Isothermal reduction of magnesium oxide with aluminum was carried out by use of an insertion tube charged with pellets composed of magnesium oxide and aluminum powders. Since the temperature in the tube increased rapidly during inserting the tube into the high temperature zone in the graphite crucible and decreased quickly during lifting up the tube to the low temperature zone, the isothermal reduction condition was essentially satisfied by the present method. The reduction rate was increased with increasing temperature, carrier gas flow rate and pellet-forming pressure. Even a little carrier gas could greatly increase the reduction rate of magnesium oxide. The XRD patterns of pellets at the different reduction stages confirmed that the aluminothermic reduction of magnesium oxide can be roughly classified into two stages. At the first stage, spinel, alumina and magnesium vapor were mainly produced; at the second stage, the spinel was further reduced by aluminum to produce alumina and magnesium vapor and the excessively added MgO was also reduced. A kinetic model is established to describe the reduction rate of magnesium oxide. The apparent rate constant in the kinetic model increases with increasing temperature, carrier gas flow rate and pellet forming pressure. The apparent activation energy of the reduction is determined to be 109 kJ/mol.
Inclusions in the commercial SUS304 slabs that differed in the aluminum content were observed to clarify the formation mechanism of spinel, MgO · Al2O3, in the stainless steels. In addition, the formation of spinel in the synthetic CaO-SiO2-Al2O3-MgO inclusions was studied. In the experiment on the synthetic inclusion, its melt was cooled at the same cooling rate at which the molten steel was cooled in CC. It was found that the spinel could precipitate from the CaO-SiO2-Al2O3-MgO melt when the composition of the synthetic inclusion was in a primary phase region of spinel. In the observation of inclusions in SUS304 slabs, the formation of spinel was influenced by the content of aluminum in the slabs. In the slab that contained 19 ppm of Al, the spinel inclusions were formed by the reaction of Al, Mg and O that were dissolved in the molten stainless steel. In the slab with 8 ppm of Al, the spinel inclusions were formed by the precipitation from the molten CaO-SiO2-Al2O3-MgO inclusions. In the slab with 6 ppm of Al, the spinel inclusion was not formed. These results revealed that spinel formed not only when the composition of steel was in the region of spinel formation in the phase stability diagram of the system of Al, Mg, Ca, Si and O in molten steel, but also when the composition of CaO-SiO2-Al2O3-MgO inclusion was in the area of primary phase of spinel in the phase equilibrium diagram of the system of CaO, SiO2, Al2O3 and MgO.
Since it has been pointed out that non-wetting behavior of liquid Zn alloy sometimes occurs on high-tensile strength steels which usually contain Si and Mn, there have been a lot of studies to improve the wettability of liquid Zn. Although those studies evaluated the wettability qualitatively by observation of the surface of galvanized steels or exfoliation test of Zn with substrate steels and so on, it is further required to evaluate the wettability of liquid Zn with steels by measuring contact angle, work of adhesion, spreading velocity etc. which are usually used to assessment of general wetting behavior. In the present work, we applied a sessile drop method to measure the change in contact angle and diameter of liquid Zn droplet wetted on steels containing Si and Mn with time to evaluate quantitatively the dynamic wetting behavior of liquid Zn with those steels.
We have been investigating the nitrogen penetration mechanism carbonitriding into high carbon chromium bearing steel JIS-SUJ2 on carbonitriding processes. Assuming that the penetration mechanisms of nitrogen and carbon are the same, we developed a calculation method in order to estimate nitrogen concentration distribution profiles, which are based on our through experimental data. In our method, the dependencies on carbon activity, partial pressures of H2 and undecomposed NH3, and diffusion coefficient of nitrogen are taken into consideration. It was confirmed that the nitrogen concentration distribution profiles predicted by our method agreed well with the experiment results under various conditions. Hence, we deduce that our method is effective in a practical range on carbonitriding processes.
In recent years, it has become important to reduce emission of CO2 from the iron and steel making process in terms of preserving ecological environment. Cementite is recognized as a new iron resource because there is no CO2 emission on the reduction of cementite to iron. In this study, a new steel making process through reduction and sintering treatment of cementite powder was proposed, and microstructure of the material obtained by this process was examined by scanning electron microscopy and EBSP analysis. The cementite powder obtained by mechanical alloying technique was compacted and then subjected to the reduction and sintering treatment in H2 gas atmosphere at 1073-1273K in order to obtain bulky steel materials. The relative density of sintered material became 95% or more when the sintering was performed at 1173K. The sintered material has ferrite-pearlite structure of the grain size of around 5 μm. This result demonstrates that the reduction and sintering of cementite is one of useful methods to fabricate fine-grained steel without CO2 emission.
To improve the delayed fracture strength of ultra high-strength low alloy TRIP-aided steels with bainitic ferrite matrix (TBF steels), the effects of aluminum content on hydrogen absorption behavior and delayed fracture properties of 0.2%C-0.5∼1.5%Si-1.5%Mn TBF steel were investigated. When aluminum was added to the TBF steel, the diffusible hydrogen increased. It was expected that the hydrogen was charged not only in retained austenite films but also on lath boundary. Delayed fracture strength of aluminum bearing TBF steels was significantly increased, compared with conventional TBF steel. This was mainly caused by (1) suppression of the stress-assisted martensite transformation resulting from the stabilized or carbon-enriched retained austenite, (2) hydrogen trapping to refined interlath retained austenite films and lath boundary, and (3) relaxation of localized stress concentration by TRIP effect of the retained austenite.
The effect of aging on microstructures and mechanical properties of 1%N-23%Cr-4%Ni-2%Mo austenitic stainless steel aged at 600∼800°C for up to 1000 h were investigated with electron microscope, analysis of extracted residues, ferrite scope, and mechanical tests such as hardness, tensile, and Charpy impact. Chromium nitrides precipitated on grain boundary at the aging temperature below 650°C, while lamellar precipitates, which were composed of plate like Cr2N and martensite, were formed above 700°C. σ phase precipitated at 700°C for 1000 h and above 750°C for longer than 100 h. Martensite was also observed around σ phase, and the total amount of martensite was 40-50% after aging at 700-800°C for 1000 h. Age hardening was observed above 700°C and hardness, which increased with aging time and temperature, reached about Hv 500 after aging at 800°C for 1000 h. Increase in tensile strength by aging was higher than that in proof strength, because of pronounced increase in work hardening rate by aging. Strengthening by aging depended mainly on the amount of lamellar precipitates and additionally on σ phase and its surrounding martensite at higher temperature. Tensile ductility and impact toughness were decreased by aging due to lamellar precipitates and σ phase.
Hydrogen penetration properties into stainless steels with and without pre-strain exposed to high-pressure hydrogen environments and effects of hydrogen and martensite on fatigue crack growth behavior of SUS304, SUS316L and SUS310S were investigated. The hydrogen penetration behavior into the austenitic stainless steels was successfully expressed by Sieverts' and Fick's laws. In SUS304, the fatigue crack growth rates in the hydrogen-exposed specimen were approximately twice as high as those in the uncharged specimen, while in SUS316L, only slight acceleration in the crack growth rate due to hydrogen was observed only when the crack length was short. Although the hydrogen content and distribution from surface to subsurface in the fatigue specimen influenced the fatigue crack growth, the method of hydrogen charge was not substantial. In the hydrogen-exposed SUS304 specimen, slip bands were less and more discrete and the crack morphology was straighter and thinner. This suggests that hydrogen caused slip localization and accordingly affecting the fatigue crack growth behavior. The estimated hydrogen penetration depths into SUS304 indicate that the presence of strain-induced martensite increases hydrogen diffusivity in the steel. In a hydrogen-exposed stable austenitic stainless steel SUS310S, slip bands were discrete and a significant number of microcracks were generated along the slip bands. It can be concluded that the accelerations in fatigue crack growth rates in the hydrogen-charged austenitic stainless steels were essentially based on slip localization due to dissolved hydrogen, and that the strain-induced martensite in the austenitic stainless steels played a role in facilitation of slip localization with hydrogen.
Precision machinery parts maker uses many grinding machines, then by-produce a lot of grinding sludge. Most of which is dumped into landfills now as industrial waste. Recycling technology which enables environmental load reduction for ISO14001 and cost reduction at the same time was investigated. One of the key technologies to establish recycling system for grinding sludge is a development of practical briquetting machine which enables low-cost, energy save and compact. At first an AC servo motor drive briquetting machine was developed to analyze the best briquettig condition. Oil-based grinding sludge contains a lot of oil in the grinding swarf, so it is difficult to squeeze out because of its high viscosity. If pressing it with high pressure at the beginning of briqueting process, grinding sludge blows out. By using a test machine, the best briquetting condition was investigated for oil-based grinding sludge. Test results pointed out that the most important factor for briquetting is that the beginning of squeezing process needs to be controlled under low pressure with appropriate speed to avoid blow-out. Based on the test results, a low-cost practical briquetting machine was developed which could contribute to grinding sludge recycling for environmental load reduction.