This is a report on discussions at VAMAS-LCF Committee of The Iron and Steel Institute of Japan. The aim of VAMAS Low Cycle Fatigue project is to examine the effects of test variables on high temperature fatigue tests. In Japan ten laboratories participated in the round robin tests. Based on the discussion of international and domestic test results at the committee, solutions were sought to certain problems on high temperature low cycle fatigue testing and recommendations for methods of testing were made. Investigation was made of the following. One is the effects of a spot-welded thermocouple on test results and alternative methods for temperature measurement, and the other is physical meaning of the number of cycles to failure in high temperature low cycle fatigue.
Superplastic deformation, especially in quasi-stable fine equiaxied grain structures, is accompanied by grain growth whose rate exceeds that which can occur without deformation. The deformation induced component of the grain growth stabilizes the deformation itself through an increase in the flow stress. An empirical expression for the grain growth is demonstrated which describes the behavior of microduplex and second-phase dispersed alloys. Finally, a new deformation model of superplasticity is proposed to explain the grain growth.
Surface tensions of liquid iron-nickel-sulphur ternary alloys as a function of the composition and the temperature have been determined by the oscillating droplet method combined with electromagnetic levitation technique. The natural frequency of the oscillating droplet was evaluated using a Fourier analyzer, and the influence of magnetic field strength on the surfac tension was also considered. The temperature dependence of surface tension for the liquid iron and iron-nickel melts containing the surface active element sulphur were found to be positive. This effect may be due to the change of the sign in the entropy terms for the bulk and surface phase caused by the reduced mobility of the surface atoms. Furthermore, iso-surface tension diagram for the iron-nickel-sulphur ternary system at 1873 K is given and discussed.
A two dimensional heat balance model has been formulated to predict the temperature profile in the sub-hearth of an iron blast furnace. The partial differential equation, constituting the model, is solved along with the boundary conditions using the Finite Element Method. From the predicted location of temperature isotherms corresponding to 1150°C, the two dimensional profile of the salamander and its depth of penetration are predicted. Effects of cooling conditions at the side walls and at the bottom of the sub-hearth on the salamander penetration and temperature profile have been discussed.
In this report, oxygen potential, PO2*, at the interface between slag and molten iron of high carbon concentration is estimated from the experimental results of the authors' previous kinetic study of phosphorus reaction. The method of the estimation of PO2* is based on an assumption that a quasi-equilibrium is established in the slag-metal system at the time point when the phosphorus reaction changes its direction from dephosphorization to rephosphorization. The estimated PO2* ranges in the order of 10–11–10–10 Pa. The PO2* increases with the increase in Fe3+/(Fe2++Fe3+) in the slag and decreases with increasing the intensity of mechanical stirring employed in the experiments. The oxygen potential of the bulk slag, PO2, s, is calculated with a regular solution model, and the comparison of the bulk slag's PO2, s with the interfacial oxygen potential, PO2*, shows that PO2* is 2-3 order or magnitude smaller than PO2, s. The slag-metal interfacial oxygen potentials, PO2*, obtained from the experimental data, are also compared with those calculated from a mathematical model which simulates the simultaneous reactions in the gas/slag/metal reaction system including Ar-O2 atmosphere. The calculated interfacial oxygen potentials agree with the experimental data reasonably well.
Kinetics of decarburization and oxygen absorption of molten iron of low carbon and oxygen concentrations has been studied. The decarburization and oxygen absorption experiments were performed by blowing Ar-O2 mixture (oxygen partial pressure 1.0×10–5<POs(atm)<5.5×10–2 and gas-flow rate 1000 (and 1900) Ncm3/min) onto the melt surface. The rates of decarburization and oxygen absorption increase with increasing oxygen partial pressure PO2>5×10–3 atm). At oxygen partial pressures of PO2<1.0×10–4 atm, the oxygen concentration is kept almost constant. The total reaction rate of oxygen with the melt (=decarburization rate+oxygen absorption rate) is larger than the rate of oxygen absorption without decarburization and smaller than the rate calculated from a reaction model of oyxgen diffusion in the gas phase. It is presumed that formation of oxide film on the melt surface has an influence on the decarburization and oxygen absorption rates. A mathematical model is proposed to explain the rates of decarburization and oxygen absorption.
The decarburization in the BOP (Basic Oxygen Process) has been modeled. The model was formulated based on the industrial data relating the temperature and the carbon content of the bath. Several formulae for calculating the variation of carbon and oxygen contents in the liquid metal as well as carbon monoxide composition in the gas phase were derived as a function of blow time, oxygen blow rate, gas purity and melt weight; these derivations were done for low (<0.3%) as well as high (>0.3%) C levels. The model was applied to a typical BOP operation and the calculated results show good agreement between predicted values and the actual operating values. It is expected that this model can be used as a reference for analyzing various steelmaking processes.
Oxidation resistance of TiAl coupons coated with IBED Si3N4 films of thicknesses of 0.5, 1 and 2 μm has been investigated by the cyclic oxidation test at 1300 K in a flow of purified oxygen at atmospheric pressure. The application of the nitride film of 0.5 μm thickness results in sufficiently good oxidation resistance; the protective scale is maintained for at least 30 cycle (600 h). The excellent oxidation resistance is attributable to the formation of a layer rich in Al2O3 and silicon compound beneath the outer TiO2 layer during the early oxidation period. However, this effect decreases as the film thickness increases. After around 10 cycle oxide scales spalled off over relatively large areas. The local fracture of the coating films during the early oxidation period was considered to be responsible for their limited effect.
Effect of dissolved oxygen concentration on fatigue crack growth behavior of a low alloy pressure vessel steel A533B CI. 1 in high temperature water was investigated. Fatigue tests were conducted at 561 K in water, of which DO concentration ranged from 1 to 8000 ppb including of BWR coolant condition. In a range of DO concentration below 1000 ppb fatigue crack growth rates (da/dN) showed independence on DO concentration, while above 1000 ppb striking DO dependence was observed, i.e. da/dN increased with increasing DO concentration. Corrosion products formed on the fracture surfaces were analyzed by an X-ray diffractometer. Magnetite was formed at all the levels of DO concentration, while hematite above 200 ppb and its X-ray diffraction intensity increased with increasing DO concentration. Effect of DO concentration on corrosion potential of the material was also investigated and discussed with the environmentally assisted cracking by DO concentration of fatigue crack growth rate in high temperature water from the viewpoint of electrochemistry.
Ti-47mol%Al alloy powder produced by plasma rotating electrode process (PREP) was consolidated by HIPing at 1423 K and 176 MPa for 10.8 ks. Isothermal hot forging (IHF) with the small samples cut from the as-HIPed compact at 1223 K and an initial strain rate of 3.8×10–4 s–1 up to 78% reduction in height was carried out. This IHF process was considered to proceed by super plastic flow. A large pancake (approximately 25 mm thick with 140 mm diameter) without any crack was successfully obtained from the as-HIPed large compact (approximately 60 mmφ with 100 mm height) by the similar 78% IHF process. The results of the tensile strain-rate-change test and elongation to failure test of the specimens machined from the pancake indicated that this material exhibits fine grain superplasticity at elevated temperature higher than 1223 K. The maximum elongation to failure value of 398% was obtained at 1323 K and an initial strain rate of 2.8×10–4 s–1.
We have investigated the effects of deformation temperature in the range from room temperature to 873 K on as-deformed and post-aging microstructures, and tensile properties after aging, for Ti-15V-3Cr-3Sn-3Al. In solution-treated conditions, slight cold deformation tends to form planar slip bands composed of extremely localized straight dislocations in β phase, because of the restriction of both cross-slip and secondary slip. The planar slip bands consequently cause the directional microstructure with α precipitates to align on the bands after aging. The tendency for straight dislocations obstinately persists even after a heavy cold deformation of about 60%. A directional distribution of rather coarse α precipitates is thereby brought about after aging in a heavily cold deformed material as well. On the other hand, cross-slip and secondary slip are both activated with increasing deformation temperature, to diminish the microstructural directionality. A fairly non-directional uniform distribution of finer α precipitates is attained through deformation at 873 K. The balance of strength and ductility of Ti-15V-3Cr-3Sn-3Al can be pronouncedly enhanced by the microstructural improvement. Cross-slip promoted at elevated temperatures was expected to be due to not only the thermal activation of itself, but also the reduction of athermal ω phase as its inhibitor.
The f.c.c. to b.c.c. martensitic transformation in small coherent Fe-Co particles has been induced by applying external stress to single crystals of a Cu-Fe-Co alloy without activating crystal dislocations. Orientation of the stress-induced martensite variants is sensitively dependent on the direction (f or f) and sense (tension or compression) of the applied stress. The experimental results can be well explained by postulating that the applied stress helps the lattice shear deformation involved in the f.c.c. to b.c.c. lattice change.