Amount Of Carbon In Solution Research Articles

Excess Solute Carbon and Retained Tetragonality in Tempered Fe-0.6C-1Mn Martensite and the Effect of Silicon Addition

The tetragonality and carbon distribution in tempered Fe-0.6C-1Mn martensite were investigated by X-ray diffraction and atom probe tomography to elucidate strain relaxation in the tetragonal lattice during tempering and its relationship with the solubility of excess carbon in martensite. Even though tetragonality (c/a) decreased with an increase in the tempering temperature, it persisted at low levels up to 400 °C. Si addition suppressed the decrease in tetragonality at 400 °C by inhibiting recovery in the dislocated matrix. Such persistence implies that dislocation migration is crucial for the complete release of tetragonal lattice strain at such a temperature, in addition to the decrease in the amount of solute carbon in martensite. A low level of tetragonality was observed for martensite containing carbon in the solid solution below the critical value of ~ 0.2 mass pct, at which a bcc structure was predicted. The amount of solute carbon after tempering was linearly correlated with tetragonality in the solute carbon content range of 0.07 to 0.6 mass pct, and the correlation coefficient was similar to those for as-quenched auto-tempered martensite and bainitic ferrite; these results indicate that the amount of excess carbon is simply determined by the amount of tetragonal lattice distortions remaining after carbide precipitation and recovery.

Influence of Microstructure on Yield Strength of Ferrite–Pearlite Steels

The mechanical properties of two-phase steels are mainly controlled with the volume fraction of the harder phase. In the previous reports, however, some different relationships between the mechanical properties and pearlite volume fraction have been demonstrated for ferrite–pearlite steels. This means that the relationship must be changed sensitively to experimental conditions so that the influence of the volume fraction of pearlite on the mechanical properties of hot-rolled ferrite–pearlite steels were experimentally investigated using Fe–C alloy with variation of carbon content.Yield strength of the air-cooled samples after hot-rolling gradually increased with the increase of pearlite volume in lower volume fraction range while dramatically in higher range. The water-cooled samples exhibited higher yield strength than that of the air-cooled samples especially in the lower pearlite volume fractions. These results indicated that the mechanical properties of the ferrite–pearlite steels having the lower pearlite fraction are determined by properties in ferrite area. This is attributed to sparse distribution of pearlite; almost all ferrite grains are surrounded by the other ferrite grains.The yield strength of ferritic steels consisted of grain refinement strengthening, precipitation strengthening by carbides and solid-solution strengthening of carbon. The solid-solution strengthening varied with change in the amount of carbon in solution and grain refinement strengthening also raised with increase in Hall–Petch coefficient by grain boundary segregation of carbon during aging.These results confirmed that the aging condition of ferrite is important for alloy designing of ferrite–pearlite steels.

Effect of prior microstructures on the behavior of cementite particles during subcritical annealing of medium carbon steels

The effect of prior microstructures on the behavior of cementite particles in conjunction with microstructural changes of the matrix during subcritical annealing was investigated by changing the initial microstructures into ferrite + coarse pearlite, ferrite + fine pearlite, bainite, and martensite, in medium carbon steels. While the coarsening of cementite particles in martensite proceeded rapidly with the growth of large cementite particles at boundaries with the dissolution of smaller particles within martensite laths, the coarsening rate of cementite particles in bainite was found to be much slower than that in martensite. This could be attributed to the thermal stability of cementite particles, the smaller amount of carbon in solution, and the lower driving force for solute diffusion due to the uniform size distribution of cementite particles in bainite. The controlling coarsening kinetics in medium carbon steels with ferrite-pearlite, bainite and martensite, were found as boundary diffusion, diffusion along dislocation, a combination of boundary diffusion and diffusion along dislocation, respectively.

Microstructure alterations at the surface of a heavily corrugated rail with strong ripple formation

The topography, morphology, structure and residual stress state at the surface of a rail with severe longitudinal ripples was characterized using a combination of spectrometry, microscopy, scattering and diffraction methods. The investigations reveal that white etching layers develop at the ripple hills, while the ripple valleys contain a severely plastic deformed layer. While the structure in the ripple valleys remains pearlite, the structure of the white etching layers on top of the ripple hills is nanocrystalline martensite containing cementite particles of a few nanometer size. The microhardness of the white etching layers on top of the ripple hills is about three times higher than the microhardness in the ripple valleys. The microhardness throughout the white etching layer is not homogeneous due to the non-uniform amount of carbon in solute solution and also due to differences in the amount and the size of the fine carbide particles. The whole running surface is under biaxial compressive residual stresses. In the ripple valleys the direction of the principal residual stresses nearly coincides with the longitudinal and the transverse direction on the rail. On the ripple hills the compressive residual stresses due to the martensite formation are significantly higher and the stress state is nearly homogeneous.

Combined effect of special grain boundaries and grain boundary carbides on IGSCC of Ni–16Cr–9Fe– xC alloys

Susceptibility to intergranular stress corrosion cracking in Ni–16Cr–9Fe– xC alloys in 360°C primary water is reduced with increasing fraction of special grain boundaries, i.e. coincident site lattice boundaries (CSLB) and low angle boundaries, and grain boundary carbides. Intergranular stress corrosion cracking (IGSCC) was investigated using interrupted constant extension rate tensile tests in a primary water environment at 360°C. Thermal–mechanical treatments were used to increase the fraction of special boundaries from approximately 20–25% to between 30 and 40%. In a carbon-doped heat, further heat treating was used to precipitate grain boundary carbides preferentially on high-angle boundaries (HAB). Orientation imaging microscopy was used to determine the relative grain misorientations and scanning electron microscopy (SEM) was used to identify specific grain boundaries after each interruption. After each strain increment, the same regions in each sample were examined for cracking. Results showed that irrespective of the microstructure condition, CSLBs always cracked less than HABs. Results also showed that IGSCC is reduced with increasing solution carbon content, and for the same amount of carbon in solution, the addition of grain boundary carbides reduced IGSCC still further. The best microstructure was the one consisting of an enhanced CSLB fraction and chromium carbides precipitated preferentially on high-angle boundaries.

Effect of Single and Double Austenitization Treatments on the Microstructure and Mechanical Properties of 16Cr-2Ni Steel

Double austenitization (DA) treatment is found to yield the best combination of strength and toughness in both low-temperature as well as high-temperature tempered conditions as compared to single austenitization (SA) treatments. Obtaining the advantages of double austenitization (DA) to permit dissolution of alloy carbides without significant grain coarsening was attempted in AISI 431 type martensitic stainless steel. Structure-property correlation after low-temperature tempering (200 °C) as well as high-temperature double tempering (650+600 °C) was carried out for three austenitization treatments through SA at 1000 °C, SA at 1070 °C, and DA at 1070+1000 °C. While the increase in strength after DA treatment and low-temperature tempering at 200 °C is due to the increased amount of carbon in solution as a result of dissolution of alloy carbides during first austenitization, the increased toughness is attributable to the increased quantity of retained austenite. After double tempering (650+600 °C), strength and toughness are mainly found to depend on the precipitation and distribution of carbides in the microstructure and the grain size effect.

High strain rate torsional behavior of an ultrahigh carbon steel (1.8 Pct C-1.6 Pct Al) at elevated temperature

The elevated temperature mechanical properties of a 1.8 pct C-1.6 pct Al ultrahigh carbon steel (UHCS-1.8C-1.6Al) is described in the temperature range from 750 °C to 1150 °C and in the strain rate range from 0.2 to 26 s−1. A torsion test apparatus was used which permitted rapid cooling (50 °C per second) immediately after fracture to establish microstructure-processing-property relations. The strength-strain rate relation of the UHCS-1.8C-1.6Al material correlates well with a lattice diffusion-controlled dislocation creep process. The present data, together with other high carbon steels data, predict that austenite containing a high amount of carbon in solution has a high stacking fault energy. The ductility of the UHCS-1.8C-1.6Al is maximum at 1050 °C. This indicates that successful deep die forging and other mechanical processing operations at high strain rates should be performed at this temperature. The microstructure of the deformed samples consisted of a matrix of pearlite with some undissolved spherical carbides when rapidly cooled from 900 °C and 1050 °C and of a thin network of proeutectoid carbides when cooled from 1150 °C. High hardnesses in the range of Rockwell C 42 to 50 are obtained for such structures.

Influence of solution treatment on the microstructure and crystallographic texture of cold rolled and recrystallised low carbon steel

The effects of solution treatment on the texture and microstructure of cold rolled (e = 80 %) and recrystallised (T = 650 °C, salt bath) low carbon steel (mass content of 0.032 % C) were studied using both specimens having a maximum amount of carbon in solution and specimens in the as-hot-rolled state. For the investigations X-ray texture measurements and TEM observations were performed. Deformation was more inhomogeneous in specimens with carbon in solution. In such specimens a strong tendency to the formation of shear bands was observed at plastic strains in excess of 50 %. The specimens with little carbon in solution showed only few shear bands. Increasing contents of carbon in solution led to a degradation of the orientation density of the typical cold rolling texture components. This was attributed to the inhomogeneity of deformation, e.g. the formation of shear bands. The highest tendency to shear band formation was observed in {111} oriented grains. The development of shear bands in the deformation microstructure was interpreted in terms of Taylor theory. The {111} recrystallisation texture of samples with carbon in solution was weaker than in conventionally prepared samples. This observation was explained in terms of random nucleation in the inhomogeneously deformed microstructure and solute drag effects.

Improvement of RGO - electrical steel by modified intermediate annealing

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A13-Type phase revealed in rapidly solidified high-carbon iron alloy

Iron-based alloys, when rapidly solidified, can contain phases, such as austenite, e phase,X phase, amorphous phase, etc. The e phase, which is reported by Ruhl and Cohen,[1] has a hexagonal close-packed structure and contains a large amount of carbon in solution. TheX phase, as well as the E phase, is a nonequilibrium phase and has a cubic structure.[2.3.4] The authors[5] showed that the Cr-Mo iron alloy with a rather high carbon content of 4.4 pet exhibits the amorphous phase. Thus, rapidly solidified iron alloys reveal many different phases. In the present work, we report that an unknown phase was revealed in a Cr-Mo iron alloy with a lower level of carbon than that reported previously.[5]

Strain hardening of low-carbon manganese steels

Low-carbon steels with a high manganese content (3–5%) are highly susceptible to strain aging, which is accompanied by hardening due to precipitation of finely dispersed cementite particles. The increase of the manganese content, due to the displacement character of the transformation of austenite, increase the amount of carbon in solution and leads to precipitation hardening similar to the intermediate transformation activated by cold plasmic deformation. The dispersed structure formed in this case ensures high strength and ductility of the material.

Design of Fe/4Cr/0.4C martensitic steels eliminating quench cracking

Inter-granular cracking in the as-quenched structures of vacuum melted Fe/4Cr/0.4C steel has been investigated in terms of the heat treatment conditions. It has been shown that amount of carbon in solution and martensite packet size are the two most important factors influencing cracking tendency. Multiple heat-treatments have been designed in order to take advantage of higher austenitizing temperature and fine austenite grain size. Elimination of quench cracking and good mechanical properties were obtained after these treatments.

Strain ageing during the fatigue of carbon steels

Ageing reactions occurring during continuous cyclic stressing of steels at about 200 c/s have been investigated principally by observing progressive changes in the temperature rises which accompany cyclic plastic deformation. Evidence of two kinds of ageing reaction has been found in prestrained carbon steels: both reactions occur rapidly during continuous cycling at room temperature and have low apparent activation energies (~0.5 eV). The first reaction resembles static strain ageing in its dependence on dissolved interstitial solute. The second, which was observed only in steels containing numerous fine carbide particles, did not appear to be controlled by the amount of carbon in solution at the outset of fatigue. It is suggested that this second reaction may involve re-solution of fine carbide particles in regions of active slip. The causes of the rapid reaction rates observed at low ambient temperatures and the low apparent activation energies of the fatigue-ageing processes are uncertain. It is unlikely that they can be sufficiently explained by localized heating effects in the centres of active slip.

Kinetics of carbon precipitation in irradiated iron

Iron containing approximately 0.01 wt.% carbon was irradiated in the BNL reactor at different times and temperatures, and the amount of carbon in solution was measured by torsion pendulum internal friction. Isothermal annealings after irradiation for 12 days at 200°K showed that the irradiation accelerated the disappearance of carbon from solution by about three orders of magnitude over the thermal rate. The extent of the accelerated decay is a function of irradiation time, and after 12 days irradiation the peak decayed to background level. The decay curves are all second order with an activation energy of the diffusion of carbon in iron. After a 4 hr irradiation at 57°C, however, the decay was accelerated by only about one order of magnitude and the decay rate is not affected by further irradiation.