Microstructure and mechanical properties of Inconel 718 electron beam welds

Abstract

Bead on plate, full penetration electron beam welds were produced in 2 mm thickness sheets of Inconel 718 in the solution treated condition. Welds were subjected to an aging treatment with and without post-weld solution treatment. Weld microstructures, high temperature tensile properties and stress rupture properties were evaluated. The as welded fusion zone showed a considerable amount of interdendritic niobium segregation and brittle intermetallic Laves phase. The tensile and stress rupture properties of the welds after post-weld aging treatment were found to be inferior in relation to the base metal. Post-weld solution treatment at 980°C was found to result in partial dissolution of Laves phase, some reduction in niobium segregation and the formation of δ phase needles around the Laves particles. The use of 980°C solution treatment was found to improve the weld properties to some extent, although not to the level of the base metal. The reasons for this behaviour are discussed, correlating microstructures, fracture features and mechanical properties.     

Bestimmung der Streckgrenze und der Hall‐Petch‐Konstanten des Vergütungsstahles 42CrMo4 unterschiedlichen Gefüges mittels Eindruckprüfungen

Determination of yield stress and Hall‐Petch coefficients of the tempering steel 42CrMo4 with different microstructure by means of indentation testing Yield stress of steels can be determined by tensile tests or by indentation testing. Indentation tests offer the advantage of measuring the yield stress of certain parts that otherwise could not be used to withdraw tensile samples. Furthermore, measurements in parts with a varying yield stress e.g. surface hardened steels can be carried out. This work investigates several methods of indentation tests to measure the yield stress of the tempering steel 42CrMo4. The yield stress of different microstructures is compared to results of tensile tests. Using the indentation test according to GOST 22762‐77 the Hall‐Petch coefficients of microstructures consisting of perlite and ferrite are determined. Thus the influence of grain size and distance of perlite lamellas on the yield stress can be taken into account.     

High cycle fatigue of welded joints with aging influence

Fatigue loading appears frequently at many machine parts and it can, under certain conditions, cause a failure. Many parts of an internal combustion engine are subjected to a raised temperature where the fatigue properties are different than they are at room temperature. The exhaust system is one of the parts that are loaded with fatigue at different temperatures, known as thermo mechanical fatigue (TMF). Welds are used frequently in the exhaust system and present critical locations for crack initiation. During loading initial crack can propagate through a section to a final failure, so if the lifetime of the exhaust system is determined by the results of a base material, it can be misjudged. In the paper it is presented that the fatigue degradation regards to the weld influence. Test sample preparation, material and special high cycle fatigue testing manner are also presented. Test specimens without weld, with weld and additionally with aging influence are used. The material exposure to increased temperature over time affects the stress–strain response and other mechanical properties. This effect is known as aging and the results of this influence on the fatigue of welded parts are presented and discussed.     

The statistical second‐order two‐scale method for mechanical properties of statistically inhomogeneous materials

A class of random composite materials with statistically inhomogeneous microstructure, for example, functionally graded materials is considered in this paper. The microstructures inside a component are gradually varying in the statistical sense. In view of this particularity, a novel statistical second‐order two‐scale (SSOTS) method is presented to predict the mechanical properties, including stiffness, and elastic limit. To develop this method, the microstructures of statistically homogeneous, and inhomogeneous materials are represented. In addition the SSOTS formulas are derived based on normalized cell depending on the position variables by a constructing way, and the algorithm procedure is described. The mechanical properties of the different inhomogeneous materials are evaluated. The numerical results are compared with the experimental findings. It shows that the SSTOS method is effective. Copyright © 2010 John Wiley & Sons, Ltd.     

Einfluss eines Gefügegradienten auf die rauhigkeitsinduzierte Rissschließung bei TIMETAL 1100

Influence of a Grain Size Gradient on Roughness Induced Crack Closure of TIMETAL 1100 Crack closure effects, which affect the crack propagation behaviour at low R‐values, become crack length dependent in gradient materials. It is not known, if the crack closure at a given ΔK is locally identical to that in a homogeneous microstructure or if closure effects are crack direction dependent. To get some insights to these questions, coarse lamellar microstructures (showing only roughness induced crack closure) with a gradient in the lamellae package sizes where produced on TIMETAL 1100 by recrystallisation. Their crack closure behaviour was compared to that of two different homogenous microstructures. The results clearly show, that crack closure effects show identical dependencies on the stress intensity and the local fracture surface roughness for all microstructures. The roughness, in turn, is uniquely determined by the sizes of the lamellae. Thus no crack direction effects where observed. It is shown that a model for roughness induced crack closure, which was developed for homogeneous microstructures, can be used to calculate the crack closure stress intensity for any gradient and loading condition.     

Temperature, stress and microstructure in 10Ni5CrMoV steel plate during air–arc cutting process

Although the air–arc cutting process has been widely used in the material processing engineering, little information about temperature, stress and microstructure in the plate air–arc cut is known. A three-dimensional finite element model including the material removal and the thermal effect of the arc is developed to study the temperature and stress fields of 10Ni5CrMoV steel plate during air–arc cutting process in this paper. The microstructures and micro-mechanical properties of the parts near the groove especially in heat affected zone (HAZ) are studied by experimental methods, and they also can be used as a method to verify the numerical results. Effects of stresses induced by air–arc cutting process on the initial residual stress fields of base materials are also researched. The results show that the cooling velocity in HAZ is higher than the one of the welding process for the same base material, and the zone with high temperature is very narrow, which means that the temperature gradients near the groove are very steep during the air–arc cutting process; this special temperature field depresses multiphase transformations and coarse microstructures. The evolution of the stress during the air–arc cutting is described, and the numerical results indicate that the characteristics of the evolution of stresses and the residual stresses distribution in the plate in air–arc cutting process seem to be similar to the ones of the butt welding for flat plates. The influences of air–arc cutting process on initial stress fields present two aspects: thermal effect and material removal effect, and the former plays a primary role. Both numerical temperature and stress fields are compared with the experimental ones. It is very important for researchers to clarify the temperatures, stresses and microstructures in the plate during air–arc cutting process, and understand fully the mechanism of influences of air–arc cutting on the plate; it is also very valuable for engineering application of the air–arc cutting process.     

Betrachtungen zur Gefügesystematik

Considerations on Mircostructural Systematics Many terms of modern engineering materials science have been defined by metal science and metallurgy. Their transmission to other materials as ceramic needs adaption. But also the original content of the terms undergoes timedepending variations according to the nowadays state-of-the-art. This is why the definition and the relation of scientific-technical terms to each other has to be reconsidered again and again. In this context related to microstructure the present paper discusses terms as adhesion, agglomeration, alloys and anisotropy, cohesion, cristallites, directionally solidified eutectics, disordered and duplex microstructures. Engineering materials, grains and grain boundaries are also put into this frame as well as heterogenity, homogenity, incoherence, inhomogenity, in-situ- and interconnected microstructures, isotropy, material and matrix microstructures. Microstructural elements and microstructural levels, microstructural modelling and microstructural parameters are related to microstructure-property-correlations and by multiphased microstructures, ordered microstructures, orientation factors, phases, pores, shape factors, single phase and synthetical microstructures as finally textures and types of microstructures the sequence of terms is completed. What they mean, how they belong together and why microstructure needs them is subject of the constribution.     

Intergranular corrosion of welds in type 405 stainless steel

A low chromium ferritic stainless steel (Type 405) with different heat treatments and weld configurations, has been investigated for susceptibility to intergranular corrosion by chemical and electrochemical potentiodynamic reactivation (EPR) tests, and the results were evaluated by weight losses, dye penetration, metallographic techniques, and the ratio of the reactivating and activating currents. Welds are susceptible to attack, particularly in the heat-affected zone. Applied stress in a U-bend increases significantly the attack on welds, and annealing eliminates the effects of stress. Results on the susceptibility of unwelded specimens in the form of flat plates and U-bends after various heat treatments are also presented and discussed.     

The role of microstructural morphology on creep properties of a [0 0 1] oriented nickel‐base single crystal superalloy

The experimentally observed microstructure of nickel‐base single crystal alloy consists of a large volume of cuboidal γ′ precipitates coherently embedded in the γ matrix. In calculation, a representative volume element is usually used to represent the whole structures due to the regular γ/γ′ topological structures. Here, three experimentally found microstructures have been extracted to generate the representative volume elements. One is constituted by one cuboidal γ′ phase surrounded by γ phase. The other two consisted of two cuboidal γ′ phases and one rectangle γ′ phase with different arrangement of the two γ′ phases. The misfit stress is taken into consideration by different thermal expansion coefficients of the two phases. The influences of different microstructures on the macro‐creep strain evolution, rafting and stress distributions are discussed.     

Solids Under Extreme Shear: Friction‐Mediated Subsurface Structural Transformations

Tribological contacts consume a significant amount of the world's primary energy due to friction and wear in different products from nanoelectromechanical systems to bearings, gears, and engines. The energy is largely dissipated in the material underneath the two surfaces sliding against each other. This subsurface material is thereby exposed to extreme amounts of shear deformation and often forms layered subsurface microstructures with reduced grain size. Herein, the elementary mechanisms for the formation of subsurface microstructures are elucidated by systematic model experiments and discrete dislocation dynamics simulations in dry frictional contacts. The simulations show how pre‐existing dislocations transform into prismatic dislocation structures under tribological loading. The stress field under a moving spherical contact and the crystallographic orientation are crucial for the formation of these prismatic structures. Experimentally, a localized dislocation structure at a depth of ≈100–150 nm is found already after the first loading pass. This dislocation structure is shown to be connected to the inhomogeneous stress field under the moving contact. The subsequent microstructural transformations and the mechanical properties of the surface layer are determined by this structure. These results hold promise at guiding material selection and alloy development for tribological loading, yielding materials tailored for specific tribological scenarios.     

The use of digital images to determine deformation throughout a microstructure Part I Deformation mapping technique

Materials with heterogeneous microstructures do not deform uniformly under stress (mechanical or environmental). A new deformation mapping technique (DMT), which compares digital images of microstructures of the same field of view before and after deformation occurs, is reported. Two digital images are required: a reference image, taken before deformation; and a deformed image, taken after deformation. The displacements of pixels required to match the deformed image to the reference image are computed, and these displacements are used to calculate the percent deformation in the two principal directions. Results are presented as either a deformation map, as a histogram, or as data files containing the displacements at the corner of each pixel. Comparison with exact solutions generated on a simulated microstructure shows that the accuracy of this technique is quite good.     

Dissimilar friction stir lap welding of AA 5754-H22/AA 6082-T6 aluminium alloys: Influence of material properties and tool geometry on weld strength

Dissimilar friction stir welding (FSW) of heat (AA 6082-T6) and non-heat (AA 5754-H22) treatable aluminium alloys, in lap joint configuration, was performed in this work. The base material plates were 1 mm thick. Welds were performed combining different plates positioning, relative to the tool shoulder, in order to assess the influence of base materials properties on welds strength. Three different tools were tested, one cylindrical and two conical, with different taper angles. Welds strength was characterized by performing transverse and tensile–shear tests. Strain data acquisition by Digital Image Correlation (DIC) was used to determine local weld properties. The results obtained enabled to conclude that the dissimilar welds strength is strongly dependent on the presence of the well-known hooking defect and that the hooking characteristics are strongly conditioned by base materials properties/positioning. By placing the AA 6082-T6 alloy, as top plate, in contact with the tool shoulder, superior weld properties are achieved independently of the tool geometry. It is also concluded that the use of unthreaded conical pin tools, with a low shoulder/pin diameter relation, is the most suitable solution for the production of welds with similar strengths for advancing and retreating sides.     

Effect of thermal treatments on the fracture behaviour of notched polyethylene terephthalate ribbons

The application of different thermal treatment procedures to thin polyethylene terephthalate (PET) sheets yields to microstructures of different molecular weight/ degree of crystallinity combination. As a consequence, variations in the mechanical properties, especially the fracture properties and the particular fracture mechanisms occur. This is demonstrated in this paper by measurements of elastic modulus, maximum stress, failure initiation energy, and total work to fracture of notched PET-ribbons. Failure mechanisms analysed by the use of optical and scanning electron microscopy vary between highly ductile via semi-brittle after crazing, to absolute brittle at very low stresses. The results are summarized in terms of a three-dimensional failure energy map divided into regions of particular failure behaviour for particular molecular weight/ degree of crystallinity combination. In addition, the typical values of material strength, defined as the product of resistance to damage initiation (maximum stress) and crack propagation (total work to failure) are given for each region. The optimum fracture resistance was achieved for PET material with moderately low molecular weight and moderately high degree of crystallinity.     

Crack path evaluation on HC and BCC microstructures under multiaxial cyclic loading

In this paper the multiaxial loading path effect on the fatigue crack initiation, fatigue life and fracture surface topology are evaluated for two different crystallographic microstructures (bcc and hc): high strength low-alloy 42CrMo4 steel and the extruded Mg alloy AZ31B-F, respectively.A series of multiaxial loading paths were carried out in load control, smooth specimens were used. Experimental fatigue life and fractographic results were analyzed to depict the mechanical behavior regarding the different microstructures.A theoretical analysis was performed with various critical plane models such as the Fatemi–Socie, SWT and Liu in order to correlate the theoretical estimations with the experimental data. A new approach based on maximum stress concentration factors is proposed to estimate the crack initiation plane, estimations from this new approach were compared with the measured ones with acceptable results. To implement this new approach a virtual micro-notch was considered using FEM. Moreover, the multiaxial loading path effect on stress concentration factors is also studied. The obtained results clearly show the effect of the applied load conditions on local microstructures response.     

Einfluß der Korngröße auf Reibung und Gleitverschleiß von TZP?ZrO2-Keramik bei Festkörper- und Mischreibung

Effect of grain size on friction and wear of TZP? ZrO₂ ceramic in unlubricated and boundary lubricated sliding contact . Microstructures with systematically between 0.30 and 1.62 μm varying average grain sizes were produced by cold isostatically pressing and sintering of commercial 3 mol.% Y₂O₃ stabilized ZrO₂ powder. Hardness and fracture toughness of the different structures as well as the amount of tetragonal, cubic and monoclinic phases were measured. Wear tests were carried out on the different self-mated microstructures in unlubricated and oil or water lubricated unidirectional and unlubricated reciprocating sliding contact, respectively. Worn surfaces were systematically analysed using scanning electron microscopy. The experimental results showed a significant influence of the average grain size as well on mechanical properties such as hardness and fracture toughness as on tribological properties. Friction and wear are discussed in relation to grain size and phase stability of the microstructures and also as a function of the different tribosystems and lubricants used.     

Mechanical performance of AA6181 refill friction spot welds under Lap shear tensile loading

This study investigates the fracture behaviour of refill friction spot welding welds under shear tensile loading. Overlap joints of 6181‐T4 aluminium were produced in 1.7‐mm sheets by varying the rotational speed from 1900 to 2900 rpm and the welding time from 2 to 3.4 s while keeping the plunge depth constant at 1.75 mm. After shear tensile tests, the samples were analysed using optical microscopy and scanning electron microscopy. The strength of the weld and its ductile/brittle behaviour are associated with the nucleation, growth and propagation of two types of cracks: circumferential cracks and annular cracks. Welds produced with longer welding times (≥3 s) and slower rotational speeds (1900 rpm) had higher strengths, low scattering and high energy absorption prior to failure, while welds produced with short welding times (2 to 2.4 s) resulted in poor joints, especially when they also used high rotational speeds.     

Anrißerzeugung in WC-Co-Hartmetall-Proben für die Messung des kritischen Spannungsintensitätsfaktors KIC

Precracking of WC-Co-Hardmetal-Specimens for Fracture-Toughness Testing The determination of a valid critical stress intensity factor K_IC requires an extremely sharp, well defined initial crack. Methods producing such a crack are well known for metallic materials, but they often can not be used with brittle materials, like cemented carbides or ceramics. Their low fracture toughness makes a controlled crack growth under pure tensile stress nearly impossible. More useful are precracking methods, utilizing a stress gradient to stop the crack at defined depth. A very simple methods uses the indentation of a hardness tester to produce a semi-elliptical surface crack, interfered with residual stresses. For different areas of application and specimen geometries, bridge indentation, wedge indentation and composite bending method produce cracks with a straight front. Also under cyclic loading, under tensile as well as under compressive stress, the creation of a sharp precrack, applicable in K_IC measurement, is possible.     

Generalized approach to estimation of strains and stresses at blunt V‐notches under non‐localized creep

Geometrical discontinuities such as notches need to be carefully analysed by engineers because of the stress concentration generated by them. Notches become even more important when the component is subjected, in service, to very severe conditions, such as high‐temperature fatigue and imposed viscoplastic behaviour such as creep. The knowledge of strains and stresses in such stress concentration zones is essential for an efficient and safe design process. The aim of the paper is to present an improvement and extension of the existing notch‐tip creep stress–strain analysis method developed by Nuñez and Glinka, validated for U‐notches only, to a wide variety of blunt V‐notches. The key in obtaining the extension to blunt V‐notches is the substitution of the Creager–Paris equations with the more generalized Lazzarin–Tovo solution, allowing a unified approach to the evaluation of linear elastic stress fields in the neighbourhood of both cracks and notches. Numerous examples have been analysed to date, and the stress fields obtained according to the proposed method were compared with appropriate finite element data, resulting in a very good agreement. In view of the promising results discussed in the paper, authors are considering possible further extension to sharp V‐notches and cracks introducing the concept of the strain energy density.     

Microstructure and properties of epitaxial layers by laser glazing

Various processes of laser surface melting are employed to modify the features of epitaxially grown microstructures in the melt. Three kinds of microstructures are demonstrated in this investigation. The one with complex microstructures possesses the best resistivity against hot stress corrosion.