Showing papers on "Hardening (metallurgy) published in 1997"

Viscoplasticity for instabilities due to strain softening and strain-rate softening

Abstract: Three viscoplastic approaches are examined in this paper. First, the overstress viscoplastic models (i.e. the Perzyna model and the Duvaut—Lions model) are outlined. Next, a consistency viscoplastic approach is presented. In the consistency model a rate-dependent yield surface is employed while the standard Kuhn—Tucker conditions for loading and unloading remain valid. For this reason, the yield surface can expand and shrink not only by softening or hardening e⁄ects, but also by softening/hardening rate e⁄ects. A full algorithmic treatment is presented for each of the three models including the derivation of a consistent tangential sti⁄ness matrix. Based on a limited numerical experience it seems that the consistency model shows a faster global convergence than the overstress approaches. For softening problems all three approaches have a regularising e⁄ect in the sense that the initial-value problem remains well-posed. The width of the shear band is determined by the material parameters and, if present, by the size of an imperfection. A relation between the length scales of the three models is given. Furthermore, it is shown that the consistency model can properly simulate the so-called S-type instabilities, which are associated with the occurrence of travelling Portevin-Le Chatelier bands. ( 1997 John Wiley & Sons, Ltd.

Press forming of a low alloy steel part with an increased ductility region

Abstract: A shaped low alloy steel part, having regions of increased ductility, is produced by heating regions of a sheet of the steel to 600-900 deg C in less than 30 seconds, before or after forming and hardening in press tooling. A shaped metal part, having regions of increased ductility, is produced from a sheet of steel of composition (by wt.) 0.18-0.3% C, 0.1-0.7% Si, 1.0-2.50% Mn, \}0.025% P, 0.1-0.8% Cr, 0.1-0.5% Mo, \}0.01% S, 0.02-0.05% Ti, 0.002-0.005% B, 0.01-0.06% Al, balance Fe and impurities by (a) heating regions of the sheet to 600-900 deg C in less than 30 sec. and then forming and hardening in press tooling; (b) press forming the sheet to an intermediate or finished product, heating regions of the product to 600-900 deg C in less than 30 sec. and then post-pressing and/or hardening in press tooling; or (c) heating the sheet uniformly to 900-950 deg C, forming and hardening the sheet in press tooling and heating regions of the sheet to 600-900 deg C in less than 30 sec.

Void growth in glassy polymers

Abstract: This paper deals with a study of voids in amorphous glassy polymers that exhibit elastic-viscoplasticity with rate dependent yield, intrinsic softening and progressive strain hardening at large strains. The study is motivated by the plastic deformation in voided polymer-rubber blends caused by cavitation of the rubber particles, and thus attempts to contribute to the understanding of the toughening mechanisms in blends. Axisymmetric cell analyses are presented to study the plastic deformation around initially spherical voids and their resulting growth in terms of size and shape up to large overall strains. This void growth is demonstrated to inherit particular properties from the typical features of plasticity in glassy polymers, viz. small strain softening and large strain hardening. The role of strain localization into shear bands and their subsequent propagation in controlling void growth is highlighted. Furthermore, an approximate constitutive model is presented for the description of the macroscopic overall behaviour of porous glassy polymers. This model includes a modification of existing porous plasticity models to account for elasticity effects on the initiation of overall plasticity, which are important in polymers because of their relatively high yield strain. Its predictions are compared with the results from the numerical cell analyses.

A finite element analysis of the squeeze flow of an elasto-viscoplastic paste material

Abstract: A finite element analysis of squeeze flow has been implemented for a material that exhibits elasto-viscoplasticity. The formulation is based upon the assumption that linear elastic deformation occurs prior to yield and that the yield surface is strain rate hardening as defined by an associated viscoplastic flow rule. Both no-slip and lubricated wall boundary conditions are considered. The numerical simulation results are compared with experimental measurements involving a model elasto-viscoplastic material for which the material parameters were derived from tensile and ram extrusion measurements. Satisfactory agreement was obtained for the compressive forces as a function of displacement, the radial displacement fields and the wall normal and shear stress distributions.

Cyclic Deformation and Fatigue of Selected Ferritic and Austenitic Steels : Specific Aspects

Abstract: Some recent work on the cyclic deformation and fatigue behaviour of selected ferritic and austenitic steels is reviewed. The steels considered are the ferritic plain carbon steels SAE 1010 and SAE 1045, the martensitically or bainitically hardened roller bearing steel SAE 52100 and metastable austenitic steels of the type AISI 304 L. The review deals with several specific aspects. In particular, experimental data are presented and discussed for the following topics: cyclic deformation (hardening/softening) and its temperature dependence, the cyclic stress-strain behaviour in single- and multiple-step tests and in the incremental step test, the characteristic microstructural changes and dislocation mechanisms, cyclic plasticity and Masing or non-Masing behaviour, some effects of mean stress, dynamic strain ageing phenomena and their effects on fatigue life in stress- and strain-controlled tests and, finally, the enhancement of the monotonic strength and the fatigue resistance of metastable austenitic steels by exploitation of the fatigue-induced martensitic deformation, in particular at low temperatures.

Micro structure and hardening of Al-based nanophase composites

Abstract: Part amorphous-part crystalline Al-Ni-Y alloys can be produced by devitrification of melt-spun fully amorphous alloys. These materials have a novel structure of nanometre-sized crystals of α-A1 in an amorphous matrix and can be regarded as nanophase composites. Devitrified microstructures are studied by X-ray diffraction and transmission electron microscopy. The significant hardening accompanying devitrification is characterised for various heat treatments, and is mainly attributed to the solute enrichment of the remaining amorphous phase. The hardening kinetics are, however, also influenced by the composition uniformity of amorphous matrix.

Mechanism of the hardening process for a hydroxyapatite cement

Abstract: The dynamics for the hydraulic process of calcium phosphate cement (CPC) were investigated by X-ray diffraction quantitative analysis. The results show that the hardening process of CPC is initially controlled by the dissolution of reactants in a 4-h period and subsequently by diffusion through the product layer of hydroxyapatite (HAP) around the grains. The compressive strength rises approximately linearly with the increase of the extent of conversion in a 4-h period, and a maximum compressive strength of about 51 MPa, which is superior to those reported by the references, is obtained in 4 h. Then the compressive strength drops a little with an increase in the extent of conversion. The final product of setting reaction is acicular HAP crystal. Crystal seed not only reduces the setting time but also drops the compressive strength. The variation of pH in CPC slurry from 7.5 to 10.5 reveals that the control step of the dissolution process in the hardening process is the dissolution of dicalcium phosphate anhydrous and the presence of crystal seed will reduce the supersaturation to produce HAP.

The rate-dependent deformation and localization of fully dense and porous Ti-6Al-4V

Abstract: The rate-dependent deformations of the titanium alloy Ti-6A1-4V are investigated using a combination of standard testing machines and compression and torsional Kolsky bars. Both homogeneous and localized deformations of both non-porous (‘fully dense’) and porous (7.6% porosity) versions of the alloy are studied. The microstructures of the two materials were made similar through heat-treatment, with the primary difference being the presence of the porosity. The fully dense Ti-6Al-4V (for this Widmanstatten microstructure) is found to show negligible strain hardening in compression (either quasistatic or dynamic), while the porous Ti-6Al-4V shows significant strain hardening in compression (both quasistatic and dynamic). The difference is believed to be because of the progressive compaction of the pores with increasing compressive strain. Both the fully dense and porous Ti-6Al-4V are observed to be rate-sensitive, with an increase in flow stress of ~ 30% over a six decade increase in strain rate. The strength-reduction due to porosity is ~ 20% for both quasistatic and dynamic strain rates. The constitutive response of the fully dense Ti-6Al-4V alloy in torsion is consistent with data in the literature, and again shows negligible strain hardening. The porous Ti-6Al-4V also shows negligible strain hardening in torsion, in contrast with the strong hardening evident in the compression data. This influence of the stress state on the behavior of the porous metal is due to the sensitivity of the pores to the mean (hydrostatic) stress. Specimens of the fully dense Ti-6Al-4V develop a deformation instability (during compression) in the form of a localized shearing failure in a plane at about 45° to the compression axis. The localized shearing failure in the fully dense metal consists of ‘deformed’ shear bands 3–10 μm in width which develop after a critical strain of 8–10%. In contrast, the porous Ti-6Al-4V specimens never fail in compression, showing homogeneous deformation even up to 30% strain. The relative stability of the porous metal is believed to be a result of the hardening mechanism afforded by the progressive compaction of the pores with increasing compressive strain. Shear bands are developed in both the fully dense and porous Ti-6Al-4V alloys after dynamic torsion. The critical shear strains to failure are ~ 14% for the fully dense metal and ~ 7% for the porous metal. Thus, the presence of the porosity destabilizes the deformation in shearing, but appears to stabilize the deformation in compression.

Low cycle fatigue behaviour in vacuum of a 316L-type austenitic stainless steel between 20 and 600°C—Part II: Dislocation structure evolution and correlation with cyclic behaviour

Abstract: In this second part, we have reported the evolutions of the dislocation structures of an austenitic stainless steel cycled in vacuum for different temperatures between 20 and 60°C. For each temperature, constant plastic strain amplitudes Δɛp/2 ranging from 6 × 10−4 to 5 × 10−3 have been considered. The dislocation evolutions have been correlated to the cyclic behaviour described in Part I. The slip character appears to be more wavy at 20 and 600°C than at intermediate temperatures, with dislocation structures composed, in the main, of cells and walls-and-channels. Between 200 and 500°C, the number of cycles to failure is higher than at 20 and 600°C and, at the same time, the dislocation arrangements are more planar from the first cycles, up to failure, but cells, walls-and-channels and tangles are less numerous. Instead, another structure appears, which we have called the corduroy structure, composed of alignments of very small defects such as loops, debris and cavities. This structure develops progressively with cycling and is all the more extended as cycling is performed at low strain amplitudes. The optimum temperature for its formation is 400°C. It is shown that the extension of the corduroy structure can be directly correlated to the amount of secondary hardening observed in vacuum between 200 and 500°C. In contrast, the degree of formation of corduroy structure is not directly correlated with fatigue-life evolution. Fatigue lifetime results from the competition between a beneficial effect (planar slip) and a detrimental one (the cyclic stress level). The planar behaviour has been associated with dislocation interactons with C and N solute atoms, which also determine dynamic strain ageing at intermediate temperatures.

Mechanical properties of pva fiber reinforced cement composites fabricated by extrusion processing

Abstract: Discontinuous polyvinyl alcohol (PVA) fiber reinforced cement matrix composites fabricated by an extrusion process were investigated. The extruded composites have exhibited a postpeak strain hardening type of response with an enhanced composite strength. Tensile strength of extruded composites was dependent on the fiber fraction used, while the flexural strength and deflection at the peak load were influenced by both the fiber fraction and the matrix composition. Larger fiber volume fraction, longer fiber length, and higher cement content lead to a higher flexural strength, a larger deflection at peak, and a higher elastic modulus. Fracture process was studied using laser Moire interferometry and scanning electron microscopy. It was observed that sequential multiple cracking was associated with the strain hardening type of response and that the spatial distribution of fibers can control the sequence of multiple cracking. Interface between fiber and matrix was also studied using continuous, aligned fibers and with the help of Moire interferometry.

Dispersion hardening of metals by nanoscaled ceramic powders

Abstract: Nanoscaled Al 2 O 3 -powders (n-Al 2 O 3 ) with a median particle diameter of 14 nm are distributed in microscaled copper powders by ball milling in a planetary mill followed by uniaxially hot pressing. The microstructures of the samples are analysed by SEM and EDX investigations and the hardness of the specimens is determined before and after heat treatments of the composites. The results on the produced Cu/n-Al 2 O 3 composites show that the nanoparticles are largely homogeneously distributed in the matrix grains, that the matrix grains are stabilised against growth even after heat treatments close to the melting temperature of Cu, and that the hardness of the sample is largely maintained after heat treatment. The results show that mechanical alloying of metal micropowders with oxide ceramic nanopowders can produce dispersion-strengthened materials. The mechanical alloying with n-powders can be easily applied to systems which cannot be dispersion-strengthened with n-oxide ceramics by other methods, e.g. internal oxidation. The last is shown by dispersion-strengthening of magnesium with n-Al 2 O 3 .

Dislocation Structure and Non-Proportional Hardening of Type 304 Stainless Steel

Abstract: This paper describes the microstructure of Type 304 stainless steel after cyclic loading at room temperature under tension-torsion nonproportional strain paths. The degree of cyclic nonproportional hardening is correlated with changes in the dislocation substructure. Dislocation cells, dislocation bundles, twins and stacking faults are all observed. The type of microstructure formed and resultant stress response is dependent on the degree of nonproportional loading and strain range. Cyclic stress range was uniquely correlated with mean cell size.

Low cycle fatigue behavior in vacuum of a 316L type austenitic stainless steel between 20 and 600°C Part I: Fatigue resistance and cyclic behavior

Abstract: In the present work, the fatigue behavior of a 316L type stainless steel was investigated in the temperature range 20–600°C. Fatigue experiments at constant plastic strain amplitude were carried out in vacuum in order to determine the intrinsic effect of temperature on fatigue lifetime and cyclic properties. A particular behavior has been pointed out at intermediate temperatures (200–400°C) at which dynamic strain ageing takes place. This behavior is characterized by a strong secondary cyclic hardening and at the same time by a significant increase of the fatigue resistance. In air the role of temperature is affected by the environment coupled effects.

Dislocation dynamics in carbon-doped titanium aluminide alloys

Abstract: Different thermal treatments were conducted on carbon containing two-phase titanium aluminide alloys to obtain solid solution and precipitation hardening effects. The strengthening mechanisms were characterized by activation parameters of the glide processes and electron microscope observations. Carbon in solid solution was found to be less efficient than carbide precipitates for hardening the material. Fine dispersions of Ti3AlC perovskite precipitates form arrays of strong glide obstacles so that perfect and twinning partial dislocations were effectively pinned. This mechanism results in a high athermal contribution to the flow stress, which significantly improves the high temperature strength of the material.

Calculation and experimental determination of dimensions of hardening and tempering zones in quenched U7A steel irradiated with a pulsed electron beam

Abstract: Based on metallographic data and microhardness depth distributions measured in preliminarily quenched and tempered U7A steel 1 (0.7% C) after irradiation with an electron beam of duration 10 −5 s, electron energy 170–180 keV and energy density 40–150 J/cm 2 , the dimensions of the hardened and tempered zones have been determined. The experimental results are compared with those obtained by solving numerically the heat equation taking into account the processes of melting, evaporation, and crystallization. Good agreement between experimental and theoretical data is observed.