Sten Johansson

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Papers by Sten Johansson

Surface Integrity and Structural Stability of Broached Inconel 718 at High Temperatures

Metallurgical and Materials Transactions, Apr 22, 2016

The current study focused on the surface integrity issues associated with broaching of Inconel 71... more 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 ˚C, 550 ˚C and 650 ˚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 α-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.

format_quoteBroaching generated high tensile residual stresses; peak tensile stress reached 760 MPa at the surface, indicative of stress distribution. format_quote

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Characterization of Austenitic Stainless Steels Deformed at Elevated Temperature

Metallurgical and Materials Transactions, Jul 17, 2017

Highly alloyed austenitic stainless steels are promising candidates to replace more expensive nic... more Highly alloyed austenitic stainless steels are promising candidates to replace more expensive nickel-based alloys within the energy-producing industry. The present study investigates the deformation mechanisms by microstructural characterization, mechanical properties and stress-strain response of three commercial austenitic stainless steels and two commercial nickel-based alloys using uniaxial tensile tests at elevated temperatures from 673 K (400 C) up to 973 K (700 C). The materials showed different ductility at elevated temperatures which increased with increasing nickel content. The dominating deformation mechanism was planar dislocation-driven deformation at elevated temperature. Deformation twinning was also a noticeable active deformation mechanism in the heat-resistant austenitic alloys during tensile deformation at elevated temperatures up to 973 K (700 C).

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Hot Corrosion Behavior of HVOF CoNiCrAlY(Hf) Coating on Ni-based Superalloys

To Increase Fatigue Strength of Grey Iron By Shot Peening

Deformation and damage mechanisms in IN792 during thermomechanical fatigue

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, May 1, 2011

The deformation and damage mechanisms arising during thermomechanical fatigue (TMF) of the polycr... more The deformation and damage mechanisms arising during thermomechanical fatigue (TMF) of the polycrystalline superalloy IN792 have been investigated. The TMF cycles used in this study are in-phase (IP) and out-of-phase (OP). The minimum temperature used in all TMF-tests is 100°C while the maximum temperature is 500° or 750°C in the IP TMF-tests and 750°, 850° or 950°C in the OP TMF-tests. The majority of the cracks are transcrystalline, except for the IP TMF-test at 750°C, where some tendency to intercrystalline crack growth can be seen. In all tests, the cracks were initiated and propagated in locations where deformation structures such as deformation bands have formed in the material. In the temperature interval 750°-850°C, twins were formed in both IP and OP TMF-tests and this behaviour is observed to be further enhanced close to a crack. Twins are to a significantly lesser extent observed for tests with a lower (500°C) and a higher (950°C) maximum temperature. Recrystallization at grain boundaries, around particles and within the deformation structures have occurred in the OP TMF-tests with a maximum temperature of 850° and 950°C and this is more apparent for the higher temperature. Void formation is frequently observed in the recrystallized areas even for the case of compressive stresses at high temperature.

format_quoteInelastic strain range correlates well with cycles to failure, indicating it is the most crucial factor for TMF life, regardless of cycle type or maximum temperature.format_quote

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Lattice strain evolution and phase interaction in a super duplex stainless steel under uniaxial loading

Lattice strain evolution and phase interaction in a super duplex stainless steel under uniaxial l... more

Fatigue crack initiation in a notched single-crystal superalloy component

Procedia Engineering, Apr 1, 2010

In this paper, the fatigue crack initiation in notched single-crystal test specimens of material ... more In this paper, the fatigue crack initiation in notched single-crystal test specimens of material MD2 is investigated and analysed. A critical plane approach is adopted, in which the total strain ranges on the discrete slip planes are evaluated. Furthermore, a Coffin-Manson type of expression is used to describe the number of cycles to fatigue crack initiation. This relation is determined from a set of smooth test specimens loaded uniaxially in the [001], [011] and [ 11] directions at 500ºC with R = 1. The numerical procedure is then applied to a series of experiments, in which notched single-crystal test specimens were exposed to uniaxial cyclic loading in the [001] direction at 500 o C with R = 0.

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Comparison of Low-Cycle Fatigue Properties of Two Coated Single Crystal Nickel-Base Superalloys, CMSX-4 and SCB

Comparison of Low-Cycle Fatigue Properties of Two Coated Single Crystal Nickel-Base Superalloys, ... more

Residual Stresses in Metallic Samples Produced byh Laser FreeForm Fabrication

Analysis of Residual Stress in Stress Harps of Grey Iron by Experiment and Simulation

Advanced Materials Research, Aug 1, 2014

Stress harps with bars of different size were used to study residual stresses due to different co... more Stress harps with bars of different size were used to study residual stresses due to different cooling rate during casting of a grey iron. Finite element (FE) simulations were performed to predict residual stresses from the casting process and the effect of a stress relieving heat treatment. Intended for validating the simulations, neutron diffraction (ND) and hole drilling methods were used to measure the residual stress distribution through the thickness and in a thin surface layer, respectively. Good agreement between the FE simulations and ND measurements is observed for the annealed harp and the normal and transverse directions of the as cast harp. Discrepancy occurs in the axial direction and especially in the side bars of the as cast harp for which the simulation shows much higher compressive residual stresses. The observed difference between the different techniques was discussed with respect to the characteristics of the different methods.

format_quoteValidation of numerical simulations showed strong agreement with neutron measurements in thinner stress harps, informing modeling accuracy.format_quote

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$Research paper thumbnail of <i>In-Situ</i> Neutron Diffraction Study of the Deformation Behaviour of Two High-Manganese Austenitic Steels$

<i>In-Situ</i> Neutron Diffraction Study of the Deformation Behaviour of Two High-Manganese Austenitic Steels

Materials Science Forum, Mar 1, 2011

In-situ neutron diffraction experiments under tensile loading were carried out to study the micro... more In-situ neutron diffraction experiments under tensile loading were carried out to study the micromechanical behaviour of two iron-manganese based steels, a TWIP (twinning induced plasticity) steel with 30 wt% Mn and a TRIP steel (transformation induced plasticity) with 20 wt% Mn. The former was loaded to 31.3% strain and the latter to 20% strain. The 30 wt.% Mn steel had a fully austenitic microstructure which remained stable over the loading range studied, while stress induced austenite to α´- and ε-martensite transformations occur in the 20 wt.% Mn steel which initially contained an α´-martensite in addition to the austenite. The evolution of lattice strains under tensile loading differs between the two steels, reflected their different plastic deformation mechanisms. A stronger grain-orientation dependent behaviour is observed during deformation for the 20 wt.% Mn in contrast to the 30wt.% Mn steel.

Effect of Cooling and Shot Peening on Residual Stresses and Fatigue Performance of Milled Inconel 718

Residual Stresses 2016, 2016

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licens... more

format_quoteTensile residual stresses were induced regardless of coolant use, with wet milling resulting in lower surface tension than dry milling.format_quote

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Influence of Cu and Zn Residues from Wire Electro‐Discharge Machining on TBC Durability

Thermal barrier coatings (TBC) are deposited on superalloys in gas turbines to prolong component ... more Thermal barrier coatings (TBC) are deposited on superalloys in gas turbines to prolong component life. Electro-discharge machining (EDM) provides a convenient way of machining superalloys prior to coating deposition. Wire-EDM may leave residues from the wire material on the cut surface. A study was performed to investigate if brass residues from the EDM wire affected TBC oxidation behavior and spallation life. Isothermal oxidation at 1050 °C and thermal cycling at 1100 °C were performed on plasma sprayed TBCs on two different EDM cut substrates. No decrease in TBC life was detected due to the brass residues on the cut surfaces.

Analysis on Microstructural Evolution of PtAl Diffusion Coating on Ni-based Superalloy Influenced by Creep Process

With superior oxidation and corrosion resistance, metallic coatings (i.e. diffusion coatings and ... more With superior oxidation and corrosion resistance, metallic coatings (i.e. diffusion coatings and MCrAlX coatings) are widely used to protect hot components made of superalloys in turbine engines. Two issues are critically important for the coating at high temperatures: thermal property related to oxidation/corrosion behavior and microstructure stability, and mechanical properties (e.g. creep and fatigue). The aim of this project is to develop better understanding of the thermal and mechanical behaviors of metallic coatings on superalloys and to improve the accuracy of prediction of their lifetime by thermodynamic modeling. The present work includes an investigation on the oxidation behavior of MCrAlX coating with a new lifetimeprediction model and a study on the influence of diffusion coatings on creep and fatigue behaviors of the superalloy IN792.Experiments on isothermal and thermal cycling oxidation were designed to investigate the oxidation behavior of a HVOF CoNiCrAlYSi coating...

081 Mode-II and Mode-III Effects of Cyclic Crack Propagation in Specimens

Crack propagation in real aircraft engine components calculated by the in-house software CRACKTRA... more Crack propagation in real aircraft engine components calculated by the in-house software CRACKTRACER3D frequently exhibits Mode-II and Mode-III effects, i.e. the cracks do not stay in-plane. Validation, however, is quite difficult since few experimental results are available. The present contribution uses mixed-mode experimental evidence from MTU and the literature in order to verify the predictions of CRACKTRACER3D. The tests include the propagation of a slanted crack in a 4-point bending specimen, a Compact Tension Shear Rotation Specimen under mode-III loading and a biaxial test of a square specimen with holes. It is shown that the results are on the conservative side due to the neglection of friction between the crack faces.

Hot corrosion behavior of HVOF-sprayed CoNiCrAlYSi coatings in a sulphate environment

Vacuum, 2015

HVOF-sprayed CoNiCrAlYSi coatings were tested at 900 °C in a hot corrosion environment containing... more HVOF-sprayed CoNiCrAlYSi coatings were tested at 900 °C in a hot corrosion environment containing sodium-potassium sulphates. The HVOF spraying caused the typical splat-on-splat structure. The results after the hot corrosion testing showed that the corrosion preferentially occurred at the coating surface and the splat boundaries. The oxidation along the splat boundaries can isolate the splat from the underlying coating matrix. In those isolated splats or coating parts, internal oxidation and nitridation of Al took place, following that the Al-depleted coating fragments were then oxidized to spinels. For those coatings which had a worse splat boundary quality (i.e. with higher porosity and intersplat oxides) or had a worse coating surface quality (i.e. with more small coating fragments therefore more interfaces), heavier corrosion attack was observed on those coatings due to the corrosion of the splats or the coating fragments. The results indicated that the as-sprayed coating quality including porosity and surface morphology was important for the hot-corrosion resistance of the coatings.

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Nano-scale characterization of white layer in broached Inconel 718

Materials Science and Engineering: A, 2017

The formation mechanism and properties of white layers created during broaching are not well inve... more The formation mechanism and properties of white layers created during broaching are not well investigated and understood to date. In the present study, multiple advanced characterization techniques with a nano-scale resolution, including transmission electron microscope (TEM), transmission Kikuchi diffraction (TKD), atom probe tomography (APT) as well as nanoindentation, 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. The TEM observations showed that the broached white layer consists of nano-sized grains, mostly in the range of 20 nm to 50 nm. The crystallographic texture detected by TKD further revealed that the refined microstructure is primarily attributed to strong shear deformation. Colocated 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 2 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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Influence of Dynamic Strain Ageing and Long Term Ageing on Deformation and Fracture Behaviors of Alloy 617

Materials Science Forum, 2016

Influences of dynamic strain ageing and long term ageing on deformation, damage and fracture beha... more Influences of dynamic strain ageing and long term ageing on deformation, damage and fracture behaviors of Alloy 617 material have been studied. Dynamic strain ageing can occur in this alloy at temperature from 400 to 700°C, which leads to a strain hardening and also an increase in fracture strain due to plastic deformation caused by twinning. Long term ageing at 700°C for up to 20 000 hours can cause different precipitation such as γ ́, M6C (Mo-rich) and M23C6 (Cr-rich) carbides. These carbides are both inter-and intra-granular particles. The long term ageing reduces the fracture toughness of the material, but the alloy can still have rather high impact toughness and fracture toughness even with an ageing at 700°C for 20 000 hour. The mechanisms have been studied using electron backscatter detection and electron channeling contrast imaging. It shows that besides dislocation slip, twinning is another main deformation mechanism in these aged Alloy 617 materials. At the crack front, pl...

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Analysis of Residual Stress in Stress Harps of Grey Iron by Experiment and Simulation

Advanced Materials Research, 2014

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Fatigue crack growth behaviour of Inconel 718 - The concept of a damaged zone caused by high temperature hold times

Fatigue crack growth testing of Inconel 718 has been carried out at the temperatures 550°C and 65... more Fatigue crack growth testing of Inconel 718 has been carried out at the temperatures 550°C and 650°C. The tests were conducted using a mix of hold times and pure cyclic loading, referred to as block tests. From the test results, the existence of an embrittled volume or damaged zone in the vicinity of the crack tip has been revealed. It has been found that the evolution of this damaged zone can be sufficiently well described using a power law function with an exponent n=0.25.

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