Showing papers on "Hardening (metallurgy) published in 1989"

Plasticity-induced martensitic transformation during cyclic deformation of AISI 304L stainless steel

Abstract: Specimens of AISI 304L stainless steel were deformed cyclically at room temperature at various plastic strain amplitudes Δϵpl/2. In addition to tests at constant plastic strain amplitudes, incremental step tests were also carried out. The cyclic deformation behaviour was investigated and related to microstructural changes observed by transmission electron microscopy (TEM). At low values of the plastic strain amplitude a saturation state was reached, whereas specimens fatigued at plastic strain amplitudes Δϵpl/2 > 0.3% exhibited, after initial cyclic hardening, an extensive secondary hardening stage which persisted until fracture. The reasons for this behaviour were clarified by TEM observations. Planar dislocation arrays with faults were observed after cycling at Δϵpl/2 = 0.02%. However, martensitic phases were not found. The specimens fatigued at Δϵpl/2 = 0.5% or in the incremental step test (0.02%<Δϵpl2<0.5%) displayed cell structures with a considerably higher number of stacking faults. Moreover, deformation-induced martensitic phases were observed and identified as ϵ- and α′-martensite by electron diffraction pattern analysis.

Cyclic hardening properties of hard-drawn copper and rail steel

Abstract: A cyclic hardening law due to Armstrong and Frederick (CEGB Report RD/B/N731, 1966) has been extended to describe plastic strain accumulation (ratchetting) in hard-drawn copper and rail steel. The four parameters of the theoretical model were determined from a single uniaxial test on each material, in which unequal tension and compression were applied. Using these parameters the model was found to give good predictions of the ratchetting rate measured in non-proportional cycles of tension-torsion-compression, which are representative of the stress cycles experienced by surface elements in rolling and sliding contact.

Inelastic deformation of porous materials

Abstract: This paper examines the structure of material constitutive laws for time-independent plastic and creeping porous bodies through the determination of bounds to the flow and strain-rate potentials. The results are a logical extension of conventional plasticity and creep formalisms for incompressible materials. It is demonstrated that the shape of the yield surface for a time-independent plastic material and the surface of constant energy dissipation rate for a creeping solid are a function of stress and void volume fraction only, and independent of material parameters apart from a weak dependence on the creep exponent, n. This condition is not satisfied by the model proposed by G urson (J. Engng Mater. Tech. Trans. ASME, 99, 2, 1977) and modifications are suggested to his model and its extension to material hardening. The predictions obtained for a creeping solid are in broad agreement with results of other studies.

Dynamic Shear Band Development in Plane Strain

Abstract: : Plane strain compression of a rectangular block is used as a model problem to investigate the dynamics of shear band development from an internal inhomogeneity. The material is characterized as a von Mises elastic-viscoplastic solid, with a hardness function that exhibits a local maximum. Regardless of whether the material is hardening or softening, plastic strain development involves the evolution of finger-like contours emanating from the inhomogeneity at 45 degrees to the compression axis. Once a given strain contour crosses the specimen, it fans out about its initial direction of propagation. For a softening solid, this fanning out ceases for some strain level greater than the strain at the hardness maximum and further straining takes place in an ever narrowing band. Many of the qualitative features of shear band development under dynamic loading conditions are the same as under quasi-static loading conditions, but a significant retardation of shear band development due to inertial effects is found.

Comparison of the microstructures and abrasive wear properties of stellite hardfacing alloys deposited by arc welding and laser cladding

Abstract: The microstructure of cobalt-based hardfacing alloys deposited by manual metal arc (MMA) welding, tungsten inert gas (TIG) welding, and laser cladding has been investigated as part of a study attempting to establish the relationship between microstructure and abrasive wear properties. For typical deposition conditions, the differences in freezing rates associated with the three processes are found to give rise to large differences in microstructure. The MMA process is found to lead to the largest degree of dilution of the hardfacing deposit; the TIG and laser deposits exhibited much lower levels of mixing with the base plate. For the deposition conditions used in this study and for the alloys examined, the scale of the microstructure decreases in the order MMA, TIG, and laser cladding, leading to an increase in the deposit hardness in the same order. It is found that with alumina as an abrasive, the wear rate persistently is higher with the MMA deposits (which have the coarsest microstructure with the lowest starting hardness), the weight loss being approximately linear with time. The laser and TIG deposits, which have more refined microstructures and slightly higher carbon concentrations, both are found to exhibit significantly lower wear rates. Initially, the TIG samples are the most resistant to abrasion, even though their microstructure compares with that of the laser samples; this is a consequence of their higher ductility associated with a lower rate of strain hardening. The laser samples, which contain a lower matrix iron concentration, strain harden more rapidly; consequently, they exhibit an initial decrease in wear rate. With the much harder silicon carbide abrasive, all samples show similar wear rates which do not decrease with time. The wear data are found to correlate with scanning and transmission electron microscopy observations, and it is possible to rationalize the interaction among microstructure, abrasive, and alloy deposition processes.

Spinodal decomposition and mechanical properties of an austenitic Fe-30wt.%Mn-9wt.%Al-0.9wt.%C alloy

Abstract: The microstructure and mechanical properties of an austenitic Fe-30wt.%Mn-9wt.%Al-0.9wt.%C alloy after aging in the temperature range between 743 and 873 K have been investigated. The occurrence of spinodal decomposition was confirmed by transmission electron microscopy observation of a modulated structure with superlattice reflections and identification of the X-ray sidebands. The rapid increase in the yield stress in the early stage of aging was proportional to the increase in the modulation amplitude and independent of the wavelength. The observed hardening has been examined using a theory dealing with the coherency strain produced by the spinodal decomposition.

Influence of Aging at 200 C on the Corrosion Resistance of Al-Li and Al-Li-Cu Alloys

Abstract: The influence of heat treatment on the microstructure and corrosion resistance of an aluminum-lithium alloy and two aluminum-lithium-copper alloys was investigated. Aging the Al-Li alloy r...

Crack tip singular fields in ductile crystals with taylor power-law hardening

Abstract: AN ASYMPTOTIC singular solution of the HRR type is presented for mode I tensile cracks in ductile single crystals. These are assumed to undergo Taylor hardening with a power-law relation between stress and str+ at sufficiently large strain. Results are given for a crack on the (010) plane wjth its tip along the [IOI] direction, and for a crack on the (101) plane with its tip along the same [IOI] direction in a fee crystal. The yield surfaces for both of these orientations are identical and thus, for the “small strain” formulation, the same macroscopic solution applies to both. The near-tip region is divided into angular sectors which are maps of successive flat segments and vertices of the yield surface. While the solution here involves 14 different sectors referring to stress states corresponding to Rat and vertex segments of the yield locus, RICE’S (Me& Mater. 6,714, 1987) asymptotic solution for theelasticcideally plastic crystals involved only 7 sectors which mapped into the vertex points of the yield surface. The perfectly plastic limit of the HRR fields here reduce to 7 stress states of RICE (1987). In this limit. the HRR displacement fields remain continuous resulting in a discontinuous yet bounded and nonzero strain held. In contrast, the elasticideally plastic solutions have discontinuous shear displacements across sector boundaries. Furthermore the contours of constant effective strain here have various peaks and troughs at sector boundaries and lean backward relative to the direction of crack growth. Conversely, in the recent finite element solutions for elastic-ideally plastic single crystals by Hawk (preliminary summary of results is included in RICE PI ul., 1111. J. F~~c/ure. in press. 1989). the plastic zones lean forward and the strain field is consistent with a Dirac singular form similar to RICE’S (1987). Thus it is conjectured that. similar to the anti-plane shear case of RICX and SAEEDVAFA (.I. Meclr. Pltxs. Solids 36. 189, 1988). the single crystal HRR fields are dominant only over part of the plastic region immediately adjacent to the crack tip, and that their domain of validity vanishes as the perfectly plastic limit is approached. RICE (1987) presented an asymptotic solution for the stress and deformation field very near the tip of a mode I crack in an ideally plastic ductile crystal. Two specific orientations, a crack on the (010) cube face with its tip along the [lOi] face diagonal and a crack on the (101) plane with its tip along the same diagonal. were considered in fee and bee crystals for both stationary and quasi-statically growing cracks. In the case of stationary cracks, the stress field was found to be piecewise constant in angular sectors mapping into vertex points of the yield surface and changed discontinuously between sectors. The displacement and strain fields were not fully determined, although it was shown that there must be a shear displacement discontinuity across the sector boundaries for elastic-plastic crystals. The full solution could only be

Fracture and fatigue crack propagation properties of hardened 52100 steel

Abstract: Good toughness in hardened 52100 ball bearing steel is important in order to prevent premature fracture during mounting or service of bearing elements. Steel cleanliness, residual copper content, and carbon content effects have been investigated in relation to fracture mechanics properties, and it was observed that only the carbon content has any relevance for the range of compositions investigated. The effect of hardening and tempering temperatures for conventional furnace-hardening techniques on toughness was investigated, theKlcbeing generally much less sensitive to these parameters than blunt notch toughness testing. Cold deformation of the material prior to martensitic hardening significantly increased the blunt notch toughness. Thermal grain refining treatments did not give the same improved blunt notch toughness as observed for prior cold deformation. Short austenitization cycles (ten seconds) for martensitic hardening resulted in microstructures with high retained austenite contents. This microstructure resulted in higher fracture toughness and retardation of the crack growth rates, the mechanism being associated with transformation toughening in the plastic zone. Inductive tempering of martensitic-hardened 52100 was observed to result in similar blunt notch toughnesses as compared to furnace tempered material of the same hardness. A poor correlation between fracture toughness and blunt notch toughness was observed, particularly for the unstable structures,i.e., microstructures with high levels of retained austenite. Fracture toughness does not represent the intrinsic toughness of high carbon martensite with related high contents of retained austenite.

Pattern forming method

Abstract: PURPOSE:To obtain a pattern of a metal oxide film, metal film, metal sulfide film or metal nitride film having an excellent characteristic, which can be easily formed even on a substrate of a large size at a low cost, by spraying a solution or injecting gas under a normal pressure so as to form the film CONSTITUTION:An ink composition is pattern-printed or deposited, followed by drying or hardening, and a pattern mask layer 3 including a filler 2 is formed by photolithography While a substrate 1 with the mask layer 3 formed thereon is heated at a temperature higher than a pyrolysis temperature of a metal compound, a solution 4 of the metal compound is sprayed or gas of the metal compound or pyrolytic gas 5 is injected onto the substrate 1 so as to thermally decompose the metal compound, followed by baking A forming film 6 of either of a metal oxide film, metal film, metal sulfide film and metal nitride film is formed on a portion, where no mask layer 3 is formed, of the substrate 1 Therefore, it is possible to obtain a method for forming a pattern of metal oxide film, metal film, metal sulfide film or metal nitride film having fineness and an excellent characteristic in a simple process

The effect of void shape on void growth and ductility in axisymmetric tension tests

Abstract: The effect of void growth and ductility in axisymmetric tension tests was examined theoretically. The inelastic response of periodic arrays of initially oblate spheroidal voids in a homogeneous isotropically hardening matrix was analyzed for loading histories representative of axisymmetric tension tests. The failure strains were taken as the point where the deformation mode became that of uniaxial strain. All subsequent deformation was localized to ligaments between the voids. The results showed that, for stress states with a low hydrostatic stress component, the void growth rates were governed primarily by the void dimension perpendicular to the tensile axis. They also indicated that the failure strains were proportional to the projected area of the voids onto a plane perpendicular to the tensile direction, in agreement with published experimental results. For stress states with higher triaxiality, the influence of the initial aspect ratio was diminished.

High Temperature Niobium Alloys

Abstract: This book covers the following areas regarding high temperature niobium alloys: atomic mass transport of interstitial solutes in niobium; electroplastic effect; dispersion strengthening; dispersion hardening by carbides; tensile behavior of tungsten/niobium composites; phase stability; mechanical properties; oxidation.

The rate dependence and microstructure of high-purity silver deformed to large strains between 0.16 and 0.30 T m

Abstract: High-purity polycrystalline silver was deformed to large strains in torsion at temperatures from 203 to 373 K (0.16 to 0.30T m)at a variety of strain rates. Saturation of the flow stress was always observed. However, saturation is achieved only after relatively large strains over which both Stage III and Stage IV hardening are observed. At 373 K, there is mechanical and micro-structural evidence of dynamic recrystallization. At ambient temperature and below, hardening processes seem to be balanced exclusively by dynamic recovery, and steady-state creep is observed. The recovered microstructure is dominated by elongated subgrains. Subgrain boundaries are well-defined, and the misorientation across the walls is relatively high. The activation energy and strain-rate sensitivity for steady-state flow were measured. Furthermore, the constant-structure strain-rate sensitivity was measured as a function of primary strain from small strain levels (e<0.01) up to steady-state values (approximately 3.0). The observed activation energies and strain-rate sensitivities are related to fundamental processes.

Hardening mechanisms in Al-Sc alloys

Abstract: The hardening mechanism in Al-Sc alloys with scandium content of 0.11 and 0.19 at% is studied. Applying theoretical results due to the yield stress and work hardening of two phase alloys as a function of volume fraction and precipitate particle size, it is shown that after ageing at above 300° C the Orowan mechanism operates in these alloys. Using the experimental results, the volume fraction and average radius of the precipitate particles are determined.

Friction effect in low load hardness testing of iron

Abstract: Vickers hardness tests were carried out on specimens of iron in a cold rolled or annealed condition to investigate apparent variation of hardness in the load range 15 g–20 kg. This hardness variation was markedly reduced by lubrication. Therefore, it is suggested that the indentation size effect in these conditions is controlled by friction. Strain hardening seemed to be an important secondary factor.MST/1023

A study on residual-stresses in laser surface hardening of a medium carbon-steel

Abstract: The transient thermal stress and the residual stress in laser surface-hardening treatment of a medium carbon steel were analysed by employing a new two-dimensional finite element model. In this formulation, a sliced solution domain, having one element in the hardening direction, was introduced to satisfy the self-equilibrium of the resultant force in the hardening direction. By using the proposed model, the thermal and residual stresses in the laser surface heat treatment were successively calculated. The thermal stress was induced mainly by the temperature gradient and the martensitic phase transformation; the phase transformation was found to have a greater influence on the residual stress than the temperature gradient. The simulation results revealed that a compressive residual stress region occurs near the hardened surface of the workpiece and a tensile residual stress region occurs in the interior of the workpiece, whereas the maximum tensile residual stress occurs along the centre of the laser scanning path in the interior region (y = 0). In comparison with the gaussian distribution of the beam power, the square beam mode results in a wider, but shallower, hardened zone. The calculation results also showed that the high-power beam with the high scanning speed is more suitable for laser surface hardening than the low-power beam with the low scanning speed if the heat input per unit length of the workpiece is maintained constant.

High.Speed Tool Steels

Abstract: HIGH-SPEED TOOL STEELS and their requirements are defined by The American Society for Testing and Materials in Specification A600-79 as follows: High-speed tool steels are so named primarily because of their ability to machine materials at high cutting speeds. They are complex iron-base alloys of carbon, chromium, vanadium, molybdenum, or tungsten, or combinations thereof, and in some cases substantial amounts of cobalt. The carbon and alloy contents are balanced at levels to give high attainable hardening response, high wear resistance, high resistance to the softening effect of heat, and good toughness for effective use in industrial cutting operations. Commercial practice has developed two groups of cutting materials:

Study of dislocation cell structures from uniaxial deformation of AISI 4340 steel

Abstract: A considerable interest has focused on the role of substructure during the uniaxial deformation of pure metals and alloys. To elucidate this role further, the evolution of dislocation cell structures during the tensile straining of AISI 4340 steel was investigated by transmission electron microscopy. It was found that substructural evolution reflects the inhomogeneous distribution of stress and strain in a composite material consisting of ferrite and pearlite. Cell formation occurs at different stages in the phases, with a more advanced state of cell development in ferrite grains indicating higher dislocation mobility. The strength of 4340 steel appears to be largely due to the pearlite. Work hardening by cell mechanisms occurs in the ferrite only after stresses exceed the pearlite strength. The dependence of dislocation cell size on flow stress σ was also studied and σ is found to be linearly related to the inverse cell size, indicating the operation of similitude in the “meshlength” theory of work hardening over most of the strain range. The breakdown of this relationship at higher strains is related to the transition to stage IV hardening behaviour, where increased dislocation mobility through profuse cross slip results in a very low hardening rate and nearly constant cell size.

Strain hardening during superplastic deformation of A1-7475 alloy

Abstract: Strain hardening dominates the deformation process in fine-grained A1-7475 alloy in the temperature range 400 to 515 °C. It is shown that anomalously low stress exponents are obtained as a result of strain hardening in strain-rate-change tests. In order to measure stress exponents in a quasi-steady state condition, the samples must be initially deformed at a relatively high stress (≍ 10 MPa) to a relatively high strain (≍ 0.5) before initiating a strain-rate-change test. Such a procedure revealed that a stress exponent about equal to two and an activation energy (141 kJ/mole) nearly equal to the activation energy for lattice diffusion are obtained. The results are interpreted in terms of a model involving grain boundary sliding accommodated by slip following the Gifkins' “core and mantle” concept. It is proposed that strain hardening is associated with the development of a boundary-dislocation structure in the mantle region in a manner similar to the development of subgrains in the core of a grain when slip is the principal deformation mode.