Spontaneous combustion property of low rank coal, an important issue in coal science, has been substantially studied. However, there are few studies discussing quantitative chemical structural changes of coal during its oxidization under 80°C.
In the present study, low rank coals of X and Y were oxidized in an adiabatic vessel where their temperature was increased due to spontaneous heating. Generated gases of H2O, CO2, and CO were analyzed by gas chromatography during the oxidization. As a result, coal Y, which was more easily oxidized than coal X, showed higher generation rate of CO2 and CO gas than coal X. Some of H2O generated during the oxidation must have been remained in coal as adsorbed water since the amount of water in the oxidized coal was higher than that before oxidation.
13C MAS NMR spectra of coal X and Y indicated that and chemical reaction producing carboxyl acid in coal Y is more activated than that in coal X. Furthermore, the number of aromatic carbon was reduced during the oxidization. It implied that the aromatic carbon might contribute to oxidization reaction. Coal Y before oxidation had smaller number of aromatic carbon per one cluster than coal X, while the structural parameters of aliphatic carbons in coal X and Y were almost the same. It is considered that smaller number of aromatic carbons more destabilize carbon radicals to generate “active” species. Such active carbon radicals are suggested to also react with oxygen in the atmosphere.
Negative effect from low coke rate operation at cohesive zone is obvious because it makes thinning of coke slit thickness. Correct knowledge about gas permeability of cohesive layer is becoming more and more important. In order to precisely understand cohesive behaviour, a softening and melting simulator under rapid heating and quenching conditions was applied for clarify a determinant factor of gas permeability behaviour. To focus on softening and melting behaviour, granulated slag particle bed layer without iron oxide was prepared as packed bed sample layer can show softening and melting. The packed bed slag samples in graphite crucible were rapidly heated up to 1200°C, and then gradually heated up to 1500°C with 10°C/min under inert gas atmosphere and 0.1 MPa load. Gas pressure drop and shrinkage degree of the sample layer were measured during the softening and melting test, and quenched sample was made at certain temperature when the maximum gas pressure drop was measured. The CT observation of the quenched sample provided 3D shape information of gas path shape in sample packed bed. Gas pressure drop was estimated with fanning's equation with the gas path information. The estimation values were shown positive correlation with measured maximum pressure drop. The CT observation also gave triple line length among molten slag, graphite, and gas. Combination the triple line length and molten slag surface tension could use for evaluation of static force balance when maximum pressure drop obtained.
Improving operation performance of blast furnaces is necessary to reduce CO2 emissions and pig iron costs. One of the important factors for the operation is keeping gas permeability well in the blast furnace. Coke fines affect gas permeability and the operation condition become worse, therefore clarification and prediction of the generation behavior of coke fines in blast furnaces are desired.
In the present study, first of all, new evaluation method was developed to quantify coke abrasion behavior. And then, we proposed the prediction expression to estimate the amount of coke fines based on the results of abrasion experiments using new method. Finally, the distribution of coke fines in the blast furnace was numerically simulated by coupled 2D blast furnace model and Discrete Element Method (DEM) using proposed equation.
The results are summarized as follows:
1) The influential factors of coke abrasion were mechanical conditions like shear distance or compressive stress, and coke quality like strength (Drum Index, DI) or porosity. The amount of coke fines increased by rising shear distance, compressive stress and porosity or decreasing DI.
2) According to numerical simulation using these models, coke fines generated around peripheral area in lower shaft, belly and bosh. In addition, the total amount of coke fines increased with the decrease of DI. These results are in accordance with the conventional knowledge.
The distribution of coke fines in the blast furnace became predictable. This evaluation will lead optimum burden distribution and ideal coke quality to achieve highly efficient operation of blast furnaces.
We investigated the reductive gasification of phosphorus compounds. As the first step, the partial P2 gas pressure (PP2) was calculated by using the data of FactSage7.2 for reductive gasification of Ca3(PO4)2, FePO4, P4O10. It was suggested that the reductive gasification of P4O10 was easiest. Moreover, CO reduction of P4O10 was more effective under 830°C and H2 reduction of P4O10 was more effective over 830°C. The weight change of phosphorus compound tablets were measured by using thermal balance furnace under 900°C in N2 or CO-N2 gas atmosphere. The gasification and reduction of Ca3(PO4)2 and Ca5(PO4)3OH tablets were not confirmed. On the other hand, the weight of red phosphorus tablets decreased from 400°C in N2 gas atmosphere. The gasification occurred. The weight of FePO4 tablets slightly decreased from 700°C in CO-N2 gas atmosphere. The gasification or reduction was suggested. The reduction of phosphorus compound in iron oxide tablets was carried out at 700°C, 900°C, 1000°C for 1 h in H2-CO2-N2 gas atmosphere. The removal ratio (Rp) was estimated by measuring concentration of Fe and P before and after reduction. The Rp of red phosphorus compound tablets, Ca5(PO4)3OH, Ca3(PO4)2 was 57%, 9%, 0%, respectively. In the red phosphorus tablets, P might be gasified.
A selective oxidation of alloying elements such as Si and Mn is generally important for the zinc wettability of high strength steels in the process of hot dip galvanizing. In the present paper, the effect of both the temperature and the dew point of recrystallization annealing process on the selective surface oxidation behavior of 2 mass% Mn added steel were investigated by glow discharged spectroscopy (GD-OES), secondary electron microscopy (SEM) and transmission electron microscopy (TEM). When the annealing temperature was below 973 K, the Mn internal oxidation did not occur and the amount of Mn external oxidation increased with increasing temperature regardless of the dew point. On the other hand, when the annealing temperature was more than 1023 K, both the external oxidation and the internal oxidation were observed. Especially, the internal oxidation occurred actively and the external oxidation is suppressed when the dew point is more than 263 K. Wagner’s theory could explain the phenomenon which the internal oxidation was occurred in the wider range of dew point as the annealing temperature increased.
The influence of pro-eutectoid cementite on fatigue crack growth behavior was investigated using various pearlitic steels containing from 0.64 to 1.21 mass%C. The fatigue crack growth rates of the hypo-eutectoid and eutectoid pearlitic steels hardly changed, while that of the 1.21 mass%C steel having a large amount of pro-eutectoid cementite (θ) was accelerated especially in the high stress intensity factor region. Scanning electron microscope (SEM) observation revealed that fatigue fracture surface in the 1.21 mass%C steel more frequently contained islanded brittle fracture surfaces than that in other steels. In the 1.21 mass%C steel, the total area fraction of brittle fracture surface was notably increased with an enhancement in maximum stress intensity factor (Kmax) due to crack extension. More detailed SEM fractographies were performed comparing between before and after etching in order to identify microstructures beneath the brittle fracture appearances on the fatigue fracture surface of the 1.21 mass%C steel. As a results, it was suggested that the pro-eutectoid θ was involved in the formation of brittle fracture. Based on these investigations, the accelerated fatigue crack growth behavior of hyper-eutectoid steel was discussed in terms of static brittle fracture induced by pro-eutectoid θ near the crack tip.
Hall-Petch slopes for each grain boundary of Fe-based alloys were measured by nanoindentation tests. Pop-in phenomenon was observed in a load-displacement curve during nanoindentation near a grain boundary, associated with a slip transfer across the boundary. A critical stress at which pop-in phenomenon takes place and a distance between the grain boundary and the indentations satisfied the Hall-Petch relationship. The Hall-Petch slope depended on alloying element and its amount, which is closely related to a segregation of the element at grain boundaries. The slope also varied with the type of grain boundary. The combination of activated slip systems in neighboring grains strongly influenced the slope.
In this paper, fatigue tests of two different structural steels utilizing DIC technique were carried to evaluate fatigue crack initiation and propagation properties based on the local strain. At the bottom of the notch, hysteresis loop was observed after plastic deformation in the first cycle. The maximum strain and strain range of the hysteresis loop depended on the steel strength. In addition, the fatigue crack initiation life estimated from the strain range was almost in agreement with the fatigue test results. In the strain measurement around the fatigue crack, reasonable results were obtained by removing the fatigue crack part from the analysis area of DIC. The opening / closing behavior at each position of the fatigue crack could be evaluated by directly measuring the opening displacement. The position of the maximum strain near the fatigue crack moved according to the crack closure, and the peak position corresponded to the tip of open crack. The strain range at the tip of fatigue crack showed a good correlation with the fatigue crack propagation rate.
In this study, we investigated the strain-history dependence of ductile fracture behavior in ferrite–pearlite (FP) two-phase steels. The mechanism of this dependence was analyzed using finite element (FE) simulation. The orthogonal strain path changes were subjected to single-phase ferrite steel, two types of FP steels with different pearlite fraction, and single-phase pearlite steel. After the tensile pre-deformation, secondary tensile deformation was applied in the same direction or orthogonal to the first direction. The results showed that the elongation was higher when the secondary deformation was applied in the direction orthogonal to the pre-deformation, compared with that in the no-path change case. The elongation improvement due to the path change was greater with higher pre-deformation and pearlite fraction. The mechanism was investigated by analyzing the localization behavior of plastic deformation in the microstructure via FE simulation using a three-dimensional heterogeneous microstructure model. The simulations showed that the deformation path change in FP steel suppressed local accumulation of damage by changing the strain localization region owing to strength heterogeneity between the two phases.
The residual stresses at a circular punched end face in tempered martensitic high-strength steel sheets were investigated using triaxial stress analysis via X-ray diffraction. The maximum principal stress and its direction were calculated from the measured nine stress components. The relationship between the directions of the maximum principal stress and hydrogen cracks was verified by generating hydrogen cracks on the punched end face in the same specimen using cathodic hydrogen charging. The direction of the cracks was perpendicular to that of the maximum principal stress. This result indicates that hydrogen embrittlement at the sheared end face is caused by the maximum principal stress. Moreover, the distribution of the residual stresses toward the thickness direction and the relationship between residual stresses and tensile strength of the specimens were investigated. The maximum principal stress on the punch side was lower than that on the dice side. Unlike the maximum principal stresses, the normal stresses did not increase monotonically with the tensile strength of the specimens. Therefore, it was concluded that investigating the maximum principal stress at any area between the dice side and a line located midway from the end face and dice side is crucial for considering the hydrogen embrittlement criteria.
The crack propagation behavior of nitrocarburized JIS SCM420 steel was investigated in a rotating bending fatigue test, focusing on crack stagnation behavior. The crack had clearly stagnated at a length of approximately 200 µm at the fatigue limit of 400 MPa, indicating that crack stagnation could control fatigue strength. The crack stagnation cannot be explained only by the change of the stress intensity factor, since the calculated value in this process increases with the depth from the notch. A large amount of plastic strain was observed around the tip of the crack by EBSD analysis. Because the stagnated position corresponds to the critical depth between the hardened and unhardened regions formed by nitrocarburizing, it can be easily deformed. Therefore, it is inferred that the crack stagnation nitrocarburized JIS SCM420 steel can be explained by a plastic-induced closure mechanism.