Carbon Diffusion Rate Research Articles

フェライト系ステンレス鋼の耐食性

The corrosion behavior of ferritic stainless steels is reviewed. The intergranular corrosion behavior of commercial purity ferritic stainless steels had been believed to be quite different from that of austenitic stainless steels. The heat-treatment that would make ferritic alloys susceptible makes the susceptibility of austenitic alloys to become minimum. Recently, however, it has been shown that the behaviors of both ferritic and austenitic stainless steels can be explained by the chromium depletion theory. The difference in their behavior described above is considered to be due to the very low solubility and rapid diffusion rate of carbon and nitrogen in the ferrite phase compared with the austenite phase. The effects of heat-treatment and alloy composition on the susceptibility to intergranular corrosion are also outlined.The increase in chromium and/or molybdenum contents of Fe-Cr alloys markedly increases the resistance to pitting and crevice corrosion in chloride environments. Ferritic stainless steels are known to be highly resistant to stress corrosion cracking, but when the alloys contain nickel or copper, they become susceptible to stress corrosion cracking. The susceptibility of ferritic stainless steels to hydrogen embrittlement is also discussed briefly.

Variation of some physical properties of steel 40Kh with the deoxidation procedure

1. The melting and deoxidation procedures affect the saturation of steel with active silicon monoxide. The degree of saturation with a surface-active impurity affects the formation of the carbide phase and the ferrite matrix. 2. Deoxidation without silicon increases the solubility of carbon in ferrite, accelerates diffusion processes, and carbon atoms are concentrated to a lesser extent on dislocations. 3. The higher diffusion rate of carbon in steel deoxidized without silicon reduces the difference in the decomposition rate of the supersaturated solid solution in the grain boundaries and within the grains, which makes the steel insusceptible to irreversible temper brittleness. 4. The low saturation of the steel with surface-active impurities of the SiO type leads to smaller concentrations of interstitial impurities on dislocations, which lowers the ductile-brittle transition temperature.

Precipitation of carbon from solid solution in vanadium

Precipitation of carbon from supersaturated solid solution in vanadium has been measured using internal friction, electrical resistance, and transmission electron microscopy. The precipitation rate is shown to fit an equationw = 1 −- exp [−(t/τ)n]. Values ofn near 0.5 imply a one-dimensional diffusional process. An Arrhenius plot of τ is consistent with diffusion of carbon, thus the precipitation rate must largely be dependent on rate of carbon diffusion. Precipitates formed in the range of aging temperatures 100° to 200°C are very finely divided; they can barely be observed by transmission electron microscopy even with prolonged aging.

Dissolution of polycrystalline silicon carbide in liquid silicon

1. A study was made of the dissolution kinetics of polycrystalline silicon carbide in liquid silicon, and the rate constants of the dissolution process were determined. 2. The dissolution rate of silicon carbide in liquid silicon is mainly determined by the diffusion rate of carbon in the latter. 3. The diffusion coefficients and solubility of carbon in liquid silicon were calculated.

Some observations on carbide precipitation and grain boundary migration in a duplex stainless steel

(c) quenched end showed s amp le s quenched without u l t r a sonic energy to be mos t ly m a r t e n s i t e . In con t ra s t , a g r ea t e r volume of aus teni te was t r a n s f o r m e d into mix tu re of cement i te and f e r r i t e in samples t r ea t ed with u l t r ason ic energy , Fig. 4. The i r r ad ia t ion of u l t r a son i c energy on the 4340 s tee l dur ing the t r a n s f o r mat ion thus apparen t ly i n c r e a s e d the diffusion ra te of carbon and lowered i ts ha rdenabi l i ty . Th i s work was supported by the U.S. Army Resea rch Office under Grant No. DA-ARO(D)-31-124(d)

Properties of vacancies in uranium carbide

From measurements of changes in electrical resistivity of uranium carbide due to quenching and annealing, a vacancy formation energy of 1.0 ± 0.1 eV and a migration activation energy of 1.45 ± 0.3 eV were determined which are attributed to C-vacancies. It is shown that in uranium carbide the migration of Cand U-vacancies occurs very likely in the sublattices of carbon and uranium, respectively. For U-vacancies a formation energy between 1.4 and 1.7 eV and a migration activation energy of 2.2 eV are estimated. Furthermore it is concluded that in the lower temperature range (< 1400 °C) the diffusion rate of carbon is determined by a single vacancy diffusion mechanism and in the higher temperature range (> 1400 °C) by a divacancy diffusion mechanism in the carbon sublattice. The diffusion of U-atoms is assumed to be determined by a single vacancy mechanism in the uranium sublattice in the entire temperature range investigated.

Vacuum carbidization of transition metals of groups IV and V

1. We investigated vacuum carbidization of transition metals of group IV (titanium, zirconium, hafnium) and group V (vanadium, niobium, tantalum). We found that the optimal processing temperatures for a high-quality case tightly adherent to the base metal are as follows: titanium, 1300°C; zirconium, 1400°C; hafnium, 1100°C; vanadium, 1300°C; niobium, 1400°C; and tantalum, 1600°C. 2. It was found that the case usually consists of two phases on metals of group V, which agrees with the respective phase diagrams. The thickness of each phase is proportional to the width of the homogenous region on the phase diagram, which indicates approximately equal diffusion rates of carbon in these phases. 3. The characteristic acicular structure of the case on vanadium is due to the crystallographic direction of the diffusion of carbon in V2C.

Influence of alloyed elements on electrolytic boriding of medium-carbon steel

1. All the alloyed elements decrease the rate of increase of the thickness of the borided layer. 2. Elements which increase the binding strength in the austenite lattice have the most important decelerating effect on the diffusion of boron. They decrease the diffusion rate of carbon and deform the lattice of the metalsolvent (γ-Fe) very little. 3. The elements which form stable borides increase the concentration of FeB in the layer and decrease the amount of the second boride (Fe2B). 4. With increasing degrees of alloying of the steel and with increasing saturation temperatures the structure of the boride needles becomes more complex.

Austenizing gray cast iron

1. Austenizing of cast iron is related to the redistribution of silicon between the austenite formed and the remaining ferrite. The most intense redistribution of silicon occurs in ferritic cast irons with large graphite inclusions. The redistribution of silicon between phases is very rapid during the α→γ transformation, because the diffusion rate of silicon in a single-phase matrix is considerably slower than the diffusion rate of carbon. Therefore, we can ascertain that the main factor slowing down the austenizing of ferritic cast iron containing large graphite inclusions is the difficulty with which the ferrite with a high concentration of silicon is carburized. 2. Austenizing of ferritic cast iron, particularly in the case of rapid heating to high temperatures, leads to a microliquation of silicon entirely different from the original one. This affects the formation of the structure of the matrix during subsequent cooling of cast iron and also repeated austenizing. 3. The heating of ferritic cast iron containing large graphite inclusions leads first to the formation of austenite with a concentration of carbon considerably lower than the eutectic concentration (0.1–0.2%). The carburizing of austenite is very slow. Therefore, most of the austenite is transformed into ferrite when cast iron is cooled in air. A considerable part of the austenite is transformed into ferrite even when small samples are quenched in brine. The austenite is rapidly carburized when pearlitic cast iron or cast iron with highly branched graphite inclusions is heated. In all cast irons the high-silicon areas of austenite are carburized slowest — the dendrite branches and the central areas of the colonies. This is due to the fact that silicon increases the activity of carbon and therefore slows down its diffusion into austenite and ferrite. Silicon increases the rate of the diffusion of carbon from ferrite or austenite.

Degassing properties of nickel

The grades of nickel used in the electronic valve industry, containing magnesium, silicon and carbon among other impurities, have been found to have gas contents in the range 1.5 to 16 ml. torr g-1, the average value being about 5.0 ml. torr g-1. The total quantity of gas evolved is limited by the amount of oxygen present in the form of oxides of the impurities, particularly magnesium. These oxides have been found, both in wire and sheet, to be concentrated near the surface. In the case of sheet material 0.150 mm thick, 80% or more of the total gas content is in a skin 15μ thick. The diffusion constant of the gas desorbed from the nickel (largely carbon monoxide resulting from the reaction of carbon with the impurity oxides) has been measured in the temperature range 850° C to 975° C and, comparing the results with the diffusion of carbon in nickel, it is concluded that the evolution rate of carbon monoxide is largely controlled by the rate of carbon diffusion. In nickel which contains no stable impurity oxides the evolution of carbon monoxide proceeds at a slower rate and is controlled by the diffusion rate of the oxygen in the nickel lattice. The differences in evolution rate of carbon monoxide from the various grades of nickel can be explained in terms of the location of the oxygen in the nickel.

Reactions of Oxygen with Pure Tungsten and Tungsten Containing Carbon

Oxygen was made to flow at constant rates through a system consisting of a tungsten ribbon, an ion gauge, and a conventional mass spectrometer. The ion gauge recorded total pressures while the spectrometer recorded partial pressures of O2, CO, CO2, H2O, and other gases. From the way these pressures changed with filament temperature and time, we computed absolute rates at which O2 was converted to WO3, CO, CO2, and O or evaporated as O2. The data show that the sticking probabilities, the adsorption and desorption energies, and the rates of reaction change drastically when all the surface tungsten atoms have made first valence bonds with the adsorbed species; they change again after second valence bonds have been completed. Other conclusions are: (1) On a clean W surface consisting of 311 and 411 planes and for oxygen concentrations less than 4×1014 atoms/cm2, the sticking probability is 0.14, the desorption energy is 4.6 ev, and the evaporation rate is proportional to θ12; for concentrations between 5–9×1014 atoms/cm2, the sticking probability is 0.04, the desorption energy is 2.3 ev, and the evaporation rate is proportional to θ22. (2) In the temperature range 1200–1600°K the rate of formation and evaporation of WO3 increases with p(O2) and with T; the activation energy is 1.1 ev. (3) From 2000–2400°K the rate of WO3 formation increases as p(O2)32 and it decreases with T. (4) Less than about 4% of O2 molecules which strike the hot surface are converted to WO3. Most of the molecules evaporate as O2. Only above 2200°K is any oxygen evaporated as O. (5) No CO or CO2 is generated if the tungsten is carbon free. (6) If the tungsten contains carbon, CO and CO2 are formed if the surface is covered with a layer of oxygen and the temperature is high enough. Every carbon atom that diffuses to the surface is oxidized. The diffusion rate of carbon is proportional to the concentration gradient and increases exponentially with (1/T); the activation energy is 2.2 ev. (7) Tungsten can be made carbon free by glowing it at 2200°K in oxygen pressures near 10−6 mm for 10–60 hr.

A dislocation-attraction model for the first stage of tempering

A dislocation-attraction model governing the flow of carbon atoms is proposed to account for the kinetics of the first stage of tempering in steel. It is predicted that the time exponent of the isothermal reaction should be 1 3 , and the activation energy for the reaction should equal that for diffusion of carbon in martensite. On the basis of previous kinetic data, a good check of the theoretical time exponent is obtained; however, it appears that the activation energy for diffusion of carbon is about 50% higher in martensite than in α-ferrite. The slower rate of carbon diffusion in martensite is explained by the availability of paths of lower barrier energy in the b.c.c. ferrite than in the b.c.t. martensite.

鋼の滲炭機構に関する研究（第1報）滲炭過程

Under the carburizing conditions as described below, all the phenomenan in this carburizing process were explained from the increased weight and by examining the surface and the section of the specimen with the electron diffraction method and a microscope. The specimen is the uniforming of carbon concentration between the surface layer and the middle part of the section arrives as soon as possible. The carburizing gas is composed of 65% nitrogen and 35% carbon monooxide, so that the surface reaction rate which is later than diffusion rate of carbon determines the carburizing rate. The result is as follows : until the speciemen is saturated with carbon from the start of carburization, the carburizing rate decreases in inverse proportion to the increased carbon concentration. As soon as the specimen is saturated at Acm line, the graphite precipitates and grows quickly on the surface of specimen. The mechanism of the graphite deposition was discussed from the graphitization theories on amorphous carbon and cast iron. In the interior of specimen, Fe3C phase grows at the grain boundary after saturation of carbon. However, when the carburizing further proceeds, Fe3C phase disappears gradually with the occurence of graphitization. On the other hand, on the surface of the specimen, after saturation of carbon, carbon is deposited, and the depositing rate becomes larger in consequence of the increased surface area by the diffusion of iron in specimen to the deposited carbon.

Diffusion Rates of Carbon in Iron-Molybdenum and Iron-Tungsten Alloys

Measurements are reported of the influence of tungsten and molybdenum on the diffusion rate of carbon in face-centered iron at 1000\ifmmode^\circ\else\textdegree\fi{}C. Both of these elements slow down the diffusion of carbon, the influence of tungsten being more than twice that of molybdenum. These results are compared with previous measurements on iron-cobalt alloys and other known data. No connection seems to exist between the variation in lattice parameter of face-centered iron and the variation in diffusion rate of carbon.