Neutron diffraction and finite element analysis of quenching residual stress of GH4169 super alloy

The quenching process is indispensable for manufacturing the superalloys. However, high residual stress is inevitably induced by the large heat gradient, which influences the mechanical properties of the alloys. Therefore, it is of great significance to accurately characterize the internal residual stress of the super alloy and establish an effective finite element modeling. The quench- induced residual stresses of GH4169 super alloy were measured using neutron diffraction stress instrument. The results show that the center of the workpiece is subjected to tensile stress in three directions (hoop, radial and axial), which is about 400MPa. The rims of workpiece are subjected to compressive stress in one or two directions (hoop or axial), which is from -300MPa to -400MPa. The temperature and strain/stress fields of the GH4169 super alloy workpiece during quenching process were simulated by a 3D finite element model. The calculated residual strain/stress at the center and rim of the workpiece are compared to those by neutron diffraction, which shows good correspondence. These data provide reliable evidences to the formation of the residual stress during quenching.     

The influence of grain boundary sulfur concentration on the intergranular brittleness of nickel of different purities

When systematically combined, tensile tests at -150 °C and analysis by AES (Auger Electron Spectroscopy) or by electron probe reveal the mechanical and chemical effects of sulfur in the grain boundaries of nickel of different purities. The sulfur concentration in the grain boundaries of nickels containing globally less than 450 ppm S appears to be the essential factor in the grain boundary brittleness of the metal. Other elements may however interfere with the sulfur in the mechanical characteristics of the metal, either by increasing or decreasing the brittleness according to the particular case. They also interfere with the sulfur by slowing down the kinetics of the approach to equilibrium. Formerly Research Associate, Département de Métallurgie, Ecole Nationale Supérieure des Mines de Saint-Etiénne Formerly Professor, Département de Métallurgie, Formerly Professor, Département de Métallurgie,     

A thermal model of laser cladding by powder injection

A two-dimensional (2-D) finite element model is presented for laser cladding by powder injection. The model simulates the quasi-steady temperature field for the longitudinal section of a clad track. It takes into account the melting of the powder in the liquid pool and the liquid/ gas free surface shape and position, which must conform to the thermal field in order to obtain a self-consistent solution. The results for an idealized problem, where there is almost no melting of the substrate material, demonstrate the linear relationship between the laser power, the processing velocity, and the thickness of the deposited layer. The calculated clad heights agreed well with the experimental values for the conditions where a cobalt-based hard-facing alloy is clad onto mild steel with a linearly focused laser source. Formerly Postdoctoral Fellow, Ecole Polytechnique Fédérale de Lausanne     

Elastoplastic finite element analysis of short-fiber-reinforced SiC/Al composites: effects of thermal treatment

Elastoplastic finite element analyses of realistic models of short-fiber-reinforced composites were extended to include the effects of prior thermal treatments on predictions of subsequent mechanical properties. Two three-dimensional models were used, one in which the fiber ends were transversely aligned and another in which they were staggered. Both models were found to be necessary for accurate predictions of the behavior of higher volume fraction composites. The temperature dependence of the yield stress of the matrix material was explicitly included in the analysis. The spatial and temporal history of calculated. The room temperature residual stresses were also predicted. Both the plastic deformation and the residual stresses in the matrix were spatially non-uniform and varied rapidly from the regions near the ends of the fiber to those near the midpoint. Predictions of subsequent tensile stress-strain properties were in good quantitative agreement with experiments. The presence of residual stresses and locally deformed regions caused the tensile behavior to differ from the compressive behavior. These differences were complex and depended on the volume fraction and aspect ratio of the reinforcement. The analyses provide detailed insight into the deformation mechanisms of these composites.     

Sheet bending and determination off residual stresses by means of FEM

The simulation of metal forming processes with the finite element method (FEM) in the sense of a “numerical experiment” is gaining more and more importance. Bending of FeP04 (St1403), × 5 CrNi 189 and AIMgSi1 sheets in V- and U-shaped dies prove the quality of this type of calculation. Of particular interest here is the determination of punch forces, strains and stresses in the workpiece. The calculation of residual stresses is important for process optimisation. Comparisons between calculated and experimentally determined results indicate the calculation qualities. There are many possibilities of influencing the simulation quality of residual stress calculations. In particular, the material model has to be optimally selected. Two combined isotropic-kinematic hardening models (from Axelsson/Samuelsson and McNamara/Sharma) implemented in the FE program to take the Bauschinger effect into consideration point out the influence of material models on the calculated residual stresses.     

Residual stresses and their effects on deformation

Residual stresses induced by thermal expansion mismatch in metal-matrix composites are studied by three-dimensional (3-D) elastic-plastic finite element analyses. Typically, the stress-free state is 150 to 300 K above room temperature. The coefficient of thermal expansion of the matrix is 3 to 5 times larger than that of the ceramic inclusion, resulting in compressive stresses of order 200 MPa in the inclusions. Both compressive and tensile stresses can be found in the matrix. Since the stress may exceed the matrix yield strength near the particles, plastic flow occurs. The authors find a significant influence of this flow on the elastic and plastic properties of the composite. The calculated residual strains in TiC particles due to thermal expansion mismatch and external loads compare well with recent neutron diffraction experiments (Bourkeet al.) The present work is the first reported three-dimensional analysis of spherical inclusions in different arrays (simple cubic (sc) and face-centered cubic (fcc)) that permit a study of particle interactions.     

Thermal residual stresses in functionally graded and layered 6061 Al/SiC materials

The thermal residual stresses that develop in spray atomized and codeposited functionally graded and layered 6061 Al/SiC metal-matrix composites (MMCs) during cooling from the codeposition temperature to ambient temperature were studied using thermo-elastoplastic finite element analysis. In an effort to investigate the effect of layered and graded structures on the residual stress distribution, the composites with homogeneous distribution of SiC particulates were also analyzed. The effect of SiC volume fraction in the SiC-rich layers and the effect of SiC-rich layer thickness on the residual stresses were investigated. Based on the present study, it was found that the residual stress distribution is very distinct for the aluminum and the SiC-rich layers in the layered materials. As the volume fraction of SiC increases in the SiC-rich layer, the magnitude of residual stresses also increases. The radial stress was found to be tensile in the aluminum layers and compressive in the SiC-rich layers. It was also found that, as the thickness of the SiC-rich layer increases, the magnitude of radial stress in the aluminum layers increases, and that in the SiC-rich layers decreases. In the graded material, the lower region of each layer exhibits tensile radial stress, and the upper region of each layer shows compressive radial stress in order to maintain continuity between layers during cooldown. In general, the layered and the graded materials have greater residual stresses and more complicated stress distribution, as compared with those in the composite materials with homogeneous distribution of SiC particulates.     

Hydride orientation and tensile properties of Zr-2.5 wt% Nb pressure tubing hydrided while internally pressurized

Abstract

An investigation was carried out to determine the orientation and the effect on tensile properties of hydrides precipitated in heat-treated Zr-2.5 wt% Nb tubing during the corrosion hydriding of internally pressurized samples. It was found that as the stress during hydriding was increased the hydrides gradually assumed a characteristic radial orientation nearly perpendicular to the hoop stress. The observed radial hydride orientation was influenced by prior β grain size, residual quenching stresses, cold-work strains and stresses applied while the pressure tube was being hydrided. Hoop stresses in excess of 20,000 psi were required to produce complete radial orientation of the hydrides.

Transverse tensile tests on the hydrided tubes were conducted using a reduced section ring technique. These tests revealed a low ductility at low temperatures with a transition in the range 60°–100°C for material with radially oriented hydrides. Low tensile ductility was observed at significantly higher temperatures when a massive hydride layer was present near the surface of the test samples.

Résumé

Dans des tubes en Zr-2.5 % poids Nb trempé et vieilli et soumis à la corrosion hydrogenante sous pression, une étude a été entreprise pour déterminer l'orientation des hydrures précipités et leurs effets sur les propriétés mecaniques en traction. Les résultants indiquent que lorsque la contrainte augmente pendant léhydrogenation, les hydrures acquièrent graduellement une orientation radiale caractéristique, à peu prés perpendiculaire à la contrainte circonférentielle. Cette orientation radiale des hydrures est influencée par la taille du grain β initial, les contraintes de trempe résiduelles, les déformations à froid et les contraintes appliquées lorsque le tube sous pression est hydrogéné. Des contraintes circonférentielles supérieures à 20,000 psi sont nécessaires pour produire une orientation parfaitement radiale des hydrures.

Des essais de traction en travers des tubes hydrogénés ont été réalisés en utilisant la technique de l'anneau à section réduite. Ces essais ont révélé une faible ductilité à basses températures avec une transition dans le domaine 60°–100°C pour le matériau à hydrures orientés radialement. Une faible ductilité en traction a été observée à des températures beaucoup plus hautes quand une couche massive d'hydrures existait près de la surface des échantillons.     

Induction Heat Treatment of Sheet‐Bulk Metal‐Formed Parts Assisted by Water–Air Spray Cooling

In order to produce components with massive secondary functional elements from sheet metal bulk forming operations, termed sheet‐bulk metal forming, can be applied. Owing to high, three‐dimensional stress and strain states present during sheet‐bulk metal forming, ductile damage occurs in the form of micro‐voids. Depending on the material flow properties, tensile residual stresses can also be present in the components' formed functional elements. During service, the components are subjected to cyclic loading via these functional elements, and tensile residual stresses exert an unfavorable influence on crack initiation and crack growth, and therefore on the fatigue life. Following the forming process, temperature and microstructurally related compressive residual stresses can be induced by local heat treating of the surface. These residual stresses can counteract potential crack initiation on the surface or in the subsurface regions. In the present study, the adjustability of the residual stress state is investigated using a workpiece manufactured by orbital cold‐forming, which possesses an accumulation of material in its edge region. Based on residual stress measurements in the workpiece's edge region using x‐ray diffractometry, it is possible to verify the compressive residual stresses adjusted by varying the cooling conditions.     

Processing of Beryllium for High Technology Applications

Unique combination of physical, mechanical and thermal properties, that beryllium possess makes it an ideal candidate material for space applications, as for examples, in Inertial Guidance Systems, Spacecraft Mechanisms, optics and Structural Element). Its inherent nuclear properties makes it attractive in nuclear programmes. Besides these, it has been a recommended choice in computers, micro electronics, audio industries and medical for high technology systems. Though beryllium is readily machinable for high precision accuracies and excellent surface finishes, yet it has tendency to chip off or edge crack and prone to residual stresses and surface damages. Additionally, inhalable beryllium dust fumes generated during machining is injurious to health. In order to achieve desired accuracies, processing of beryllium calls for appropriate fixturing, adoption of proper cutting parameters, toolings, operation sequencing and proper handling, and job analysis. This paper takes a stock of the present technology and other processes of forming beryllium into shapes.     

Characterization of Flame Cut Heavy Steel: Modeling of Temperature History and Residual Stress Formation

Heavy steel plates are used in demanding applications that require both high strength and hardness. An important step in the production of such components is cutting the plates with a cost-effective thermal cutting method such as flame cutting. Flame cutting is performed with a controlled flame and oxygen jet, which burns the steel and forms a cutting edge. However, the thermal cutting of heavy steel plates causes several problems. A heat-affected zone (HAZ) is generated at the cut edge due to the steep temperature gradient. Consequently, volume changes, hardness variations, and microstructural changes occur in the HAZ. In addition, residual stresses are formed at the cut edge during the process. In the worst case, unsuitable flame cutting practices generate cracks at the cut edge. The flame cutting of thick steel plate was modeled using the commercial finite element software ABAQUS. The results of modeling were verified by X-ray diffraction-based residual stress measurements and microstructural analysis. The model provides several outcomes, such as obtaining more information related to the formation of residual stresses and the temperature history during the flame cutting process. In addition, an extensive series of flame cut samples was designed with the assistance of the model.     

Fracture of Precipitation-Hardened Copper Alloy Produced by Semi-Continuous Casting at Elevated Temperature

Abstract

In semi-continuously cast slabs of precipitation-hardened copper alloy, internal cracks are often observed after homogenization, although those defects can not be found out after casting. It is considered that, before stress relief takes place during homogenization, thermal stresses generated in the slab during casting will exceed the fracture stress. It is of interest to understand how internal stresses of slabs will affect the fracture during homogenization. Fracture temperatures of the material under static tensile stresses are investigated in an attempt to demonstrate the fracture evolved during homogenization. © 2000 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.

Résumé

Dans les brames d'alliage de cuivre durci par précipitation et coulées en semi-continu, on observe souvent des fissures internes après homogénéisation, bien qu'on ne trouve pas ces défauts après la coulée. On considère que les ontraintes thermiques générées dans la brame lors de la coulée vont excéder la contrainte de rupture lors de l'homogénéisation, avant que l'effet de relaxation puisse se produire. Il est intéressant de comprendre de quelle manière les contraintes internes des brames affecteront la rupture lors de l'homogeneisation. On a etudie les températures de rupture du matériel sous contraintes statiques en traction dans une tentative de démontrer que la rupture a évolué lors de l'homogénéisation. © 2000 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.     

MICROSTRUCTURE AND MECHANICAL BEHAVIOUR OF Ni-ALLOYED AUSFORMED AUSTEMPERED DUCTILE IRON

Abstract

The abrasion wear behaviour of ausformed austempered ductile iron (AADI) as well as conventional austempered ductile iron (ADI) was investigated. The effects of both Ni content and rolling reduction during the ausforming process on wear resistance of AADI and ADI were studied thoroughly. The ausforming process has created a finer and more homogeneous ausferrite structure that has a direct influence on the mechanical properties of AADI. Maximum hardness and tensile strength were obtained with a 35% rolling reduction. On the other hand, ductility and impact strength were reduced with increasing rolling reduction during the ausforming process. AADI showed superior abrasion wear resistance due to its finer and harder structure.

On a investigué le comportement de résistance à l’abrasion de la fonte ductile austénitoformée, à trempe étagée bainitique (AADI) ainsi que de la fonte ductile conventionnelle à trempe étagée bainitique (ADI). On a étudié à fond l’effet de la teneur en Ni ainsi que de la réduction par laminage lors du traitement d’austénitoformage sur la résistance à l’usure de l’AADI et de l’ADI. Le traitement d’austénitoformage créait une structure ausferritique plus fine et plus homogène qui a une influence directe sur les propriétés mécaniques de l’AADI. On a obtenu une dureté et une résistance à la traction maximales avec une réduction par laminage de 35%. D’un autre côté, la ductilité et la résistance à l’impact étaient réduites avec une augmentation de la réduction par laminage lors du traitement d’austénitoformage. L’AADI a montré une résistance supérieure à l’abrasion, grâce à sa structure plus fine et plus dure.     

Multiple-cracking phenomenon of the galvannealed coating layer on steels under thermal and tensile stresses

The multiple-cracking phenomenon of the Fe-Zn intermetallic coating layer on the hot-dip galvannealed (GA) steels under thermal and tensile stresses was studied experimentally by tensile tests and analytically by means of the finite-element analysis. The multiple cracking of the coating layer had occurred in the as-supplied samples, and it progressed with increasing applied strain. Based on the calculated dependence of the stress of the coating layer on the crack spacing and applied strain, the multiple cracking in the as-supplied samples was accounted for by the thermally induced residual stress, and the further multiple cracking with increasing applied strain was accounted for by the increased stress of the coating layer. The experimentally observed decrease of the average crack spacing with increasing applied strain was described well, and the tensile strength of the coating layer was estimated to be 260 MPa, by application of the calculated relation between the increased stress of the coating layer and applied strain. The influences of the thickness of the coating layer and the substrate material on the multiple cracking were discussed based the stress analysis. It was revealed that the thinner the coating layer and the higher the flow stress of the substrate, the higher the stress of the coating layer becomes and, therefore, the smaller the crack spacing becomes.     

Phase composition and residual stresses in thermal barrier coatings

The phase composition and the residual stresses in multilayer thermal barrier coatings, which consist of an external ZrO₂–8Y₂O₃ ceramic layer, an intermediate gradient (metal ceramic) layer, and a transient metallic NiCrAlY sublayer, are studied. It is shown that an increase in the specific volume of the metallic sublayer as a result of the formation of thermal growing oxide Al₂O₃ generates high compressive stresses in this sublayer. The ceramic layer undergoes tensile stresses in this case. A method is proposed to estimate the stresses in gradient coatings from X-ray diffraction results.     

Microstructure and mechanical property correlations in AA 6061 aluminium alloy friction stir welds

The influence of friction stir welding on the microstructure development and its role on residual stress distribution in the weldment and mechanical properties has been investigated. The study also focused on the impact of post weld heat treatment on the microstructure and mechanical properties as well as on residual stress distribution. The weld nugget region contained fine equiaxed grains as a result of thermo-mechanical working. Hardness survey showed that nugget region is soft due to precipitates dissolution. Weld joint exhibited lower strength as compared to the parent metal. Post weld Solution Treatment and Aging (STA) of longitudinal welds resulted in strength and ductility equivalent to that of parent metal while transverse weld tensile strength and ductility were lower than that of parent metal even after post weld STA. Residual stress distribution profiles across the weld region are asymmetric with respect to weld centerline, with the largest residual; stress gradients occurring on the advancing side of the weld. Within the region inside the shoulder diameter, residual stress is entirely compressive. Welds exhibited tensile residual stresses in post weld STA condition     

Effect of damaged liners and roping on classification performed by 6 inch hydrocyclone

Abstract

Mineral processing plants use hydrocyclones to classify coarse and fine particles. The hydrocyclone is a robust piece of equipment whose operation can be hampered by mechanical and operation problems. This paper assesses the effect of mechanical defects and roping on the partition curve of a 15 cm hydrocyclone. Two types of liner damages in the cylinder of the hydrocyclone were tested and not found to deteriorate significantly the classification of particles in the unit. Results obtained at the transition from spay to rope discharge show that a roping condition does not impact significantly the quality of the hydrocyclone classification.

Les usines de traitement de minerais utilisent des hydrocyclones pour classifier les particules selon leur poids. L’hydrocyclone est une pièce d’équipement robuste dont l’opération peut être entravée par des problèmes mécaniques et de fonctionnement. Cet article évalue l’effet des bris mécaniques et de décharge en boudin sur la courbe de efficacité d’un hydrocyclone de 15 cm. Deux types de dommages au revêtement du cylindre de l’hydrocyclone ont été évalués et les résultats ne montraient pas de détérioration significative de la classification des particules dans l’unité. Les résultats obtenus lors de la transition de décharge en parapluie à décharge en boudin montrent que la condition en boudin n’a pas d’impact important sur la qualité de la classification de l‘hydrocyclone.     

The influence of austenite grain size and its distribution on chip deformation and tool life during machining of AISI 304L

In this article, the influence of austenite grain size and its distribution on chip deformation and tool life during machining of AISI 304L austenitic stainless steel bar is examined. Hot-forged bar and the quenched bars (at different quenching temperatures, 1050 °C, 1100 °C, 1150 °C, and 1200 °C) are machined at a high cutting speed. It was noted that the inhomogeneous distribution of grain size in the surface area, within a depth of 15 mm of the workpiece, resulted in tool edge breakage and lower tool life when machining the hot-forged bar compared with all of the quenched bars. In addition, a slight decrease in tool life was observed as the grain size increased in the quenched bars. The chip studies revealed that a higher segment height ratio of chip was gained when machining the hot-forged bar, compared to machining the quenched bars, due to the inhomogeneous distribution of grain size. Moreover, the thickness of the secondary shear zone was reduced as the grain size increased. Interestingly, it was noticed that the chip work hardened during the machining process due to strain-induced twinning and ɛ martensite transformation. The studies of tool wear and failure revealed that a crack was initiated on the flank face at the interface between the deposited workpiece and the tool substrate when machining the hot-forged bar. This crack was formed due to either the thermal and mechanical fatigue or plastic deformation of the tool substrate. The fatigue crack propagated into the tool substrate through the decohesion of interface between carbides. The criterion of tool life when machining all of the quenched bars was normal flank wear. Based on the studies of chip deformation and the mechanisms for tool wear and failure, the effects of austenite grain size and its distribution on tool life were explained.     

Temperature and thermal stresses in work rolls during the hot rolling of strip

In this paper the solution of the unsteady, three-dimensional heat transfer in work rolls was derived by using Bessel functions and the δ-function and expressed by an infinite series. In this manner other problems of heat transfer in solid cylinders and hollow cylinders can also be calculated. The temperature profile of the work roll due to the distribution of the temperature in the axial direction can cause unregular pressure distribution between work roll and backup roll and can influence the quality of strip. Therefore, the thermal crowns must be considered at various moments during rolling. The thermal crown can be determined by using the FE-Method or others. The reason for the fire-cracking in the roll surface can be determined as the thermal shock load in the contact zone, in which the sharp compressive stresses lead to local plastic deformation. The local plastic deformations are followed by residual deformations. Therefore, residual tensile stresses occur in the cooling zone. On each revolution, the surface undergoes plastic strain in compression and in tension. The result is thermal fatigue. The fine network of cracks in the roll surface can result in a sharp stress concentration which is dangerous for the rolls loaded with a bending moment. The maximum tensile stresses due to the temperature distribution occur in the roll core, which usually do not lead to damages of the rolls.     

Macroscopic and microscopic evolutions of a shot-peened layer during isothermal recovery

The present study deals with the recovery of work-hardened layers, which are often encountered in mechanical parts. Two simple cases are studied for the 38Cr4Mo steel: the shot-peened layer of semi-infinite bodies and a ground surface. The latter case is applied to in situ tensile samples using the gold microgrids technique developed at PMTM Laboratory in order to determine some possible dimensional changes. Both work-hardened layers show the existence of macroscopic straining during the recovery. For the only shot-peened layer, a phenomenological study provides the evolution of the residual stresses and the integral breadth depth profiles using X-ray diffractometry. A method is proposed to determine the recovery strain field from the residual stresses evolution. As the only possible strain on a semi-infinite body lies along the normal axis to the surface, calculated recovery strains are compared with the dimensional measurements. On the microstructural viewpoint, the modified Warren-Averbach’s analysis is quickly presented and is performed to analyze the X-ray peak broadening through the affected layer of a shot-peened sample. Several corrections and deconvolution calculations of the X-ray profiles finally lead to the second and third types of broadening effects and their microstructural interpretations.