Showing papers on "Hardening (metallurgy) published in 1981"

Automatic method for fabricating a three‐dimensional plastic model with photo‐hardening polymer

Abstract: A new method for automatic fabrication of a three‐dimensional plastic model is presented. A solid model is fabricated by exposing liquid photo‐hardening polymer to ultraviolet rays, and stacking the cross‐sectional solidified layers. Three types of equipment were constructed, their operational conditions were investigated, and some solid models were fabricated. A transparent plastic model whose internal structure was visible from the outside of the model was obtained. The operations are simple and can be easily automated. The present method is useful in displaying three‐dimensional shapes.

Cyclic deformation and fatigue behaviour of α-iron mono-and polycrystals

Abstract: The reported studies are based on a series of cyclic deformation tests that were conducted at room temperature on decarburized high-purity α-iron specimens in mono-and polycrystalline form. The experimental data cover plastic strain ranges Δe pl in the regime 10−4 ≲ Δe pl ≲ 10−2 and variations in cyclic plastic strain rates έ pl between ∼10-5 and ∼10−2 s−1. In the case of single crystals, the effect of solute carbon (∼30 wt. ppm) was investigated as well. The mechanical data were supplemented by detailed studies of the dislocation arrangements by transmission electron microscopy and of the surface patterns by scanning electron and optical microscopy. Detailed accounts are given of the following topics: cyclic hardening and saturation, dislocation mechanisms, shape changes due to asymmetric slip of serew dislocations, cyclic stress-strain response and fatigue crack initiation. Under conventional conditions of “high” έ pl (≲10−4 s−1) the fatigue behaviour of α-iron at room temperature reflects the low mobility of the screw dislocations which is characteristic of the lowttemperature mode of deformation of body-centred cubic (b.c.c.) metals. As a consequence the behaviour exhibits significant differences with respect to that of fatigued face-centred cubic (f.c.c.) metals such as: strongly impeded dislocation multiplication below Δe pl ∼ 5 × 10−4, appreciable secondary slip at higher Δe pl leading to a cell structure (persistent slip bands do not form), shape changes due to asymmetric slip of screw dislocations and a relatively high effective stress level. The reduction of έ pl and the presence of solute carbon atoms modify this behaviour significantly, making it more similar to that of f.c.c. metals. In all cases it was found that only the athermal component of the peak (saturation) stress but not the latter itself represents a suitable measure of the properties of the dislocation substructure. On the basis of the cyclic deformation behaviour and of observations of trans-and intergranular fatigue crack initiation it was concluded that the fatigue limit of α-iron is an intrinsic property of the b.c.c. structure whose characteristics, however, are affected sensitively by interstitial impurity content and by the strain rate of the fatigue test.

An anisotropic, critical state model for soils subject to cyclic loading

Abstract: This Paper applies and extends an anisotropic hardening model to the study of inelastic, undrained clay response under cyclic loading conditions. Besides kinematic and isotropic hardening rules, a cyclic degradation parameter is introduced which grows with accumulated deviatoric deformation and induces additional growth of pore pressure and softening of the material. As a result, strain amplitudes and accumulated strains increase in the course of cyclic loading. A modified two-surface model and a model with an infinite number of hardening surfaces are discussed and applied in the study of undrained, cyclic response of normally consolidated and overconsolidated clays in triaxial tests. The number of cycles to failure, pore pressure variation, growth of accumulated strain and the static strength after cyclic loading are predicted by these models and compared with experimental data. Cette communication a trait au developpement d'un modele d'ecrouissage durcissant anisotrope et son application a l'etude du co...

Recrystallization and aging effects associated with the high temperature deformation of waspaloy and inconel 718

Abstract: Cylindrical samples of Waspaloy and Inconel 718 were hot compressed, using a computerized Instron machine. The test program covered strain rates from 10•4 s•1 to 1 s•1 temperatures ranging from 875 °C to 1220 °C and deformations up to strains of 0.7. Interrupted tests were also carried out to determine the nature of the static softening and hardening processes. Dynamic recrystallization, partial or complete, was observed at temperatures above 950 °C. At 950 °C and below, dynamic recovery was the process controlling the deformation. Static softening was found to take place both by recovery and by recrystallization. Yield points were detected in Waspaloy under certain conditions as well as in Inconel 718. For Waspaloy the yield drops occurred in the vicinity of 1100 °C, and a deviation from the normal behavior in the stress-temperature curve was seen in the same temperature range. The mechanism responsible for the occurrence of the yield drops, which in turn is related to the deviation in the σvsT curve, is believed to be short range ordering of the γ’ forming elements. For Inconel 718, elements such as Co, Cr and Fe may be causing short range ordering, but the locking mechanism may also be associated with the precipitation of carbides or other intermetallic phases on the dislocations.

On the Characterization of Strain-Hardening in Plasticity.

Abstract: : In the context of a purely mechanical, rate-type theory of elastic-plastic materials and utilizing a strain space formulation, this paper is concerned mainly with developments pertaining to strain-hardening behavior consisting of three distinct types of material response, namely hardening, softening and perfectly plastic behavior. It is shown that such strain-hardening behavior may be characterized by a rate-independent quotient of quantities occurring in the loading criteria of strain space and the corresponding loading conditions of stress space. With the use of special constitutive equations, the predictive capability of the results obtained are illustrated for strain-hardening response and saturation hardening in a uniaxial tension test.

Endochronic Description of Cyclic Hardening Behavior for Metallic Materials

Abstract: The cyclic hardening behavior of metallic materials at a prescribed constant plastic strain rate is investigated. The development is within the framework of the endochronic theory of viscoplasticity. It has been shown that a cyclic steady state can be attained theoretically, in agreement with the experimental finding. A general expression for describing the stress-strain relation for all cycles of a fatigue test with fixed total strain amplitude is presented and the peak stress of each cycle at different specified total strain level is also investigated.

Evaluation of the marginal integrity of ceramometal restorations: Part II

Abstract: lhe fit of a ceramometal casting deteriorates during the firing of a porcelain veneer.lm4 This distortion phenomemon has been investigated by many researchers,5-‘4 but quantitatively, the contributing factors remain uncertain. According to the literature, the factors responsible for the distortion include (1) porcelain contraction (porcelain has expansion properties quite different from the alloys), (2) contamination of the casting that reduces the melting temperature, (3) grain growth of the alloy, (4) plastic flow of the alloy, (5) progressive reduction in the resilience of the metal caused by hardening and/or rigidity of the porcelain, (6) design of the metal substructure, and (7) inadequate support of the metal framework during firing. Regardless of the specific factor involved, the common denominator is elevated temperature. It therefore follows that the higher the melting temperature of the ceramic alloy, the more resistant it must be to temperature-related distortion. Nickel-chromium alloys are able to withstand temperature and stress-related distortion better than their gold-based counterparts.‘5*2’ Commonly held beliefs that nickel-chromium cannot be cast as accurately as gold22,23 have been challenged. lo, ‘L*’ From a marginal integrity viewpoint, &rating et al.” showed that a nickel-chromium alloy can be cast as accurately as semiprecious or precious ceramometal alloys. They also found that metal distortion, at least on a marginal level, is not a significant factor between alloys if a single unit of 0.4 mm thickness is used. In contrast, Buchanan et aLI found that the marginal opening changed more in the specimen made from the higher strength metal than it did in those made from lower strength metal. It is imperative to distinguish between incomplete seating of castings that is due to contamination and that due to marginal

Constitutive Equations of Elastoplastic Materials With Anisotropic Hardening and Elastic-Plastic Transition

Abstract: Constitutive equations of elastoplastic materials with anisotropic hardening and elastic-plastic transition are presented by introducing three similar surfaces, i.e., a loading surface on which a current stress exists, a subyield surface limiting a size of the loading surface and a distinct-yield surface representing a fully plastic state. The assumption of similarity of these surfaces leads the derived equations to remarkably simple forms. Also a more general rule of the kinematic hardening for the distinct-yield surface is incorporated into the constitutive equations. While they seem to be applicable to various materials, special constitutive equations of metals, for example, are derived from them and are compared with experimental data on a cyclic uniaxial loading of aluminum. A close correlation between theory and experiment is observed in this comparison.

Surface Hardness and Abrasive Wear Resistance of Ion-Implanted Steels

Abstract: The hardnesses of nitrogen-implanted steel surfaces have been measured with an abrasive wear technique capable of characterizing surface layers as thin as 25 nm. Treated steel disks and reference disks were abraded with 1–5 μm diamond, and relative wear resistances were calculated from the mass losses. Surface hardness was obtained from a relationship between wear resistance and hardness. The surface of a hardened and tempered carbon steel implanted with nitrogen ions (1017/cm2) was significantly harder than with other treatments including quench hardening and nitriding. The hardness decreased to the bulk value over a depth corresponding to the initial implantation depth. Nitorgen-implanted stainless-steel surfaces wore faster than un-implanted ones, possibly due to interference with transformation hardening which normally occurs during wearing. This “softening” effect persisted to depths several times the depth of implantation, and may help to explain the reduction of sliding wear produced by the implant...

Microstructural instabilities during superplastic forging of a nickel-base superalloy compact

Abstract: The high temperature flow behavior of a nickel-base superalloy powder compact, prepared by hot isostatic pressing has been examined by means of uniaxial compression testing in terms of the microstructures developed during plastic flow. The tests were done isothermally at 1050 and 1100 °C and at constant true strain rates between 10-5 s-1 and 1 s-1. The fine grained compact exhibits some degree of superplasticity which always increases with compressive flow as the grain structure is refined. The faster the rate of deformation, the finer is the grain size produced at high strains, when steady state conditions of flow appear to develop. By deforming to different strains at a given strain rate or into the steady state regimes at various strain rates, grain sizes in the range 1 to 5 pun were produced. By unloading and restraining the test pieces in situ, the effect of grain size on the onset of plastic flow has been examined and the yield stress observed to increase with grain size. It is shown that, in this material, hardening or softening occurs during flow depending on the size of the initial grains. The changes in microstructure and flow stress observed during deformation are analyzed and the potential offered for control of the microstructure during isothermal forging is discussed.

Plastic relaxation of the transformation strain energy of a misfitting spherical precipitate: linear and power-law strain hardening

Abstract: Complete solutions to the displacement, stress and strain fields, plastic zone size and misfit energy are calculated for an isotropic misfitting spherical precipitate under the assumptions of von Mises’ yield criterion and incremental plasticity. Analytical solutions are obtained for the case of linear strain hardening while a numerical technique is necessary for the case of power-law hardening. Large changes in the stress field in the regions surrounding the precipitate are observed when contrasted with the elastic state. The energy of the relaxed state is found to be a strong function of the strain-hardening parameter as is the plastic work done during the relaxation process. The plastic zone size, however, is not strongly dependent upon the strain-hardening parameter and for a homogeneous precipitate is independent of it.

Strain hardening and dynamic recovery

Abstract: Work hardening and creep of pure metals are surveyed on the basis of the correlation between the main features of structure evolution and macroscopic behavior. At low strains dislocation storage follows geometrical rules resulting in a stage of athermal hardening. In the subsequent stage the hardening coefficient becomes temperature dependent. This can be described consistently by the increasing influence of a thermally activated dynamic-recovery component which superimposes upon the athermal hardening component. In this regime dislocations arrange in a cellular pattern, i.e., cell walls at low temperatures and subboundaries at high temperatures. As strain continues the development of the cellular structure appears to be disturbed by structural instabilities and inhomogeneities of deformation, such as dynamic recrystallization, micro-bands and shear bands. The influence of these processes on work hardening makes it necessary to distinguish four different stages of work hardening and has important consequences for the interpretation and evaluation of steady state flow.

Cyclic deformation of Nb single crystals I. Influence of temperature and strain rate on cyclic hardening, shape changes and stress asymmetry

Abstract: Niobium single crystals with single glide orientation have been cyclically deformed in tension-compression, at total axial strain amplitudes between 0.3 × 10−3 and 6.0 × 10−3. The cyclic hardening, the changes in shape of the crystals and the asymmetry of the hysteresis loop, have been studied at different temperatures (between 250 K and 354 K) and different strain rates (between 3.0 × 10−5 ' s−1 and 1.2 × 10−2 s−1). The cyclic stress-strain curves exhibit four different hardening regions, the main characteristic being the existence of a “plateau” region at low strain amplitudes where a limited amount of hardening or softening is observed. The length of this plateau increases as the temperature is decreased or the strain rate increased, and an interpretation is given in terms of the relative absence of dislocation-dislocation interactions. The shape of the cyclic stress-strain curves is found to depend on the cyclic strain history of the crystals, especially at the small strain amplitudes. The ch...

On the dispersion hardening potential of interphase precipitation in micro-alloyed niobium steel

Abstract: In an experimental high-strength bw-alloy (HSLA) steel micro-alloyed with 0.13wt% Nb, microhardness of the ferrite phase has been measured as a function of the particle spacing across interphase precipation sheets of niobium carbonitrides. The dispersion geometry was controlled by isothermal transformation treatments using different temperatures and time intervals, and evaluated quantitatively by dark-field transmission electron microscopy. The results were in reasonable agreement with the Orowan—Ashby model of dispersion hardening, suggesting that the sheet spacing may represent the effective geometrical parameter responsible for dispersion hardening due to interphase precipitation.

Anisotropic hardening theory and the Bauschinger effect

Abstract: A review of anisotropic hardening theory is presented with particular reference to the Bauschinger effect in reversed torsion and anisotropic yield loci in σ, τ space associated with plastic shear strain history.The Bauschinger effect is obtained experimentally from a series of torsion tests on En3B steel tubes prestrained to a maximum of 10 per cent plastic shear strain. The effect, measured from the stress in reversed torsion at the proportionality limit, is analysed from the theory. It is shown to be consistent with experimental observations made on the translation and contraction of an initial yield locus, that are in marked contrast to the rigid translation of kinematic hardening rules.The degree of shear prestrain is shown to considerably influence the magnitude of the effect, an observation in full support of a theoretical Bauschinger parameter. The present test data together with existing published data for commercially pure aluminium 1100-F and the aluminium alloy Noral 19 S confirm that ...

Metastable Austenite Phase in Rapidly Quenched Fe-Cr-C Alloys

Abstract: By rapid quenching technique, metastable austenitic alloys with high strength and hardness have been found in Fe-Cr-C ternary system. This formation range is limited to about 1.0-2.2wt%C and 7-30wt%Cr. The austenite phase has ultra-fine grains of about 0.2μm in diameter. Their Vickers hardness, 0.2% proof stress and tensile fracture strength increase with the amounts of carbon and chromium, and the maximum values reach about 630DPN, 1500 and 1550MPa, respectively. These alloys are so ductile that no crack is observed even after closely contacted bending test. In addition, the changes in microstructure and mechanical properties of the tempered austenitic alloys have been investigated and it has been observed that a large secondary hardening occurs at about 870K due to phase transformation from austenite to equilibrium structure of ferrite and M7C3 pearlite. Thus the present alloys may be attractive as a fine gauge high-strength wire or plate.

Hardening mechanisms of ionic crystals on {110} and {100} slip planes

Abstract: Ionic crystals of the NaCl structure can be deformed on several non-equivalent crystallographic planes. The hardness varies with the type of slip system. This plastic anisotropy depends on temperature, divalent doping and ionicity of the material as is shown by measurements of the critical resolved shear stress for slip on {110} and {100} planes. For a series of NaCl-type crystals hardening mechanisms are discussed based on Peierls and dislocation-solute interactions. The plastic anisotropy is of substantial help in the analysis of these interactions.

Some Factors Influencing the Pitting, Micro-Pitting (Frosted Areas) and Slow Speed Wear of Surface Hardened Gears

Abstract: Surface hardness and hardening depth are the most important influence factors for the load capacity of surface hardened gears. Besides the structure of the hardening pattern, it’s carbon content, amount of retained austenite, residual stress and white layer (nitrided gears) have a considerable influence. Lubricant and material of the mating gear as well as roughness of the tooth flanks have a different influence on pitting, micro-pitting and wear.