Toru Takayama

Ammonia gas nitriding of Fe-18Cr-9Ni alloy at lower than 823 K

An Fe-18Cr-9Ni alloy, which it had not previously been possible to nitride at temperatures below 873 K, was found to form nitrides in an ammonia gas atmosphere at temperatures as low as 823 K after annealing at low hydrogen pressure at 1473 K. Microstructure and hardness were examined on cross-sections of the nitrided specimens. An internal nitriding layer had formed beneath an external nitriding layer on the specimen surface. Vickers hardness was above 1000 throughout the internal nitriding layer. The nitrides formed at the specimen surface and in the internal nitriding layer were identified using grazing incidence X-ray diffraction and ordinary X-ray diffraction methods, respectively. The external nitriding layer, which was about 6 to 10 μm thick, formed on the surface, which consisted of ɛ-Fe2−3N, γ′-Fe4N, and CrN. Two types of chromium nitride were precipitated by ammonia gas nitriding of the present alloy: CrN in the external nitriding layer and Cr2N in the internal nitriding layer.

In situ QXAFS observation of the reduction of Fe2O3 and CaFe2O4

In situ QXAFS studies of the reduction of α-Fe2O3 and CaFe2O4 were conducted to determine their reduction kinetics and mechanisms. The reduction of α-Fe2O3 involved two steps, the first being a very fast process in which Fe3+ was reduced to Fe2+ and the second being the reduction of Fe2+ to Fe metal over a longer period. In contrast, the reduction of Fe in CaFe2O4 was a single first-order reaction, although an induction period was clearly observed at the beginning of the reduction process. The reduction processes were successfully studied using a combination of in situ QXAFS spectra at the Ca and Fe K-edges.

Hot V-Bend Formability of Galvannealed Boron Steel for Hot Stamping

The hot stamping process has been widely applied to the production of automotive body parts because the stamped panels with both high strength and good shape-accuracy are easily obtained. For the purpose of omitting the de-scalling treatment, the galvannealed (GA) boron steel for hot stamping was developed. When the steel is hot formed with the existence of liquid zinc, brittle fracture occurs by the infiltration of liquid zinc into the solid metal. This phenomenon is well known as Liquid Metal Embrittlement (LME). In this study hot V-bend test was carried out to estimate LME behaviour for GA boron steel in terms of suitable conditions of both heating and forming processes. First, LME disappeared with an increase of dwell time in gas furnace. In this heating condition, the dwell time that LME disappeared was 225 s. And the depth of LME crack became small as dwell time, and LME crack of 10 micrometer size was also observed in the dwell time of 210 s just before liquid zinc would be completely depleted from the Г contents index IГ based on X-ray diffraction pattern analysis for Г-phase of V-bent test specimen. In addition, in such condition, LME did not occur when a bending strain was small of 0.05 true strain. The overbend crush test for V-bent test specimen was carried out to investigate the influence of LME crack on the crash performance. Consequently it turned out that LME crack of about 10 micrometer did not have much influence on the crash performance. And this method is supposed to be very simple and valuable as a quality verification test for LME of the hot stamped parts in production.