Temper embrittlement of Ni-Cr steel by Sn

Tin-induced temper embrittlement of 3.5 pct Ni, 1.7 pct Cr steels containing 0.2 and 0.4 pct C was studied by means of notched-bar testing, scanning electron fractography, and Auger electron spectroscopy of isothermally aged specimens. The ductile-brittle transition temperature varied linearly with Sn concentration on grain boundaries at a rate which increased with hardness of the steel. The rate of approach to the steady state level of embrittlement was faster in the higher C steel. The grain boundary concentrations of Ni and Sn bear a unique relationship in a manner analogous to the behavior found previously in Sb-doped steels. The potency of Sn as an embrittling element is somewhat less than that of Sb, but considerably greater than that of P. Formerly Research Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia,PA Formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania H. C. Feng Formerly with the Research Staff, LRSM, University of Pennsylvania     

Carbon solubility as carbide in calcium silicate melts

R.A. Berryman, Formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto,     

Modeling of base metal dissolution behavior during transient liquid-phase brazing

Detailed finite difference modeling of dissolution during transient liquid-phase (TLP) brazing of nickel using Ni-11 wt pct P and Ni-14 wt pct Cr-10 wt pct P filler metals is carried out assuminginfinite andfinite heating rates between the filler melting temperature (T _m) and the brazing temperature (T _b). Two conditions are modeled during brazing, namely, where the liquid width due to base metal dissolution isunrestrained (variable) and where the liquid width is keptconstant due to expulsion of excess liquid during the brazing process. The time required for completion of dissolution is proportional to the square of the filler metal thickness when an infinite heating rate betweenT _m andT _b is assumed. This occurs for both for constant and unrestrained liquid width situations. Also, the advance of the solid-liquid interface is approximately related to the reciprocal of the square root of the solute diffusivity in the liquid, except in the final stages of the dissolution process. When a multicomponent filler metal composition (Ni14 wt pct Cr-10 wt pct P) is used, the time for completion of dissolution exceeds that when using a simple binary filler metal composition. The phosphorous concentration in the liquid approaches that of the liquidus line when the heating rate betweenT _m andT _b decreases and thin filler metals are used. This tendency can be estimated using a homogenizing index (z) which combines the effects of filler metal thickness, diffusivity in the liquid, brazing temperature and heating rate, and a distribution index(a). When thick filler metals are used, lower heating rates produce more dissolution at temperatures betweenT _m andT _b and less dissolution at the final brazing temperature. Conversely, heating slowly when using thin filler metals produces less base metal dissolution (at any temperature) and negligible dissolution at the final brazing temperature. At very low heating rates, solidification can occur while heating fromT _m toT _b. Formerly Visiting Research Scientist, Department of Metallurgy and Materials Science, University of Toronto Formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto     

Effect of oxygen on hydrogen cracking in high-strength weld metal

The effect of oxygen content on the susceptibility of high-strength weld metal to hydrogen cracking is examined. Increasing oxygen content had a detrimental effect on the cracking susceptibility of weld metal containing a dψusible hydrogen content of 4.7 ppm. In weld metal containing a much lower dψusible hydrogen content (0.87 ppm), increasing weld metal oxygen content had no detrimental effect on hydrogen cracking susceptibility. These results are explained by a model which proposes that hydrogen cracking occurs when a critical oxide inclusion density promotes intergranular fracture at prior austenite grain boundaries and when a critical level of hydrogen is present in the weld metal. For the same level of hydrogen (moisture) contamination, high-strength weld metals containing oxygen contents greater than 200 ppm will be much more susceptible to hydrogen cracking than deposits made using inert gas-shielded or vacuum-operated welding processes. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto     

Effect of oxygen on hydrogen cracking in high-strength weld metal

The effect of oxygen content on the susceptibility of high-strength weld metal to hydrogen cracking is examined. Increasing oxygen content had a detrimental effect on the cracking susceptibility of weld metal containing a dψusible hydrogen content of 4.7 ppm. In weld metal containing a much lower dψusible hydrogen content (0.87 ppm), increasing weld metal oxygen content had no detrimental effect on hydrogen cracking susceptibility. These results are explained by a model which proposes that hydrogen cracking occurs when a critical oxide inclusion density promotes intergranular fracture at prior austenite grain boundaries and when a critical level of hydrogen is present in the weld metal. For the same level of hydrogen (moisture) contamination, high-strength weld metals containing oxygen contents greater than 200 ppm will be much more susceptible to hydrogen cracking than deposits made using inert gas-shielded or vacuum-operated welding processes. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto     

The effect of an air environment on the creep and rupture behavior of a nickel-base high temperature alloy

The behavior of wrought and cast polycrystalline Udimet 700 tested in static tension at 1200 K (1700†F) has been determined. An air environment decreases rupture life and ductility, except in very coarse-grained cast specimens, because of premature failure by stress-assisted grain boundary oxidation and cracking. In very coarse-grained cast specimens greater life and ductility are found in air than in vacuum, presumably due to the paucity of transverse grain boundaries and to some type of surface hardening effect. Wrought specimens exhibit greater grain boundary sliding, hence more cracking, shorter life, and lower ductility, than cast specimens in any environment. This is attributed to differences in grain boundary topography. Formerly Research Fellow, Department of Metallurgy and Materials Science and LRSM, University of Pennsylvania, Philadelphia     

Reducibility of laterite ores

The reducibility of several types of lateritic nickel bearing ores was investigated. The ores were reduced with hydrogen over a temperature range of 673 to 1273 K and reaction times from 5 to 80 minutes. The fraction of nickel, iron, and cobalt reduced to the metallic state was determined by leaching the reacted samples with a bromine-methanol solution followed by atomic absorption analysis for the individual elements. The reducibility of nickel increased with increasing iron concentration of the ore. Increased reduction temperature greatly raised the amount of nickel reduced for ores with high iron concentrations. The cobalt reducibility decreased with increasing iron concentration of the ore. Changes in reduction temperature affected cobalt reduction less than nickel reduction. The observed changes in reducibility have been attributed to the formation of phases which incorporate nickel and cobalt. The major ore components were plotted on the ternary phase diagram of the SiO₂+(Al₂O₃)-MgO-FeO system. It is shown how this plot can be used to predict the reducibility of different types of lateritic ores. Formerly with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada     

The influence of intercritical heat treatment on the temper embrittlement susceptibility of aP-Doped Ni-Cr steel

Temper embrittlement of a Ni-Cr steel doped with 0.06 wt pct P aged at 480°C after an intercritical heat treatment (IHT) has been compared with that of the conventionally heat treated steel with a range of prior austenite grain sizes. The IHT virtually eliminated the embrittlement susceptibility, but low temperature brittle fracture was still intergranular. It appears that most of the benefit of IHT in this steel comes from microstructural refinement; however, IHT also reduced the amount of segregation of Ni and P to grain boundaries. This is believed to be connected with partitioning of Cr during IHT. Formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania Formerly on the Research Staff, LRSM, University of Pennsylvania     

Interface sliding,migration, and cracking during fatigue deformation of a superplastic aluminum-zinc eutectoid alloy

A superplastic aluminum-zinc eutectoid alloy was fatigue tested at 100 °C and 200 °C at different constant plastic strain amplitudes and strain rates. During fatigue deformation, the average peak stress increased with increasing strain rate and grain size and decreasing temperature. It was almost independent of the plastic strain amplitude. To detect interfacial sliding, interphase boundary migration, and intergranular cracking, selected areas on surfaces were examined before fatigue deformation and re-examined after fatigue deformation. Interface sliding, which was almost reversible, occurred on (Al)/(Al) and (Zn)/(Zn) grain boundaries and on (Al)/(Zn) interphase boundaries. Grains appeared to slide in groups. Cracks followed grain and interphase boundaries. Along an intergranular crack, most interfaces were (Zn)/(Zn) grain boundaries and (Al)/ (Zn) interphase boundaries. Grains deformed to accommodate interfacial sliding. The absence of slip lines suggested that diffusional creep made a significant contribution to deformation in grains of the zinc-rich phase. Deformation of the aluminum-rich phase involved the glide and climb of dislocations. J. W. BOWDEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto.     

Grain-boundary relaxations in an Fe-Ni-Cr alloy

Grain boundary internal friction spectra have been studied in a stabilized and an unstabilized Fe-20 pct Cr-25 pct Ni alloy. The effect of grain size and of prior creep on the relaxation phenomena have been studied. It has been found that the spectrum consists of three constituent peaks occurring in the temperature regions of 630°, 690°, and 770°C at 1 Hz. The relaxation strength and the width (seen through the β factor) of each of the peaks have been observed to decrease and the peak temperature to increase with increasing grain size. It has been suggested that the relaxation processes at the grain boundaries are associated with the solvent iron, solute chromium, and solute nickel atoms, respectively. Deformation by creep increases the peak height, width, and temperature of the peaks in the stabilized alloy, but has no effect on the grain-boundary relaxation spectrum of the unstabilized alloy. From the study of stabilized and unstabilized alloys it can be implied that the dislocation structure at the grain boundaries influences the relaxation strength of the grain-boundary peaks. Formerly Graduate Student, Department of Metallurgy and Materials Technology, University of Surrey, United Kingdom     

The effect of fatigue deformation on microstructural evolution in a superplastic aluminum-zinc eutectoid alloy

The microstructure of a superplastic aluminum-zinc eutectoid alloy that had been fatigue tested at 100 °C and 200 °C was examined. At 100 °C, in the aluminum-rich phase, precipitate-free zones (PFZs) formed beside (Al)/(Zn) interphase boundaries because of interphase boundary migration. Interphase boundary migration was due to phase growth, which proceeded more rapidly during fatigue deformation than during annealing. At 100 °C and 200 °C, PFZs beside (Al)/(Al) grain boundaries were asymmetrical owing to grain boundary migration. The precipitation of the equilibrium zinc-rich phase in the aluminum-rich phase proceeded more rapidly during fatigue deformation than during annealing. J. W. BOWDEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON.     

Void growth and coalescence in constrained silver interlayers

The mechanisms of void growth and coalescence during fracture of thin Ag interlayers were studied by tensile testing and metallographic examination. No measurable void growth was observed in the deformed interlayers prior to fracture. The fracture surface dimple size, however, increased with increasing interlayer diameter-to-thickness ratio(D/T). The experimental results suggest that fracture in the constrained Ag interlayers occurred by void initiation at silicon oxide inclusions followed immediately by void coalescence. The highly triaxial stress state in the interlayer promoted void coalescence by plastic instability and accounts for the observed change in fracture surface dimple size withD/T. An expression, based upon a slip-line field model of the deformation zone between neighboring voids, is presented which relates the dimple size to the average inclusion diameter and the stress state in the interlayer. The predictions of the expression are in broad agreement with the experimental data. R.J. KLASSEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto. G.C. WEATHERLY, formerly with the Department of Metallurgy and Materials Science, University of Toronto.     

Convective heat-transfer measurements in liquid metals under different fluid flow conditions

In this study an experimental method to measure convective heat transfer characteristics in liquid metals is presented. This method involves the immersion into a metal bath of a solid specimen whose melting point is equal to or lower than that of the metal or alloy in the metal bath, and which will not react chemically with the liquid metal or alloys used. The specimen should have a hollow bore whose opening is held above the surface of the liquid metal; immersion continues until such a time as the liquid metal penetrates the hollow bore. The apparent weight of the specimen is monitored to determine the rate at which the net downward force changes. Experimental results are reported for liquid aluminum, liquid copper, and liquid steel. Those experimental results were conducted under different fluid flow conditions. The applicability of this method to liquid slags is also discussed. Formerly Postdoctoral Research Fellow with the Department of Metallurgy and Materials Science, University of Toronto     

Grain boundary void nucleation in astroloy produced by room temperature deformation and anneal

Dyson and co-workers have shown that the creep life of a nickel base superalloy can be greatly shortened if the material is strained at room temperature before the creep test is carried out. They found that a prestrain followed by a short annealing time produces small grain boundary cavities, and it is the presence of these prenucleated voids which so seriously degrades service life at elevated temperatures. The present work explores the relationship between microstructure and prestrain void nucleation. Samples of the nickel base superalloy astroloy were given various heat treatments which led to significantly different microstructures. It was found that voids resulting from a prestrain-anneal treatment form preferentially at the ends of carbides on grain boundaries oriented roughly parallel to the prestrain tensile axis or rolling direction. Void spacing in the various microstructures is proportional to (but larger than) carbide spacing. The growth of these cavities during annealing is attributed to the presence of tensile residual stresses arising from the difference in deformability between grain boundary regions and the relatively soft matrix. Formerly Postdoctoral Fellow with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL Formerly Postdoctoral Fellow with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL     

Arsenic activities in molten copper and copper sulfide melts

In recent years, the concentration of the group Va elements such as arsenic, antimony, and bismuth has been increasing in copper concentrates. The elimination and recovery of these elements during the copper smelting process have presented serious problems. While the distribution of minor elements has been studied extensively, very little knowledge exists on the activities of these minor elements in copper mattes. Consequently, in this study the activities of arsenic were measured to determine activity coefficients of arsenic in the dilute solution region of molten copper, in Cu₂S saturated copper, and in copper mattes equilibrated with copper at 1423 K by a mass spectrometric Knudsen effusion technique. Formerly with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada     

The oxidation of liquid Ni3S2

Using a levitation technique, molten nickel sulfide droplets were oxidized at temperatures above 1500°C under oxygen potentials varying from 5 to 40 pct in He gas. To analyze the results, the oxidation process was divided into two stages. The first stage corresponded to the desulfurization of Ni₃S₂ by oxidation of the dissolved sulfur. In the second stage, a small amount of desulfurization, oxidation of nickel vapor and absorption of oxygen gas into the droplet occurred. Both stages were found to be controlled by mass transfer of oxygen within the gas boundary layer. Under conditions of high oxygen potential, a halo appeared around the levitated droplet during the initial period. This halo disappeared during desulfurization, but reappeared towards the end of the oxidation period. Formerly Post-Doctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto.     

A noncrystalline Pt-Sb phase and its equilibration kinetics

Utilizing splat cooling, a metastable noncrystalline phase was obtained in Pt-Sb alloys with 30 to 43 pct Sb and in Pt-Si alloys with the eutectic composition Pt-68 Si. These nonequilibrium structures can be retained indefinitely at room temperature. The kinetics of the decomposition of the metastable noncrystalline phase in a Pt-34 pct Sb alloy was studied in detail by X-ray diffraction, electron diffraction, and electron transmission microscopy. The decomposision starts at an appreciable rate at about 210°C, and progresses gradually with the precipitation of relatively equiaxed grains of crystalline phases. From isothermal kinetic data, average activation energies from 55.5 to 49.5 (±4) kcal per mole were derived for the overall decomposition process. Formerly Research Assistant, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass. Formerly Research Associate, Department of Metallurgy and Materials Science, M.I.T.     

Liquid- solid interface migration at grain boundary regions during transient liquid phase brazing

A two-dimensional (2-D) finite difference model has been used to analyze the effect of grain boundary regions on the migration of the liquid-solid interface during transient liquid phase (TLP) brazing of Ni with Ni-11 wt pct P filler metal. This work has been carried out to explain the differences observed between actual and calculated completion times for isothermal solid- ification during TLP brazing and the faster isothermal solidification rates when brazing fine- grained nickel-base material. Modeling considers the situation where the grain boundary intersects the liquid-solid interface at right angles. Four factors are considered in addition to solute diffusion in solid and liquid phases,viz., (1) high diffusivity at the grain boundary region, (2) the balance between the grain boundary energy and the liquid-solid interfacial energy, (3) the interfacial energy due to the curvature of the liquid-solid interface, and (4) diffusional flow along the liquid-solid interface (produced by the gradient of solute chemical potential resulting from factors (2) and (3)). Increased solute diffusivity at the grain boundary region has a neg- ligible effect on migration of the liquid-solid interface in the bulk region and shifts the interface at the grain boundary region in a direction opposite that observed in actual brazed samples. On the other hand, when factors (2) through (4) above are taken into account, the liquid-solid interface in the region of the grain boundary is displaced in the same direction as in the ex- perimental results and liquid penetration comparable with the experimental results occurs at the grain boundary region. Factors (2) through (4) accelerate the isothermal solidification process in the bulk region in accordance with actual experimental test results. Formerly Visiting Scientists Formerly Visiting Scientists     

The formation of austenite in a low-alloy steel

Austenite formation from different microstructural conditions has been studied in an Fe-lV-0.2C steel. The starting microstructures of ferrite, bainite, and martensite, and the morphology of austenite formation have been examined in detail by light microscopy and transmission electron microscopy. Retained austenite in quenched martensitic areas has been used to establish the crystallographic relationships during austenite nucleation, and to indicate the behavior of the initial vanadium carbide dispersion present in the ferrite starting microstructure. Limited measurements were also made of the kinetics of austenitization. Formerly Research Student, Department of Metallurgy and Materials Science, University of Cambridge. Formerly of Royal Society, Warren Research Fellow, Department of Metallurgy and Materials Science, University of Cambridge.     

Temper embrittlement of Ni-Cr steel by antimony: III. Effects of Ni and Cr

The individual effects of Ni and Cr on temper embrittlement in a Ni-Cr steel doped with Sb were studied by notched-bar testing and Auger electron spectroscopy. Nickel is required for significant segregation of Sb to grain boundaries (and concomitant embrittlement). The addition of Cr somehow makes Ni more effective in this respect; segregation of Cr was not detected. Precipitation of Cr-rich carbides, which depletes the α matrix of Cr, reduces the embrittlement susceptibility. The amount of segregated Ni is a function only of the amount of segregated Sb; it is independent of Cr or C content of the steel. Formerly Research Fellow at Department of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia, PA 19174.