Stress-relieved Conditions Research Articles

The effects of stress relieving heat treatment on the microstructure and residual stress of Inconel 718 fabricated by laser metal powder bed fusion additive manufacturing process

In this study, the microstructure and residual stress of the Inconel 718 parts, which were fabricated by laser metal powder bed fusion additive manufacturing process, in as-fabricated and stress-relieved conditions were investigated by metallographic analysis and microhardness test. Fine equiaxed and columnar cellular dendrites were revealed in the Transverse plane, Frontal plane, and Sagittal plane, respectively. Laves phases were found in the interdendritic regions and interlayers. After the stress relief heat treatment, the Laves phases has partly solved into the γ matrix and the microstructure became relatively homogeneous, which leads to the significant increase (∼19 %) in the microhardness. Residual stress was unevenly distributed in the parts, and no notable difference was found between the Frontal plane and Sagittal plane. The maximum absolute compressive residual stress dropped from 378.4 MPa to 321 MPa after stress relief due to the homogenization of the microstructure.

An experimental study of residual stress and direction-dependence of fatigue crack growth behaviour in as-built and stress-relieved selective-laser-melted Ti6Al4V

Selective-laser-melting (SLM) is a powder-bed fusion additive-manufacturing process that has the potential to deliver three-dimensional complex parts with mechanical properties comparable or superior to parts produced via traditional manufacturing using cast and wrought alloys. Concerns for metallic parts built via SLM are the process-induced residual stresses, and anisotropic mechanical properties. This paper investigates the effect of residual stresses on the fatigue crack growth rate of SLM Ti6Al4V in as-built and stress-relieved conditions. Neutron diffraction and the contour method are employed to measure residual stresses in compact-tension samples. Neutron diffraction results are in good agreement with the contour method. It was found that tensile stresses are present at the notch root and the free edge areas, and compressive stress is seen in the middle of the sample. The tensile stresses in the as-built condition resulted in a higher fatigue crack growth rate. After stress relieving by heat treatment, the tensile residual stress diminished by around 90%, resulting in decreased crack growth rate. The build direction was seen to affect the crack growth rate, although the trend was different between the as-built and stress-relieved conditions.

Experimental tests and fatigue strength assessment of a scotch yoke valve actuator

Aim of this work is the fatigue assessment of a main component, termed scotch yoke, of a valve actuator used for oil & gas, power and chemical industries, in order to comply with its heavy-duty applications. To do this, full-scale specimens of the scotch yoke made of structural steel have been fatigue tested under nominal axial loading. All specimens have been tested under stress-relieved conditions by adopting a nominal load ratio R=-1. After experimental tests, the fatigue crack paths have been analysed by means of liquid penetrant inspections. The fatigue strength class of the considered scotch yoke has been determined by statistically re-analysing the experimental results, expressed in terms of range of the nominal applied load, and it has been compared with the design condition required by the relevant European Standard, EN 15714-3/4. Finally, two methodologies for fatigue strength assessment of the considered scotch yokes have been proposed, which are based on experimental fatigue data derived from smooth or sharp V-notched specimens, respectively, made of the same yoke material. The assessment capability of the proposed methodologies has been evaluated and discussed by comparing theoretical estimations with the experimental fatigue results of the scotch yokes.

Fatigue properties of a titanium alloy (Ti–6Al–4V) fabricated via electron beam melting (EBM): Effects of internal defects and residual stress

A clear understanding of the fatigue properties of Ti–6Al–4V manufactured with electron beam melting (EBM) is needed to ensure performance in critical applications in the medical device and aerospace industries. In this work, the effects of residual stress and internal defects (pores and voids) on fatigue properties of EBM Ti–6Al–4V material in as-built, stress-relieved, and hot isostatic pressed (HIPed) conditions were evaluated. Conventional techniques were used to measure the chemical composition and quantify microstructures, and neutron scattering was utilized to measure residual stresses. Post-processing did not alter chemical composition. Compared to the as-built condition, microstructure was unchanged for stress-relieved material and coarser for HIPed material. No significant residual stresses were measured for any of the three conditions. This indicates build platform and layer preheating lead to sufficient process temperatures to achieve full stress relief in-situ. The fatigue strengths at 107cycles measured for the as-built and stress-relieved conditions were statistically similar and were measured to be 200–250MPa. A significantly higher fatigue strength at 107cycles of 550–600MPa was measured for the HIPed condition. The increase in fatigue endurance limit was attributed to a reduction in internal porosity and void content.

Experimental and simulation analysis of texture formation and deformation mechanism of rolled AZ31B magnesium alloy under dynamic loading

Magnesium alloys components are potentially used under shock loading conditions. Considering the fact that deformation behavior is completely different under high strain rate conditions compared to quasi-static conditions, it is very important to evaluate the dynamic mechanical response and deformation mechanisms of magnesium alloys under impact loading. In this research, texture formation, microstructural changes and dynamic deformation behavior of rolled AZ31B in the tempered H24 stress-relieved conditions, shock-loaded under compressive dynamic loading, were investigated. Texture measurements showed that the as-received AZ31B alloy had a strong (00.2) basal texture. Four groups of cylindrical samples were cut in the rolling direction (RD), in 45 degrees to the RD, in the transverse direction (TD) and in the direction perpendicular to the RD-TD plane. Dynamic shock loading of the test samples were conducted using Split Hopkinson Pressure Bar at room temperature at strain rates ranging from 600 to 1100 s(-1), while loading direction was parallel to the longitudinal axis of the cylindrical samples. After high strain rate deformation, although the loading direction was different, a strong (00.2) basal texture was observed in all samples. It was inferred that increasing the strain rate led to an increase in strength and ductility, but to a decrease in twinning fraction, indicating the possible activation of non-basal slip systems. Besides, a high degree of mechanical anisotropy was found for all strain rates used such that the lowest strength was registered for the samples cut along the direction parallel to the rolling direction. A viscoplastic self-consistent model with a tangent approach was used to corroborate the experimental textures and possible deformation mechanisms by simulation.

Microstructural evolution in and flow properties of Zr–2.5Nb pressure tube material at elevated temperature

Cold worked and stress relieved Zr–2.5wt%Nb alloy was annealed at 900°C for 5–90min and quenched in water, which resulted in formation of α′ martensite and primary α phase. Reheating of the same samples to 700°C leads to α′→αZr+βZr transformation following JMAK kinetics (n=0.55 and k=0.34). Tensile samples in annealed condition were deformed at 625–700°C over strain rates of 2×10−5–1×10−2s−1. With the increase in annealing time, strain rate sensitivity and activation energy for deformation were found to change from 0.42 to 0.20 and 181.4–685kJ/mol respectively. The maximum peak stress (136.5MPa) obtained at 700°C from present results suggests higher load bearing ability of the Zr–2.5Nb alloy in annealed and quenched condition compared to that of cold-rolled and stress-relieved condition.

Development and Characterization of Microstructure and Mechanical Properties of Heat-Treated Zr–2.5Nb Alloy for AHWR Pressure Tubes

Abstract Zr–2.5Nb Alloy is used in a cold-worked and stress-relieved (CWSR) condition as pressure tubes in Indian pressurized heavy water reactors (IPHWR). The life-limiting factor for pressure tubes is in-reactor dimensional changes caused by irradiation growth and creep resulting in diametral expansion and axial elongation. Recent work has indicated that heat-treated Zr–2.5Nb alloy may exhibit lower in-reactor dimensional changes vis-à-vis CWSR pressure tubes. The advanced heavy water reactor (AHWR) specification demands more stringent operating condition for the pressure tubes and it was decided to develop a new fabrication route for heat-treated Zr–2.5Nb alloy for possible application as pressure tubes in AHWR. In this work, the microstructural characterization and tensile properties evaluation was carried out using samples obtained from various stage of fabrication of water-quenched and aged Zr–2.5Nb alloy following a route similar to Fugen pressure tubes is discussed. The microstructure was characterized using optical and transmission electron microscopy. The chemical composition, morphology, and location of the precipitates formed during aging were studied using thin-foil electron microscopy and carbon extraction replica. Samples with its axes parallel to longitudinal (L) or transverse (T) direction of rolled plate were machined from quenched, cold-rolled, and aged material and were tested in tension at 25°C and 300°C. The microstructure and the tensile strength of the alloy processed in the present investigation were comparable to that of heat-treated pressure tubes used in Reaktor Bolshoy Moshchnosti Kanalniy (RBMK) and Fugen reactors.

Microstructural evolution and interfacial crystallography in Cr–Cr2Ta

The microstructural features of a Cr–Ta alloy containing 10at% Ta have been studied in the as-cast and stress-relieved condition, and after long-term annealing at 1300 °C and 1500 °C. In the as-cast and stress-relieved state, the microstructure consisted of dendrites of the Cr2Ta Laves phase and a lamellar eutectic mixture of Cr(Ta) solid solution and Cr2Ta. After protracted annealing at 1300 °C, the lamellar eutectic was observed to spherodise, producing a microstructure consisting of a Cr(Ta) matrix, within which fine particles of Cr2Ta were embedded. This was accompanied by solid state precipitation of fine Cr2Ta particles within the Cr(Ta) matrix. In addition, the Cr2Ta phase transformed progressively from the hexagonal C14 polytype, found in the as-cast and stress-relieved condition, to either the cubic C15 polytype or intermediate, faulted, hexagonal-based structures. The cubic Laves phase was seen to contain either coarse or fine twin bands. An orientation relationship between the cubic Laves phase and the Cr(Ta) matrix, clearly different from ones previously reported elsewhere was observed. A significant density of dislocations was also observed in the matrix, which persisted even after annealing.

The Effect of Microstructure on Delayed Hydride Cracking Behavior of Zircaloy-4 Fuel Cladding—An International Atomic Energy Agency Coordinated Research Program

Abstract The rate of delayed hydride cracking (DHC) has been measured in Zircaloy-4 fuel cladding in several metallurgical conditions using the pin-loading tension technique. In light water reactor (LWR) cladding in the cold-worked and cold-worked and stress-relieved conditions, the cracking rate followed Arrhenius behavior up to about 280 °C, but at higher temperatures the rate declined with no cracking above 300°C. Non-LWR cladding appeared to behave in the same manner. In LWR cladding in the recrystallized condition, the cracking rate was highly variable because it depended on KI within the test range up to 25 MPa√m, whereas in the other LWR claddings, cracking rate was independent of KI, indicating that KIH was below 11 MPa√m. The main role of microstructure was to control the material strength; the cracking rate increased as the strength increased. Although all the claddings had a radial texture, it did not protect the cladding from DHC. The DHC fracture surface consisted of flat broken hydrides, often in arcs, but no striations were observed, except in one specimen subjected to thermal cycles.

Modified Wöhler curve method, theory of critical distances and Eurocode 3: A novel engineering procedure to predict the lifetime of steel welded joints subjected to both uniaxial and multiaxial fatigue loading

In order to propose a procedure suitable for assessing steel welded joints by post-processing simple linear-elastic finite element (FE) models, this paper summarises an attempt to formalise a novel approach based on the use of the modified Wöhler curve method (MWCM) applied along with the theory of critical distances (TCD). This engineering procedure was initially calibrated by using the standard curves supplied by Eurocode 3. Subsequently, an unifying value for the multiaxial critical distance was calculated by taking full advantage from the notch-stress intensity factor (N-SIF) approach. The accuracy and reliability of the devised method was systematically checked by using a large amount of data taken from the literature and generated by testing steel welded details both under uniaxial and multiaxial nominal loading. In particular, both standard cruciform joints and structural details having complex geometries were considered and the devised procedure was applied to specimens tested both in as-welded and in stress-relieved condition. This approach was seen to be highly accurate, giving estimates falling within the widest standard scatter band between the two used to calibrate the approach itself. In particular, our method proved to be capable of correctly taking into account the scale effect as well as the degree of non-proportionality of the stress field damaging the fatigue process zone. Such results are very interesting and promising, especially in light of the fact that the numerical effort which has to be made to apply the proposed approach is the same as the one needed to apply other existing linear-elastic approaches.

CARLSON ALLOY 904L

Abstract Carlson Alloy 904L is a 21Cr-24Ni-4.5Mo-1.5Cu low-carbon austenitic alloy that resists corrosion in a wide range of both oxidizing and reducing environments. It has good pitting and crevice-corrosion resistance and eliminates stress-corrosion cracking in most common chloride environments. The alloy resists intergranular corrosion in postwelded or stress-relieved conditions because of its very low carbon content. This datasheet provides information on composition, physical properties, hardness, and tensile properties. It also includes information on corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: SS-844. Producer or source: G.O. Carlson Inc.

Relationships between thermoelectric power (TEP) values and crystallographic texture of zircaloy-4 plates

In zirconium base alloys, Thermoelectric Power (TEP) measurements are known to be sensitive to the concentration of alloying elements, such as iron and chromium. in solid solution. In this paper, the relationship between the TEP data and the oxygen content of Zy-4, for concentrations ranging from 100 to 4000 ppm, has been established, not only to link the oxygen content to the TEP but also to separate this effect from the relationship established between the texture of polycrystalline Zircaloy-4 samples (with an α hexagonal crystallographic structure) and the TEP data, which is the main objective of the present paper. To study the relationship between texture and TEP data, a Zy-4 plate heat treated at three different temperatures and showing three particular crystallographic textures (Stress-Relieved, Fully Annealed and Beta-Treated metallurgical conditions) has been used. The texture of the samples has been characterised by x-ray diffraction techniques using pole figures measurements and the calculation of the Orientation Distribution Function (ODF). The relationship between TEP data and texture has been performed by using the Kearns factor, f d , which is an anisotropic coefficient taking into account the volume fraction of axes with respect to the macroscopic reference system (Rolling Direction RD, Transverse Direction TD and Normal Direction ND). On one hand, the TEP measurements are quasi linearly correlated to the Kearns anisotropic coefficients f d , on the other hand, the study of the influence of oxygen content on TEP has shown that the texture effect seems to be reduced when the oxygen content is low. One can make the assumption that crystallographic orientation effects on TEP could be partly explained by the interactions of electrons with oxygen atoms located in preferential planes (octaedric sites). This study shows that, when the texture is the only varying parameter of the microstructure, TEP measurement is an easy and fast experimental device which can provide a good estimate of the anisotropy of the material. TEP measurements can also give a fairly local information of the texture. Finally, it is shown that this technique can be applied to estimate the texture of Zircaloy-4 tubes.

Evaluation of oxide dispersion strengthened (ODS) molybdenum and molybdenum–rhenium alloys

Oxide dispersion strengthened (ODS) molybdenum alloys being developed for high temperature applications possess excellent high temperature strength and creep resistance. In addition they exhibit a ductile-to-brittle transition temperature (DBTT) in the worked and stress-relieved condition under longitudinal tensile load well below room temperature. However, in the recrystallized condition, the DBTT may be near or above room temperature, depending on the volume fraction of oxide dispersion and the amount of prior work. Dilute rhenium additions (7 and 14 wt%) to ODS molybdenum were evaluated to determine their effect on low temperature ductility. The addition of 7 wt% rhenium to the ODS molybdenum did not significantly enhance the mechanical properties. However, the addition of 14 wt% rhenium to the ODS molybdenum resulted in a DBTT well below room temperature in both the stress-relieved and recrystallized condition. Additionally, the tensile strength of ODS Mo-14 Re is greater than the base ODS molybdenum at 1000–1250°C.

Low cycle fatigue properties and microscopic deformation structure of Zircaloy-4 in recrystallized and stress-relieved conditions

Low cycle fatigue (LCF) tests have been performed on Zr-4 in recrystallized (RZ) and stress-relieved (SR) conditions at RT and 400°C. Results show that Zr-4/RZ displays cyclic hardening at RT and 400°C, whereas Zr-4/SR exhibits cyclic hardening at RT, but cyclic softening at 400°C. Fatigue lifetime curves as a function of plastic strain range imply that Zr-4/RZ has higher lifetime than that of Zr-4/SR at RT and in the low-cycle region at 400°C, and the LCF properties at 400°C are superior compared to those at RT for Zr-4 in both RZ and SR conditions. TEM examination shows that typical dislocation configuration contains some parallel dislocation lines in Zr-4/RZ, and some dislocation lines are visible between the channels of elongated original grain boundaries for Zr-4/SR at RT. Elongated dislocation cells plus dislocation dipole perpendicular to the cell boundary are observed in Zr-4/RZ, whereas rectangle cells in Zr-4/SR at 400°C. Trace analysis indicates that prismatic and pyramidal slips are possible cyclic deformation modes for Zr-4 in different heat treatment conditions and at different temperatures. Finally, the relationships between LCF properties and microscopic deformation mechanisms are discussed.

The effect of undercut and residual stresses on fatigue behaviour of misaligned butt joints

A numerical model is developed to predict the combined effect of a weld toe undercut, residual stresses and misalignment on the fatigue strength and fatigue life of butt welded joints subjected to pulsating tensile loading. Linear Elastic Fracture Mechanics (LEFM), Finite Element Analysis (FEA) and superposition approaches have been used for the modelling. It has been found that the effect of undercut is the most significant geometrical effect affecting the fatigue behaviour of butt welded joints and is governed by the effect of other weld geometry parameters. The reduction of fatigue life and fatigue strength, in comparison to flush-ground welded plate, caused by an introduction of weld toe undercut is twice that due to welded joint without undercut. It means that by elimination of undercut, fatigue strength can be improved up to 50% of the level that can be obtained by flush-ground welded plate. It is also found that a presence of misalignment of 5% of plate thickness and undercut of 2% of plate thickness are fairly representative for the lower boundary of S- N curves of butt joints in a real situation. Fatigue life can be decreased by up to 10 and 100 times compared to perfect stress-relieved conditions of aligned and misaligned joints, respectively, once high tensile residual stresses of yield stress magnitude are present. As a result, the slope of S- N is no longer constant and the effect of misalignment is more significant than that of undercut. However, when the magnitude of tensile residual stress becomes smaller or it changes into compressive stresses the effect of undercut will play a dominant role in comparison with that of misalignment. The effect of misalignment may be beneficial for high cycle fatigue life of the order of greater than 2 × 10 5 cycles, especially when compressive residual stresses are introduced into the weld toe surface by using surface treatment (i.e. shot peening, hammering). In this case, the improvement of fatigue strength of misaligned joints and undercut-free joints can be 50% more effective than that of aligned undercut joints.

Effect of refractory alloying additions on mechanical properties of near-stoichiometric NiAl

Alloying effects have been investigated in near-stoichiometric NiAl for the purpose of improving it mechanical properties. NiAl alloys containing up to 3% of Mo, W, Nb, Ta, Zr and Hf were prepared by are melting and drop casting, followed by hot extrusion at temperatures to 1000 °C. The mechanical properties of the NiAl alloys were characterized by tensile and creep testing at temperatures to 1000 °C. Among the alloying elements, Mo is found to be most effective in improving the room temperature ductility of NiAl alloys in stress-relieved conditions. Alloying with 1.0 ± 0.6% Mo almost doubles the room temperature tensile ductility of NiAl and triples its yield strength at 1000 °C. The creep properties of Mo-modified NiAl alloys can be further improved by alloying with up to 1% Nb, Ta and Zr. Alloying with 1% MoZr reduced the creep rate and extended the rupture life of NiAl by more than 5 orders of magnitude. Transmission electron microscope and electron microprobe studies show that the solubility of Mo, Nb and Zr is low, less than 0.1 at.%. Both coarse and fine precipitates enriched with refractory elements are observed, with fine particles about (0.1 μm) distributed within grains. The mechanical properties of these NiAl alloys are discussed in terms of both particle strengthening and solid solution hardening.

Ti-3Al-2.5V

Abstract Ti-3A1-2.5V is a near-alpha titanium alloy offering 20-50% higher tensile properties than the strongest commercially pure grade of titanium at both room and elevated temperatures. Normally furnished in the annealed, or in the cold-worked stress-relieved condition, Ti-3A1-2.5V titanium alloy features excellent cold formability and good notch tensile properties, as well as corrosion resistance in many environments. This datasheet provides information on composition, physical properties, elasticity, and tensile properties. It also includes information on corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: Ti-95. Producer or source: Titanium alloy mills.

Effect of Heat Input and Microstructure on Pitting Corrosion in AISI 316L Submerged Arc Welds

Abstract The pitting resistance of AISI 316L stainless steel (SS) weld metal in the as-welded and stress-relieved conditions was evaluated as a function of heat input. The results obtained indicated a decrease in the value of critical pitting potential in acidic chloride medium as the heat input was increased. However, on stress-relief heat treatment, the critical pitting potential remained unchanged with varying heat input, though its magnitude was much lower than that of the as-welded alloy. The deterioration in the pitting resistance on increasing heat input is discussed on the basis of enhanced segregation of solute elements like chromium and molybdenum to delta-ferrite and the presence of M23C6 carbides at the austenite/ferrite interface.

The Fatigue Behavior of Large-Scale As-Welded and Stress-Relieved Tubular T Joints

The changing strain distribution within large-scale tubular T joints, subjected to fully reversed axial brace fatigue loading, were measured in as-welded and stress-relieved conditions. Initial static loading enabled estimation of stress concentration factors and comparisons with the predictions of available published formulas. Dynamic load amplitudes were selected to give approximately equal hot-spot strains in all joints, thus facilitating direct comparison between specimens with different dimensions. Post-weld heat treatment increased the cycles needed for crack initiation and also reduced the crack propagation rates.

Modification of the structure of cold-worked Zr-2.5 wt% Nb nuclear reactor pressure tube material

The metallography and mechanical properties of cold-worked Zr-2.5 wt% Nb alloy nuclear reactor pressure tubing are described in relation to the phases present after various heat treatments in the ( α + β Zr) and ( α + β Nb) phase regions. A heat treatment is recommended that produces room-temperature mechanical properties that are almost as good as those of current pressure tubing, which is in the cold-worked plus stress-relieved condition, but that will have enhanced microstructural stability at higher operating temperatures.