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1.
A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of iron, iron-0.48 wt pct molybdenum and iron-1.16 wt pct molybdenum specimens which were equilibrated with a series of methane-hydrogen gas mixtures of constant composition. The equilibria investigated involved the austenite phases of the alloys at 783, 813 and 848‡C. The experimental results permit direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of the enthalpy and entropy of solution of carbon. The results are compared with the experimental measurements of a number of other investigators. The results are in excellent agreement with those of Smith and Schenck and Kaiser for the Fe-C system at 800‡C, and indicate -H C /M values of 9700 ± 500 cal/mole for pure Fe, 10,030 ± 500 cal/mole for an Fe-0.48 wt pct Mo alloy, and 10,150 ± 500 cal/mole for an Fe-1.16 wt pct Mo alloy. The effect of molybdenum in austenite is to decrease the activity coefficient of carbon in austenite.  相似文献   

2.
A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of α-Fe, Fe-0.48 wt pct Mo and Fe-1.16 wt pct Mo specimens which were equilibrated with a series of methane-hydrogen gas mixtures. The equilibria investigated involved the ferrite phases of the alloys between 682 and 848‡C. The experimental results permitted direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of other thermodynamic functions, without the necessity for any correction factors. The results have been compared with the experimental measurements of a number of other investigators. In ferrite, the partial molar enthalpy and entropy of solution of carbon are found to be 26,800 cal/mole, (112, 130 J/mole), and 30.59 cal/K-mole, (127.99 J/K-mole) respectively at temperatures below about 727‡C. Above this temperature, the values are 25,200 cal/mole and 29.13 cal/K-mole, respectively. The addition of molybdenum results in an increase in these properties below 727‡C and a decrease in the values above 753‡C, and the changes are found to be proportional to the molybdenum content. The solubility of carbon in α-Fe is found to be 0.0176 wt pct at the eutectoid temperature. Molybdenum increases the solubility relative to the Fe-C system at temperatures above the eutectoid and decreases it below the eutectoid.  相似文献   

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Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Somein situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M3C filMs upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe3C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The sit11Ation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which o°Curs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can o°Cur in the same steels, depending on the tempering conditions.  相似文献   

6.
An original method of evaluation of the cluster population in carbon martensite has been developed. Using this method, it is shown that Kurdjumov’s model of carbon redistribution within the different octahedral site sublattices can quantitatively account for both observed normal and abnormal tetragonality in carbon martensite. It is also shown that the existence of the internal strains in martensite constitutes a necessary and sufficient condition for the energetic preference of tetrahedral over the cubic lattice. The presence of the residual tetragonal distortion in the quasi-cubic phase of k-martensite is associated with the presence of the mixed clusters formed of the atoms belonging to O c sublattice as well as to remaining ones. By using a computer simulation of the dynamical behavior of carbon martensite approaching the thermodynamical equilibrium, it was found that the ultimate state of this system is strongly beyond the thermal equilibrium. Even after long-term aging, the free energy is far beyond the minimum value allowed for this system. The reason for such a behavior and the possible aging processes proceeding in this system are discussed at the molecular level. All of the ordering parameters are affected by the aging process. The evolution proceeds in the distinctly different time intervals for different parameters. At first, the long-range ordering parameter that determines the tetragonality of martensite evolves and reaches the stable value. In the next stage, the formation and then disintegration of two-particle clusters occurs. Disintegration of two-particle clusters coincides with the stage when three-particle cluster formation occurs at a high rate. Threeparticle clusters also disintegrate when some time elapses. The same pattern repeats regarding four-, five-, six-, seven-, and eight-particle clusters. To simplify the calculations, the nine-particle clusters are assumed to be the largest possible and are identified with an existence of superstructure. The formation of 100 pct of nine-particle clusters with no contribution of free atoms in an alloy ceases all aging processes. The evolution of these processes is illustrated graphically in the time range from 16 seconds to 1500 years, as estimated on the basis of experimental data.  相似文献   

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The thermodynamics of carbon and oxygen solutions in manganese melts is studied. An equation for the temperature dependence of the activity coefficient of carbon in liquid manganese is obtained (γ C(Mn) 0 = ?1.5966 + (1.0735 × 10?3)T). The temperature dependence of the Gibbs energy of the reaction of carbon dissolved in liquid manganese with the oxygen of manganese oxide is shown to be described by the equation ΔG T 0 = 375264 ? 184.66T(J/mol). This reaction can noticeably be developed depending on the carbon content at temperatures of 1700–1800°C. The deoxidation ability of carbon in manganese melts is shown to be much lower than that in iron and nickel melts due to the higher affinity of manganese to both oxygen and carbon. Although the deoxidation ability of carbon in manganese melts increases with temperature, the process develops at rather high carbon contents in all cases.  相似文献   

9.
The thermodynamics of carbon in manganese and ferromanganese melts were studied to predict the refining limit of carbon during the decarburization of molten ferromanganese. The equilibrium carbon content in a Mn-C melt was determined by the C-CO equilibrium in the presence of pure solid MnO at 1673 to 1773 K. The activities of manganese and carbon in the Mn-C melt were then calculated from the experimental results, the equilibrium constant for the reaction, and the Gibbs-Duhem equation integrated by the Belton-Fruehan treatment. The standard free-energy change of carbon dissolution in the manganese melt was determined to be 41,700 — 59.6 T J/g · atom, with the standard state taken as 1 wt pct carbon in solution. The effect of iron on the activity coefficient of carbon in ferromanganese was determined by measuring the carbon solubility in Mn-Fe melts. The first- and second-order interaction parameters between carbon and iron in ferromanganese melts were determined. The activity coefficient of carbon in the ferromanganese alloy melt can be expressed as
where the interaction parameters are independent of temperature in the temperature range of 1673 to 1773 K. The thermodynamic parameters determined in the present study could predict the equilibrium carbon content in the ferromanganese melt accurately for various melt compositions and CO partial pressures.  相似文献   

10.
Supersaturation and precipitation of cementite and/or graphite in austenite due to electrotransport were studied. Two types of experiments were made with alloys of relatively high initial carbon concentration (1.3 wt. pct); one in the two-phase (austenite plus cementite) region at 850° and the other in the single phase (austenite) region at 927° C. In the two-phase experiments the formation of cementite and/or graphite was observed at the high-carbon end of the specimen. In the single-phase case, only a continuous layer of graphite was formed at the cathode end of the specimen. The present research suggests that electrotransport can be used to investigate processes involving supersaturation, such as precipitation from solid solution. T. Okabe, formerly Postdoctoral Fellow, Department of Metallurgical and Materials Engineering, University of Florida, Gainesville, Florida  相似文献   

11.
The effect of prior deformation on the processes of tempering and austenitizing of lath martensite was studied by using low carbon steels. The recrystallization of as-quenched lath martensite was not observed on tempering while the deformed lath martensite easily recrystallized. The behavior of austenite formation in deformed specimens was different from that in as-quenched specimens because of the recrystallization of deformed lath martensite. The austenitizing behavior (and thus the austenite grain size) in deformed specimens was controlled by the competition of austenite formation with the recrystallization of lath martensite. In the case of as-quenched (non-deformed) lath martensite, the austenite particles were formed preferentially at prior austenite grain boundaries and then formed within the austenite grains mainly along the packet, block, and lath boundaries. On the other hand, in the case of lightly deformed (30 to 50 pct) lath martensite, the recrystallization of the matrix rapidly progressed prior to the formation of austenite, and the austenite particles were formed mainly at the boundaries of fairly fine recrystallized ferrite grains. When the lath martensite was heavily deformed (75 to 84 pct), the austenite formation proceeded almost simultaneously with the recrystallization of lath martensite. In such a situation, very fine austenite grain structure was obtained most effectively.  相似文献   

12.
The influence of boron on the isothermal decomposition of Fe-Ni6-C0.12 (wt pct) steels has been investigated. The isothermal γ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history. Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied. Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates were identified as Fe23(B, C)6 (FCC structure with a ≈ 10.6?). The grain boundary Fe23(B, C)6 were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe23(B, C)6 precipitates with respect to the γ → proeutectoid ferrite reaction is discussed.  相似文献   

13.
The influence of boron on the isothermal decomposition of Fe-Ni6-C0.12 (wt pct) steels has been investigated. The isothermal γ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history. Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied. Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates were identified as Fe23(B, C)6 (FCC structure with a ≈ 10.6Å). The grain boundary Fe23(B, C)6 were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe23(B, C)6 precipitates with respect to the γ → proeutectoid ferrite reaction is discussed.  相似文献   

14.
The recent extensive data for the variation with temperature and composition of the activity of carbon in austenite has been analyzed using a quasichemical model for interstitial solid solutions. The most important parameter in the theoretical activity equation is the pairwise binding energy Δε between two nearest-neighbor solute atoms. A computer fitting method has shown that the new activity data for austenite, which spans a temperature range from 900° to 1400°C and compositions ranging from very dilute up to the limiting composition of the phase boundary, are compatible with the theoretical model with a Δε-value of -2 kcals per mole which is independent of both temperature and composition.  相似文献   

15.
The effects of second-phase particles formed by the addition of vanadium, nitrogen, and aluminum on the austenite grain coarsening behavior of 0.15 pct carbon steels were studied. The oxidation and etching technique has been adopted to reveal the prior austenite grain boundaries. The specimens were austenitized at intervals of 50°C within the range of 900°C to 1150°C under high vacuum (<10−4 torr) for half an hour, toward the end of which they were oxidized for about one minute by introducing oxygen at about 250 mm Hg to reveal the grain boundaries, and then quenched into iced water. The variation of prior austenite grain size with temperature in these steels indicates that vanadium carbonitride, V(C, N), is much more effective in austenite grain refinement than vanadium carbide, VC, at all temperatures. The effect of vanadium carbonitride in austenite grain refinement is more or less the same as that of aluminum nitride. AlN, at temepratures below 1000°C, but this effect of vanadium carbonitride in austenite grain refinement decreases with increasing temperature. Above 1000°C, aluminum nitride is a much better grain refiner than vanadium carbonitride. The presence of the V (C, N) and AlN particles in the same steel causes moderate grain growth of austenite. MD. Mohar Ali Bepari, formerly with the Department of Metallurgy, The University of Sheffield, Sheffield, England, is Associate Professor of Department of Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh.  相似文献   

16.
利用Gleeble-3800热/力模拟试验机,研究了1种低碳微合金钢的形变奥氏体连续冷却转变行为以及终轧温度、终轧变形量对相变组织形态的影响。结果表明,实验钢在研究的冷速范围内均可得到一定量的贝氏体组织。随着冷速的增加,冷速达到50℃/s时,贝氏体组织由粒状贝氏体逐渐过渡到板条状贝氏体。在较快的冷速下,随终轧温度的降低或终轧变形量的减小,贝氏体转变开始与结束温度均降低,获得的贝氏体具有不同的组织形态。  相似文献   

17.
The effect of composition on the dynamic recrystallization behavior of plain carbon steels has been examined in the temperature range 850 to 1300 °C and the strain-rate range 6 × 10-6 to 2 × 10-2 s-1. With increasing solute content, the onset of dynamic recrystallization is delayed and the rate of recrystallization is decreased. At 1000 °C the elements can be ranked in order of increasing effectiveness as C, Ni, Mn, Si, P. At higher temperatures the effect of composition is complicated by its effect on grain coarsening, which itself postpones the onset and slows the rate of recrystallization. Similarly, for steels in which precipitation may occur, a fine-grain structure can promote the earlier onset and faster rate of recrystallization. Thus, out of an examination of the effect of composition comes an appreciation of the influence of initial grain size. Another factor which can complicate any explanation of the compositional dependence of dynamic recrystallization is grain-boundary segregation, which is probably responsible for the strong retarding influence of phosphorus.  相似文献   

18.
The scavenging effect of precipitated austenite in a low carbon, commercial Fe-5.5Ni cryogenic alloy was investigated through observation of the dissolution of cementite precipitates during intercritical tempering and study of the associated change in Charpy impact toughness. Cementite precipitates initially located along prior austenite grain boundaries were gradually dissolved into reverted austenite as the intercritical tempering proceeded. The austenite tends to form at or around the carbide particles and may be catalyzed by their presence. The Charpy impact energy is changed through both a decrease in the ductile-brittle transition temperature and an increase in the upper shelf energy. The latter effect is specifically associated with the dissolution of the carbides which act as preferential void nucleation sites in the untempered alloy.  相似文献   

19.
The resolved shear stress and strain-rate sensitivity of Fe-Mo and Fe-Re alloys, well within the solid solution range, were measured versus temperature and compared with previous results for Fe-Ti alloys. The strain-rate sensitivity-temperature curves shifted systematically to lower temperatures as solute was added. The resolved shear stress was divided into two parts, a dynamic part which is a function of strain-rate and a static part which is not. The strain-rate sensitivity data were inverted to give the dynamic part of the flow stress and this was subtracted from the resolved shear stress to give the static part of the flow stress. Significant solid solution softening occurred in the dynamic part of the flow stress at 166 and 196 K. This is due to intrinsic effects and not to scavenging of im-purities. The static part of the flow stress is made up of two parts, a peaking effect super-imposed on normal solid solution hardening.  相似文献   

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