an investigation of the influence of strain gradient on springback is indispensable. To achieve this, microbending tests assisted by RH were conducted at different temperatures ranging from 298 to 723 K in the present study. 0.05 mm-thick pure Ti foils with varying grain sizes of 2.7, 14.7, and 24.5 μm were used. As results, normalized bending moment decreased with increasing temperature and with increasing grain size. The less strain gradient of the foils with larger grain size and at elevated temperatures was confirmed to be the reason according to a theoretical analysis of springback using the constitutive model considering statically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs). The predicted normalized bending moment by theoretical calculation showed good agreement with experimental results at the temperature of 573 K or higher but not at the temperature lower than 573 K. It was found that the springback of the foils was influenced by the strain gradient at low temperatures. Furthermore, the size effects caused by strain gradient reduced as the bending temperature increases.

Abstract: The deep drawability and bendability in hot stamping of ultra-high strength steel parts were examined. Although the cold drawability is greatly influenced by the blank shape, the limiting drawing depths for the square and circular blanks were equal for hot stamping because of small flow stress. In hot hat-shaped bending using draw-and form-type tools, the effect of the blankholder force generated with the draw-type tools on the springback was small, and the seizure for the form-type tools was smaller than that of the draw-type tools. Since both edges in contact with the electrodes are not heated for resistance heating, cracks were caused at the edges for resistance heating in the transversal directions in hot stamping of an S-rail with form-type tools, and thus it is required to control deformation of the non-heating zones.

Abstract: A hot spline forming process of die-quenched gear drums using resistance heating of a side wall of a cup formed by cold deep drawing and ironing was developed. The side wall having uniform cross-sectional area is resistance-heated by passage of the current in the axial direction, the heated side wall of the drawn cup is ironed and is finally die-quenched. The gear drum was successfully formed and the hardness was between 400 and 500 HV. Not only the formability was improved but also the formed dram was hardly oxidised because of rapid resistance heating.

Abstract: Non-isothermal resistance heating in the hot stamping of quenchable steel sheets was developed to produce ultra-high strength steel formed parts with tailored properties. The heating temperature of parts is related with width of samples heated by resistance heating. With the same input energy, the strength in the narrow portions is high owing to the high energy density and that in the wide portions is low owing to the low energy density. Hat-shaped products having a tensile strength arrange from 600 MPA to 1800 MPa were formed. The tempering treatment on the directly hot-stamped boron steel resulted in better mechanical properties and higher formability index. The SEM figures indicates that the nano-carbide formation during the tempering treatment were suggested as the evident reasons for the occurrence of the mentioned robust properties. Finally the combination of temperature 250 ℃ and holding time 45 min can achieve the best comprehensive mechanical properties.

Abstract: In order to reduce the energy consumption and working procedure for manufacturing thin-walled titanium alloy components, a hot stretch-creep forming (SCF) technique via resistance heating is proposed to form the hard-to-form material. Firstly, the principle of hot SCF was introduced that the technique can manufacture titanium alloy components precisely because the residual stress relaxes in the material during the dwelling time. Secondly, the hot SCF equipment via resistance heating was developed, which makes use of electrical current for heating the titanium materials to improve their ductility. Then based on a set of optimized process parameters, including forming temperature, stretch velocity, post stretch percentage and dwelling time, a comparative experment of single curvature Ti-6Al-4V alloy sheet components was conducted via this innovative technology to examine the hot SCF equipments and the effect of creep forming. Results show that the component precision of the hot SCF is more precise than that of the hot stretch forming (SF) due to the creep in the stress relaxation period. Comparing to the as-received material, the yield strength of the material suffered hot SF decreases a little; while the strength suffered hot SCF increases a little. This novel technique shows a promising future to manufacture titanium sheet or profile components precisely as a feasible and cost-effective way.

Abstract: To enhance the heating efficiency and the formability of AZ31 magnesium alloy, the assistance heating method is adopted during the hot bulging process. The free bulging test of coarse grained and fine grained AZ31 magnesium alloy sheet was carried out. During the forming process, the effects of pulse current on the formed sheet combine both thermoelectricity and electro-plasticity. Directional and asymmetrical deformation of the coarse-grained AZ31 magnesium alloy sheet by the effect of pulse current is observed and analyzed. But in the same processing condition, the deformation of fine grained AZ31 magnesium alloy sheet is symmetrical. To investigate the influence of electron wind force on pulse current auxiliary bulging process, the simulation of AZ31 magnesium alloy pulse current auxiliary bulging based on the electron wind force value calculated by mathematical model was performed using the Marc software. The bulging simulation result showed a special phenomenon that the shape of the bulging specimen is unsymmetrical and the dome deviated from the symmetry axis to the side of positive pole. The simulation results were basically consistent with the experimental results. The deformation properties, microstructure characteristics, dislocation movement of the AZ31 magnesium alloy during gas bulging processing by the pulse current are investigated.

Abstract: First, with a view of raising the operation rate of the press used in hot stamping and press quenching process, the influence of completion timing of press quench on shape quality and hardness property of the product was examined by an arc-shaped hot bending test with practical hot stamping steel sheet, and recommendable measures were suggested. In addition, by comparing the test results with those of austenitic stainless steel sheet without martensitic transformation, it was implied that good forming performance in the practical steel sheet was closely relating to its martensitic transformation. Then, in this connection, the influence of shape retaining force in the press quench operation on the shape quality was examined and the relevant features were revealed. Finally, with regard to the relation of the good shape quality in hot stamped and press-quenched steel sheet part with martensitic transformation, its mechanism was considered from the point of view of minimization of the internal work accompanying the anisotropic martensitic transformation with volume expansion.

Abstract: A novel heat assisted microbending system was developed by applying the resistance heating technology. To clarify the effect of heating on springback for ultra-thin metal foils, high precision microbending test was carried out under the different temperature of room temperature (RT), 300oC and 600oC. As-received rolled and electrochemically polished pure titanium foils with thickness of 0.1, 0.05 and 0.02mm were used. To evaluate the springback in each thickness, springback angle of pure titanium foils with different thicknesses was measured under the same maximum bending strain. As results, the springback angle was decreased with increasing the process temperature for all materials and thickness conditions. Furthermore, the thinner the foil, the larger the springback angle, irrespective of the temperature conditions. The relationship between the heating temperature and the decreasing ratio of the springback angle was experimentally demonstrated.