Showing papers on "Hardening (metallurgy) published in 1999"

Precipitation hardening in metals

Abstract: Precipitation hardening has long been used to increase the strength of commercial alloys, such as quenched and tempered steels and the duralumin type aluminium alloys. The theoretical treatments of precipitation hardening are briefly considered. The equations for strengthening by ‘hard’ indeformable particles and by ‘soft’ deformable particles are presented, and the implications are discussed. These lead to the concept of an optimum particle size for a given system, but the optimum can vary from system to system depending upon the particle characteristics. A broad comparison is made between the increments in strength that occur due to precipitation in commercial alloys and the predictions of the theories; an important contribution to these increments in strength is shown to derive from variations in the volume fraction of precipitated particles that can be employed in the various systems.

Microstructural evolution of 6063 aluminum during friction-stir welding

Abstract: The microstructural distribution associated with a hardness profile in a friction-stir-welded, age-hardenable 6063 aluminum alloy has been characterized by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). The friction-stir process produces a softened region in the 6063 Al weld. Frictional heating and plastic flow during friction-stir welding create fine recrystallized grains in the weld zone and recovered grains in the thermomechanically affected zone. The hardness profile depends greatly on the precipitate distribution and only slightly on the grain size. The softened region is characterized by dissolution and growth of the precipitates during the welding. Simulated weld thermal cycles with different peak temperatures have shown that the precipitates are dissolved at temperatures higher than 675 K and that the density of the strengthening precipitate was reduced by thermal cycles lower than 675 K. A comparison between the thermal cycles and isothermal aging has suggested precipitation sequences in the softened region during friction-stir welding.

Atomic-scale simulations of the mechanical deformation of nanocrystalline metals

Abstract: Nanocrystalline metals, ie, metals in which the grain size is in the nanometer range, have a range of technologically interesting properties including increased hardness and yield strength We present atomic-scale simulations of the plastic behavior of nanocrystalline copper The simulations show that the main deformation mode is sliding in the grain boundaries through a large number of uncorrelated events, where a few atoms (or a few tens of atoms) slide with respect to each other Little dislocation activity is seen in the grain interiors The localization of the deformation to the grain boundaries leads to a hardening as the grain size is increased (reverse Hall-Petch effect), implying a maximum in hardness for a grain size above the ones studied here We investigate the effects of varying temperature, strain rate, and porosity, and discuss the relation to recent experiments At increasing temperatures the material becomes softer in both the plastic and elastic regime Porosity in the samples result in a softening of the material; this may be a significant effect in many experiments

Strain gradient plasticity effect in indentation hardness of polymers

Abstract: Plasticity in material is typically described as a function of strain, but recent observations from torsion and indentation experiments in metals suggested that plasticity is also dependent on strain gradient. The effects of strain gradient on plastic deformation in thermosetting epoxy and polycarbonate thermoplastic were experimentally investigated by nanoindentation and atomic force microscopy in this study. Both thermosetting and thermoplastic polymers exhibited hardening as a result of imposed strain gradients. Strain gradient plasticity theory developed on the basis of a molecular kinking mechanism has predicted strain gradient hardening in polymers. Comparisons made between indentation data and theoretical predictions correlated well. This suggests that strain gradient plasticity in glassy polymers is determined by molecular kinking mechanisms.

Evolution of dislocation structure induced by cyclic deformation in a directionally solidified cobalt base superalloy

Abstract: Cyclic deformation has been carried out on a directionally solidified cobalt base superalloy at room temperature in air under the control of different total strain amplitudes. Dislocation structures of different stages in the cyclic stress response curve were studied by TEM. Observations show that a large number of stacking faults and fault intersections are formed in the initial hardening stage, and this leads to the initial hardening of the alloy. However, formation of hexagonally close packed (hcp) zones and twins is proposed to be the cause of cyclic saturation. Models for both the transition from stacking faults to a hcp layer structure and to twins are proposed.

Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304

Abstract: Cylindrical specimens of the austenitic stainless steel AISI 304 were shot peened or deep rolled with different peening intensities, and rolling pressures, respectively. The resulting near surface properties were characterized by cross sectioning transmission electron microscopy (TEM), residual stress and phase analysis as well as interference line half-width and microhardness measurements. Cyclic deformation curves were obtained by hysteresis measurements under stress control with zero mean stress. The microstructural alterations in the fatigued surface regions were again characterized by the above mentioned methods. The investigations revealed that both shot peening and deep rolling lead to a complex near surface microstructure, consisting of nanocrystalline regions, deformation bands and strain induced martensitic twin lamellae with high dislocation densities in the austenitic matrix. These microstructural changes severely influence the cyclic deformation behaviour: Plastic strain amplitudes and cyclic creep were drastically decreased by shot peening and especially by deep rolling. Both surface finishing methods were found to decrease crack initiation and propagation rate. Remarkably, the initial residual stress profile and surface strain hardening were not completely eliminated even by applying high cyclic stress amplitudes. This is due to the fact that during cyclic loading dislocation cell structures were only formed in greater depths whereas the nanocrystalline layer remained stable. In the case of deep rolled surfaces, the martensitic layer was even increased by fatigue-induced martensite formation.

Hardening of new resin cements cured through a ceramic inlay.

Abstract: This study investigated the degree of hardening achieved through self-curing only and through dual-curing a group of eight new resin-based cements In addition, the effect of ceramic inlay thickness on cement hardness was determined Disk specimens measuring 6 mm in diameter and 25 mm thick were prepared from eight cements: Adherence, Choice, Duolink, Enforce, Lute-It, Nexus, Resinomer, and Variolink Eight specimens were prepared from each material; half were self-cured, while the remainder were dual-cured Knoop hardness measurements were then made at 1-hour, 1-day, and 1-week intervals In addition 12 specimens of the same dimensions were prepared from each cement and were dual-cured through ceramic spacers of varying thickness (1-6 mm) Hardness measurements were made as above ANOVA showed significant differences in hardness of self-cured versus dual-cured specimens for all cements (P < 00001) Significant differences were also found in the hardness of specimens dual-cured through ceramic spacers 2-3 mm in thickness or more compared with those that were dual-cured without spacer It is concluded that for some materials self-curing alone was not adequate to achieve sufficient hardening; cement hardness was significantly reduced when ceramic inlay thickness was 2-3 mm or more

Grind-Hardening: A Comprehensive View

Abstract: The invention of advanced grinding processes enabling the surface hardening of steel parts was described for the first time in 1994 [1]. In such operations, named grind-hardening the dissipated heat in grinding is utilized to induce martensitic phase transformations in the surface layer of components. A grinding process then becomes a heat treatment operation like induction or flame hardening. The fundamentals of this new process, which had been developed up to first industrial applications, will be illustrated in this paper. Especially the impact of different grinding parameters on the structure and the achievable hardness penetration depth are discussed in detail.

Effect of Volume Fraction of Constituent Phases on the Stress-Strain Relationship of Dual Phase Steels

Abstract: Ferrite-martensite and ferrite-bainite dual phase steels (DP-steels) were prepared by applying accelerated cooling (AcC) process on a linepipe steel. Their stress-strain relationships were predicted by micromechanics. In the predictions, the stress-strain relationships of the constituent phases whose chemistries were determined by microscopic examinations and some thermodynamic data were used. The effect of volume fraction of the constituent phases on the stress-strain relationships of the DP-steels was also examined. According to the applied model, a simple stress-strain curve can be divided into three stages. As a result of this investigation, work hardening takes place in stage II and at the beginning of stage III. Further, in stage II, the hardening rate is strongly dependent on the volume fraction of the harder phase. In stage III, the hardening rate for each DP-steel is smaller than that in stage II and is related to the difference in tensile strength between the harder and the softer phases. Furthermore the second investigation by means of FEM analysis was carried out in order to evaluate the influence of variation of the volume fraction of the harder phase on the stress-strain behavior of a DP-steel. Tensile tests showed that by increasing the amount of the harder phase (bainite) in the DP-steel, Luders elongation disappears. According to the results obtained by the FEM calculations, the stress-strain behavior is related to the microstructure, such as volume fraction and shape of the grains in the DP-steel.

Composition, microstructure, hardness, and wear properties of high-speed steel rolls

Abstract: The effects of alloying elements on the microstructural factors, hardness, and wear properties of four high-speed steel (HSS) rolls fabricated by centrifugal casting were investigated. A hot-rolling simulation test was carried out using a high-temperature wear tester capable of controlling speed, load, and temperature. The test results revealed that the HSS roll containing a larger amount of vanadium showed the best wear resistance because it contained a number of hard MC-type carbides. However, it showed a very rough roll surface because of cracking along cell boundaries, the preferential removal of the matrix, and the sticking of the rolled material onto the roll surface during the wear process, thereby leading to an increase in the friction coefficient and rolling force. In order to improve wear resistance with consideration to surface roughness, it is suggested that a reduction in the vanadium content, an increase in solid-solution hardening by adding alloying elements, an increase in secondary hardening by precipitation of fine carbides in the matrix, and formation of refined prior austenite grains by preaustenitization treatment be employed to strengthen the matrix, which can hold hard carbides in it.

Physical Hardening of Asphalt Binders Relative to Their Glass Transition Temperatures

Abstract: Physical hardening (physical aging) is a process that occurs below room temperature in asphalt binders. Physical hardening causes time-dependent isothermal changes in the rheological behavior and specific volume of asphalt binders. The process is reversible: when the asphalt binder is heated to room temperature or above, the effect of physical hardening is completely removed. Physical hardening for amorphous materials is generally reported as occurring below the glass transition temperature (Tg), but this is not the case for asphalt binders, in which physical hardening is observed both above and below Tg. The glass transition temperature of asphalt binders is measured by using three different techniques: dilatometry, differential scanning calorimetry, and rheological considerations (peak in the loss modulus versus temperature). These three techniques give roughly equivalent estimates of the glass transition temperature. The behavior of physical hardening in asphalt binders is somewhat different than that ...

Origin of the initial rapid age hardening in an Al-1.7 at.% Mg-1.1 at.% Cu alloy

Abstract: The origin of the rapid hardening which occurs during the initial ageing stage of an Al-1.7 at.% Mg-1.1 at.% Cu alloy has been investigated by the threedimensional atom-probe (3DAP) and transmission electron microscopy techniques. Although the rapid hardening reaction occurs within 1 min at 150 degrees C, no evidence for solute clusters or Guinier-Preston-Bagaryatsky (GPB) zones was detected using the 3DAP after this ageing time. When a short-term aged specimen is deformed and then aged at 150 degrees C, further rapid hardening occurs. This suggests that the rapid hardening is associated with a solutedislocation interaction. Uniform dispersions of Cu-Mg co-clusters do not occur until closer to the end of the hardness plateau and it is thought that these evolve into GPB zones during the second stage of the age-hardening process.

Low-temperature dynamic precipitation in a supersaturated Al± Zn± Mg alloy and related strain hardening

Abstract: Dynamic precipitation during low-temperature deformation of a supersaturated Al-Zn-Mg alloy has been studied by small-angle X-ray scattering and in-situ resistivity in the temperature range 4.2–300 K. At 300 K, dynamic precipitation is observed both for the material strained in the form of a solid solution and for the material strained in the partially aged state, that is containing Guinier-Preston (GP) zones. In the first case, dynamic precipitation occurs by nucleation of precipitates 5 A in diameter, and in the second case it occurs by coarsening of existing precipitates. In all cases the dynamic precipitation results in an unusual-strain hardening behaviour in which the work-hardening rate is very high and essentially temperature independent.

Effects of strain rate and temperature on the stress–strain response of solder alloys

Abstract: To ensure reliable design of soldered interconnections as electronic devices become smaller, requires greater knowledge and understanding of the relevant mechanical behavior of solder alloys than are presently available. The present paper reports the findings of an investigation into the monotonic tensile properties of bulk samples of three solder alloys; a lead–tin eutectic and two lead-free solders (tin–3.5 copper and a tin–3.5 silver alloy). Temperatures between−10 and 75°C and strain rates between 10−1 and 10−3 s−1 have been studied. Both temperature and strain rate may have a substantial effect on strength, producing changes well in excess of 100%. Strength is reduced by lowering strain rate and increasing temperature, and Sn–37 Pb is usually most sensitive to the latter. Expressions for strain and strain rate hardening have been developed. The Sn–0.5 Cu alloy is usually the weakest and most ductile. Sn–37 Pb is strongest at room temperature but with increasing temperature and lower strain rates it becomes inferior to Sn–3.5 Ag. Ductility changes with temperature and strain rate for all three alloys are generally small with inconsistent trends. The role of such data in stress analysis and modeling is considered and the paramount importance of employing data for conditions appropriate to service, is emphasized.

On cavitation, post-cavitation and yield in amorphous polymer-rubber blends

Abstract: The deformation behaviour of amorphous polymer–rubber blends is investigated in terms of an axisymmetric unit cell model containing an initially spherical rubber particle. The behaviour of the rubber is described by an incompressible non-Gaussian network theory, while for the matrix we adopt a recent large strain elastic–viscoplasticity model that incorporates the intrinsic softening upon yield and the subsequent progressive orientation hardening typical for amorphous glassy polymers. Guided by simple analytical estimates, cavitation of the rubber particle is interpreted in terms of the unstable growth of a pre-existing small void. It is shown that cavitation and yield are essentially coupled processes. On the macroscopic scale, both are softening mechanisms : If macroscopic yield takes place before the limit stress for cavitation is reached, cavitation is prohibited. Furthermore, and contrary to common belief, it is found from the interfacial stress history that, using realistic material parameters, the rubber particle continues to significantly affect plasticity in the matrix in the post-cavitation regime, i.e. after it has cavitated, so that cavitated particles cannot always be considered to be equivalent to particle-sized voids.