Stress Relief Heat Treatment Research Articles

Investigation of the Application of a C-ring Geometry to validate the Stress Relief Heat Treatment Simulation of Additive Manufactured Austenitic Stainless Steel Parts via Displacement

AbstractDirected energy deposition is a metal additive manufacturing process that builds parts by joining material in a layer-by-layer fashion on a substrate. Those parts are exposed to rapid thermo-cycles which cause steep stress gradients and the layer-upon-layer manufacturing fosters an anisotropic microstructure, therefore stress relief heat treatment is necessary. The numerical simulation can be used to find suitable parameters for the heat treatment and to reduce the necessary efforts to perform an effective stress relieving. Suitable validation experiments are necessary to verify the results of the numerical simulation.In this paper, a 3D coupled thermo-mechanical model is used to simulate the heat treatment of an additive manufactured component to investigate the application of a C-ring geometry for the distortion-based validation of the numerical simulation. Therefore, the C-ring samples were 3D scanned using a structured light 3D scanner to quantify the distortion after each process step.

Adhesion of HVOF-Sprayed WC-Co Coatings on 316L Substrates Processed by SLM

Different studies have been demonstrated that the surface integrity of substrate bulk materials to be coated has a significant impact on the adhesion of thermally sprayed coatings. It is known that the surface integrity of parts processed by selective laser melting (SLM) differs from those obtained from bulk materials. Although 316L stainless steel is among the most investigated material for SLM, the adhesion of thermally sprayed coatings on 316L stainless steel substrates processed by SLM has not been studied yet. This study aims at evaluating the effect of various mechanical pre-treatments onto 316L stainless steel substrates processed by SLM and their effect on the adhesion of high velocity oxy-fuel (HVOF)-sprayed WC-Co coatings. To differentiate between topographical effects and residual stress-related phenomena, a stress-relief heat treatment of the SLM substrates served as a reference throughout the investigations. The differently pre-treated SLM substrates were investigated with regard to the surface roughness and residual stresses. For the HVOF-sprayed SLM composites, Vickers interfacial indentation tests were conducted to assess the resulting coating adhesion. The findings demonstrated that the HVOF-sprayed WC-Co coatings predominantly exhibit good adhesion to the SLM 316L substrates. However, it was found that the stress state in the SLM 316L substrate surface is more likely to affect the adhesion of the WC-Co coating, while the substrate surface roughness showed a marginal effect.

Wire-Based Additive Manufacturing of Ti-6Al-4V Using Electron Beam Technique.

Electron beam freeform fabrication is a wire feed direct energy deposition additive manufacturing process, where the vacuum condition ensures excellent shielding against the atmosphere and enables processing of highly reactive materials. In this work, this technique is applied for the α + β-titanium alloy Ti-6Al-4V to determine suitable process parameter for robust building. The correlation between dimensions and the dilution of single beads based on selected process parameters, leads to an overlapping distance in the range of 70–75% of the bead width, resulting in a multi-bead layer with a uniform height and with a linear build-up rate. Moreover, the stacking of layers with different numbers of tracks using an alternating symmetric welding sequence allows the manufacturing of simple structures like walls and blocks. Microscopy investigations reveal that the primary structure consists of epitaxial grown columnar prior β-grains, with some randomly scattered macro and micropores. The developed microstructure consists of a mixture of martensitic and finer α-lamellar structure with a moderate and uniform hardness of 334 HV, an ultimate tensile strength of 953 MPa and rather low fracture elongation of 4.5%. A subsequent stress relief heat treatment leads to a uniform hardness distribution and an extended fracture elongation of 9.5%, with a decrease of the ultimate strength to 881 MPa due to the fine α-lamellar structure produced during the heat treatment. Residual stresses measured by energy dispersive X-ray diffraction shows after deposition 200–450 MPa in tension in the longitudinal direction, while the stresses reach almost zero when the stress relief treatment is carried out.

Effects of stress-relief heat treatment on the microstructure and fatigue property of a laser additive manufactured 12CrNi2 low alloy steel

In this study, the impact of stress-relief heat treatment on the microstructure and fatigue property of a laser melting deposited (LMD) 12CrNi2 low alloy steel was analyzed. Microstructural evolution of processed LMD 12CrNi2 steel by heat treatment was studied using electron backscatter diffraction (EBSD) and X-ray diffraction (XRD), aiming to establish a rational microstructure-fatigue property relationship. The results showed that the main phase of the samples before and after heat treatment was α-Fe phase, whereas the lattice parameter slightly decreased after annealing, as a result of stress relief and carbide formation in the heat-treated samples. Meanwhile, the increase of the hard and diffusely distributed carbides inside the matrix was the main contributor for the higher material strength. Based on the EBSD analysis, the heat treatment decreased grain boundary misorientation leading to a corresponding reduction in the dislocation density, and increased the content of high-angle grain boundaries (HAGBs) promoting the toughness of the material. Furthermore, Schmid factor results revealed that the heat-treated samples exhibited a relatively high ductility resulting from the activation for more slip systems during the cyclic deformation. The stress-relief heat treatment significantly improved the fatigue performance of samples, which was attributed to the enhanced strength and ductility owing to the related microstructure change and the relief of residual stresses.

Effect of Stress-Relieving Heat Treatment on the High Strain Rate Dynamic Compressive Properties of Additively Manufactured Ti6Al4V (ELI)

A study was undertaken on the compressive high strain rate properties and deformation behaviour of Direct Metal Laser-Sintered (DMLS) Ti6Al4V (ELI) parts in two separate forms: as-built (AB) and stress relieved (SR). The high strain rate compression tests were carried out using a Split Hopkinson Pressure Bar test system at ambient temperature. The average plastic strain rates attained by the system were 400 s−1 and 700 s−1. Comparative analyses of the performance (flow stresses and fracture strains) of AB and SR specimens were carried out based on the results obtained at these two plastic strain rates. Microstructural analyses were performed to study the failure mechanisms of the deformed specimens and fracture surfaces. Vickers microhardness test values were obtained before and after high strain rate compression testing. The results obtained in both cases showed the strain rate sensitivity of the stress-relieved samples to be higher in comparison to those of as-built ones, at the same value of true strain.

Corrosion–microstructure interrelations in new low-lead and lead-free brass alloys

ABSTRACTStress-relieving heat treatment has been reported to deplete the corrosion resistance of new low-lead and lead-free brass alloys. How the heat treatment, processing and material composition relates to the microstructure and corrosion performance is not well understood. The present study aims to fill this knowledge gap by mapping stress-relieving annealing conditions and different standardised compositions to their respective microstructures and dezincification performance. It was found that loss of corrosion resistance was only the most severe for alloys with higher aluminium and iron content. These alloys displayed significant precipitation of intermetallic aluminium arsenide particles on grain boundaries, twins and lead particles, as well as the formation of β-phase along grain boundaries.This paper is part of a Thematic Issue on Copper and its Alloys.

Experimental Study of the Effects of Welding Depth and Heat Treatment on Residual Stresses in a Cracked Rotor

One of the important issues in power plants is the occurrence of cracks on the steam turbine rotor. Removing cracks by machining is a common repair method to prevent crack propagation and failure. However, excess machining and reducing the rotor radius is limited. Therefore, welding the machined zone is a good way to repair the rotor. In this paper, the effects of welding depth and heat treatment on residual stresses of a laboratory sample of a welded power plant rotor have been investigated. For this purpose, a minimized model of the turbine rotor made of VCN200 with two V-shaped grooves has been prepared and then the grooves have been, respectively, filled by 7- and 32-pass welding after preheating. Afterward, residual stresses in different rotor zones (welded and non-welded zones), before and after stress relieving heat treatment, have been measured by the hole drilling method. Results showed the presence of residual stresses in all areas of the rotor before heat treatment, i.e., tensile residual stresses in non-welded and 7-pass welded areas and compressive residual stresses in 32-pass welded area. The results also showed stress relieving heat treatment led to a significant reduction in residual stresses in all studied areas.

Investigation on the Case-Hardening Behavior of Additively Manufactured 16MnCr5

Additive manufacturing (AM) technologies, such as laser-based powder bed fusion of metals (PBF-LB/M), allow for the fabrication of complex parts due to their high freedom of design. PBF-LB/M is already used in several different industrial application fields, especially the automotive and aerospace industries. Nevertheless, the amount of materials being processed using AM technologies is relatively small compared to conventional manufacturing. Due to this, an extension of the material portfolio is necessary for fulfilling the demands of these industries. In this work, the AM of case-hardening steel 16MnCr5 using PBF-LB/M is investigated. In this context, the influences of different processing strategies on the final hardness of the material are studied. This includes, e.g., stress relief heat treatment and microstructure modification to increase the resulting grain size, thus ideally simplifying the carbon diffusion during case hardening. Furthermore, different heat treatment strategies (stress relief heat treatment and grain coarsening annealing) were applied to the as-built samples for modifying the microstructure and the effect on the final hardness of case-hardened specimens. The additively manufactured specimens are compared to conventionally fabricated samples after case hardening. Thus, an increase in both case-hardening depth and maximum hardness is observed for additively manufactured specimens, leading to superior mechanical properties.

Titanium Ti6Al4V alloy reinforced by the addition of nano yttria stabilized zirconia fabricated by selective additive manufacturing: microstructure and mechanical investigations

Additive manufactured Ti6Al4V reinforced with nano yttria-zirconia (nYSZ) parts were fabricated using selective laser melting technology (SLM). The as-received Ti6Al4V powder and two powder mixtures of Ti6Al4V mixed with several nYSZ contents (1wt% and 2.5wt%) were prepared and then SLM processed. Parts were further subjected to a stress relief heat treatment. Besides, hot isostatic pressure (HIP) was used in order to eliminate residual porosities. The pycnometer-based technique was used to measure the mass density. XRD and EBSD analysis were performed to investigate the influence of nYSZ additions on the microstructure and subsequent mechanical properties via microhardness and compression tests. It was found that addition of nYSZ increases the density of the reinforced parts and produces a fine α martensite phase. Besides, the grain size was refined compared to that of heat treated Ti6Al4V. As a consequence, a significant increase in both the hardness and the compressive strength for the reinforced Ti6Al4V were obtained while the elongation to failure was kept. These improved mechanical properties are discussed in relation to the effect of nYSZ addition, which includes latice distortions and strengthening from grain size refinement and/or α formation.

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.

Multi-Laser Powder Bed Fusion Benchmarking\u2014Initial Trials with Inconel 625

Production rate is an increasingly important factor in the deployment of metal additive manufacturing (AM) throughout industry. To address the perceived low production rate of metal AM systems based on single-laser powder bed fusion (L-PBF), several companies now offer systems in which melting has been parallelised by the introduction of multiple, independently controlled laser beams. Nevertheless, a full set of studies is yet to be conducted to benchmark the efficiency of multi-laser systems and, at the same time, to verify if the mechanical properties of components are compromised due to the increase in build rate. This study addresses the described technology gaps and presents a 4-beam L-PBF system operating in “single multi” (SM) mode (SM-L-PBF) where each of the four lasers is controlled so that it melts all of a particular components’ layers and produces specimens for comparison with standard L-PBF specimens from the same machine. That is all four lasers making all of some of the parts were compared to a single-laser manufacturing all of the parts. Build parameters were kept constant throughout the manufacturing process and the material used was Inconel 625 (IN625). Stress-relieving heat treatment was conducted on As-built (AB) specimens. Both AB and heat-treated (HT) specimen sets were tested for density, microstructure, tensile strength and hardness. Results indicate that the stress-relieving heat treatment increases specimen ductility without compromising other mechanical properties. SM-L-PBF has achieved a build rate of 14 cm3/h when four 200 W lasers were used to process IN625 at a layer thickness of 30 μm. An increase in the build rate of 2.74 times (build time reduction: 63%) has been demonstrated when compared to that of L-PBF, with little to no compromises in specimen mechanical properties. The observed tensile properties exceed the American Society for Testing Materials (ASTM) requirements for IN625 (by a margin of 22 to 26% in the 0.2% offset yield strength). Average specimen hardness and grain size are in the same order as that reported in literatures. The study has demonstrated that a multi-laser AM system opens up opportunities to tackle the impasse of low build rate in L-PBF in an industrial setting and that at least when operating in single mode there is no detectable degradation in the mechanical and crystallographic characteristics of the components produced.

The Influence of Residual Stresses Induced by Near-Net-Shape Blanking Processes on the Fatigue Behavior under Bending Loads

Gears manufactured by blanking can be found in many different products like hammer drills or automobiles. Here, only the functional surface, the clean shear, can be used to transmit torque. Therefore, parts in serial production demand a high amount of clean shear, so the required torque can be transmitted with a minimal thickness and part weight. To achieve this, these parts are usually manufactured by fineblanking or related Near-Net-Shape Blanking processes (NNSBPs). Furthermore, the gears are subjected to cyclic loading which can, especially in the highly stressed tooth root, lead to tooth breakage. The effect of different process variants and process parameters on the residual stresses and the fatigue behavior under a pulsating bending load has not been investigated yet. Due to the potential of endurance improvement of blanked gears, this topic is addressed in this paper. To accomplish this, C-shaped profiles are manufactured by five different Near-Net-Shape blanking processes. The investigated processes are fineblanking, precision blanking with and without blank holder, and blanking with a small die clearance with and without a v-ring. The sheet metal material, S355MC (material number 1.0976) with a thickness of 6 mm, is first subjected to a stress relief heat treatment to minimize residual stresses induced by the specimen preparation and to ensure a defined initial residual stress state. After blanking, the residual stresses of the parts are measured. Finally, fatigue strength tests are carried out under a pulsating bending load on the C-shaped profiles with shear-cut edges. The results show that the residual stress state, as well as the part’s fatigue behavior are strongly influenced by the chosen blanking process.

Effects of Stress-Relieving Heat Treatment on Impact Toughness of Direct Metal Laser Sintering (DMLS)-Produced Ti6Al4V (ELI) Parts

The impact toughness of as-built (AB) and stressed-relieved (SR) direct metal laser sintering-produced Ti6Al4V (ELI) was investigated using the standard Charpy impact test over the temperature range of − 130°C to 250°C. Stress-relieving heat treatment was conducted at 650°C for a soaking period of 3 h in argon gas atmosphere. The results showed improvements in the impact toughness after stress-relieving heat treatment. Stress relieving also shifted the established ductile-to-brittle transition temperature to lower temperatures. Comparative analysis of the impact toughness values for AB and SR specimens at ambient temperature showed them to be 48% and 22% lower than recommended values for use in aircraft structures, respectively.

Determination of the influence of a stress-relief heat treatment and additively manufactured surface on the fatigue behavior of selectively laser melted AISI 316L by using efficient short-time procedures

To use Additive Manufacturing (AM) technology for production of safety critical structural components, it is indispensable to investigate thoroughly the materials AM specific microstructure as well as the resulting mechanical and especially cyclic properties. Besides their special microstructure, AM materials show a highly inhomogeneous distribution of residual stresses as well as numerous process induced microstructural notches, e.g. pores in the materials volume and high roughness of additively manufactured surfaces. Therefore, in production of AM components stress-relief heat treatments as well as surface post-processing are commonly used. In the present work, the influence of stress-relief heat treatment as well as the additively manufactured surface condition on the cyclic properties was investigated at selectively laser melted (SLM) specimens made of AISI 316L stainless steel. Specimens were manufactured in horizontal as well as vertical building direction, leading to layer planes oriented parallel and perpendicular to the loading direction, respectively. To reduce the high material and time effort in fatigue investigations, the short-time methods PhyBaLCHT, which is based on cyclic indentation tests and enables a characterization of the materials defect tolerance, as well as load increase tests (LITs) were used for qualitative analyzes of the cyclic deformation behavior. Moreover, the physically based lifetime calculation approach (PhyBaLLIT) was applied for quantitative determination of the fatigue behavior of heat treated specimens, showing excellent correlation to additional constant amplitude tests (CATs), performed for validation. While the heat treatment does not significantly influence the fatigue behavior of vertically built specimens, an increase of fatigue strength could be observed for horizontal building direction, which is mainly caused by an improved defect tolerance. The higher defect tolerance was determined with PhyBaLCHT and could be proved by fracture mechanics considerations using the area-concept. Furthermore, the results show a significantly lower fatigue lifetime for additively manufactured surface condition compared with polished samples, which is more pronounced for vertically built specimens, because of the higher surface roughness of specimens manufactured in this building direction. Moreover, a double-staged S-Nf curve could be observed for specimens with additively manufactured surface, which correlates to the cyclic deformation behavior observed in LITs and CATs.

Effect of simulated stress-relieving heat treatment on microstructure and tensile properties of CLAM steel

Post weld heat treatment (PWHT) is indispensable in manufacturing process of the test blanket modules for international thermonuclear experimental reactor (ITER). Multiple PWHTs are required to reduce the residual stress that comes from the concentrated welded seams of the test blanket modules. As the number of treating cycles increases, considerable strength degradation of the base metal occurs. In this work, simulated stress-relieving heat treatment (SSRHT) was performed to investigate the effect of multiple PWHTs on the microstructure and tensile properties of China low activation martensitic (CLAM) steel. The results showed that its hardness decreased by 31 HV and yield strength decreased by 146 MPa for a 10-cycle treating at room temperature. The softening rate decreased and the ductility increased with increasing number of cycles at all temperatures. This phenomenon could be explained by the alterations in the martensitic lath width, volume fraction of the precipitates and the particle size. Considering the service safety of the blanket components fabricated with CLAM steel, the safety factors should be higher than 1.33, 1.29 and 1.28 for SSRHT within 10 cycles at room temperature, 450 °C and 550 °C, respectively.

Phase Fraction and Evolution of Additively Manufactured (AM) 15-5 Stainless Steel and Inconel 625 AM-Bench Artifacts.

A proper understanding of the structure and microstructure of additively manufactured (AM) alloys is essential not only to the prediction and assessment of their material properties, but also to the validation and verification of computer models needed to advance AM technologies. To accelerate AM development, as part of the AM-Bench effort, we conducted rigorous synchrotron-based X-ray scattering and diffraction experiments on two types of AM alloys (AM 15-5 stainless steel and AM Inconel 625). Taking advantage of the high penetration of synchrotron hard X-rays, we determined the phases present in these alloys under different build conditions and their statistically meaningful phase fractions using high-resolution X-ray diffraction. Using in situ multi-scale X-ray scattering and diffraction, we quantitatively analyzed the phase evolution and development of major precipitates in these alloys as a function of time during stress relief heat treatments. These results serve to validate AM microstructure models and provide input to higher-level AM processing and property models to predict the material properties and performances.

Microstructural Changes During Stress Relief Heat Treatment of Inconel 625–A106 Carbon Steel Joint

The present study investigates the effect of stress relief treatment at different temperatures (900, 1040, and 1200 °C) on the microstructure of Inconel 625 and A106 carbon steel weld joints. The results of the study show that, due to the generation of heat by welding, the bainite phase was formed in the heat-affected zone of A106 alloy. By applying stress relief treatment, even at the lowest temperature, the bainite phase was removed from the microstructure and the ferritic–pearlitic structure was formed. As the temperature of stress relief treatment increased, the grain size of the samples and the ratio of pearlite structure increased in the matrix. The morphology of the weld metal was converted from the columnar dendritic to an island state. Gradually, by increasing the temperature up to 1200 °C, austenitic grain boundary appeared in the microstructure. The SEM results confirmed the presence of complex molybdenum–niobium–rich carbides among secondary phase particles in the weld zone. In addition, an increase in the heat treatment temperature caused an increase in the concentration and dimensions of the carbides in the heat-affected zone, weld metal, and base metal of Inconel 625.

A study on the effect of stress relief heat treatment on the microstructure and mechanical properties of dissimilar GTAW weld joints of Inconel 625 and A106 carbon steel

In this study, the effect of stress relief heat treatment at different temperatures (900 °C, 1040 °C, and 1200 °C) on the mechanical properties and microstructure of Inconel 625 and A106 GTAW weld joint (using ER-625 as filler rod) has been investigated. The results showed that bainite phase was formed at the HAZ of A106 alloy which resulted in the brittleness of the alloy and hardness increase in the non-heat treated sample (187 and 210 HV at HAZ of A106 and Inconel, respectively). By applying the stress relief process, bainite phase was omitted from the microstructure and ferrite-pearlite structure was formed. By increasing the process temperature to 1200 °C, the austenitic grain boundaries appeared in the microstructure. EDS analysis approved the presence of molybdenum and niobium rich complex carbides in the welding metal microstructure. The results of tensile test also showed that without stress relief process, the sample ductility decreased by ∼27%. By applying the stress relief process, the ductility of samples obviously improved and the sample which was stress relieved at 1040 °C showed the best tensile properties (UTS of ∼493 MPa and ductility of ∼29%). By increasing the stress-relief temperature to 1200 °C, the strength (UTS of ∼443 MPa) and hardness of the sample decreased due to the growth of the grain size.

Impact properties of half stress-relieved and hot isostatic pressed Ti–6Al–4V components fabricated by laser powder bed fusion

This study focuses on the impact properties of Ti–6Al–4V produced by LPBF using un-notched Charpy impact testing. The effects of the build orientation, impact orientation, and half stress-relief and hot isostatic press (HIP) heat treatments on the impact resistance were investigated. The impact properties of HIPed samples were significantly greater than those of stress-relieved ones. Microstructural analysis and fractography revealed that process defects such as porosity, and columnar grain boundaries play an important role in crack initiation and propagation in the stress-relieved samples. In the HIPed samples the orientation of prior-β grain boundaries with respect to the impact loading, and impact energy absorption by microstructural modification from the high strains promoting formation of globular α were the main contributors in the impact resistance of the samples. Consequently, the vertically-built HIPed samples demonstrated the greatest impact energy from the combination of the perpendicular prior-β grain boundaries to the impact and highest observed α globularization.

Microstructure-topology relationship effects on the quasi-static and dynamic behavior of additively manufactured lattice structures

This study demonstrates a relationship between manufacturing variables including design topology and post-processing heat treatment on the porosity distribution, quasi-static, and dynamic behavior of additively manufactured lattice structures (AMLS). Lattice structures were manufactured out of Inconel 718 using selective laser melting technique with four different topologies. The effect of heat treatment on the porosity size and distribution was examined using X-ray computed tomography for as-built (AB), stress relieved (SR), and hot isostatic pressed (HIP) plus solution aged (SA) heat-treatment conditions. It was noticed that reduction of porosity in the as-built samples, as a result of SR, was greater compared the porosity reduction due to the subsequent HIP plus SA. Quasi-static and dynamic loading was conducted and it was found that the deformation trends of each topology were independent of the strain rate. It was also found that the stress relieving heat treatment process enhances the quasi-static and dynamic flow stress after yielding. However, further heat-treating, including HIP and SA, for the same topology were not as effective as the initial SR process. Furthermore, the validity of digital image correlation in measuring average global strain and the validity of using a Kolsky bar for measuring dynamic mechanical behavior of AMLS are discussed.