Textural changes during recovery annealing of a heavily cold-rolled Fe–Mn–Al–Si–C alloy

A heavily cold-rolled (80%) high manganese austenitic steel was subjected to isochronal and isothermal heat treatments, in order to study the texture development during the recovery stage. Progress of recovery was negligible after annealing at 500°C for 60?min, but annealing for 60?min at 800°C led to complete recrystallisation, and appearance of the final texture was very similar to that of a copper type deformation texture. Isothermal heat treatments at 600°C revealed that at the very early stage of recovery, there was a weakening of brass type deformation texture, but it was followed by a sharpening of Bs component. At the same time, there was a shift of peak intensity towards Bs/Goss location. After this stage of intermediate sharpening, the texture sharpness experienced a continuous weakening with further progress of recovery.     

Mechanical Properties of β–Ti-35Nb-2.5Sn Alloy Synthesized by Mechanical Alloying and Pulsed Current Activated Sintering

A developed Ti-35?pct Nb-2.5?pct Sn (wt pct) alloy was synthesized by mechanical alloying using high-energy ball-milled powders, and the powder consolidation was done by pulsed current activated sintering (PCAS). The starting powder materials were mixed for 24 hours and then milled by high-energy ball milling (HEBM) for 1, 4, and 12 hours. The bulk solid samples were fabricated by PCAS at 1073?K to 1373?K (800 °C to 1100 °C) for a short time, followed by rapid cooling to 773?K (500 °C). The relative density of the sintered samples was about 93?pct. The Ti was completely transformed from ?? to ??-Ti phase after milling for 12 hours in powder state, and the specimen sintered at 1546?K (1273 °C) was almost transformed to ??-Ti phase. The homogeneity of the sintered specimen increased with increasing milling time and sintering temperature, as did its hardness, reaching 400?HV after 12 hours of milling. The Young??s modulus was almost constant for all sintered Ti-35?pct Nb-2.5?pct Sn specimens at different milling times. The Young??s modulus was low (63.55 to 65.3 GPa) compared to that of the standard alloy of Ti-6Al-4V (100 GPa). The wear resistance of the sintered specimen increased with increasing milling time. The 12-hour milled powder exhibited the best wear resistance.     

On the Behaviour of Hematite and Graphite Blend during Ball Milling and Heating Up Reduction of Milled Mixture

The effect of ball milling under argon and air atmospheres on the reaction behaviour of the mixture of sintered hematite and graphite was investigated. Thermo‐gravimetry / differential thermal analysis (TG‐DTA) was adopted to determine the effect of milling time on the reduction process during heating up under Ar atmosphere. The samples were heated at a constant heating rate of 10 °C/min from room temperature up to 1100 °C and maintained at this temperature for 30 minutes. TGL (thermo‐gravimetry loss) curves showed a decrease of onset temperature of reduction with increase of milling time. XRD patterns of milled samples at room temperature revealed that the peaks of graphite disappeared after 48 hours milling. This represents the transformation of crystalline structure of graphite to the amorphous structure. By increasing the milling time to 72 hours, magnetite peaks appeared in the XRD pattern as a result of reduction of hematite with graphite during milling. However, the amount of magnetite formed during milling process increased as milling proceeded. The powders milled under Ar atmosphere became more active than the powders milled under air and consequently the carbothermic reduction of hematite in powders milled under Ar atmosphere was observed at lower temperatures compared with air‐milled powders. It was observed that the reduction time of hematite in powder mixture was decreased with increase of sintering time of hematite prior to milling.     

Reduction Behavior of Panzhihua Titanomagnetite Concentrates with Coal

The reduction behavior of the Panzhihua titanomagnetite concentrates (PTC) briquette with coal was investigated by temperature-programmed heating under argon atmosphere in a vertical tube electric furnace. The mass loss behavior of the PTC-coal mixture was checked by thermogravimetric analysis method in argon with a heating rate of 5 K (5 °C)/ min. It was found that there are five stages during the carbothermic reduction process of the PTC. The devolatilization of coal occurred in the first stage, and reductions of iron oxides mainly occurred in the second and third stages. The reduction rate of iron oxide in the third stage was much higher than that in the second stage because of the significant rate of carbon gasification reaction. The iron in the ilmenite was reduced in the fourth stage. In the final stage, the rutile was partially reduced to lower valence oxides. The phase transformation of the briquette reduced at different temperatures was investigated by X-ray diffraction (XRD). The main phases of sample reduced at 1173 K (900 °C) are metallic iron, ilmenite (FeTiO₃), and titanomagnetite (Fe_3–x Ti _x O₄). The traces of rutile (TiO₂) were observed at 1273 K (1000 °C). The iron carbide (Fe₃C) and ferrous-pseudobrookite (FeTi₂O₅) appeared at 1473 K (1200 °C). The titanium carbide was found in the sample reduced at 1623 K (1350 °C). The shrinkages of reduced briquettes, which increased with increase in the temperature, were found to depend greatly on the temperature. With increasing the reduction temperature to 1573 K (1300 °C), the iron nuggets were observed outside of the samples reduced. The nugget formation can indicate a new process of ironmaking with titanomagnetite similar to ITmk3 (Ironmaking Technology Mark 3).     

Continuous reduction of the oxide scale of hot-rolled steel strip in hydrogen

The continuous reduction in the oxide scale of hot-rolled steel strip in H₂–N₂ atmosphere was simulated in laboratory. Scale specimens were reduced in 20% H₂–N₂ or 50% H₂–N₂ atmosphere. The sample weight losses were measured after soaking at 550, 700 and 800°C. In both atmospheres, specimen reduced at 700°C showed the minimum weight loss after soaking for 240?s. At 700 and 800°C, higher hydrogen concentration accelerated the reaction in the beginning of soaking, but had little effect once the dense-reduced iron layer formed. While at 550°C, the reduced iron kept growing in porous structure and the weight loss rate increased significantly in higher H₂ concentration.     

Synthesis of Ni–TiC Composite by Ball Milling and Heat Treatment of NiO/TiO2/C Powder Mixture

In this study, a Ni–TiC composite was synthesized by the carbothermic reduction of activated NiO/TiO₂/C powder mixture. The effect of 0-, 2-, 5-, and 10-h ball milling of the sample on the reduction process was investigated. Results of XRD pattern from milled samples showed that no reaction occurred between NiO, TiO₂, and carbon due to milling. FESEM images for samples milled for 2, 5, and 10 h revealed the fine distribution of the brittle oxide particles in the matrix of graphite, as a ductile phase. Particle size reduction was also noticeable, especially in the case of oxide particles. By increasing the milling time, agglomeration of particles was also observed. Results of the thermogravimetry analysis of milled and un-milled mixtures showed the onset temperature of reduction for NiO decreased from 867 °C in the un-milled sample to 582, 571, and 560 °C in samples activated for 2, 5, and 10 h, respectively. Also, the onset temperature of the reduction of TiO₂ decreased from 1029 for the sample milled for 2 h to 1019 and 1004 °C for samples milled for 5 and 10 h, respectively. The XRD pattern of the heat-treated milled powder mixture for 1 h under vacuum proved that Ni–TiC composite was formed at 1100 °C. It was found that bulk Ni–TiC composite could be synthesized by the heat treatment of activated powder at 1300 °C. Formation of TiC quadrilateral particles as reinforcement in the continuous matrix of Ni was evident at this temperature. Furthermore, the best morphology with the most appropriate particle size distribution was observed in the sample milled for 2 h.

     

In‐Situ Observation of the Formation of MnS during Solidification of High Sulphur Steels

A Confocal Scanning Laser Microscope equipped with a gold image furnace was used to directly observe the precipitation of MnS during solidification of high sulphur steels under isothermal conditions in the temperature region 1440 to 1480°C on the free surface of the steel melt. For the case of Al‐killed steels, below 1480°C MnS particles were found to precipitate with Fe forming simultaneously around them. This MnS containing structure continued to grow rapidly (264 μm/s) as a surface film. The film gradually changed, as the level of S in the melt decreased, into a eutectic structure (with lamella spacing of 2 μm) as predicted by thermodynamics. In Si‐ killed steels there was significantly lower tendency to form MnS both in terms of time until precipitation occurred and growth rate.     

Effect of sintering temperature on the microstructure and mechanical properties of Ti(C, N)-based cermets

Ti(C, N)-based cermets are produced by vacuum sintering at 1420, 1430, and 1440°C. The effect of sintering temperature on the microstructure and properties of Ti(C, N)-based cermets is studied. The microstructure and fracture morphology are investigated with a scanning electron microscope; mechanical properties such as transverse rupture strength and hardness are measured. The results show that, with increasing sintering temperature, the microstructure of cermets became uniform, and the rim phase structure is gradually completed. When the sintering temperature reaches 1440 °C, the rim phase becomes thicker and more brittle, grains start growing, and the mechanical properties decrease, indicating that a sintering temperature of 1440°C is too high.     

Role of Microstructure and Temperature on the Tensile Fracture Behavior of Oxide Dispersion Strengthened 18Cr Ferritic Steel

The tensile fracture behavior of oxide dispersion strengthened 18Cr (ODS-18Cr) ferritic steels milled for varying times was studied along with the oxide-free 18Cr steel (NODS) at 25, 200, 400, 600, and 800 °C. At all the test temperatures, the strengths of ODS–18Cr steels increased and total elongation decreased with the duration of milling time. Oxide dispersed 18Cr steel with optimum milling exhibited enhanced yield strength of 156 pct at room temperature and 300 pct at 800 °C when compared to oxide-free 18Cr steel. The ductility values of ODS-18Cr steels are in the range 20 to 35 pct for a temperature range 25 to 800 °C, whereas NODS alloy exhibited higher ductility of 37 to 82 pct. The enhanced strength of ODS steels when compared to oxide-free steel is due to the development of ultrafine grained structure along with nanosized dispersion of complex oxide particles. While the pre-necking elongation decreased with increasing temperature and milling time, post-necking elongation showed no change with the test temperature. Fractographic examination of both ODS and NODS 18Cr steel fractured tensile samples, revealed that the failure was in ductile fracture mode with distinct neck and shear lip formation for all milling times and at all test temperatures. The fracture mechanism is in general followed the sequence; microvoid nucleation at second phase particles, void growth and coalescence. The quantified dimple sizes and numbers per unit area were found to be in linear relation with the size and number density of dispersoids. It is clearly evident that even nanosized dispersoids acted as sites for microvoid nucleation at larger strains and assisted in dimple rupture.

     

The effect of quench rate on the martensitic transformation in Fe−C alloys

The effect of very high quench rates on the transformation kinetics of a series of Fe?C and low-alloy steels and the morphology of an Fe?14Ni-0.76C alloy was investigated. TheM _S temperatures of the Fe?C and Fe?C?X alloys increased between 90° and 122°C in a sigmoidal fashion over a quench rate range from 2,750° to 24,800°C per sec. The sensitivity of theM _s temperature to the quench rate from the austenitizing temperature to 315°C was shown to be related to the influence of the third alloying element on the diffusivity of carbon in austenite. Transmission electron microscopy and optical metallography showed that the morphology of an Fe?14Ni?0.76C martensite is changed from a lath structure in slow quenched samples to a plate structure in fast quenched samples. The substructure of the untransformed austenite adjacent to the martensite plates changed from planar dislocation arrays to dislocation tangles with increased quench rate. These results were explained using a model for ferrous martensite strengthening based upon the extent of carbon segregation to imperfections in the austenite during cooling.     

Influence of deformation on the structure and mechanical and corrosion properties of high-nitrogen austenitic 07Kh16AG13M3 steel

The correlation has been studied between the structure of a high-nitrogen austenitic Cr-Mn-N steel formed in the process of combined hardening treatment, including cold plastic deformation (CPD), and its mechanical and corrosion properties. The structure and properties of commercial high-nitrogen (0.8% N) 07Kh16AG13M3 steel is analyzed after rolling by CPD and aging at 500 and 800°C. It is shown that CPD of the steel occurs by dislocation slip and deformation twinning. Deformation twinning and also high resistance of austenite to martensitic transformations at true strains of 0.2 and 0.4 determine the high plasticity of the steel. The contribution of the structure imperfection parameters to the broadening of the austenite lines during CPD is estimated by X-ray diffraction. The main hardening factor is stated to be lattice microdistortions. Transmission electron microscopy study shows that heating of the deformed steel to 500°C leads to the formation of the intermediate CrN phase by a homogeneous mechanism, and the intermtallic χ phase forms along the austenite grain boundaries in the case of heating at 800°C. After hardening by all investigated technological schemes, exception for aging at 800°C, the steel does not undergo pitting corrosion and is slightly prone to a stress corrosion cracking during static bending tests, while aging at 800°C causes pitting corrosion at a pitting formation potential E _pf = ?0.25 V.     

A study on the microstructural evolution of Al-25 at. pct V-12.5 at. pct M (M = Cu,Ni, Mn) powders by planetary ball milling

The alloying behavior of Al-25 at. pct V-12.5 at. pct M (M = Cu, Ni, Mn) by planetary ball milling of elemental powders hours as been investigated in this study. In Al₃V binary system, an amorphous phase was produced after 6 hours and the amorphous phase was mechanically crystallized after 20 hours. The large difference in the diffusivities between Al and V atoms in Al matrix results in the formation of the amorphous phase when the homogeneous distribution of all the elements in a powder was achieved at 6 hours. According to thermal analyses, the amorphous phase in the binary Al₃V was crystallized at 350 °C. The addition of ternary elements (Cu, Ni, Mn) increased the activation energy for the crystallization to D0₂₂ phase by interfering with the diffusion process. Therefore, ternary element addition improved the thermal stability of the amorphous structures. The amorphous phase in the 12.5 at. pct Ni added Al₃V was crystallized to D0₂₂ phase at 540 °C. The mechanical crystallization of the amorphous phase in the ternary element-added Al-V system either occurred later or was not observed during ball milling up to 100 hours. It is thought that the amorphous intermetallic compacts could be produced more easily in ternary element-added alloys by using an advanced consolidation method.     

Deterioration of electromotive force of chromel-alumel thermocouples in reducing atmospheres at high temperatures

The deterioration of electromotive force (emf) of Chromel-Alumel (CA) thermocouples in 80 pct H₂ + 15 pct CO + 5 pct CO₂ has been analyzed in terms of the corrosion behavior of Chromel. Emf of the CA thermocouple deteriorated drastically in 80 pct H₂ + 15 pct CO + 5 pct CO₂. After exposure for about 1000 hours at 900 °C, the decrease of emf was about 16 mV. The deterioration process could be separated into three terms. The first term, which has the smallest time constant of about 20 hours, was attributed to carbon deposition on the Chromel surface in the temperature range of 600 to 700 °C. The second term, which has a time constant of about 100 hours, was attributed to the severe internal oxidation of chromium in the temperature range of 500 to 800 °C. The third term, having the largest time constant of several thousand hours, might be attributed to the moderate and gradual preferential oxidation of chromium in Chromel in the range 800 to 900 °C. Boron nitride (BN) coating on CA thermocouples could reduce this deterioration of emf; the decrease of emf was improved to about 3 °C during 700 hours test at 900 °C.     

Structure-property changes during hardening and tempering of new ultra high strength medium carbon low alloy steel

Abstract

A medium carbon low alloy steel, electroslag refined, modified AFNOR 15CDV6, has been developed for satellite launch vehicle and related applications. Conventionally processed (without electroslag refining) mostly bainitic AFNOR 15CDV6 (with 0·15 wt-% carbon and ~ 3·5 wt-% other alloying elements) has a yield strength of ~ 800 MPa. Electroslag refining, coupled with increased carbon (0·29 wt-% carbon, but no change in percentage of other alloying elements), increased the yield strength to about 1300-1400 MPa, without sacrificing ductility. The microstructure of the modified grade was martensitic. Martensite in the as hardened state was mostly in the form of laths, although ~20% plate martensite was also observed. Until 150°C tempering, no noticeable loss of tetragonality was observed, while the unit cell parameter c/a ratio dropped to almost 1 after 300°C tempering. The interesting observation at 150°C tempering was the predominant presence of fine rodlike ? carbide, which may also explain the increased yield strength. Tempering above 150°C converted the ? carbide to cementite, relatively thicker precipitates of similar morphology. At higher tempering temperatures, no evidence of spheroidisation of cementites was noted. The highest tempering temperatures of 500 and 600°C resulted in two marked changes in the microstructure: the appearance of M₂₃C₆ type (Cr, Fe and Mo bearing) carbides, and the appearance of, in some regions of the microstructure at least, a relatively 'recovered' lath structure. Misorientation among adjacent laths, nearly constant at 8-9° until 450°C tempering, increased noticeably, to 13 and 16°, after the respective tempering temperatures of 500 and 600°C.     

Fatigue crack growth through

Fatigue cracks were grown at 25 °C and 800 °C in a titanium aluminide alloy heat-treated to give a γ+ α ₂ lamellar microstructure. These lamellae, having widths of =0.5 to 2 μm, were in colonies approximately 1.2 mm across. Crack growth was observed and photographed under high resolution conditions using a loading and heating cyclic stage for the scanning electron microscope. Stereoimaging was used to measure displacements around crack tips, from which crack opening displacements and strains were derived. Cracks were found to grow about 10 times faster at 25 °C than at 800 °C, and the threshold stress intensity for fatigue crack growth was lower at 25 °C. Strain to fracture the lamellae was determined as ≈0.08, while fatigue crack tips could sustain up to 0.3 strain at 25 °C and 0.5 strain at 800 °C. The lamellar micro- structure was found to have a strong influence on crack tip behavior.     

Metastable crystalline and amorphous structures formed in the Cu-W system by vapor deposition

The possibility of producing nonequilibrium amorphous and crystalline phases in the Cu-W system is of interest because, under equilibrium conditions, no mutual solubility is expected between Cu and W. Triode sputtered coatings (45 to 150 μm thick, produced at deposition rates between 20 and 150 Å/s) consisted of amorphous and metastable crystalline phases. The latter remained decomposition-resistant on heating to various temperatures between 340 °C and 600 °C (the maximum temperature of exposure). The amorphous phase in such coatings crystallized on heating into a metastable body-centered cubic (bcc) phase, and the crystallization temperatureT _x was found to decrease across the phase diagram from 450 °C to 340 °C as the percentage of W increased from 26 to 60 at. pct. Samples containing amorphous phase regions, when subjected to heating between 150 °C and 250 °C, showed an unusual rapid precipitation of Cu at the sample surface, indicating an easy diffusion of the Cu component. This occurred without crystallization of the remaining slightly tungsten-enriched amorphous matrix. Microhardness measurements in sputtered two-phase amorphous and bcc regions have shown that in alloys of the same composition, the amorphous phase was always softer than the bcc solid solution phase. X-ray, microprobe, and optical evidence suggests that the amorphous films deposited at very low temperatures(i.e., at liquid N₂) may subsequently undergo a phase separation upon heating to room temperature and prior to crystallization. Earlier work and present studies of vapordeposited alloys in this system confirm that the observed phases and microstructures can be related to free energy trends estimated from thermodynamic considerations and to specific deposition parameters, such as the substrate temperature and the deposition rates, which influence the kinetics.     

Analysis and Characterization of Phase Evolution of Nanosized BaTiO3 Powder Synthesized Through a Chemically Modified Sol-Gel Process

In the current research, a cost-effective and modified method with a high degree of reproducibility was proposed for the preparation of fine nanoscale and high-purity BaTiO₃. In contrast to the other established methods, in this research, carbonate-free BaTiO₃ nanopowders were prepared at a lower temperature and in a shorter time span. To reach an in-depth understanding of the scientific basis of the proposed process, an in-detail analysis was carried out for characterization of nanoscale BaTiO₃ particles via differential thermal analysis (DTA)/thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques aided by theoretical calculations. The effects of the temperature and time of calcination process on the preparation mechanism, phase transformation, tetragonality, and particle size of BaTiO₃ were examined. The reaction that results in the formation of barium titanate initiated at approximately 873?K (600?°C) and seemed to be completed at approximately 1073?K (800?°C) and the polymorphic transformation of cubic to tetragonal initiated at approximately 1173?K (900?°C). It seemed to be completed at approximately 1373?K (1100?°C). According to the reaction mechanism, the formation of BaTiO₃ in the initial stage of the interfacial reaction between BaCO₃ and TiO₂ depends on the BaCO₃ decomposition. In the second stage, the BaTiO₃ formation is controlled by barium diffusion through the barium titanate layer. In this stage, in contrast to the literature, no secondary phase was detected. The overall characterizations showed the temperature is more effective than time on the progress in process of preparation because of its diffusion-controlled nature.     

Corrosion Behavior of Alloy 625 in PbSO4-Pb3O4-PbCl2-ZnO-10?Wt?Pct CdO Molten Salt Medium

Corrosion behavior and degradation mechanisms of alloy 625 under a 47.288 PbSO₄-12.776 Pb₃O₄-6.844PbCl₂-23.108ZnO-10CdO (wt pct) molten salt mixture under air atmosphere were studied at 873?K, 973?K, and 1073?K (600?°C, 700?°C, and 800?°C). Electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) measurements, and potentiodynamic polarization techniques were used to evaluate the degradation mechanisms and characterize the corrosion behavior of the alloy. Morphology, chemical composition, and phase structure of the corrosion products and surface layers of the corroded specimens were studied by scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and X-ray map analyses. Results confirmed that during the exposure of alloy 625 to the molten salt, chromium was mainly dissolved through an active oxidation process as CrO₃, Cr₂O₃, and CrNbO₄, while nickel dissolved only as NiO in the system. Formation of a porous and nonprotective oxide layer with low resistance is responsible for the weak protective properties of the barrier layer at high temperatures of 973?K and 1073?K (700?°C and 800?°C). There were two kinds of attack for INCONEL 625, including general surface corrosion and pitting. Pitting corrosion occurred due to the breakdown of the initial oxide layer by molten salt dissolution of the oxide or oxide cracking.     

On the Solidification and Phase Stability of a Co-Cr-Fe-Ni-Ti High-Entropy Alloy

In the present study, a Co1.5CrFeNi1.5Ti0.5 high-entropy alloy has been investigated for its high-temperature microstructural stability. This material is shown to possess mainly a face-centered cubic (FCC) structure; the η phase is present at the interdendritic region in the as-cast condition, and it is stable between 1073 K and 1273 K (800 °C and 1000 °C); γ′ particles are found throughout the microstructures below 1073 K (800 °C). Segregation analysis has been conducted on a single crystal sample fabricated by a directional solidification process with a single crystal seed. Results show that Co, Cr, and Fe partition toward the dendritic region, while Ni and Ti partition toward the interdendritic areas. Scheil analysis indicates that the solid–liquid partitioning ratio of each element is very similar to those in typical single crystal superalloys.     

Creep rupture and creep behaviour of martensitic X 18 CrMoVNb 11.1 type steel at elevated temperatures and after a temperature transient

Martensitic CrMoVNb 1.4914 type steel, which is at present being tested as a material for fuel element wrapper tubes, was subjected to tests in order to find out the impact on the original hardening and tempering strength of brief temperature rises up to 975°C. T-transients in the range between 800 and 900°C (20–36 min > A_c1b) do not exert a pronounced influence on creep-rupture strength; merely the times up to ≤ 1 % creep strain are clearly reduced, as is indicated by creep-rupture tests at 650°C. There is a more pronounced influence on creep rupture and creep behaviour if the transient extends into the 975°C region and is subsequently held in the range of 600–750°C, where transformation to the pearlite stage occurs. High creep stability is seen at holding temperatures of < 600 and > 400°C. The explanation is furnished by the findings obtained in isothermal creep-rupture tests above A_c1b (800–925°C). Extensive metallographic examination confirms the structural changes expected from the IT-diagram.