Thermal dependence of austempering transformation kinetics of compacted graphite cast iron

The evolution of the relative fraction of high-carbon austenite with austempering time and temperature was analyzed in a compacted graphite (CG) cast iron (average composition, in wt pct: 3.40C, 2.8Si, 0.8Mn, 0.04Cu, 0.01P, and 0.02S) at five different austempering temperatures between 573 and 673 K. Samples were characterized by Mössbauer spectroscopy, hardness measurements, and optical microscopy. During the first stage of transformation, the kinetics parameters were determined using the Johnson-Mehl’s equation, and their dependence with temperature in the range from 573 to 673 K indicates that the transformation is governed by nucleation and growth processes. The balance between growth-rate kinetics and nucleation kinetics causes the kinetics parameter (k) to have a maximum at ≈623 K of 3.9×10^?3(s^?1). The evolution of the C content in the high-carbon austenite was found to be controlled by the volume diffusion of carbon atoms from the ferrite/austenite interface into austenite, with a dependence of t ^0.40±0.05 on the austempering time (t).     

Influence of Isothermal Treatment on MnS and Hot Ductility in Low Carbon,Low Mn Steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 × 10^?4 s^?1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The higher-carbon plain C-Mn steel only displayed a narrow trough less than the Ae₃ temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility.     

Calorimetry Investigation of Phase Stability and Thermal Property in Ti-5 % Ta Alloy

An accurate measurement of enthalpy increment H _T–H _298.15 has been made for Ti-5 % Ta alloy in the temperature range of 463–1,257 K using drop calorimetry. This temperature interval covers the low temperature α + β two phase and also the α(hcp) → β(bcc) transformation domain (1,083 K ≤ T ≤ 1,183 K), in which the enthalpy versus temperature variation exhibited a clearly delineated inflection. The drop calorimetry data has been phenomenologically modelled to obtain the transformation enthalpy, Δ°H _tr ^α→β as 66 J g^?1. Further, in the α → β diffusive transformation zone the transformation kinetics has been quantitatively modelled in terms of Kolmogorov–Johnson–Mehl–Avrami model of diffusion limited phase transformation, to obtain the effective activation energy as 284 ± 10 kJ mol^?1.     

Non-steady-state rates of dissolution of iron and cobalt in liquid copper under static conditions

The horizontal bottom face of cylindrical iron or cobalt is exposed to liquid copper, and their dissolution rates are determined at 1473 to 1476 (±10) K and 1573 to 1576 (±10) K for the Cu-Fe system and at 1473 to 1475 (±10) K for the Cu-Co system. The decrease in height of the cylinder, z (m), is proportional to the square root of time, √t(s^1/2), as is expected in mass transfer controlled by non-steady-state diffusion. The observed value of α (m · s^?1/2) in the equationz = α√t is 1.95 × 10^?6 (m · s^?1/2) in the lower temperature range and 3.53 × 10^?6 (m · s^?1/2) in the higher temperature range for the Cu-Fe system, and 2.66 × 10^?6 (m · s^?1/2) for the Cu-Co system. Under fixed experimental conditions, the value of α calculated on the basis of the expression that the rate of diffusion in liquid is proportional to the activity gradient of a diffusing substance is in agreement with the observed value, but that calculated on the basis of Fick’s first law is 1.9 to 2.7 times as great as the observed value. For this expression to be valid it is necessary that the ratio of the phenomenological coefficient defined by irreversible thermodynamics to the activity is independent of the concentration. An estimate of the density gradients caused by dissolution suggests that no natural convection occurs in the vicinity of the solid-liquid interface.     

On the Stability of Reversely Formed Austenite and Related Mechanism of Transformation in an Fe-Ni-Mn Martensitic Steel Aided by Electron Backscattering Diffraction and Atom Probe Tomography

The stability of reversely formed austenite and related mechanism of transformation were investigated against temperature and time in an Fe-9.6Ni-7.1Mn (at. pct) martensitic steel during intercritical annealing at a dual-phase (α + γ) region. Dilatometry, electron backscattering diffraction (EBSD), atom probe tomography (APT), and X-ray diffraction (XRD) were used to characterize the mechanism of reverse transformation. It was found that under intercritical annealing at 853 K (580 °C), when the heating rate is 20 K/s (20 °C/s), reverse transformation takes place through a mixed diffusion control mechanism, i.e., controlled by bulk diffusion and diffusion along the interface, where Ni controls the diffusion as its diffusivity is lower than that of Mn in the martensite and austenite. Increasing the intercritical annealing to 873 K (600 °C) at an identical heating rate of 20 K/s (20 °C/s) showed that reverse transformation occurs through a sequential combination of both martensitic and diffusional mechanisms. The transition temperature from diffusional to martensitic transformation was obtained close to 858 K (585 °C). Experimental results revealed that the austenite formed by the diffusional mechanism at 853 K (580 °C) mainly remains untransformed after cooling to ambient temperature due to the enrichment with Ni and Mn. It was also found that the stability of the reversely formed austenite by martensitic mechanism at 873 K (600 °C) is related to grain refinement.     

Interfacial Molten Reactivity of Aluminum in Contact with (0001) Al2O3 and (111) Al2MgO4 Single Crystal Surfaces

The present work studies (0001) Al₂O₃ and (111) Al₂MgO₄ wetting with pure molten Al by the sessile drop technique from 1073 K to 1473 K (800 °C to 1200 °C) under Ar at PO₂ 10^?15 Pa. Al pure liquid wets a smooth and chemically homogeneous surface of an inert solid, the wetting driving force (t,T) can be readily studied when surface solid roughness increases in the system. Both crystals planes (0001) Al₂O₃ and (111) Al₂MgO₄ have crystallographic surfaces with identical O^?2 crystalline positions however considering Mg²⁺ content in Al₂MgO₄ structure may influence a reactive mode. Kinetic models results under similar experimental conditions show that Al wetting on (0001) Al₂O₃ is less reactive than (111) Al₂MgO₄, however at >1273 K (1000 °C) (0001) Al₂O₃ transformation occurs and a transition of wetting improves. The (111) Al₂MgO₄ and Al system promotes interface formations that slow its wetting process.     

Impact Response and Microstructural Evolution of 316L Stainless Steel under Ambient and Elevated Temperature Conditions

The impact response and microstructural evolution of 316L stainless steel are examined at strain rates ranging from 1?×?10³ to 5?×?10³?s^?1 and temperatures between 298?K and 1073?K (25?°C and 800?°C) using a split Hopkinson pressure bar and transmission electron microscopy (TEM). The results show that the flow behavior, mechanical strength, and work-hardening properties of 316L stainless steel are significantly dependent on the strain rate and temperature. The TEM observations reveal that the dislocation density increases with increasing strain rate but decreases with increasing temperature. Moreover, twinning occurs only in the specimens deformed at 298?K (25?°C), which suggests that the threshold stress for twinning is higher than that for slip under impact loading. Finally, it is found that the volume fraction of transformed ???? martensite increases with increasing strain rate or decreasing temperature. Overall, the results suggest that the increased flow stress observed in 316L stainless steel under higher strain rates and lower temperatures is determined by the combined effects of dislocation multiplication, twin nucleation and growth, and martensite transformation.     

Vacancy versus pipe diffusion mechanisms for subgrain growth in OFHC copper

Oxygen free high conductivity (OFHC) copper wires have been annealed in a temperature range of 443–473 K. The subgrain size (D) in these wires have been observed to follow a parabolic growth law (D² = d² + K · t) with respect to annealing time (t) at a given temperature, where d is the subgrain size in the cold worked condition and K is a temperature dependent constant. It has been found that K decreases as the amount of prior cold work increases while it increases as the annealing temperature increases. The temperature dependence of K has been analysed in terms of both vacancy and pipe diffusion mechanisms. The pipe diffusion mechanism dominates the subgrain growth process at lower recovery temperatures while the conventional vacancy mechanism prevails at higher recovery temperatures.     

Thermodynamics of oxygen solutions in Fe-Mn melts

Oxygen solutions in Fe-Mn melts are analyzed thermodynamically. The composition of the oxide phase is determined, and the equilibrium oxygen concentrations in Fe-Mn melts are calculated over a wide composition range. The oxide phase mainly contains MnO: even at a molar fraction of manganese of 0.02 in the melt, the molar fraction of manganese oxide in the slag is more than 0.9. This is due to a much higher oxygen affinity of manganese as compared to iron; that is, manganese additives to iron considerably decrease the oxygen solubility. When the manganse content in the melt is 19.32%, the oxygen solubility curve has a minimum corresponding to an oxygen concentration of 5.136 × 10^?3%. However, a further increase in the managanese content results in an increase in the oxygen concentration in the melt. In liquid manganese, the oxygen saturation concentration at 1873 K is 0.0472%. The interaction parameter e _o(Mn) ^o (?0.207) and the activity coefficient γ _o(Mn) ^o (1.131 × 10^?4) have been calculated for the first time.     

Beitrag zur Herstellung von hochreinem Eisen

Ausgehend von Elektrolyteisen mit ≦ 5 · 10^?3 % C, ≦ 5 · 10^?3% Si, ≦ 5 · 10^?3% Mn, ≦ 5 · 10^?3% P und ≦ 5 · 10^?3% S und von Reinsteisen in Stangenform mit 3 · 10^?3% C, 5 · 10^?4% Si und 3 · 10^?4% Mn durch Glühen bei 1350 °C 24 h unter gereinigtem Wasserstoff, Schmelzen im Hochvakuum, horizontales induktives Zonenschmelzen und Zonenschmelzen im Elektronenstrahlofen ohne Tiegel Herstellung von sehr reinem Eisen. Vergleich der Reinheit durch Messung der Rekristallisationstemperatur, der Koerzitivfeldstärke und des Verhältnisses des Widerstandes bei 300°K zu dem bei 4,5 °K. Chemische Zusammensetzung des zonengeschmolzenen Eisens.     

Fracture Behavior of High-Nitrogen Austenitic Stainless Steel Under Continuous Cooling: Physical Simulation of Free-Surface Cracking of Heavy Forgings

18Mn18Cr0.6N steel was tension tested at 0.001 s^?1 to fracture from 1473 K to 1363 K (1200 °C to 1090 °C, fracture temperature) at a cooling rate of 0.4 Ks^?1. For comparison, specimens were tension tested at temperatures of 1473 K and 1363 K (1200 °C and 1090 °C). The microstructure near the fracture surface was examined using electron backscatter diffraction analysis. The lowest hot ductility was observed under continuous cooling and was attributed to the suppression of dynamic recrystallization nucleation.     

Phase equilibria and X-ray diffraction investigation of the system Cu−Fe−O

An equilibria investigation for the system Cu?Fe?O determined nine regions of stability which were investigated in the oxygen pressure range of 1×10^?4 to 5×10^?1 atm (P _total=1 atm) and at temperatures of 1173° to 1250°K. Oxygen dissociation pressures were determined for three univariant, four bivariant, and two trivariant equilibria. X-ray examination of selected equilibrated samples established conclusively that 1) cuprous ferrite (delafossite) is a stoichiometric compound, 2) cupric ferrite is a solid solution, and 3) solubility of delafossite in cuprous oxide is negligible. A relationship of lattice parameters with compositional variations was determined for the solid solution. Tetragonal-to-cubic spinel crystal transformation of cupric ferrite solid solution occurs with ac/a ratio at or slightly greater than 1.028. A volume discontinuity accompanies the tetragonal-to-cubic spinel transformation, which is a first-order type of thermodynamic transition.     

Martensitic Phase Transformation from Non-isothermally Deformed Austenite in High Strength Steel 22SiMn2TiB

Non-isothermal compressive deformation was performed on high strength steel 22SiMn2TiB for the study of martensitic phase transformation from deformed austenite. The transformation start temperature M _s decreased with the increase of deformation from 0 to 50 pct, and the variation of deformation rate (0.1 and 10 s^?1) and the appearance of deformation-induced ferrite and bainite showed no influence on the change of M _s temperature. The deformation at both the rates increased the volume fraction of retained austenite; however, the carbon content of retained austenite decreased at 10 s^?1 and remained basically unchanged at 0.1 s^?1. The yield strength of austenite at M _s temperature and the stored energy in deformed austenite were experimentally obtained, with which the relationships between the change of M _s temperature and the thermodynamic driving force for martensitic phase transformation from deformed austenite were established by the use of the Fisher-ADP–Hsu model. And finally, the transformation kinetics was analyzed by the Magee–Koistinen–Marhurger equation.     

The influence of growth rate and temperature on high cycle fatigue of Al−Al3Ni

High-cycle fatigue tests have been conducted on specimens of an Al?Al₃Ni eutectic alloy, unidirectionally solidified at selected rates from 1.39×10^?4 cm/s to 0.3 cm/s. Tests were conducted in air at 298, 458 and 683 K. Room temperature fatigue lives were independent of growth rate at low solidification rates (1.39×10^?4–8.33×10^?3 cm/s, but were markedly improved in samples grown at 0.3 cm/s. Materials grown at 8.33 × 10^?3 cm/s exhibited fatigue lives similar to those of the lower growth rates, despite gross misalignment due to cellular growth. At 0.5T _m (458 K) and 0.75T _m (683 K), the fatigue lives of the material grown at low solidification rates were dependent on growth rate. The dependence of fatigue life on growth rate at elevated temperatures appears to be due primarily to differences in cyclic creep rates as a result of varying interfiber spacings. Crack initiation and propagation mechanisms were established by metallographic and fractographic examination. Dislocation substructure-fiber interactions were studied by transmission electron microscopy.     

A Stacking Fault Energy Perspective into the Uniaxial Tensile Deformation Behavior and Microstructure of a Cr-Mn Austenitic Steel

A Cr-Mn austenitic steel was tensile strained in the temperature range 273 K (0 °C) ≤ T ≤ 473 K (200 °C), to improve the understanding on the role of stacking fault energy (SFE) on the deformation behavior, associated microstructure, and mechanical properties of low-SFE alloys. The failed specimens were studied using X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. The SFE of the steel was estimated to vary between ~ 10 to 40 mJ/m² at the lowest and highest deformation temperatures, respectively. At the ambient temperatures, the deformation involved martensite transformation (i.e., the TRIP effect), moderate deformation-induced twinning, and extended dislocations with wide stacking faults (SFs). The corresponding SF probability of austenite was very high (~10^?2). Deformation twinning was most prevalent at 323 K (50 °C), also resulting in the highest uniform elongation at this temperature. Above 323 K (50 °C), the TRIP effect was suppressed and the incidence of twinning decreased due to increasing SFE. At elevated temperatures, fine nano-sized SF ribbons were observed and the SF probability decreased by an order (~10^?3). High dislocation densities (~10¹⁵ m^?2) in austenite were estimated in the entire deformation temperature range. Dislocations had an increasingly screw character up to 323 K (50 °C), thereafter becoming mainly edge. The estimated dislocation and twin densities were found to explain approximately the measured flow stress on the basis of the Taylor equation.     

Developing the Processing Maps Using the Hyperbolic Sine Constitutive Equation

Hot compression tests were performed on a duplex stainless steel at temperatures ranging from 1223 K to 1473 K (950 °C to 1200 °C) and strain rates from 0.001 to 100 s^?1. The constitutive analysis of flow stress was carried out using the hyperbolic sine function, and the material constants were determined at two typical strains of 0.3 and 0.7. The power dissipation map, instability map, and processing map for the material were developed for strains of 0.3 and 0.7. The developed processing maps were based on the hyperbolic sine as well as the conventional power-law constitutive equations. The efficiency of power dissipation (η) varied from 12 to 60 pct over the studied temperature and strain rate. The highest value of η was obtained at strain rates below 0.01 s^?1, whereas the lowest value of η was observed at the intermediate strain rates. The instability region in sin h-based processing map was only observed in the range of 1423 K to 1473 K (1150 °C to 1200 °C) and at a strain rate of 100 s^?1, while the conventional processing map did not predict any instability region. Optical microscopy observations were more consistent with the results of the sin h-based processing map and indicated that the instability regime at high temperatures and high strain rates was due to the development of adiabatic shear bands.     

Mechanism of Dynamic Strain Aging in a Niobium-Stabilized Austenitic Stainless Steel

Dynamic strain aging (DSA) behavior of a niobium (Nb)-stabilized austenitic stainless steel (TP347H) was studied from room temperature (RT) to 973 K via tensile testing, transmission electron microscopy (TEM), and internal friction (IF) measurements. The DSA effect is nearly negligible from 573 K to 673 K, and it becomes significant at temperatures between 773 K and 873 K with strain rates of 3 × 10^?3 s^?1, 8 × 10^?4 s^?1, and 8 × 10^?5 s^?1, respectively. The results indicate that a dislocation planar slip is dominant in the strong DSA regime. The Snoek-like peak located at 625 K is highly sensitive to the diffusion of free carbon (C) atoms in solid solution. C-Nb octahedrons are formed by C chemical affinity to substitutional Nb solute atoms. Octahedron structure is very stable and captures most free C atoms and inhibits DSA at low tensile test temperatures of 573 K to 673 K. At high test temperatures in the range from 773 K to 873 K, C-Nb octahedrons break up and release free C and Nb atoms, resulting in the stronger Snoek-like peak. The interaction between C atoms and dislocations is responsible for DSA at low temperatures ranging from 573 K to 673 K. At higher temperature of 773 K to 873 K, the Cr and Nb atoms lock the dislocations, and this formation contributes to DSA.     

Effect of austenitisation temperature on austenite transformations in 0·7%C Cr–Mo PM steel

Abstract

The effect of austenitisation temperature on austenite transformations on 0·7%C Astaloy CrL steel was studied by dilatometry. The steel has a good hardenability, forming martensite at most of the austenitisation temperatures and cooling rates investigated. Only on cooling from 1073 K, austenite transforms into bainite completely at 3 K s^?1 and partially at 12·5 K s^?1. The effect of austenitisation temperature on the prior austenitic grain size is quite poor because of the pinning effect of pores. The martensite start temperature M_s increases slightly with the austenitisation temperature up to 1173 K and decreases at 1523 K. This trend is due to the presence of nanometric carbides (Cr₂₃C₆), which were detected at TEM. They dissolve almost completely in austenite at 1523 K only, increasing the stability of austenite against the martensitic transformation. The effect of temperature in the range from 1073 K up to 1523 K is poor. As a consequence, the microstructural characteristics of hardened steels are very similar.     

Load Partitioning and Strain-Induced Martensite Formation during Tensile Loading of a Metastable Austenitic Stainless Steel

In-situ high-energy X-ray diffraction and material modeling are used to investigate the strain-rate dependence of the strain-induced martensitic transformation and the stress partitioning between austenite and α′ martensite in a metastable austenitic stainless steel during tensile loading. Moderate changes of the strain rate alter the strain-induced martensitic transformation, with a significantly lower α′ martensite fraction observed at fracture for a strain rate of 10^?2 s^?1, as compared to 10^?3 s^?1. This strain-rate sensitivity is attributed to the adiabatic heating of the samples and is found to be well predicted by the combination of an extended Olson–Cohen strain-induced martensite model and finite-element simulations for the evolving temperature distribution in the samples. In addition, the strain-rate sensitivity affects the deformation behavior of the steel. The α′ martensite transformation at high strains provides local strengthening and extends the time to neck formation. This reinforcement is witnessed by a load transfer from austenite to α′ martensite during loading.     

An EBSD Study of the Deformation of Service-Aged 316 Austenitic Steel

Electron backscatter diffraction (EBSD) has been used to examine the plastic deformation of an ex-service 316 austenitic stainless steel at 297 K and 823 K (24 °C and 550 °C) at strain rates from 3.5 × 10^?3 to 4 × 10^?7 s^?1. The distribution of local misorientations was found to depend on the imposed plastic strain following a lognormal distribution at true strains <0.1 and a gamma distribution at strains >0.1. At 823 K (550 °C), the distribution of misorientations depended on the applied strain rate. The evolution of lattice misorientations with increasing plastic strain of up to 0.23 was quantified using the metrics kernel average misorientation, average intragrain misorientation, and low angle misorientation fraction. For strain rate down to 10^?5 s^?1, all metrics were insensitive to deformation temperature, mode (tension vs compression), and orientation of the measurement plane. The strain sensitivity of the different metrics was found to depend on the misorientation ranges considered in their calculation. A simple new metric, proportion of undeformed grains, is proposed for assessing strain in both the aged and unaged materials. Lattice misorientations develop with strain faster in aged steel than in unaged material, and most of the metrics were sensitive to the effects of thermal aging. Ignoring aging effects leads to significant overestimation of the strains around welds. The EBSD results were compared with nanohardness measurements, and good agreement was established between the two techniques of assessing plastic strain in aged 316 steel.