Papers by Author: Hisao Fujikawa

Abstract: Among the high temperature damages, this paper was discussed on the cases in which the materials were damaged mainly by the effect of environmental factors. That is, high temperature oxidation, steam oxidation, molten salt corrosion, high temperature particle erosion and corrosion, high temperature sulfurization, carburizing, metal dusting, nitriding, high temperature chloride corrosion and so on were introduced using my research data. Finally, anticipation to future research of high temperature corrosion was maintained.

Abstract: Nitriding treatment is well known as one of the corrosion protection methods for steels as well as a way to prevent wear and fatigue. Initially, salt bath nitrocarburizing was popular, but recently, gas nitriding, gas nitrocarburizing, plasma nitriding and so on have come to be used more often because of their superior nitriding ability. In the case of nitriding, only nitrogen (N) diffuses into the steel, but in the case of nitrocarburizing, both nitrogen and carbon (C) diffuse into the steel. General speaking, nitriding includes all the treatments mentioned above. The corrosion behavior of nitride carbon steels has been understood mainly by salt bath or gas nitrocarburizing treatments^1)-4).However, recently, nitriding is mainly applied to parts for things such as automobiles which need protection from wear and fatigue, and is seldom used for parts which need corrosion resistance. The present paper is to remind researchers again that nitrided steels show good corrosion resistance.Therefore, the comparison of various thicknesses of nitride layers as well as the comparison between nitride layers on steel has been carried out in this examination, using the salt spray corrosion test method. The effect of oxidation treatment after nitriding was also investigated.

Abstract: Three studies on the oxidation behaviour of austenitic stainless steels were described in the present paper. (1) High temperature oxidation behaviour and its mechanism in austenitic stainless steels with high silicon: Sulfur contained as impurity in steel showed a harmful influence to the oxidation resistance of 19Cr-13Ni-3.5Si stainless steels. It was found that the abnormal oxidation was caused from the surroundings of MnS inclusions. (2) Effect of a small addition of yttrium on high temperature oxidation resistance of Si-containing austenitic stain less steels: The oxidation resistance of 19Cr-10Ni-1.5Si steels was improved remarkably even with only 0.01%Y addition, which is the same concentration as added for de-oxygenation. Y was enriched at the grain boundary of oxide scale and metal-oxide interface. It was suggested that Y-containing steels shoed good oxidation resistance, because the enriched Y at the grain boundary and metal-oxide interface prevented the diffusion of iron and oxygen ions through the oxide scale. (3) Effect of grain size on the oxidation behaviour of austenitic stainless steels: Type 304, 316 and 310 steels with finer grain size showed better oxidation resistance than those with coarser grain size at 850°C. The oxide scale of steels with coarser grain size easily spalled during the cooling process.

Abstract: The effect of different carbonitride forming elements on the oxidation performance of a 17Cr steel has been investigated. Whilst C and N are shown to have deleterious effects on the formation of protective oxides on unstabilised steel, a relationship has been found between the strength of the carbonitride former and the relative improvement in the oxidation performance. Furthermore, the appearance of the austenite phase has harmful influence on the oxidation resistance of the ferritic steel because the diffusion rate of Cr, Fe and so on is much slower in the austenitic structure than in the ferritic structure. The improvement is the most pronounced when the stabilizing element is Zr. The formation of the scale observed using transmission electron microscopy has been shown to be strongly controlled by the inward diffusion of the oxidant.

Abstract: The steam oxidation behavior and the oxide scale structure of Ni-based alloys containing 20 to 25 mass% Cr content were studied. The oxidation test was carried out from 550 to 700°C at 50°C intervals for 100 hours in steam. All Ni-based alloys used showed good steam oxidation resistance; higher Cr containing alloys have a little better oxidation resistance than lower Cr containing alloys. The kinetics of the oxidation rate of all Ni-based alloys used was estimated as the temperature parameter in a 100 hour test and as the temperature and time parameters, respectively. These equations could be useful for industrial applications. The oxide scale of Alloy 625 was composed of two layers. The outer layer was composed of needle-like oxides and the inner layer was composed of isometric oxides. The oxide scale was composed of Cr2O3 type. The Cr/Ni ratio in the oxide scale at 700°C was by one order larger than that at 650°C. The oxidation resistance of Ni alloys is maintained by a uniform Cr2O3 layer which is composed of high Cr content.

Abstract: The nitrided layer formed on Type 304 steel after gas nitriding was studied using TEM and so on. (1) Dependency of the nitriding temperature and time on the depth of the nitrided layer was studied. The following equation was obtained. L2=8.7*108*EXP(-146,000/RT)*t (2) Nitrided layer formed at both 570°C for 3 hrs and 410°C for 48 hrs had high density of dislocation, stacking fault and lattice strain. (3) Nitrided layer formed at 570°C was mainly composed of є-Fe3N, and had much Fe4N and CrN. (4) On the other hand, the nitrided layer formed at 410°C was mainly composed of S-phase, є-Fe3N was not detected and Fe4N and CrN were very little. N content in the nitrided layer formed at 410°C was about 7-8 mass%. (5) Nitrided layer formed at 410°C showed good corrosion resistance.

Abstract: Steam oxidation behavior of a new Ni-based single crystalline superalloy (TMS-75) developed as the high strength material for the advanced gas turbine which would be cooled by steam was studied in comparison with Inconel 625 alloy which is a representative Ni based superalloy and 12Cr ferritic steel which is conventionally used in boiler tubes and main steam pipes and steam turbines. TMS-75 alloy showed worse oxidation resistance in steam than Inconel 625 alloy, but it had much better oxidation resistance in steam than 12Cr steel. Also, the oxide scale of Inconel 625 alloy is very thin and is mainly composed of Cr2O3. However, the outer scale of TMS- 75 alloy was mainly composed of NiO with Co and a little Al and other elements. The Ni/Co ratio in the oxide scale was a little higher in the outer layer than in the inner layer.

Abstract: Air Water Incorporated Japan (AWI) has succeeded in developing and commercializing new surface treatment technologies. One of them is the NV Nitriding process. Through an activation treatment using fluorine-based gas for the surface treatment of metals before the nitriding treatment, it is possible to remove the oxide film, such as the passive film, formed on stainless steels and Ni-based alloys and also to nitride these alloys on a mass-production scale. The activation effect afforded by fluoride enables nitriding at a temperature about 150°C lower than other conventional processes, achieving minimal thermal deformation and strain. Also, with this activation process, the range of selection of nitriding conditions such as temperature, gas composition and heat pattern has been considerably extended to enable the formation of a nitride layer optimum for the required characteristics of the treated metals. That is, this process can control the thickness of the nitride compound layer. NV Pionite treatment also has been developed as the soluble carbon diffusion process by which to ensure high corrosion resistance and high hardness in austenitic stainless steels using the above activation treatment.