Stress Relief Heat Treatment Research Articles

Effect of Deformation Structure of AISI 316L in Low-Temperature Vacuum Carburizing

The effect of plastic deformation applied to AISI 316L in low-temperature vacuum carburizing without surface activation was investigated. To create a difference in the deformation states of each specimen, solution and stress-relieving heat treatment were performed using plastically deformed AISI 316L, and the deformation structure and the carburized layer were observed with EBSD and OM. The change in lattice parameter was confirmed with XRD, and the natural oxide layers were analyzed through TEM and XPS. In this study, the carburized layer on the deformed AISI 316L was the thinnest and the dissolved carbon content of the layer was the lowest. The thickness and composition of the natural oxide layer on the surface were changed due to the deformed structure. The natural oxide layer on the deformed AISI 316L was the thickest, and the layer was formed with a bi-layer structure consisting of an upper Cr-rich layer and a lower Fe-rich layer. The thick and Cr-rich oxide layer was difficult to decompose due to the requirement for lower oxygen partial pressure. In conclusion, the oxide layer is the most influential factor, and its thickness and composition may determine carburizing efficiency in low-temperature vacuum carburizing without surface activation.

Pure molybdenum manufactured by Laser Powder Bed Fusion: Thermal and mechanical characterization at room and high temperature

As a refractory material, molybdenum is regarded with high interest for high-temperature applications. The concurrent use of powder bed Additive Manufacturing (AM) technology can provide significant design and production advantages. In this work, the Laser Powder Bed Fusion (LPBF) process parameters tuning lead to produce almost fully-dense AM Mo blocks (density of 99.5 ± 0.5%). Fine-tuning of the parameters, also involved the Single Scan Tracks analysis, are aiming at the continuous and homogeneous melt-pools production. High-density Mo specimens have been characterized at room- and high-temperature in terms of thermal and mechanical properties, then compared with conventionally manufactured Mo samples. The thermal diffusivity measurement at room temperature allowed to verify that the thermal conductivity value of AM Mo is approximately half of standard Mo. Stress relieving heat treatment improves the thermal conductivity approximately by 13%. The estimation of emissivity and thermal conductivity carried out in the 600÷1600 °C temperature range led to a similar result. The Vickers microhardness measured on fully dense specimens (212 ± 18 HV 0.15 ) is similar to commercially available Mo. Tensile tests have been performed at both room temperature and 600 °C. The effect of building direction and post-processing machining of AM specimens have been also investigated for tests at room temperature. Although the AM samples exhibited a very similar density to standard Mo, the AM Mo mechanical properties resulted generally lower.

Investigation of strengthening mechanisms in an additively manufactured Haynes 230 alloy

There are abundant studies on additive manufacturing of Ni alloys, such as Inconel 718. However, the studies on the microstructure and the deformation behavior of solid solution strengthened Ni alloys, such as Haynes 230, are limited. Here we compare the microstructure and the mechanical behavior of as-printed and stress-relieved Haynes 230 Ni alloys prepared by laser powder bed fusion. Transmission electron microscopy analysis revealed M23C6 carbides decorated at the dislocation cell walls in the as-printed material transformed to well-aligned arrays of M6C nanoprecipitates separated by an array spacing of 500 nm after stress-relief (heat treatment). The nanoprecipitates in stress-relieved alloy have formed numerous aligned orientation relationships with the matrix despite their large lattice mismatch. Post-tension microscopy analyses demonstrated high-density dislocation networks in the as-printed samples, whereas high-density stacking faults and nanoscale deformation twins were observed in the stress-relieved counterparts. This study provides insights for understanding the influence of nanoprecipitates on the deformation mechanisms of additively manufactured Ni alloys.

Monitoring residual strain relaxation and preferred grain orientation of additively manufactured Inconel 625 by in-situ neutron imaging

Microstructures produced by Additive Manufacturing (AM) techniques determine many characteristics of components produced by this particular manufacturing technique. Residual stress and texture are among those characteristics, which need to be optimized to meet dimensional and strength requirements. Post-build heat treatments are necessary to relief residual stresses, homogenize the microstructure and to develop the necessary microstructures (precipitation or solution hardening) for strength. Our focus in this study is the measurement of residual stresses during a vacuum stress relief (VSR) heat treatment. We employ the energy-resolved neutron imaging to investigate, in-situ, the uniformity of the as-built texture and to map the residual strain distributions within samples of Inconel 625 (IN625). The samples used in this study were printed using the powder-bed metal laser melting additive manufacturing technique. Strain and texture variations were measured at room temperature for the as-built samples as well as their changes during stress relief annealing at 700 °C and 875 °C in a vacuum furnace. The uniformity of crystalline plane distribution, from which texture can be inferred, was imaged with sub-mm spatial resolution for the entire sample area exceeding several square centimeters. Despite the limited accuracy of in-situ lattice strain reconstruction during sample annealing, the results indicate that most of the strain relaxation occurs at 700 °C within the first 2–3 h. Relaxation of the strain at 875 °C happened at a much faster rate. In addition, energy-resolved neutron imaging demonstrates the possibility to correlate the uniformity of grain orientation and the degree of texture variations with printing parameters, in particular the laser translation speed. Simulations of the AM build process were carried out to predict the residual stress distributions within the samples as a function of the build process parameters. Comparisons of the simulation results to the room temperature experimental measurements show good agreements when the simulation data are averaged over the volume of the part confirming the usefulness of the experiments for validating simulation results. The neutron imaging technique described in this study can be helpful for the evaluation of bulk crystallographic characteristics in AM printed materials and can be used as a guide for developing the heat treatment cycle and for validations of VSR simulation results.

Research regarding strip cladding of heat-resistant SA 387 Gr.11Cl.2 steel type

The continuous development of industrial processes in the petrochemical field requires the use of materials with anticorrosive properties, able to withstand working conditions at high temperatures but also in highly corrosive environments. The basic materials with anticorrosive properties have a high cost price which leads to the need to find alternative manufacturing solutions. In such situations, the use of bimetal plates or the option of welding cladding with an anticorrosive layer of the base material is used. The paper analyses the possibility of strip cladding using the submerged arc welding process of a reactor made of SA 387Gr11Cl.2 basic material, a heat-resistant alloy steel. To achieve the deposition of the appropriate anticorrosive layer (stainless steel type AISI 347) it was necessary to use a buffer layer made of austenitic stainless-steel type 309L. In order to eliminate the internal tensions appeared after the cladding process, the sample was subjected to a stress relieving heat treatment. After the heat treatment, the sample was subjected to non-destructive examinations (visual and penetrant testing) but also to destructive examinations specific to the approval tests of cladding technology (Metallographic tests, Vickers hardness test, bend test, ferrite number, chemical composition). Metallographic tests of cross-sectional phase structures were carried out in macroscopic and microscopic terms. In the area of the deposited material can be observed a typical austenitic casting structure resulting from the use of a high linear energy, specific to the automatic submerged arc welding. Following the performance of hardness measurements made in the characteristic areas of the deposition, it can be seen that there was a slight increase in the values obtained in the heat affected zone, without exceeding the prescribed values for the materials used. Following the analysis of the results obtained from the specific examinations and tests to the plated samples, it can be seen that they are within the limits prescribed by the standards in force regarding the approval of the cladding technology used.

Solid-State Transformation of an Additive Manufactured Inconel 625 Alloy at 700 °C.

Inconel 625, a nickel-based superalloy, has drawn much attention in the emerging field of additive manufacturing (AM) because of its excellent weldability and resistance to hot cracking. The extreme processing condition of AM often introduces enormous residual stress (hundreds of MPa to GPa) in the as-fabricated parts, which requires stress-relief heat treatment to remove or reduce the internal stresses. Typical residual stress heat treatment for AM Inconel 625, conducted at 800 °C or 870 °C, introduces a substantial precipitation of the δ phase, a deleterious intermetallic phase. In this work, we used synchrotron-based in situ scattering and diffraction methods and ex situ electron microscopy to investigate the solid-state transformation of an AM Inconel 625 at 700 °C. Our results show that while the δ phase still precipitates from the matrix at this temperature, its precipitation rate and size at a given time are both smaller when compared with their counterparts during typical heat treatment temperatures of 800 °C and 870 °C. A comparison with thermodynamic modeling predictions elucidates these experimental findings. Our work provides the rigorous microstructural kinetics data required to explore the feasibility of a promising lower-temperature stress-relief heat treatment for AM Inconel 625. The combined methodology is readily extendable to investigate the solid-state transformation of other AM alloys.

Bayesian inference of elastic constants and texture coefficients in additively manufactured cobalt-nickel superalloys using resonant ultrasound spectroscopy

Bayesian inference with sequential Monte Carlo is used to quantify the orientation distribution function coefficients and to calculate the fully anisotropic elastic constants of additively manufactured specimens from only the experimentally-measured resonant frequencies. The parallelizable and open-source SMCPy Python package enabled Bayesian inference within this new modeling framework, resulting in an order of magnitude reduction of the computation time for an 8-core machine. Residual stress-induced shifts on the resonant frequencies were explicitly accounted for during the Bayesian inference, enabling the estimation of their effect on the resonant frequencies without a stress-relief heat treatment. Additively manufactured cobalt-nickel-base superalloy (SB-CoNi-10C) specimens were sectioned at multiple inclinations relative to the build direction and scanned with resonant ultrasound spectroscopy to demonstrate characterization of any arbitrarily textured cubic microstructure through the resonant frequencies. The orientation distribution function coefficients of the textured polycrystalline microstructure were estimated in tensorial form to calculate both the 2nd order Hashin-Shtrikman bounds and the self-consistent estimate of the elastic constants, enabling accurate determination of all 21 possible independent elastic constants through the convergence constraints of the texture. Pole figures generated directly from the calculated texture coefficients showed good agreement with experimentally measured textures.

Effect of Stress Relieving Heat Treatment on Tensile and Impact Toughness Properties of AISI 316L Alloy Manufactured by Selective Laser Melting Process

Effect of Stress Relieving Heat Treatment on Tensile and Impact Toughness Properties of AISI 316L Alloy Manufactured by Selective Laser Melting Process

Experimental Characterisation and Numerical Modelling of Residual Stresses in a Nuclear Safe-End Dissimilar Metal Weld Joint

In this study, a mock-up of a nuclear safe-end dissimilar metal weld (DMW) joint (SA508-3/316L) was manufactured. The manufacturing process involved cladding and buttering of the ferritic steel tube (SA508-3). It was then subjected to a stress relief heat treatment before being girth welded together with the stainless steel tube (316L). The finished mock-up was subsequently machined to its final dimension. The weld residual stresses were thoroughly characterised using neutron diffraction and the contour method. A detailed finite element (FE) modelling exercise was also carried out for the prediction of the weld residual stresses resulting from the manufacturing processes of the DMW joint. Both the experimental and numerical results showed high levels of tensile residual stresses predominantly in the hoop direction of the weld joint in its final machined condition, tending towards the OD surface. The maximum hoop residual stress determined by the contour method was 500 MPa, which compared very well with the FE prediction of 467.7 Mpa. Along the neutron scan line at the OD subsurface across the weld joint, both the contour method and the FE modelling gave maximum hoop residual stress near the weld fusion line on the 316L side at 388.2 and 453.2 Mpa respectively, whereas the neutron diffraction measured a similar value of 480.6 Mpa in the buttering zone near the SA508-3 side. The results of this research thus demonstrated the reasonable consistency of the three techniques employed in revealing the level and distribution of the residual stresses in the DMW joint for nuclear applications.

Heat Treatments for Stress Relieving AlSi9Cu3 Alloy Produced by Laser Powder Bed Fusion.

The present work explores the effect of a stress relieving heat treatment on the microstructure, tensile properties and residual stresses of the laser powder bed fused AlSi9Cu3 alloy. In fact, the rapid cooling rates together with subsequent heating/cooling cycles occurred during layer by layer additive manufacturing production make low temperature heat treatments desirable for promoting stress relaxation as well as limited grain growth: this combination can offer the opportunity of obtaining the best compromise between high strength, good elongation to failure and limited residual stresses. The microstructural features were analysed, revealing that the high cooling rate, induced by the process, caused a large supersaturation of the aluminum matrix and the refinement of the eutectic structure. Microhardness versus time curve, performed at 250 °C, allowed to identify a stabilization of the mechanical property at a duration of 25 h. The microstructure and the mechanical properties of the samples heat treated at 25 h and at 64 h, considered as a reference for the conventionally produced alloy, were compared with the ones of the as-built alloy. Finally, it was shown that a 59% reduction of the principal residual stresses could be achieved after the 25 h-long treatment and such evolution was correlated to the mechanical behaviour.

Effect of Heat Treatment Methods on the Fatigue Behaviour of Hammermill: A Review

The production of reliable and sustainable hammermill involves heat treatment which is part of overall cost of production. Heat treatment has engrossed a lot of interests since it is one of the main production methods to improve corrosion resistance of coatings, enhancing malleability, hardness and strength to lessen mechanical or chemical damage. Nevertheless, various heat treatment procedures had resulted to hammermill failure as a result of material wear, erosive wear, impact wear, machine clogging and shock loading wear which eventually affects the efficiency of the hammermill. The study thus engrossed on some particular thermal treatment on mild steels and their impacts on hammermill applications. Progressive loss of material (wear) is a common failure that leads to hammermill fatigue due to cyclic stress posed on the hammers based on literature reviews. The mode of quenching and especially the discrepancy in the concentration affects the efficiency of the crushing unit. The fatigue performance of materials is being enhanced due to stress-relief heat treatment which makes it possible to increase the strength and ductility of the samples. However, the utilization of experimental model and simulation approach for stress prediction for the study is further recommended.

Plasticity and fracture behavior of Inconel 625 manufactured by laser powder bed fusion: Comparison between as-built and stress relieved conditions

In this study, the influence of stress relief on the plasticity and fracture behavior of Inconel 625 fabricated through laser powder bed fusion additive manufacturing (AM) was investigated. The as-built versus stress relieved microstructures were compared, showing similar grain structures but the presence of ~10 vol % δ phase in the stress relieved condition, and no δ phase in the as-built condition. Mechanical tests under plane strain tension were performed on the stress relieved samples, and an anisotropic plasticity model was calibrated and validated using finite element simulations. Uniaxial and notched tension tests were performed on both as-built and stress relieved samples to probe the effect of stress relief on stress state- and direction-dependent fracture behavior. It was found that on average, the fracture strain of the stress relieved samples along the build direction was 30% higher than that along the perpendicular build direction in the stress state range studied, and the stress relief heat treatment resulted in a 45% decrease in fracture strain. The fracture strain in stress relieved samples was more strongly dependent on stress state than in as-built samples.

Influence of Compositionally Graded Interface on Microstructure and Compressive Deformation of 316L Stainless Steel to Al12Si Aluminum Alloy Bimetallic Structures.

Bimetallic structures offer properties that can be customized based on applications, manufactured in one operation. Such manufacturing options are fascinating, as joining two metallic materials, especially for two dissimilar metals without significant defects, is challenging. In this study, 316L stainless steel (SS) to Al12Si aluminum alloy structures were processed, tailoring the compositionally graded interface on a SS 316 substrate using a directed energy deposition (DED)-based additive manufacturing (AM) process. Applying such a compositionally graded transition for joining two dissimilar metals could mitigate the mismatch of mechanical and thermal properties. This study's objective was to understand the processing parameters that influence the properties of AM processed SS 316L to Al12Si bimetallic structures. Two different approaches were used to fabricate these bimetallic structures. The results showed no visible defects on the as-fabricated samples using four layers of Al-rich mixed composition as the transition section. The microstructural characterization showed a unique morphology in each section. The microstructural variation was caused due to various processing parameters such as laser power, powder feed rate, and laser scan speed. FeAl, Fe2Al5, and FeAl3 intermetallic phases were formed at the compositionally graded transition section. After stress relief heat treatment of the bimetallic samples, diffused intermetallic phases were seen at the compositionally graded transition. At the interface, as processed bimetallic structures had a microhardness value of 834.2 ± 107.1 HV0.1, which was a result of the FeAl3 phase at the compositionally graded transition area. After heat treatment, the microhardness value reduced to 578.7 ± 154.1 HV0.1 because of a more Fe-dominated FexAly phase formation. Compression test results showed that the non-HT and HT SS 316L/Al12Si bimetallic structures had a similar maximum compressive strength of 299.4 ± 22.1 MPa and 270.1 ± 27.1 MPa, respectively. These results demonstrated that the applied heat treatment conditions only had a minor impact on samples' compression strength.

Thermo-mechanical modelling of stress relief heat treatments after laser-based powder bed fusion

Laser-based powder bed fusion, due to its layer-by-layer nature, results in a unique stress profile in a part after the primary production process. The residual stresses are typically tensile near the top, while they are compressive near the bottom of the part. When it is removed without proper precautions, the part will bend excessively. In order to alleviate this deformation, a stress relief heat treatment can be applied. In this paper, such a stress relaxation heat treatment is modelled to investigate the effect of the post-processing parameters. The model uses an Arrhenius-type creep equation to simulate the influence of the heat treatment temperature and dwell time on the stress field in a relatively simple cantilever beam produced in Ti-6Al-4V. Via validation of the simulations, the effect of the heat treatment is shown to be represented accurately. The validated model is used to predict the deformation that results from the residual stresses after heat treating the part under various conditions. The results from the simulations ultimately allow choosing the optimal heat treatment conditions to obtain a given reduction in the residual stress level, while reducing the need for extensive experimental investigations.

Evaluation of Dislocation Densities in Various Microstructures of Additively Manufactured Ti6Al4V (Eli) by the Method of X-ray Diffraction.

Dislocations play a central role in determining strength and flow properties of metals and alloys. Diffusionless phase transformation of β→α in Ti6Al4V during the Direct Metal Laser Sintering (DMLS) process produces martensitic microstructures with high dislocation densities. However, heat treatment, such as stress relieving and annealing, can be applied to reduce the volume of these dislocations. In the present study, an analysis of the X-ray diffraction (XRD) profiles of the non-heat-treated and heat-treated microstructures of DMLS Ti6Al4V(ELI) was carried out to determine the level of defects in these microstructures. The modified Williamson–Hall and modified Warren–Averbach methods of analysis were used to evaluate the dislocation densities in these microstructures. The results obtained showed a 73% reduction of dislocation density in DMLS Ti6Al4V(ELI) upon stress relieving heat treatment. The density of dislocations further declined in microstructures that were annealed at elevated temperatures, with the microstructures that were heat-treated just below the β→α recording the lowest dislocation densities.

Tensile Notch Sensitivity of Additively Manufactured IN 625 Superalloy

The notch sensitivity of additively manufactured IN 625 superalloy produces by laser powder bed fusion (LPBF) has been investigated by tensile testing of cylindrical test pieces. Smooth and V-notched test pieces with four different radii were tested both in as-built state and after a stress relief heat treatment for 1 h at 900 °C. Regardless of the notch root radius, the investigated alloy exhibits notch strength ratios higher than unity in both as-built and in stress-relieved states, showing that the additive manufactured IN 625 alloy is not prone to brittleness induced by the presence of V-notches. Higher values of notch strength ratios were recorded for the as-built material as a result of the higher internal stress level induced by the manufacturing process. Due to the higher triaxiality of stresses induced by notches, for both as-built and stress-relieved states, the proof strength of the notched test pieces is even higher than the tensile strength of the smooth test pieces tested in the same conditions. SEM fractographic analysis revealed a mixed mode of ductile and brittle fracture morphology of the V-notched specimens regardless the notch root radius. A more dominant ductile mode of fracture was encountered for stress-relieved test pieces than in the case of the as-built state. However, future research is needed to better understand the influence of notches on additive manufactured IN 625 alloy behaviour under more complex stresses.

Electron Backscattered Diffraction to Estimate Residual Stress Levels of a Superalloy Produced by Laser Powder Bed Fusion and Subsequent Heat Treatments.

Metal Additive Manufacturing and Laser Powder Bed Fusion (LPBF), in particular, have come forth in recent years as an outstanding innovative manufacturing approach. The LPBF process is notably characterized by very high solidification and cooling rates, as well as repeated abrupt heating and cooling cycles, which generate the build-up of anisotropic microstructure and residual stresses. Post-processing stress-relieving heat treatments at elevated temperatures are often required in order to release some of these stresses. The effects of 1 h–hold heat treatments at different specific temperatures (solutionizing, annealing, stress-relieve and low-temperature stress-relieve) on residual stress levels together with microstructure characterization were therefore investigated for the popular Alloy 625 produced by LPBF. The build-up of residual stress is accommodated by the formation of dislocations that produce local crystallographic misorientation within grains. Electron backscattered diffraction (EBSD) was used to investigate local misorientation by means of orientation imaging, thereby assessing misorientation or strain levels, in turn representing residual stress levels within the material. The heavily constrained as-built material was found to experience full recrystallization of equiaxed grains after solutionizing at 1150 °C, accompanied by significant drop of residual stress levels due to this grains reconfiguration. Heat treatments at lower temperatures however, even as high as the annealing temperature of 980 °C, were found to be insufficient to promote recrystallization though effective to some extent to release residual stress through apparently dislocations recovery. Average misorientation data obtained by EBSD were found valuable to evaluate qualitatively residual stress levels. The effects of the different heat treatments are discussed and suggest that the peculiar microstructure of alloys produced by LPBF can possibly be transformed to suit specific applications.

Analysis of post-processing influence on the geometrical and dimensional accuracy of selective laser melting parts

PurposeThis study aims to analyze the effect of the different common post-processes on the geometrical and dimensional accuracy of selective laser melting (SLM) parts.Design/methodology/approachAn artefact has been designed including cubic features formed by planar surfaces orientated according to the machine axes, covering all the X-Y area of the working space. The artefact has been analyzed both geometrically (flatness, parallelism) and dimensionally (sizes, distances) from coordinate measuring machine measurement results at three stages, namely, as-built, after sand-blasting and after stress-relieving heat treatment.FindingsResults from the SLM machine used in this study lead to smaller parts than the nominal ones. This effect depends on the direction of the evaluated dimension of the parts, i.e. X, Y or Z direction and is differently affected by the sandblasting post-process (average erosion ratio of 68, 54 and 9 µm, respectively), being practically unaltered by the HT applied after.Originality/valueThis paper shows the influence, from a geometric and dimensional point of view, of two of the most common post-processes used after producing SLM parts, such as sand-blasting and stress-relieving heat treatment, that have not been considered in previous research.

Heat Treatment Response of Selectively Laser Melted AlSi10Mg

Abstract The age-hardenable cast alloy AlSi10Mg is the most widely used alloy for additive manufacturing of aluminium components by means of selective laser melting. Due to the rapid solidification, the material exhibits a fine cellular microstructure, composed of a supersaturated Al-matrix and a network of silicon along the cell boundaries. The temperature of the building platform as well as the built time both have an influence on the level of precipitation in the material and this in turn affects the heat treatment response of AlSi10Mg in as-built condition. Material built on a cold platform can be strengthened by direct artificial ageing and shows only a small loss in strength after a stress relief heat treatment. Material built on a preheated platform has the highest strength in as-built condition and subsequent artificial ageing or stress relieving causes softening of the material. A condition which is truly independent of the platform temperature can only be reached by applying a solution heat treatment followed by quenching. Unlike castings, which need a long-term solution heat treatment to reach optimum mechanical properties, the selectively laser melted material shows the best mechanical properties in T6-condition after a solution heat treatment of short duration

Heat Treatment Response of Selectively Laser Melted AlSi10Mg

Abstract The age-hardenable cast alloy AlSi10Mg is the most widely used alloy for additive manufacturing of aluminium components by means of selective laser melting. Due to the rapid solidification, the material exhibits a fine cellular microstructure, composed of a supersaturated Al-matrix and a network of silicon along the cell boundaries. The temperature of the building platform as well as the built time both have an influence on the level of precipitation in the material and this in turn affects the heat treatment response of AlSi10Mg in as-built condition. Material built on a cold platform can be strengthened by direct artificial ageing and shows only a small loss in strength after a stress relief heat treatment. Material built on a preheated platform has the highest strength in as-built condition and subsequent artificial ageing or stress relieving causes softening of the material. A condition which is truly independent of the platform temperature can only be reached by applying a solution heat treatment followed by quenching. Unlike castings, which need a long-term solution heat treatment to reach optimum mechanical properties, the selectively laser melted material shows the best mechanical properties in T6-condition after a solution heat treatment of short duration