Cr Wt Research Articles

Simulating Time Temperature Transformation Diagram of Steel Using Artificial Neural Network

Design and development of steel is essentially governed by the Time-Temperature-Transformation (TTT) diagram. The diagram predicts the phase evolution during isothermal transformation schedules for a given chemistry. Selection of chemistry for obtaining a desired microstructure in steel under isothermal schedule needs determination of the TTT diagrams either by extensive experimental exercise or by rigorous thermodynamic calculations. Artificial neural network (ANN) technique has recently been employed as a versatile tool to predict the CCT diagrams of steels. The present work aims to identify a favorable composition capable of yielding an ultrafine bainitic microstructure by isothermal holding of austenite at low homologous temperature. To achieve this, TTT diagrams of varying compositions have been predicted a priori to reduce the required experimental trials. The exercise has led to the development of bainitic microstructure of nanoscale dimension in steel having 0.7C-2.0Mn-1.5Si-0.3Mo-1.5Cr (wt%). Experimental trial with the predicted composition of bainitic steel resulted into attractive combination of strength and ductility.

State of Chromium in CrSiBEA Zeolite Prepared by the Two-Step Postsynthesis Method: XRD, FTIR, UV−Vis, EPR, TPR, and XAS Studies

CrxSiBEA zeolites (x = 0.2, 0.9, and 3.4 Cr wt %) are prepared by a two-step postsynthesis method that allows controlling the introduction of chromium ions in the zeolite structure. The introduction of Cr ions in vacant T-sites and their reaction with silanol groups are evidenced by XRD and FTIR, respectively. It generates Bronsted acidic sites as shown by FTIR of pyridine adsorbed as a molecular probe. The state of chromium species is characterized by DR UV−vis, EPR, TPR, and XAS. The combined use of these techniques allows one to show that the state of chromium species depend on the treatment condition of CrSiBEA samples. In the as-prepared CrxSiBEA samples chromium species are mainly present as isolated mononuclear octahedral Cr(III) species, as evidenced by DR UV−vis and XAS spectroscopy. In calcined CrxSiBEA samples isolated mononuclear tetrahedral Cr(VI) and octahedral Cr(III) species are present as shown by DR UV−vis. After further outgassing under vacuum, part of the isolated mononuclear tetrahedr...

Surface composition and structure of Ni–Cr sputtered coatings exposed in air at room temperature

Nichrome coatings have been deposited using the d.c. sputtering of the target (80% Ni–20% Cr wt.) by Ar+ ions at a working pressure of 10−1 Pa and at room temperature. After the air exposure of samples at room temperature, their phase composition and structure was studied by X-ray diffraction, while the surface elemental composition was determined by low energy ion scattering (LEIS) and Auger electron spectroscopy (AES). AES has been also used for the bulk composition analysis. There is a significant difference between the compositions of the surface and of the bulk. Surface segregation of chromium was observed using both LEIS and AES, and it was found that the chromium concentration increases with that of oxygen adsorbed at the surface during the air exposure. The nichrome surface composition is qualitatively the same as in the case of its exposure to O2 at room temperature, but significantly different from that of the thermally annealed nichrome to 400°C in air.

Mössbauer spectroscopic studies of Fe-20 wt.% Cr ball milled alloy

Interesting differences were noticed in the alloying process during ball milling of Fe-10 wt.% Cr and Fe-20 wt.% Cr alloys by 57Fe Mossbauer spectroscopic studies. In both cases, there is almost no diffusion of Fein Cr or vice versa up to 20 h of milling time. As the powders are milled for another 20 h substantive changes occur in the Mossbauer spectra showing atomic level mixing. But the two. compositions behave differently with respect to alloying. Fe-20 wt.% Cr sample does not differ much in the hyperfine field distribution as it is milled from 40 to 100 h. On the other hand, the hyperfine field distribution keeps on changing with milling time for Fe-10 wt.% Cr sample even up to 100 h of milling. The average crystallite size is found to be 7.5 nm for Fe-10 wt.% Cr and 6.5 nm in Fe-20 wt.% Cr after milling.

Segregation and eutectic formation in solidification of Fe-1C-1 5-Cr steel

The microstructure and solute segregation have been investigated in a continuously cast bloom and laboratory cast ingot of Fe–1C–1.5Cr (wt-%) steel. Eutectic carbide formation was observed only in the centreline region in the continuously cast bloom. In both specimens, the maximum chromium level detected was 3% in the columnar and 5% in the equiaxed region, while the minimum remained at 1.2% in both regions. The corresponding segregation ratios (Cmax/Cmin) were 2.5 and 5, in agreement with many previous studies. By numerical modelling of microsegregation it has been shown that the equilibrium partition coefficient of chromium kCr, which changes with carbon content, has a significant effect on chromium distribution during solidification. The carbon distribution may be taken to be in equilibrium during solidification, while that of chromium develops a concentration gradient in the solid. Numerical predictions of segregation behaviour, assuming local equilibrium at the liquid/solid interface, backdiffusion in the solid and complete mixing in the residual liquid, are consistent with experimental results in the columnar and equiaxed regions. The conclusion that eutectic carbide observed in the centreline region must have resulted from macrosegregation is supported by an estimate of the composition of the enriched liquid.

Alumina-supported chromium-based mixed-oxide catalysts in oxidative dehydrogenation of isobutane to isobutene

The effect of various metal additives on the catalytic performance of a chromium-based alumina-supported catalyst in oxidative dehydrogenation of isobutane was investigated in the low reaction temperature range 200–350 °C and at atmospheric pressure. Various parameters such as Cr wt.% in Cr–M–O system, different iC 4H 10:O 2 ratios at a total flow rate of 75 cm 3/min, and contact time on the activity and selectivity of the catalysts were varied in this study. For the binary catalysts, the addition of Co and Ni enhanced the overall activity while W and Mo improved the catalyst selectivity to iC 4H 8. The presence of V and Li deteriorated significantly the catalyst activity and selectivity. Increasing the percent of Cr in Cr–M–O increased iC 4H 10 and O 2 conversion at the expense of selectivity. Tertiary systems such as Cr–W–Co–O, Cr–W–Ni–O and Cr–Ni–Co–O gave a much better overall performance compared with Cr–O alone. In particular, Cr–W–Co–O improved the overall activity of Cr–O catalyst by 17% while its iC 4H 8 selectivity improved by 15%. The low temperature activity of these catalysts makes them important candidates for further investigations.

An APFIM and TEM Study of Ni4Mo Precipitation In a Commercial Ni-28% Mo-1.4 % Fe-0.4% Cr Wt. % Alloy

Abstract Ni-Mo alloys containing at least 26 wt.% Mo have a negligible corrosion rate in boiling 10% hydrochloric acid and are therefore used in corrosive environments. A series of commercial Ni-Mo alloys has been developed with subtle variations in chemical composition. These alloys usually contain ∼28 wt.% Mo with additions of up to 5 % Fe and Cr. A significant amount of research has been performed to understand the microstructure and properties of these alloys, although most of the effort has concentrated on the Ni-Mo binary system. In some alloys with low Fe and Cr contents, a severe embrittlement problem has been observed due to the formation of the Ni4Mo (D1a-ordered) phase within the microstructure. The calculated section of the ternary Ni-Mo-Fe phase diagram in Fig. 1 illustrates that the addition of Fe stabilises Ni3Mo with respect to the brittle Ni4Mo phase.

Influence of heat treatlment on shape memory properties of a Cu-Al-Be alloy containing chromium

In this paper, by comparing the shape memory behaviour, phase structure, substructure of martensite, and the recovery behaviour of quenched in vacancies from 20 to 200°C of direct quenched and step quenched Cu-11.58AI-0.41Be-0.2Cr (wt-%) alloy, the effect of heat treatment on shape memory properties was analysed. It was found that the Cu-Ar-Be-Cr alloy tested exhibited low starting temperatures and very high final temperatures of shape recovery. The shape memory properties of the step quenched Cu-AI-Be-Cr alloy were better than those of the direct quenched Cu-AI-Be-Cr alloy. It was also found that the crystal structure, substructure of martensite, and vacancy concentration all affected the shape memory properties, and that a mixed mechanism was responsible for the stabilisation of martensite. Some abnormal defects such as non-basal plane planar defects and dislocations were observed in the martensite obtained by direct quenching, which is one of the reasons for the stabilisation of the martensite. The recovery behaviour of quenched in vacancies revealed that the vacancy concentration mainly affects the starting temperature of shape recovery.

ESCA studies of Cr-Co alloys

ESCA examination of films formed on Cr-Co alloys after immersion in 0.1M NaCl for 24 h has shown that the thickness of passive films decreased with an increase in chromium content. Surface films consisted of chromium and cobalt oxides as Cr2O3 and CoO. The amount of CoO in the surface film of the alloy was decreased with an increase in chromium but Cr2O3 was found at a greater depth in the passive film at any composition. Cr2O3 was a major component of the surface film when the chromium content in the alloy was 10% or higher. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate. Both Co-10 wt.% Cr and Co-30 wt.% Cr alloys investigated showed a lower corrosion rate than the Co-5 wt.% Cr alloy. Corrosion rate measured could be correlated to the surface film composition and structure as determined by ESCA.

Internal-external transition for the oxidation of Fe-Cr-Ni austenitic stainless steels in steam

Several Fe-Cr-Ni austenitic stainless steels (Cr wt.%: 13–25, Ni wt.%: 15) were oxidized in steam for 1000 hr at 500–900°C. The oxide scales were examined and categorized with respect to the chromium concentration and the grain size of the base metal. Experiments showed three conditions for the critical bulk Cr concentration and the oxidation temperature at which the oxidation behavior changed drastically. Metallographic examination showed that two of these three conditions resulted from the internal-external transition of Cr2O3 either on the metal surface or along the grain boundaries of the base metal. Attempts were made to interpret these conditions from the available oxidation theories. Atkinson's treatment was employed with some modification to incorporate the grain-boundary diffusion of Cr in the base metal. The calculation basically explained the internal-external transition for the oxidation of these steels.

Effect of presulfidation on the oxidation behavior of Co-based alloys. Part I. Presulfidation at sulfur partial pressures above the dissociation pressure of cobalt sulfide

The effects of presulfidation in H2-H2S atmospheres of sulfur activity sufficient to form cobalt and chromium sulfides on the oxidation rates of Co-Cr binary alloys containing 0–25 wt.% Cr and Co-25 wt.% Cr alloys containing 0–2 wt.% C have been investigated. Presulfidation increases the oxidation rate, but the effect is not very dramatic. Carbon additions to the Co-25 wt.% Cr alloy progressively increase the oxidation rate, but not to as great an extent as a simple model based on the reduction of the chromium activity in the alloy. Sulfur released from the preformed sulfides by oxidation diffuses into the alloy precipitating fresh sulfides, there appears to be no outward diffusion of sulfur through the oxide scale. These internal sulfides have a liquid-like morphology in cobalt-base alloys when the oxidation is carried out at 1000°C, as compared to 800°C in corresponding nickel-base alloys. When the sulfide layer produced during the presulfidation is thin, so that oxidation destroys the continuous sulfide layer, the subsequent scale morphologies after oxidation exhibit many features similar to samples subjected to hot corrosion in environments containing sodium sulfate.

Shock deformation of k-state in Ni-Cr alloys

Abstract The change of electrical resistivity was used to study the ordering kinetics and the anomalous resistivity behaviour in Ni-22 wt. % Cr and Ni-30 wt. % Cr (Ni-22 Cr and Ni-30 Cr) before and after shock deformation. Ordering was found to occur in two stages: the attainment of an equilibrium domain size, and the increase of long-range order (LRO) within the domains. Specimens of Ni-22 Cr and Ni-30 Cr in the ordered and disorderd states were subjected to explosive shock loading at shock pressures between 90 and 300 kbars. In the pressure range of 200 to 300 kbars, the resistivity of the ordered alloys decreased sharply. The resistivity of the shock-deformed initially quenched alloys decreased continuously between 90 and 300 kbars. The decrease in resistivity with shock deformation for the quenched alloys was related to the destruction of short-range order (SRO).