Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are unintentionally formed and emitted from various industrial thermal processes. So far, many researchers have examined formation/emission behavior and inhibition of PCDD/Fs. Generally, following three routes were pointed out as major formation pathways of PCDD/Fs: (I) through aromatic precursors such as chlorobenzenes and chlorophenols, (II) decomposition reaction of organic compounds having large molecular sizes such as polycyclic aromatic hydrocarbons (PAHs) and (III) via de novo synthesis from macromolecular carbon. In the incineration and other thermal processes, de novo synthesis is now regarded as an important route for PCDD/Fs formation. This reaction occurs on macro structural carbon under oxidation atmosphere at relatively low temperature. Especially, the properties of a small quantity of residual carbon particles, "soot" which suspended the exhaust gas and metallic chlorides seem to have large effect on formation of PCDD/Fs. The present paper introduces the topics concerning PCDD/Fs formation and carbon oxidation. The mechanism of PCDD/Fs formation, the effect of metallic chlorides and various reactions relating to PCDD/Fs formation are reviewed. Further, the mechanism of carbon oxidation at low temnerature and the effect of metallic comnounds are discussed.
According to the pulverized coal injection (PCI) to blast furnace (BF), the fine generation of coke in the lower part of BF especially around raceway has come to an important problem for stable operation. To clarify the mechanism of the fine generation from coke, the pore structure change before and after the gasification experiment has been investigated in previous study. In this study, the open and the closed pore were identified using SEM and the optical microscope observation. In the case of one directional penetration of fluorescent resin, the closed pore was about 5%, however, the actually existing closed pore would be less value, when the resin penetrated from many directions. The graphitized region in the coke was defined, where it was a dense and rectangle area and it included an anisotropic texture such as Mosaic, Fibrous and Leaflet texture. The micro-pore less than 10 μm mainly existed in the graphitized structure. The graphitized structure has low reactivity and remains until high temperature region more than 1500°C. From these results, it was found that the micro-pore less than 10 μm does not relate to the gasification reaction.
A numerical analysis and an experiment were carried out in order to make clear the effects of operational factors on the behaviors of powder in a ladle impinged upon molten steel in a vacuum vessel and the penetration ratio of the powder into molten steel blasted through a top lance in an RH degasser. The powder behavior in the ladle was numerically calculated by the equation of motion of the powder in the field of fluid flow given by numerical analysis of Navier-Stokes equation, and was compared with the results of a cold experiment of the RH degasser. Penetration ratio was estimated from the velocity profile of a free jet of gas and powder blasted through the top lance and verified by the hydrogen behavior of molten steel on which pulverized Ca(OH)2 was blasted in the RH degasser. The resident times of powder in the ladle decrease as the circulating flow rate and the powder size increase. The floating powders are found near down-leg for larger powder size, whereas they are sucked through up-leg when powder size becomes smaller. Therefore, optimal powder size exists to spread powders in the whole free surface of molten steel in the ladle. The yields of powder into molten steel increase with hard blow of gas and by increasing powder size. It is necessary to design the procedure of powder blasting in an RH degasser by considering the effects of cutoff of molten slag on molten steel and the yield of powder as well as the reaction rate between powder and molten steel.
Effects of air bone salts, ultra violet (UV) irradiation, acid rain, impurity elements and TiC precipitated in surface layers of titanium on discoloration of titanium sheets were investigated to clarify the mechanism of discoloration of titanium in atmospheric environments. Air bone salts and UV irradiation did not enhance discoloration of titanium, but synthetic acid rain below pH 4.5 induced discoloration. The discoloration was largely enhanced by precipitation of TiC in surface layers of titanium sheets. It is considered that dissolved Tin+ from TiC by acid rain would deposit on titanium surface, inducing interferential color. Resistance to discoloration evaluated in accelerated discoloration tests was found to be remarkably improved by decreasing precipitation of TiC in surface layers of titanium sheets.
The effects of the isothermal austempering conditions were investigated to clarify the mechanism of retained austenite formation in C-Mn-Si steel. The volume fraction of retained austenite increases with increases in the carbon content in the austenite and the volume fraction of austenite during austempering treatment. As the bainite transformation progresses, the carbon content of the austenite increases along with it, while the fraction of austenite that has not been transformed to bainite decreases. As a result, the volume fraction of retained austenite peaks during the austempering treatment. A high austempering temperature and large initial austenite grain size enhance bainite transformation, and the austempering time decreases when the volume fraction of the retained austenite peaks. A small initial austenite grain size leads to increases in the volume fraction of the retained austenite at the same level of the solute carbon content.
The effects of segregation elements and precipitation elements on the hot-ductility of laboratory melted PC Permalloy were investigated by high temperature tensile test. A mechanism of deterioration of the hot-ductility of PC Permalloy has been also discussed. Decrease in S didn't improve the hot-ductility markedly. Addition of 0.0020.004% B with 23 ppm S was effective on improving the hot-ductility while 0.004% B added alloy with 1618 ppm S showed a ductility trough. Ca added alloy with 18 ppm S showed good hot-ductility and the addition of Mg also improved the hot-ductility. It was suggested by Auger Electron Spectroscopy that hot-ductility was greatly affected by S segregation at grain boundary. The mechanism of improvement in the hot-ductility of PC Permalloy by the addition of B was thought to be attributed to the effect of site-competition segregation effect of B and S at grain boundary. Improvement of hot-ductility by the addition of Ca and Mg were thought to be related with the formation of sulfides which were stable below solidus temperature of PC Permalloy.
Change in the orientation of equiaxed grains and columnar grains during hot rolling was investigated for 16%Cr ferritic stainless steel with about 50% γ-phase from 1050 to 1160°C. In comparison between equiaxed grains and columnar grains 60 seconds after hot rolling, the peak angle of the orientation distribution of columnar grains was from 25 to 30 degrees, whereas that of equiaxed grains was from 40 to 45 degrees that was close to ideal random orientation distribution. In the equiaxed grains immediately after hot rolling, recrystallized grains around <111>//ND nonrecrystallized grains did not have particular orientation. On the other hand, recrystallied grains around <001>//ND nonrecrystallized grains formed <001>//ND orientation. It was cleared that equiaxed grains that did not have particular orientation in cast slab formed gradually <001>//ND orientation by repeated hot rolling.
Fatigue tests in 3%NaCl solution have been performed on stainless steels with different volume fraction of ferritic (α)/austenitic (γ) phases in order to clarify the influence of corrosive environment on crack initiation and growth behavior. Specimens with five different volume fractions of ferritic/austenitic phases, i.e. SUS304 (100%γ), SUS329J4L (50%γ, 28%γ, 12%γ), SUS444 (100%α), were prepared. Cracks were generated predominantly from slip band within γ grains in SUS304 and SUS329J4L and within α grains in SUS444, which was the same behavior as in laboratory air. Fatigue strengths in 3%NaCl solution slightly decreased for all materials compared to those in laboratory air. In early crack growth region, crack growth rates in 3%NaCl solution were faster than in laboratory air. In SEM observation, the brittle feature was seen on the fracture surfaces near the crack initiation site. Environmental effect was also found in high ΔK region for long cracks, where crack growth was enhanced in 3%NaCl solution.
Relation between magnetic properties and fatigue substructure in fatigued SM490YA steel was investigated. Coercive field Hc was sensitive to dislocation density, rapidly increased with increasing number of fatigue cycles at the initial stage and reached in a saturation at the middle and last stages of metal fatigue. Coefficient of magnetic susceptibility, c, increased over all stages of the fatigue tests and was sensitive to degradation in comparison with Hc. These magnetic-structure-sensitive properties would be useful for nondestructive evaluation of fatigue damage in low carbon steels.