| 1. |
- Chen, Zhe, et al.
(författare)
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Nano-scale characterization of white layer in broached Inconel 718
- 2017
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - Amsterdam : Elsevier BV. - 0921-5093 .- 1873-4936. ; 684, s. 373-384
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Tidskriftsartikel (refereegranskat)abstract
- The formation mechanism of white layers during broaching and their mechanical properties are not well investigated and understood to date. In the present study, multiple advanced characterization techniques with nano-scale resolution, including transmission electron microscopy (TEM), transmission Kikuchi diffraction (TKD), atom probe tomography (APT) as well as nano-indentation, have been used to systematically examine the microstructural evolution and corresponding mechanical properties of a surface white layer formed when broaching the nickel-based superalloy Inconel 718.TEM observations showed that the broached white layer consists of nano-sized grains, mostly in the range of 20–50 nm. The crystallographic texture detected by TKD further revealed that the refined microstructure is primarily caused by strong shear deformation. Co-located Al-rich and Nb-rich fine clusters have been identified by APT, which are most likely to be γ′ and γ′′ clusters in a form of co-precipitates, where the clusters showed elongated and aligned appearance associated with the severe shearing history. The microstructural characteristics and crystallography of the broached white layer suggest that it was essentially formed by adiabatic shear localization in which the dominant metallurgical process is rotational dynamic recrystallization based on mechanically-driven subgrain rotations. The grain refinement within the white layer led to an increase of the surface nano-hardness by 14% and a reduction in elastic modulus by nearly 10% compared to that of the bulk material. This is primarily due to the greatly increased volume fraction of grain boundaries, when the grain size was reduced down to the nanoscale.
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| 2. |
- Chen, Zhe, et al.
(författare)
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Surface Integrity and Structural Stability of Broached Inconel 718 at High Temperatures
- 2016
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 47A:7, s. 3664-3676
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Tidskriftsartikel (refereegranskat)abstract
- The current study focused on the surface integrity issues associated with broaching of Inconel 718 and the structural stability of the broached specimen at high temperatures, mainly involving the microstructural changes and residual stress relaxation. The broaching operation was performed using similar cutting conditions as that used in turbo machinery industries for machining fir-tree root fixings on turbine disks. Thermal exposure was conducted at 723 K, 823 K, and 923 K (450 A degrees C, 550 A degrees C, and 650 A degrees C) for 30, 300, and 3000 hours, respectively. Surface cavities and debris dragging, sub-surface cracks, high intensity of plastic deformation, as well as the generation of tensile residual stresses were identified to be the main issues in surface integrity for the broached Inconel 718. When a subsequent heating was applied, surface recrystallization and alpha-Cr precipitation occurred beneath the broached surface depending on the applied temperature and exposure time. The plastic deformation induced by the broaching is responsible for these microstructural changes. The surface tension was completely relaxed in a short time at the temperature where surface recrystallization occurred. The tensile layer on the sub-surface, however, exhibited a much higher resistance to the stress relief annealing. Oxidation is inevitable at high temperatures. The study found that the surface recrystallization could promote the local Cr diffusion on the broached surface.
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| 3. |
- Chen, Zhe, et al.
(författare)
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Surface Integrity and Fatigue Performance of Inconel 718 in Wire Electrical Discharge Machining
- 2016
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Ingår i: 3RD CIRP CONFERENCE ON SURFACE INTEGRITY. - : Elsevier BV. ; , s. 307-310
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Konferensbidrag (refereegranskat)abstract
- This paper presents a study to characterize the surface integrity in wire electrical discharge machining (EDM) of Inconel 718 and investigate its effect on the fatigue performance of the alloy in a four-point bending fatigue mode at room temperature. The EDM process generates a rough recast surface with multi-types of defects. Surface craters, micro-cracks and micro-voids within the recast layer have been found to be most detrimental from the point of view of fatigue as they could provide many preferential initiation sites for fatigue cracks. As a consequence, the specimens with an EDM cut surface show an approximately 30% decrease in fatigue life compared to those with a polished surface, and multiple crack origins were observed on the fracture surface. The high tensile residual stresses generated on the EDM cut surface, on the other hand, are also believed to be partly responsible for the loss in fatigue life of the alloy machined by EDM.
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| 4. |
- Gustafsson, David, et al.
(författare)
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Influence of high temperature hold times on the fatigue crack propagation in Inconel 718
- 2011
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Ingår i: International Journal of Fatigue. - : Elsevier. - 0142-1123 .- 1879-3452. ; 33:11, s. 1461-1469
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Tidskriftsartikel (refereegranskat)abstract
- High temperature fatigue crack growth in Inconel 718 has been studied at the temperatures 450 degrees C, 500 degrees C, 550 degrees C and 650 degrees C. The tests were conducted both without hold times and with hold times of different lengths and with a mix of both. Focus has been on quantifying the effect the hold time has upon the crack growth rate and how much it damages the material. Furthermore, it has been investigated how this damage influences the actual cracking behavior, i.e. where in the loading cycle the damage contributes most to the crack growth. This damage is related to the concept of a damaged zone in front of the crack tip. The size of the damaged zone has been derived from the tests and a microscopy study to confirm the findings has also been carried out. It is found that the concept of a damaged zone can be a successful explanatory model for the observed crack growth behavior under high temperature hold time.
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| 5. |
- Tofique, Muhammad Waqas, 1986-, et al.
(författare)
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ECCl/EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels
- 2017
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Ingår i: International Journal of Fatigue. - London : Elsevier. - 0142-1123 .- 1879-3452. ; 100, s. 251-262
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Tidskriftsartikel (refereegranskat)abstract
- Fatigue test data of duplex stainless steel grades, LDX 2101 and 2304 SRG, in the Very High Cycle Fatigue (VHCF) regime is presented. Fatigue testing was conducted using ultrasonic fatigue test equipment operating at 20 kHz under fully reversed tension-compression load condition. Scanning Electron Microscope (SEM) analysis of the fracture surfaces and external surfaces of failed specimens was conducted. Electron Channelling Contrast Imaging (ECCI) and Electron Back Scattered Diffraction (EBSD) studies of the axially cut surface of the failed specimens was done to analyse the accumulation of plastic fatigue damage and fatigue crack growth in the grains adjacent to the external surface and crack initiation site. Transmission Electron Microscope (TEM) analysis of thin foils cut from failed specimens of LDX 2101 was carried out to examine the effect of fatigue loading on dislocation structure. SEM studies of the Crystallographic Growth Region (CGR) showed features like grain boundaries and fatigue striations on the fracture surfaces. SEM analysis of the external surfaces of fatigue loaded specimens showed inhomogeneous accumulation of plastic fatigue damage. ECCl/EBSD analysis showed Persistent Slip Bands (PSBs) in ferrite grains in LDX 2101 grade but no PSBs were observed in any grains of 2304 SRG specimens. The barrier effect of grain and phase boundaries on short fatigue crack propagation was observed. TEM analysis of thin foils cut from the failed specimens of LDX 2101 showed stacking faults in austenite grains and they were seen to stop at the grain and phase boundaries. (C) 2017 Elsevier Ltd. All rights reserved.
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| 6. |
- Chen, Zhe
(författare)
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Surface Integrity and Fatigue Performance of Nickel-based Superalloys
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to global warming, the demand for more efficient gas turbines has increased. A way to achieve this, is by increasing the operating temperature of the gas turbine. Therefore, nickel-based superalloys have been developed to withstand these extreme temperatures and loads, especially in the hot sections. Today, the way of operating land-based gas turbines is changing. Instead of running for long periods of time, the operation is becoming more flexible, with ever-increasing cyclic loads and number of start and stop cycles. To handle the increased stress and cycles, component resistance to fatigue failures need to be improved.Surface integrity is critical to fatigue performance, since fatigue cracks normally initiate at a surface. Machining changes the surface integrity which can result in worse fatigue resistance. The work presented in this Ph.D. thesis was conducted in collaboration with Siemens Industrial Turbomachinery AB in Finspång, Sweden. Surface integrity changes which are induced during machining and their effects on fatigue performance have been studied on the alloy Inconel 718. Inconel 718 is a widely-used nickel-based superalloy for high temperature applications in modern gas turbines.Broaching, milling, and wire electrical discharge machining, related to component manufacturing in turbo machinery industries, were included in this study. Machining induced surface defects provide preferential sites for fatigue crack initiation which influence the fatigue performance of the alloy. If compressive residual stresses are induced during machining, they benefit the fatigue life by retarding fatigue crack initiation away from surface regions. Shot peening was performed on machined Inconel 718, by which high compressive residual stresses are deliberately induced. It results in enhanced fatigue performance.The high temperatures in gas turbines generally deteriorate the surface integrity. Recrystallization often occurs in the highly deformed surface layer. Microstructural degradation, in the form of α-Cr precipitates, have also been frequently observed in the deformed surface and sub-surface microstructure. Oxidation at elevated temperatures degrades the surface integrity and thereby also the fatigue performance. Fatigue cracks are preferably initiated at oxidized surface carbides, if thermal exposure has been made prior to the test. It is even worse when high temperatures relax the beneficial surface compression induced by shot-peening and thereby lowering the fatigue resistance.Machinability of a newly developed nickel-based superalloy, AD 730TM, and the surface integrity induced during turning have also been studied in this thesis project. AD 730TM is a candidate for turbine disc applications with an operating temperature above 650 °C. At such high temperatures, Inconel 718 is no longer stable and its mechanical properties start to degrade.To summarize, the results from this thesis work show the importance of understanding surface integrity effects for fatigue applications, especially in harsh environments. Moreover, the knowledge gained through this work could be used for surface enhancement of turbine components which are subjected to a high risk of fatigue failure. These will contribute to more efficient and flexible power generation by gas turbines.
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| 7. |
- Ahmad, Maqsood, et al.
(författare)
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Bending Fatigue Behavior of Blast Cleaned Grey Cast Iron
- 2017
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Ingår i: Residual Stresses 2016: ICRS-10, Materials Research Proceedings 2 (2016). - : Materials Research Forum LLC. - 9781945291166 ; , s. 193-198
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Konferensbidrag (refereegranskat)abstract
- This paper presents a detailed study on the effect of an industrial blast cleaning process on the fatigue behavior of a grey cast iron with regard to the residual stresses and microstructural changes induced by the process. A comparison was also made to the effect of a machining operation which removed the casting skin layer. The blast cleaning process was found to greatly improve the fatigue resistance in both the low and high cycle regimes with a 75% increase in the fatigue limit. Xray diffraction measurements and scanning electron microscopic analyses showed that the improvement was mainly attributed to compressive residual stresses in a surface layer up to 800 μm in thickness in the blast cleaned specimens. The machining also gave better fatigue performance with a 30% increase in the fatigue limit, which was ascribed to the removal of the weaker casting skin layer.
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| 8. |
- Zhou, Jinming, et al.
(författare)
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Analysis of subsurface microstructure and residual stresses in machined Inconel 718 with PCBN and Al2O3-SiCw tools
- 2014
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Ingår i: Procedia CIRP. - : Elsevier BV. - 2212-8271. ; 13, s. 150-155, s. 150-155
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Konferensbidrag (refereegranskat)abstract
- Subsurface microstructural alterations and residual stresses caused by machining significantly affect component lifetime and performance by influencing fatigue, creep, and stress corrosion cracking resistance. Assessing the surface quality of a machined part by characterizing subsurface microstructural alterations and residual stresses is essential for ensuring part performance and lifetime in aero-engines and power generators. This comparative study characterizes and analyzes subsurface microstructural alterations and residual stresses in Inconel 718 subjected to high-speed machining with PCBN and whisker-reinforced ceramic cutting tools. Effects of cutting tool materials and microgeometry on subsurface deformation, microstructural alterations, and residual stresses were investigated. Surface and subsurface regions of machined specimens were investigated using X-ray diffraction, electron channeling contrast imaging, and electron back-scatter diffraction to characterize microstructural alterations and measure deformation intensity and depth. (C) 2014 The Authors. Published by Elsevier B.V.
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| 9. |
- Calmunger, Mattias, 1986-
(författare)
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On High-Temperature Behaviours of Heat Resistant Austenitic Alloys
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Advanced heat resistant materials are important to achieve the transition to long term sustainable power generation. The global increase in energy consumption and the global warming from greenhouse gas emissions create the need for more sustainable power generation processes. Biomass-fired power plants with higher efficiency could generate more power but also reduce the emission of greenhouse gases, e.g. CO2. Biomass offers no net contribution of CO2 to the atmosphere. To obtain greater efficiency of power plants, one option is to increase the temperature and the pressure in the boiler section of the power plant. This requires improved material properties, such as higher yield strength, creep strength and high-temperature corrosion resistance, as well as structural integrity and safety.Today, some austenitic stainless steels are design to withstand temperatures up to 650 °C in tough environments. Nickel-based alloys are designed to withstand even higher temperatures. Austenitic stainless steels are more cost effective than nickel-based alloys due to a lower amount of expensive alloying elements. However, the performance of austenitic stainless steels at the elevated temperatures of future operation conditions in biomass-red power plants is not yet fully understood.This thesis presents research on the influence of long term high-temperature ageing on mechanical properties, the influence of very slow deformation rates at high-temperature on deformation, damage and fracture, and the influence of high-temperature environment and cyclic operation conditions on the material behaviour. Mechanical and thermal testing have been performed followed by subsequent studies of the microstructure, using scanning electron microscopy, to investigate the material behaviours.Results shows that long term ageing at high temperatures leads to the precipitation of intermetallic phases. These intermetallic phases are brittle at room temperature and become detrimental for the impact toughness of some of the austenitic stainless steels. During slow strain rate tensile deformation at elevated temperature time dependent deformation and recovery mechanisms are pronounced. The creep-fatigue interaction behaviour of an austenitic stainless steel show that dwell time gives shorter life at a lower strain range, but has none or small effect on the life at a higher strain range.Finally, this research results in an increased knowledge of the structural, mechanical and chemical behaviour as well as a deeper understanding of the deformation, damage and fracture mechanisms that occur in heat resistant austenitic alloys at high-temperature environments. It is believed that in the long term, this can contribute to material development achieving the transition to more sustainable power generation in biomass-red power plants.
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| 10. |
- Ceschini, Lorella, et al.
(författare)
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Effect of Microstructure and Overaging on the Tensile Behavior at Room and Elevated Temperature of C355-T6 Cast Aluminum Alloy
- 2015
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Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 83, s. 626-634
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Tidskriftsartikel (refereegranskat)abstract
- The present study was focused on the microstructural and mechanical characterization of the Al–Si–Cu–Mg C355 alloy, at room and elevated temperature. In order to evaluate the influence of microstructural coarseness on mechanical behavior, samples with different Secondary Dendrite Arm Spacing (SDAS) (20–25 μm for fine microstructure and 50–70 μm for coarse microstructure), were produced through controlled casting conditions. The tensile behavior of the alloy was evaluated at T6 condition and at T6 with subsequent high temperature exposure (41 h at 210 °C, i.e. overaging), both at room and elevated temperature (200 °C). Microstructural investigations were performed through optical and electron microscopy.The results confirmed the important role of microstructure on the tensile behavior of C355 alloy. Ultimate tensile strength and elongation to failure strongly increased with the decrease of SDAS. Larger SDAS, related to lower solidification rates, modify microstructural features, such as eutectic Si morphology and size of the intermetallic phases, which in turn influence elongation to failure. Overaging before tensile testing induced coarsening of the strengthening precipitates, as observed by STEM analyses, with consequent reduction of the tensile strength of the alloy, regardless of SDAS. A more sensible decrease of tensile properties was registered at 200 °C testing temperature.
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