Pct Cr Research Articles

Analysis of the composition of α plates isothermally formed in titanium binary alloys

The solute distribution within and near the proeutectoid α plates which were isothermally formed from the β solid solution in Ti-V, Cr, and Fe alloys was measured in a scanning transmission electron microscope (STEM) equipped with an energy dispersive X-ray (EDX) analyzer. In the Ti-4.9 at. pct Cr and 5.1 at. pct Fe alloys in which all measurements were made above the calculated To temperature (the maximum temperature at which the diffusionless β to α transformation can occur), the solute concentration in the α plates was close to final equilibrium as long as the analyzed plates had grown to the sizes comparable to the STEM spatial resolution (∼50 nm). In the Ti-5.4 at. pct V and 2.6 at. pct Fe alloys, the solute concentration in the α plates was already close to equilibrium at very short reaction times at temperatures more than 100 °C below To. The ledgewise diffusion growth mechanism can roughly account for the thickening behavior of these plates, consistent with the results of the composition analysis. In the Cr alloy, a microstructure resembling the lower bainite in steels, i.e., nonlamellar distribution of TiCr2 particles in thick α plates, was formed by coalescence of solute-depleted thin α plates which were formed in aggregates (in sheaves) at an earlier stage of growth.

A reassessment of the Cr-Fe-Ni system

The phase diagram and thermodynamic properties of the Cr-Fe-Ni system were reinvestigated using a large amount of experimental information. Ternary parameters for the liquid, α, and γ phases were evaluated, and satisfactory agreement was obtained with experimental data. Several isothermal and vertical sections, as well as various projections, are presented, and comparisons between experimental information and calculated results from different assessments are discussed. It is also noted that it is not possible to get good agreement at the same time for liquidus temperatures and distribution coefficients above 15 wt pct Cr within the present thermodynamic models. The present assessment reproduces the distribution coefficients better than previous assessments.

Overall reaction kinetics and morphology of austenite decomposition between the upper nose and the Ms of a hypoeutectoid Fe-C-Cr alloy

The overall transformation kinetics and microstructures of an Fe-0.13 pct C-2.99 pct Cr alloy were investigated above and below the bay temperature(Tb) with optical and electron metallography. Most of the kinetic and microstructural aspects of transformation are similar to their counterparts in Fe-C-Mo alloys. Transformation stasis (= incomplete transformation) was observed at all temperatures studied belowTb. However, nonmonotonic behavior of the percent transformation at stasis as a function of undercooling belowTb was found in the present alloy and attributed to a wider temperature interval belowTb in which the Widmanstatten structure is largely suppressed. Also, unlike Fe-C-Mo alloys, fibrous carbides form below as well as aboveTb; in both Mo and Cr alloys, however, interphase boundary carbides appear in association with proeutectoid ferrite both above and belowTb. The solute drag-like effect, somewhat weaker in Fe-C-Cr than in Fe-C-Mo alloys, provides a qualitative explanation for most of the kinetic and microstructural observations made.

Effect of thermal conditions and alloying constituents (Ni, Cr) on macrosegregation in continuously cast high-carbon (0.8 Pct), low-alloy steel

The aim of this study was to investigate the influences of heat transfer, thermal gradient, solidification rate, and the addition of up to 1 pct Ni and 1 pct Cr on the solidification and macrosegregation of high-carbon (0.8 pct), low-alloy steel. Sixteen 13.6-kg laboratory ingots were horizontally and unidirectionally cast in a static moll, fitted on one face with a water-cooled copper chill to simulate a continuous casting mold. Thermocouples, placed in the chill and the mold, were used to calculate heat flux, thermal gradient, and solidification rate. The ingots were examined with respect to macro- and microstructures, distribution of phases, dendrite arm spacing, and solute element distribution. The extent of macrosegregation of carbon and sulfur was determined by wet chemical analysis of drillings, and a TANDEM VAN DE GRAAFF accelerator was used for A1, Si, P, V, S, C, Mn, Ni, and Cu. The extent of macrosegregation of these elements was correlated with heat transfer and thermodynamic distribution coefficient data.

The Kemi stratiform chromitite deposit, northern Finland

The Kemi chromite deposit is hosted by an early Proterozoic (2.44 Ga) layered intrusion. The lower part of the intrusion is composed of peridotitic and pyroxenitic cumulates and chromitite layers. Olivine and chromite have not been observed in the upper part, where plagioclase is the predominant cumulus phase occurring either alone or together with pyroxenes. The intrusion was metamorphosed during the Svecokarelidic orogeny. Mafic minerals of the lower and upper parts of the complex are completely altered to chlorite, serpentine, talc, amphiboles, and carbonates, whereas the middle part is well preserved. The chromite grains, too, suffered from alteration, but the cores of the grains still exhibit primary compositions.A continuous chromitite layer can be traced for the entire length of 15 km of the intrusion. It varies in thickness from a few millimeters to as much as 90 m in the central part of the intrusion. The mineable thicker part of the main chromitite layer is about 4.5 km long. The main chromitite layer is overlain by a sequence of thin chromite-rich layers which continues upward to the stratigraphic level of 500 m above the basal contact of the intrusion. The upper part of the main chromitite is layered, but the lower part is more massive and brecciated.The ore reserves for open-pit mining are ca. 40 million metric tons with an average grade of 26.6 percent Cr 2 O 3 and a Cr/Fe ratio of 1.53. The whole area has additional reserves of about 110 million metric tons of ore. The ore zone is intensely fractured. The host rocks have undergone alteration, which has degraded the quality of the ore.The present surface section of the Kemi intrusion is interpreted as a cross section typical of a funnel-shaped intrusion and plunging about 70 degrees to the northwest. The cumulate sequence, especially the main chromitite layer, is at its thickest in the middle of the intrusion, and the magmatic conduit which fed the intrusion is also thought to have been located just below this thickening.It is suggested that the chromitite was deposited when the input of magma into the Kemi intrusion was contaminated by salic material from the underlying basement complex. This magma mixed with the fresh input of primitive magma resulting in chromite saturation. The chromite crystals formed during mixing in a plume according to the model of Huppert et al. (1986) and accumulated preferentially around the magmatic conduit. Evidence of contamination is found in chromite grains in the form of small inclusions, rich in alkalies, which are thought to represent trapped droplets of the contaminant salic melt.

Electrochemistry of Multiphase Nickel‐Base Alloys in Aqueous Systems

A study has been conducted to evaluate the effect of galvanic coupling between precipitated phases, matrix, and near grain boundary chemistries in nickel‐base alloys. Corrosion potential, potentiodynamic polarization behavior, galvanic effects, and passivation kinetics have been determined for grain boundary γ′, Laves phase, and Ni‐Cr‐Fe matrices for a wide range of chromium contents in aqueous systems over the temperature range 30°–288°C. Electrochemical activity of γ′, Laves phase, and sensitized chemistries is significantly higher than the matrix and varies with composition and temperature. Galvanic couples, , and Laves/matrix in a simulated crack tip environment exhibit higher current density than a sensitized chemistry (6.3 weight percent Cr)/matrix couple. The results of the study show that the electrochemical relationships between grain boundary phases and their couples with the matrix can provide for an extremely local (i.e., at the crack tip) source of anodic and cathodic processes. A correlation between grain boundary galvanic coupling and environmentally assisted intergranular cracking in alloy X‐750 has been made. At low temperatures, around 100°C, hydrogen embrittlement is proposed as the dominant mechanism. At high temperature it is suggested that both hydrogen embrittlement and stress corrosion cracking are viable mechanisms for crack propagation. A schematic model is presented which describes the relationship between embrittlement mechanisms and local electrochemistry at the crack tip.

Transport of Nickel and Oxygen during the Oxidation of Nickel and Dilute Nickel/Chromium Alloy

films have been grown on high‐purity Ni and Ni 0.1 weight percent (w/o) Cr alloy, at 700°C and 0.21 bar, by sequential oxidation in and . The distributions of oxygen, Ni, and Cr isotopes in the films were analyzed using SIMS. For both materials, the rate controlling transport process was found to be the outward diffusion of Ni through the . Inward oxygen gas transport also occurs and is responsible for the growth of new oxide within the films and for the growth of the inner layer of the well‐developed duplex films which grow on the alloy. A mechanism whereby oxygen gas penetrates the films through fissures to form new oxide in available space is proposed to explain the observations and the growth of duplex films.

Thermodynamics of Ca-CaF2 and Ca-CaCI2 systems for the dephosphorization of steel

Calcium is soluble in halide salts which can be used to remove phosphorus from steel as a phosphide ion. The activity and activity coefficient of calcium phosphide, and the equilibrium phosphorus distribution ratio between Ca-CaF2 and Ca-CaCl2 fluxes and pure solid iron were measured as a function of the Ca composition in the flux at 1350 °, 1400 °, and 1450 °. The Ca-Ca halide fluxes were equilibrated with pure solid iron and a Ag-Ca alloy in an iron crucible under an Ar atmosphere. The Ag-Ca alloy was used to maintain a constant chemical potential of calcium. Phosphorus distribution between between these fluxes and solid pure iron increased with increasing calcium activity and decreasing temperature. The activity coefficient of γCa 1.5 P was calculated to be 36.6 at 1350 ° and 11.0 at 1450 ° for a calcium activity of 0.2 (wt pct Ca = 2.5) in the Ca-CaF2; the activity coefficient increases with increasing Ca in the flux. In addition, the activity of Ca in the Ca-Ca halide fluxes was determined. The equilibrium phosphorus distribution ratio between Ca-Ca halide systems and molten chromium steel was calculated as functions of Cr and C contents of the metal and calcium activity in the flux at 1600 °C by using γCa 1.5 P obtained in the present work. This ratio was found to be about 20 for 18 pct Cr stainless steel at 1600 °.

A M�ssbauer investigation of natural troilite from the Agpalilik meteorite

Troilite close to FeS, with 0.17 weight percent Cr as main impurity, was obtained from the Agpalilik meteorite. Powder Mossbauer spectroscopy was made in the temperature range 77–645 K. The full Hamiltonian was applied in the fittings. Assuming the asymmetry parameter η to be constant on passing from the high-temperature NiAs-type structure to the medium-temperature MnP-type structure yields a quadrupole splitting (dq=0.5e2 qQ(1+(η2)/3)1/2) value of −0.25(2) mm/s for these phases. In low-temperature troilite |dq|=0.85 mm/s at room temperature. The combinations of (η, θ, φ) in troilite giving identical spectra range from (0, 49°, -) to (1, 45°, 50°) for negative V zz or from (0.3, 57°, 78°) to (1, 58°, 54°) for positive V zz . Assuming a negative V zz and B‖c gives a θ value in agreement with the shortest Fe-S join being the V zz orientation. The magnetic spin flip of 90° is proposed to occur in the MnP-phase only. The MnP phase-troilite transition occurs at lower temperatures and is more sluggish than in pure FeS.

The Electrochemical Stability and Calculated Free Energies of PtCr Alloys

In this investigation the calculated free energies and electrochemical stability of the system are presented as well as the calculated free energies of the , , and the systems. These alloy systems are currently of interest as fuel cell electrodes providing dealloying does not occur and cause degradation of performance. The calculations indicate that the and systems are the most stable with respect to their elements, followed by and finally by the system. These results are consistent with the experimental data of previous investigations involving the compositional stability in a fuel cell environment for , , and . The calculations are compared with the experimental electrochemical results of this investigation and are found to support the experimental findings in the low chromium concentration samples but do not support the findings for the higher chromium concentration alloys. Experimental results indicated that the samples of 0–35 atom percent (a/o) Cr were electrochemically very stable although the 25 a/o disordered specimen did undergo some minor changes during cycling. The 50 and 80 a/o Cr samples were quite unstable during cycling, yielding a large surface area increase and a depleted Cr layer. The Cr sample was also quite unstable as seen by the surface pitting which occurred.

The wetting characteristics and surface tension of some ni-based alloys on yttria, hafnia, alumina, and zirconia substrates

Sessile drop experiments were carried out in order to measure surface tensions and to investigate wetting characteristics of some Ni-based alloys on various ceramic substrates. The liquid-vapor surface tension (γLV) was found to be 1.764 N/m for pure Ni, 1.45 ± 0.11 N/m for Ni-20 pct Cr, 1.29 ± 0.06 N/m for Ni-20 pct Cr-1 pct Al, and 1.31 ± 0.09 N/m for Ni-20 pct Cr-4 pct Al. The commercial alloys UD520, UD718, UD720, and WASPALOY* showed non-wetting behavior on zirconia but wetting tendency on alumina substrates. Ni-20 pct Cr-1 pct Al showed non-wetting behavior on alumina, hafnia, and yttria substrates whereas Ni-20 pct Cr and Ni-20 pct Cr-4 pct Al were observed to be non-wetting on hafnia but wetting on yttria and alumina substrates. All the systems that exhibited wetting behavior were found to be non-wetting in the beginning; however, wet-ting improved with time. The wetting characteristics were apparently related to impurification of droplets during measurements, which is reflected in the solidification structure, rather than to the presence of oxides on the surface.

Structure and mechanical properties of unidirectionally solidified Fe-Cr-C and Fe-Cr-X-C alloys

In this work four different microstructures were obtained by unidirectional solidification of Fe-Cr-C eutectic alloys. Conditions for zone coupled growth were determined in alloys containing approximately 30 wt pct chromium. Furthermore, mechanical testing indicated that the maximum strength was exhibited by Fe-30Cr-C alloys with cerium or titanium additions. These alloys had the largest volume fraction of eutectic fibers and their ultimate tensile strength was of the order of 3250 MPa. Correlations between the rate of crystal growth(u) and fiber spacing (λ) or tensile strength(Rm) were found and an expression of the typeRm = Aλ-b2 was obtained whereb 2 varied between 0.283 and 0.685. Finally, manganese or chromium (35 wt pct Cr) additions did not lead to appreciable improvements in composite strength for this alloy system.

The Influence of Implanted Yttrium on the Microstructures of Chromia Scales Formed on a Co‐45 Weight Percent Cr Alloy

During the early stages of oxidation of a Co‐45 weight percent Cr alloy that has been implanted with a high dose of Y, some of the Y is incorporated into the film that forms on the alloy surface, and some diffuses back into the alloy. The yttrium in the alloy is incorporated into the oxide film during subsequent oxidation. The latter process occurs slowly, and accounts for the present of Y in the oxide near the alloy/oxide interface after 25h of oxidation at 1000°C. The Y in the oxide is present both as Y cations, which segregate to oxide grain boundaries and which account for most of the Y in the oxide, and as particles of . The particles dissociate during growth of the oxide film to yield and additional Y cations, which also segregate to oxide grain boundaries. Segregation of Y cations to oxide grain boundaries causes a change in the growth mechanism of the oxide, which causes a change in the microstructure of the oxide. It also inhibits grain growth by a solute‐drag effect, thereby yielding an oxide with a much finer grain size.

Grain Boundary Segregation of Yttrium in Chromia Scales

Implantation of a Co‐45 weight percent Cr alloy with a high dose of yttrium causes the oxidation rate of the alloy to decrease by about 100 times at 1000°C in pure oxygen. Also, the mechanism of growth of the scale changes from predominant cation diffusion to predominant anion diffusion, both of which occur along grain boundaries in the oxide. Segregation of yttrium ions to grain boundaries in the scales has been measured using EDX analysis in a STEM, and the reduction in oxidation rate and the change in growth mechanism have been attributed to segregation of yttrium at the grain boundaries. Less than 2 volume percent of particles have been found in a scale after 25h of oxidation at 1000°C, and it has been concluded that these particles do not have a significant effect on the oxidation mechanisms.

Improved 12 Percent Cr Rotor Forgings for Advanced Steam Turbines

For future advanced steam turbines of 4500 psi and 1100°F, a high-temperature rotor is one of the most important components to be developed. We estimated that the usable temperature limit of existing 12Cr steel was approximately 1075°F, and then developed improved 12Cr steels. Prototype rotors were made from three types of developed steels, and were put to various evaluation tests. All three steels had creep-rupture strength exceeding that of existing 12Cr steels and were satisfactory in quality and in the other mechanical properties.

Effect of Surface‐Applied Reactive Element Oxide on the Oxidation of Binary Alloys Containing Cr

The influence of surface‐applied Ca, Ce, Hf, La, Y, and Zr nitrate‐converted oxides on the oxidation behavior of Co‐15 weight percent (w/o) Cr, Co‐25 w/o Cr, and Ni‐25 w/o Cr alloys at 1000° and 1100°C in 1 atm has been studied. The surface oxides appeared to be most beneficial on the established forming alloy. Surface‐applied , , and were particularly effective in reducing the growth rate of the scale and improving the scale adhesion. The presence of these surface oxides also prevented base metal oxide formation and changed the growth direction of the scale. All of these observed effects were similar to those found when the reactive element oxides were present within the alloys. However, the presence of surface made the oxide scales very nonadherent causing a breakaway behavior at the early stage of the oxidation process. None of the surface‐applied oxides showed any effects on the forming alloy, and they acted as a semibarrier on the borderline former. Unlike the case of dispersoids present in the alloy, these surface‐applied oxides were unable to promote a continuous layer at alloy Cr levels lower than those normally required to form the scale.

The mechanism of intergranular cracking of Ni-Cr-Fe alloys in sodium tetrathionate

A series of electrochemical, immersion, and constant extension rate tests was conducted on samples of Ni-16Cr-9Fe in sodium tetrathionate at room temperature. Samples were heat treated to produce severe chromium depletion at the grain boundaries. Titrimetric analysis of the tetrathionate solution, before and after exposure to a sensitized alloy, under an applied cathodic current shows that the tetrathionate ion is reduced. The species primarily responsible for the observed IGA in immersion tests and IG cracking in constant extension rate tests is the tetrathionate ion, S4O6 -, although elemental S also causes shallow IGA. The mechanism responsible for the observed IGA and IG cracking in sensitized Ni-Cr-Fe alloys is stress assisted intergranular attack with the effect of stress being purely mechanical in nature. The degree of IGA and IG cracking is directly related to the grain boundary chromium content. Samples with less than 5 wt pct Cr at the grain boundary are rapidly attacked while those with 8 wt pct Cr are less susceptible and 12 wt pct Cr renders the grain boundary immune to attack. Lower extension rates and higher Na2S4O6 concentrations represent more aggressive conditions for attack.

Phase relationships in the Fe−Cr−C system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the Fe-Cr-C system. The investigation consisted of measurements of tie-lines and the liquidus surface of the liquid-delta (bcc) and liquid-gamma (fcc) equilibria in the Gibbs triangle, bounded by 0 to 1.4 wt pct C and 0 to 25 wt pct Cr (bal. Fe). The peritectic surface of the three-phase equilibrium was also measured. The temperature ranged from 1811 to about 1750 K. The tie-lines were obtained from liquid-solid equilibrium couples, and the liquidus and peritectic surfaces, by differential thermal analysis (DTA). A statistical procedure was applied to determine from the experimental results the parameters required for a thermodynamic model of the system. Calculations by the model are in good agreement with the experimental results. As a consequence the model can be used to interpolate and extrapolate properties and compositions of phases in equilibrium in the system within the composition and temperature field investigated.

Microstructure-composition relationships and Ms temperatures in Fe-Cr-Mn-N alloys

Microstructure-composition relationships and Ms temperatures have been determined in high purity nitrided Fe-Cr-Mn alloys, as part of a program to develop improved corrosion-abrasion resistant steels with unstable austenitic microstructures. Compositions in the range 8 to 12 pct Cr, 0 to 10 pct Mn, and 0 to 0.6 pct N were investigated by a resistivity technique to determine Ms temperatures and by X-ray diffraction and metallography to determine constitution. Hardness measurements were also made. At the low alloy end of the range, microstructures after annealing and air cooling are fully martensitic while at the high alloy end they are fully austenitic. At intermediate compositions, mixed martensite-austenite microstructures (with epsilon present as a minor phase in some cases) and unstable austenitic microstructures are obtained. The austenitic alloys contain a high density of stacking faults and the unstable austenitic alloys transform to martensite on deformation. At low N contents (up to at least 0.25 pct N) the Ms-composition relationship is linear and described by: Ms = 555 - 9(Cr - 8) - 40Mn - 450N [1] where Ms is in °C and Cr, Mn, and N are the weight percentages of these elements. At higher N contents, the Ms generally falls more rapidly with increasing nitrogen content. Nitrogen solubility at 1050 °C exceeds about 0.3 pct in all alloys and increases with increasing Cr and Mn content. In commercial purity steels, unstable austenitic microstructures are expected to be obtained in compositions around 10 to 14 pct Cr, 8 to 12 pct Mn, and 0.1 to 0.3 pct N when the total level of these elements is selected to ensure the Ms is below room temperature.

Structure and properties of corrosion and wear resistant Cr-Mn-N steels

Steels containing about 12 pct Cr, 10 pct Mn, and 0.2 pct N have been shown to have an unstable austenitic microstructure and have good ductility, extreme work hardening, high fracture strength, excellent toughness, good wear resistance, and moderate corrosion resistance. A series of alloys containing 9.5 to 12.8 pct Cr, 5.0 to 10.4 pct Mn, 0.16 to 0.32 pct N, 0.05 pct C, and residual elements typical of stainless steels was investigated by microstructural examination and mechanical, abrasion, and corrosion testing. Microstructures ranged from martensite to unstable austenite. The unstable austenitic steels transformed to α martensite on deformation and displayed very high work hardening, exceeding that of Hadfield’s manganese steels. Fracture strengths similar to high carbon martensitic stainless steels were obtained while ductility and toughness values were high, similar to austenitic stainless steels. Resistance to abrasive wear exceeded that of commercial abrasion resistant steels and other stainless steels. Corrosion resistance was similar to that of other 12 pct Cr steels. Properties were not much affected by minor compositional variations or rolled-in nitrogen porosity. In 12 pct Cr-10 pct Mn alloys, ingot porosity was avoided when nitrogen levels were below 0.19 pet, and austenitic microstructures were obtained when nitrogen levels exceeded 0.14 pct.