Daido Steel (Japan)

The effects of intergranular W-rich compounds (W6C/W12C and µ phases) on the creep properties of a γ’-precipitation-strengthened, wrought Ni-Co-based superalloy were examined. Two microstructures with different intergranular precipitates (but the same volume fraction and size of intragranular γ’ precipitates) were prepared by adjusting aging conditions, leading to two distinct grain-boundary precipitation microstructures, with grain-boundaries exhibiting: (i) W-rich compounds (W6C/W12C and µ), or (ii) W-free compounds (Cr23C6 and γ’ phases, as in conventional Ni-based alloys). In compressive creep tests, lower minimum creep rate and longer 1% creep life were observed for alloys with the second grain-boundary microstructure (with Cr23C6 and γ’) than for the first grain-boundary microstructure (with W6C/W12C and µ). On the other hand, in tensile creep tests, longer creep rupture life was observed for alloys with the first grain-boundary microstructure than for the second, especially at higher stresses. Based on the grain boundary precipitation strengthening mechanism, it is suggested that the difference in compressive minimum creep rate between both microstructures can be explained by the difference in grain-boundary coverage. In tensile creep tests, the creep rate due to cavity and crack growth appears to be the dominant factor in the overall creep rate at higher stresses, consistent with Stress Assisted Grain-Boundary Oxidation (SAGBO). It appears that the grain-boundary precipitation of W6C/W12C and µ phases is more effective in suppressing SAGBO than grain-boundary precipitation of Cr23C6 and γ’.

Cold tool steels contain relatively larger amount of C and Cr to utilize carbide for increase of hardness and wear-resistance. However, in the large-scale ingots, coarsening of carbides caused by microsegregation would degrade steel properties. Also, micro-porosity generated during ingot casting affects the risk of cracking and the occurrence of unclosed defects in the subsequent forging process. Therefore, it is important to predict and control its occurrence. In order to investigate the effect of solidification conditions on the formation of carbides and micro-porosity of 8Cr type tool steel (Fe-1C-8Cr-2mass%Mo), unidirectional solidification experiment with different solidification conditions were conducted. As a result, primary carbides in the interdendritic zone were formed and estimated to be M7C3 and M6C. The distribution coefficients were analyzed using random sampling method. The solid fraction, at which dendrite growth terminated and carbides began to crystallize, changed to higher solid phase fraction as the pull-down speed decreased. G/√R was calculated using the temperature gradient G and cooling rate R, and the relationship between the micro-porosity area fraction was investigated. The results showed that porosity occurrence was minimal in the range of 1.6 to 7.0 (K-s)1/2/mm. In the larger range of G/√R, the porosity formation is considered to be caused by insufficient supply of liquid phase due to higher solid phase ratio and lower permeability to the final solidification zone when the eutectic phases start to crystallize.

A mechanical tensioner system for a timing chain of an internal combustion engine consists of the leaf spring coiled on the backup spring and requires a stable friction resistance despite operating a starved lubricated condition. An effectiveness of the surface texturing to the leaf spring surface on stabilizing the friction resistance has been found. The present study describes a selection of the manufacturing process for the surface texturing consisting of aligned bumps. Electrochemical polishing, a laser abrasion and a micro milling using various cutting edges processes were applied. The residual stress and the hardness distribution measurement were connected to the endurance experiment. Results showed that the large tensile residual stress was distributed accompanied with excess hardening after the laser abrasion and that it was difficult to assemble the tensioner system. A small compressive residual stress accompanying without the excess hardening was found on textured surface using the micro milling with a ball end mill.

In this study, I investigated the mechanisms for improving the seizure resistance of nitrocarburizing, sulphonitriding, and oxynitriding steels, which are known for their excellent seizure resistance. The aim was to derive design guidelines for further seizure-resistant surface treated steel materials. A roller-on-disk friction test and surface analysis of wear scars using SEM/EDS were conducted to evaluate seizure resistance from the perspectives of friction coefficient, surface pressure, adhesion resistance, and frictional heat generation. As a result, it was revealed that sulphonitriding steel showed the most significant friction reduction effect, reducing friction by 23% compared to carburizing steel under ATF lubrication. EDS analysis revealed that sulphonitriding and oxynitriding steels exhibited superior adhesion resistance compared to nitrocarburizing and carburizing steels. Additionally, the frictional heat generation was calculated from the friction coefficient and surface pressure. This revealed that sulphonitriding steel generated less frictional heat compared to other surface treated steels, suggesting that sulphonitriding steel has superior seizure resistance compared to other surface treated steels.

Neodymium-iron-boron magnet is an essential material for the traction motors of green vehicles because it exhibits the highest maximum energy product, (BH)max, out of all permanent-magnet materials. However, heavy rare-earth elements such as dysprosium and terbium, which are scarce resources, are added to these magnets to improve their heat resistance. To address this resource problem, considerable efforts have been made to reduce the composition of heavy rare-earth elements in these magnets without causing a significant reduction in coercivity. Hot-deformed Nd-Fe-B magnets are a category of Nd-Fe-B magnets where precious materials such as heavy rare-earth elements can be eliminated or reduced to maintain high coercivity owing to their fine microstructure. Although they are not often used for the fabrication of high-performance magnets due to their complicated production process and the difficulty in controlling their fine microstructure, after the rare-earth crisis in 2020, these magnets have begun to attract attention as a material that could increase coercivity when controlling their microstructures. This paper provides an overview of hot-deformed magnets and the efforts made to improve their properties by controlling their microstructures.

Fully dense isotropic Sm-Fe-N magnets were successfully fabricated using the shock-wave consolidation method. Deterioration of the hard magnetic properties of the Sm-Fe-N by the impact load during the consolidation process was not observed, and consequently the fully dense magnet showed high (BH)<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> of 116-144 kJ/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (14.5-18 MGOe) which is the highest level as the isotropic magnet. Its temperature coefficient of B<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> was 0.066 %/°C, and that of coercivity was 0.36 %/°C. The irreversible flux loss of the sample with L/D of 0.78 after exposed at 150 °C was small value of 5-10 %. The fully dense Sm-Fe-N magnet without coating showed a good corrosion resistance, and it had enough mechanical strength. From these results, the practicality of the fully dense Sm-Fe-N magnet made by shock wave consolidation has been confirmed.

In the long history of permanent magnet research for more than 100 years, three-dimensional magnetic microscopy has been eagerly awaited to elucidate the origin of the magnetic hysteresis of permanent magnets. In this study, we succeeded in observing the three-dimensional magnetic domain structure of an advanced high-coercivity Nd-Fe-B-based permanent magnet throughout the magnetic hysteresis curve using a recently developed hard X-ray magnetic tomography technique. Focused-ion-beam-based three-dimensional scanning electron microscopy was employed to study the relationship between the observed magnetic domains and the microstructure of the magnet for the same observing volume. Thermally demagnetized and coercivity states exhibit considerably different magnetic domain structures but show the same periodicity of 2.3 μm, indicating that the characteristic length of the magnetic domain is independent of the magnetization states. Further careful examination revealed some unexpected magnetic domain behaviors, such as running perpendicular to the magnetic easy axis and reversing back against the magnetic field. These findings demonstrate a wide variety of real magnetic domain behaviors along the magnetic hysteresis inside a permanent magnet.