| 1. |
- Lundberg, Mattias, 1985-
(författare)
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Residual stresses, fatigue and deformation in cast iron
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The complex geometry of cylinder heads in heavy-duty diesel engines makes grey iron or compact graphite iron a preferred material choice due to its price, castability, thermal conductivity and damping capacity. Today’s strict emission laws have increased the demands on engine performance and engine efficiency. This means that material properties such as fatigue resistance need to be improved. Shot peening is often used to improve the fatigue resistance of components and the benefits of shot peening are associated with the induced compressive surface stresses and surface hardening. How different shot peening parameters can affect fatigue strength of grey and compact graphite iron has been investigated within the project underlying this thesis. To do this, X-ray diffraction (XRD) was utilized for residual stress measurements, scanning electron microscopy (SEM) for microstructural characterizations and mechanical fatigue testing for mechanical quantifications. The ultimate aim of this work has been to increase the fatigue resistance of cast iron by residual stress optimization.XRD measurements and SEM examinations revealed that the shot peening parameters shot size and peening intensity significantly influence residual stresses and surface deformation. Residual stress profiles, similar to the one general considered to improve the fatigue strength in steels, were obtained for both grey and compact graphite iron. Uniaxial push-pull fatigue testing on grey iron with these shot peening parameters reduced the fatigue strength with 15–20 %. The negative effect is likely related to surface damage associated with over peening and relatively high subsurface tensile residual stresses. With very gentle shot peening parameters, the uniaxial fatigue strength were unaltered from the base material but when subjected to bending fatigue an increase in fatigue strength were observed. An alternative way to increase the fatigue strength was to conduct a 30 min annealing heat treatment at 285 XC which increased the fatigue strength by almost 10 % in uniaxial loading. The improvement could be an effect of favourable precipitates forming during the annealing, which could hinder dislocation movement during fatigue.Measuring residual stresses using XRD and the sin2 -method demands accurate X-ray elastic constants (XEC) for meticulous stress analysis. The XEC referred to as 1~2s2 should therefore always be calibrated for the specific material used. The experiments conducted revealed that the XEC value is independent of the testing method used in this work. A small correction from the theoretical value should be applied when the material contains small amounts of residual stresses. The amount of residual stresses has a great impact on the XEC and thus on the stress analysis. Concluding that proper analysis of residual stresses in cast iron is not straight forward.
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| 2. |
- Azeez, Ahmed, et al.
(författare)
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Characterisation of Deformation and Damage in a Steam Turbine Steel Subjected to Low Cycle Fatigue
- 2019
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Ingår i: Structural Integrity Procedia. - : Elsevier BV. ; , s. 155-160
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Konferensbidrag (refereegranskat)abstract
- The increased use of renewable energy pushes steam turbines toward a more frequent operation schedule. Consequently, components must endure more severe fatigue loads which, in turn, requires an understanding of the deformation and damage mechanisms under high-temperature cyclic loading. Based on this, low cycle fatigue tests were performed on a creep resistant steel, FB2, used in ultra-supercritical steam turbines. The fatigue tests were performed in strain control with 0.8-1.2 % strain range and at temperatures of 400 °C and 600 °C. The tests at 600 °C were run with and without dwell time. The deformation mechanisms at different temperatures and strain ranges were characterised by scanning electron microscopy and by quantifying the amount of low angle grain boundaries. The quantification of low angle grain boundaries was done by electron backscatter diffraction. Microscopy revealed that specimens subjected to 600 °C showed signs of creep damage, in the form of voids close to fracture surface, regardless of whether the specimen had been exposed to dwell time or been purely cycled. In addition, the amount of low angle grain boundaries was lower at 600 °C than at 400 °C. The study indicates that a significant amount of the inelastic strain comes from creep strain as opposed to being all plastic strain.
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| 3. |
- Azeez, Ahmed, et al.
(författare)
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Low cycle fatigue life modelling using finite element strain range partitioning for a steam turbine rotor steel
- 2020
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Ingår i: Theoretical and applied fracture mechanics (Print). - : Elsevier. - 0167-8442 .- 1872-7638. ; 107
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Tidskriftsartikel (refereegranskat)abstract
- Materials made for modern steam power plants are required to withstand high temperatures and flexible operational schedule. Mainly to achieve high efficiency and longer components life. Nevertheless, materials under such conditions experience crack initiations and propagations. Thus, life prediction must be made using accurate fatigue models to allow flexible operation. In this study, fully reversed isothermal low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done under strain control with different total strain ranges and temperatures (20 °C to 625 °C). Some tests included dwell time to calibrate the short-time creep behaviour of the material. Different fatigue life models were evaluated based on total life approach. The stress-based fatigue life model was found unusable at 600 °C, while the strain-based models in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life prediction, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges at 500 °C. This allows for the formulation of life curves based on either plasticity-dominated damage or creep-dominated damage. At 600 °C, creep dominated while at 500 °C creep only dominates for higher strain ranges. The deformation mechanisms at different temperatures and total strain ranges were characterised by scanning electron microscopy and by quantifying the amount of low angle grain boundaries. The quantification of low angle grain boundaries was done by electron backscatter diffraction. Microscopy revealed that specimens subjected to 600 °C showed signs of creep damage in the form of voids close to the fracture surface. In addition, the amount of low angle grain boundaries seems to decrease with the increase in temperature even though the inelastic strain amplitude was increased. The study indicates that a significant amount of the inelastic strain comes from creep strain as opposed of being all plastic strain, which need to be taken into consideration when constructing a life prediction model.
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| 4. |
- Azeez, Ahmed, et al.
(författare)
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Low Cycle Fatigue Modelling of Steam Turbine Rotor Steel
- 2019
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Ingår i: 9th International Conference Materials Structure & Micromechanics of Fracture (MSMF9). - : Elsevier. ; , s. 149-154, s. 149-154
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Konferensbidrag (refereegranskat)abstract
- Materials in steam turbine rotors are subjected to cyclic loads at high temperature, causing cracks to initiate and grow. To allow for more flexible operation, accurate fatigue models for life prediction must not be overly conservative. In this study, fully reversed low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done isothermally, within temperature range of room temperature to 600 °C, under strain control with 0.8-1.2 % total strain range. Some tests included hold time to calibrate the short-time creep behaviour of the material. Different fatigue life models were constructed. The life curve in terms of stress amplitude was found unusable at 600 °C, while the life curve in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life model, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges. This allowed the formulation of life curves based on plasticity or creep domination, which showed creep domination at 600 °C, while at 500 °C, creep only dominates for higher strain range.
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| 5. |
- Belgacem, Sirine Ben, et al.
(författare)
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Enhancing thermal energy storage properties of blend phase change materials using beeswax
- 2024
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Ingår i: Environmental Science and Pollution Research. - : Springer Science and Business Media LLC. - 0944-1344 .- 1614-7499.
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Tidskriftsartikel (refereegranskat)abstract
- This study aims to use beeswax, a readily available and cost-effective organic material, as a novel phase change material (PCM) within blends of low-density polyethylene (LDPE) and styrene-b-(ethylene-co-butylene)-b-styrene (SEBS). LDPE and SEBS act as support materials to prevent beeswax leakage. The physicochemical properties of new blended phase change materials (B-PCM) were determined using an X-ray diffractometer and an infrared spectrometer, confirming the absence of a chemical reaction within the materials. A scanning electron microscope was used for microstructural analysis, indicating that the interconnection of the structure allowed better thermal conductivity. Thermal gravimetric analysis revealed enhanced thermal stability for the B-PCM when combined with SEBS, especially within its operating temperature range. Analysis of phase change temperature and latent heat with differential scanning calorimetry showed no major difference in the melting point of the various PCM blends created. During the melting/solidification process, the B-PCMs possess excellent performance as characterized by W70/P30 (112.45 J.g−1) > W70/P20/S10 (94.28 J.g−1) > W70/P10/S20 (96.21 J.g−1) of latent heat storage. Additionally, the blends tend to reduce supercooling compared to pure beeswax. During heating and cooling cycles, the B-PCM exhibited minimal leakage and degradation, especially in blends containing SEBS. In comparison to the rapid temperature drop observed during the cooling process of W70/P30, the temperature decline of W70/P30 was slower and longer, as demonstrated by infrared thermography. The addition of LDPE to the PCM reduced melting time, indicating an improvement in the thermal energy storage reaction time to the demand. According to the obtained findings, increasing the SEBS concentration in the composite increased the thermal stability of the resulting PCM blends significantly. Despite the challenges mentioned earlier, SEBS proved to be an effective encapsulating material for beeswax, whereas LDPE served well as a supporting material. Leak tests were performed to find the ideal mass ratio, and weight loss was analyzed after multiple cycles of cooling and heating at 70 °C. The morphology, thermal characteristics, and chemical composition of the beeswax/LDPE/SEBS composite were all examined. Beeswax proves to be a highly effective phase change material for storing thermal energy within LDPE/SEBS blends.
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| 6. |
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| 7. |
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| 8. |
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| 9. |
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| 10. |
- 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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| 11. |
- Calmunger, Mattias, 1986-
(författare)
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Temperaturpåverkan på egenskaperna hos högtemperaturtåliga austenitiska rostfria stål KME 701
- 2018
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Rapport (refereegranskat)abstract
- Den globala ökningen av energianvändning och sammanhängande ökning i CO2-utsläpp vid förbränning har skärpt kraven på energileverantörer att i större utsträckning använda hållbara biobränslen samt att höja verkningsgraden på energiomvandlingsprocesserna. Detta kan uppnås genom att höja tryck och temperatur i biomasseldade förbränningspannor. Sådana omställningar leder oftast till nya utmaningar kopplade till materialegenskaper. I framtiden kommer behovet av reglerkraft att öka för att kompensera för väderbaserade energianläggningar, såsom sol- och vindkraft. Detta leder till att anläggningarna måste stoppas och startas betydlig oftare än nu. Det skapar ett behov av provningsmetoder som tar hänsyn till cykliska mekaniska och temperaturbaserade laster. Tillsammans med att framtidens material måste tåla högre temperaturer och tuffare miljöer, relaterat till bränsleflexibiliteten, innebär detta att befintliga austenitiska rostfria stål måste förbättras. Inte bara genom en ökning av andelen nickel och andra verksamma legeringselement utan även genom att generera ny kunskap om hur de mekaniska egenskaperna påverkas av den tuffare högtemperatursmiljön.Syftet med detta projekt var att utvärdera mekaniska beteenden relaterade till kombinerad cyklisk och statisk belastning, långtidsåldring samt cyklisk mekanisk och temperaturbelastning vid höga temperaturer. Detta uppnåddes genom att:Utvärdera kryp-utmattningsinteraktion beteendet hos pannmaterial.Utvärdera den strukturella stabiliteten hos de austenitiska rostfria stålen efter långtidsåldring vid hög temperatur.Utvärdera termomekaniska utmattningsegenskaper hos pannmaterial.Utvärdera spänningsrelaxation sprickningsbeteenden hos pannmaterial.Mekanisk provning enligt ovan har utförts och analyserats vid Linköpings universitet samt Sandvik Materials Technology för att få en ökad förståelse för hur mekaniska egenskaper påverkas av den tuffare högtemperatursmiljön som framtidens biomasseldade pannor utgör. Detta kan användas i materialutveckling samt vidare för att förbättra konstruktionen av framtidens biomasseldade pannor.Resultaten visade att:De undersökta pannmaterialen uppvisar kryp-utmattningsinteraktion skador och längre cykliskt liv är relaterat till högt krypmotstånd.Austenitiska rostfria stål uppvisar försprödning på grund av intermetalliska utskiljningar efter långtidsåldring vid höga temperaturer.De austenitiska rostfria stålen med högst högtemperaturshållfasthet uppvisade bäst termomekaniska utmattningsegenskaper.Mer metodutveckling och undersökning krävs för att utvärdera spänningsrelaxation sprickningsbeteendet hos pannmaterialen.
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| 12. |
- Calmunger, Mattias, 1986-, et al.
(författare)
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Thermomechanical Fatigue of Heat Resistant Austenitic Alloys
- 2023
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Ingår i: Procedia Structural Integrity. - : Elsevier. - 2452-3216. ; 43, s. 130-135
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Tidskriftsartikel (refereegranskat)abstract
- Rising global energy consumption and the increase in emissions of greenhouse gases (e.g. CO2) causing global warming, make need for more sustainable power generation. This could be accomplished by increasing the efficiency of the biomass-fired power plants, which is achieved by increasing the temperature and pressure. In addition, flexible generation of power is critical if only renewable power generation is to be achieved and this will increase the number of start-and stop cycles. Cyclic condition in a long-term high temperature environment is an operation process that such materials must withstand, in order to satisfy the needs for future power generation.Commonly austenitic stainless steel are used for critical components of power plants. Because of future change in operating conditions, further investigations are needed to verify that the demands on safety for cyclic long-term usage is fulfilled. This work includes investigation of two commercial austenitic steels: Esshete 1250 and Sanicro 25. The materials were exposed to thermomechanical fatigue (TMF) in strain control under In-Phase and Out-of-Phase conditions and main testing temperature ranges of 100-650°C and 100-800°C respectively. Some of the specimens were pre-aged to simulate prolonged service condition. Mechanical test data were obtained and analysed in order to define the TMF performance of the investigated alloys. The differences in performance were discussed in relation to mechanical and microstructural characterization.
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| 13. |
- Gebeyaw, Getiye Wodaje, et al.
(författare)
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Effect of spatial-temporal behavior of a newly developed cooling system on carbon and stainless steel bar properties
- 2023
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This report summaries the work within the project ”Effect of spatial-temporal behavior of a newly developed cooling system on carbon and stainless-steel bar properties”. The project was conducted from 2020-01-01 to 2022-12-31 and was co-produced by SSAB, Outokumpu and University of Gävle (UoG). The Knowledge Foundation, SSAB, Outokumpu and UoG financed the project.For the Swedish steel companies SSAB and Outokumpu producing special steels, it is very important to be able to control the cooling process in order to produce steel bars with excellent properties. Both steel companies also want to be able to control the cooling process so that the excellent steel properties become even over the bars’ spatial configuration.The aim of the present project is to reveal the spatial-temporal behavior of a newly developed cooling technology in order to produce steel bars with excellent properties and to control the phase transformation to achieve optimal performance of the steel bars.By using the special test rig at the UoG, detail temperature measurement mapping, invers solution and direct numerical simulation, the present project has identified and quantified several important aspects related to the quenching process, operating conditions, and temperature field development within the investigated products. The result from the proposed cooling process provides an outstanding cooling rate that is very crucial to obtain the required steel phase and thus the correct properties of the bar with different sizes. Results from this study have also shown that the cost per kg product can be reduced by tunning the process parameters such as soaking time and bar temperature before starting the cooling process.In addition, both experimental and numerical results of the material investigation show that the cooling technology has resulted in the desired phase transformation and subsequently the desired steel phases and material properties. The results show that the cooling technology and the control of the cooling parameters can be used to optimize the material properties of the bar materials.These good results and conclusions have been obtained via the deep collaboration between the SSAB, Outokumpu and UoG. The co-production, starting in the steering group planning the work along with the combination of research conducted at UoG and at the companies, have led to a successful project with great knowledge transfer in all direction during the duration of the project.
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| 14. |
- Lindström, Thomas, et al.
(författare)
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Fatigue Behaviour of an Additively Manufactured Ductile Gas Turbine Superalloy
- 2020
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Ingår i: Theoretical and applied fracture mechanics (Print). - : Elsevier. - 0167-8442 .- 1872-7638. ; :108
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Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing (AM) offers new possibilities in gas turbine technology by allowing for more complex geometries. However, the fatigue performance, including crack initiation and crack propagation of AM gas turbine material, is not fully known. In addition, AM materials shows anisotropic properties due to the columnar grain growth in the building direction during the AM process, which needs to be accounted for. Also, an AM component often solidifies with a cellular dendritic structure during the manufacturing process. In the present study, the bulk material of an AM adopted nickel-based superalloy based on Hastelloy X was subjected to low-cycle fatigue (LCF) loading at room temperature. The LCF tests were conducted in strain control on additively manufactured smooth bars,with two different build orientations (with an angle of 0° and 90° relative to the building platform). The LCF results showed that the major part of the fatigue life is spent in the crack initiation phase, namely 78% to 99% of the total fatigue life. Based on the experiments, a model to predict the crack initiation life was developed that takes the anisotropic material behaviour into account. The last part of the fatigue life, the crack propagation phase, was studied on a microstructural level, where initial fractography of the ruptured LCF specimens revealed that the dendritic structure was visible on the fracture surface. It was noted that the dendritic structure could easily be mistaken for regular striations although they represent a different fracture mechanism. The fracture surfaces were therefore cross sectioned and possible correlations between fracture surface characteristics and underlying microstructure were studied using electron backscatter diffraction and electron channelling contrast imaging. The outcome showed that the dendritic structure had some effect on the LCF crack propagation behaviour by interdendritic tearing, which was discussed.
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| 15. |
- Nordström, Joakim, 1971-
(författare)
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Deformation twinning in corrosion-resistant nickel alloys : with a rising nickel content
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Sanicro 28 and Alloy 625 are corrosion-resistant nickel alloys with a fully austenitic structure and a very low carbon content, which means they are both well suited for cold working. Since the millennium shift deformation twinning has been a live research issue as it enhances strength and ductility simultaneously. As nickel has been pointed out as a high stacking fault energy element and deformation twinning should be promoted by a low stacking fault energy level they have been considered as opposite poles. Nonetheless, it is known since long that deformation twins can emerge in high stacking fault face centred cubic elements at low temperatures.In this thesis, we have investigated deformation twinning behaviour in corrosion-resistant nickel alloys. The objective is trying to distinguish between deformation twinning in TWIP steel and corrosion resistant nickel alloys regarding for instance size and bundles.Interrupted uniaxial tensile tests have been performed at several cold working temperatures for the alloys: Sanicro 28 (31% nickel) and Alloy 625 (61% nickel). The microstructure has been characterized in homogeneous deformation volume, by scanning electron microscopy electron backscattering diffraction and electron channelling contrast imaging, transmission electron microscopy and X-ray diffraction. In one investigation fracture behaviour has also been studied with secondary electrons. Ab initio calculations, crystal plasticity modelling and DAMASK simulations have been performed to support emphasizing active deformation mechanisms.It has been revealed that deformation twinning can occur in high Ni alloys. With increasing deformation twinning levels, the diffuse necking decreases. Ab initio calculations indicates that the initiation of deformation twins cannot be determined solely by the stacking fault energy. Distinct features were discovered at low strains that could be rejected from being neither deformation twins nor stacking faults. Level of texture increases with increasing strain and decreasing temperature and the texture modes are changed with decreasing temperature.
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| 16. |
- Nordström, Joakim, 1971-, et al.
(författare)
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Temperature study of deformation twinning behaviour in Nickel-base Superalloy 625
- 2024
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Ingår i: Materials Science & Engineering. - 0921-5093 .- 1873-4936.
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Tidskriftsartikel (refereegranskat)abstract
- Deformation behaviour in the Nickel-base superalloy 625 has been studied by tensile testing at four temperatures: 295, 223, 173 and 77 K. The microstructure has been investigated using TEM, FIB-SEM, EBSD and ECCI techniques. Deformation in the alloy turns out to be a competitive course of events between at least two deformation mechanisms, namely dislocation slip and deformation twinning. Slip is the predominant deformation mechanism at higher temperatures. While at 77 K, deformation induced twinning gives an extra degree of freedom as one of the main deformation mechanisms, i.e., the material shows a twin induced plasticity, TWIP, behaviour. Ab initio calculations indicate that the influence of cryogenic/sub-zero temperatures on the stacking fault energy of this alloy can be limited and that the formation of deformation twins cannot be determined solely by the stacking fault energy. The results implies that it is the critical strain and strain hardening rate that influences the deformation twinning onset and twinning rate.
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| 17. |
- Nordström, Joakim, 1971-, et al.
(författare)
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Temperature study of deformation twinning behaviour in nickel-base Superalloy 625
- 2024
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 907
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Tidskriftsartikel (refereegranskat)abstract
- Deformation behaviour in the Nickel-base superalloy 625 has been studied by tensile testing at four temperatures: 295, 223, 173 and 77 K. The microstructure has been investigated using TEM, FIB-SEM, EBSD and ECCI techniques. Deformation in the alloy turns out to be a competitive course of events between at least two deformation mechanisms, namely dislocation slip and deformation twinning. Slip is the predominant deformation mechanism at higher temperatures. While at 77 K, deformation induced twinning gives an extra degree of freedom as one of the main deformation mechanisms, i.e., the material shows a twin induced plasticity, TWIP, behaviour. Ab initio calculations indicate that the influence of cryogenic/sub-zero temperatures on the stacking fault energy of this alloy can be limited and therefore the formation of deformation twins cannot be determined solely by the stacking fault energy. The results implies that critical strain and strain hardening rate influences the deformation twinning onset and twinning rate.
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| 18. |
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| 19. |
- Norman, Viktor, 1988-, et al.
(författare)
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An Accelerated Creep Assessment Method Based on Inelastic Strain Partitioning and Slow Strain Rate Testing
- 2021
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Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 205
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Tidskriftsartikel (refereegranskat)abstract
- A new accelerated creep assessment method to evaluate the creep performance of metals and alloys from high-temperature tensile tests, i.e. slow-strain-rate testing (SSRT), is proposed and evaluated. The method consists of decomposing the inelastic strain into a plastic and creep component by adopting general assumptions on the inelastic strain behaviour of materials, formulated using a state variable formalism and verified by tensile tests with intermediate dwell times at constant stress. Either, the plastic and creep strain components are considered non-interacting and additive, as observed in the stainless steel AISI 316L at 600 °C. Or, as in the case of the ductile cast iron EN-GJS-SiMo5-1 at 500 °C and the nickel-base superalloy Hastelloy X at 800 °C, the components are considered unified, meaning that the effect of inelastic straining is the same irrespective of whether it is caused through creep at constant stress or by plastic deformation due to an instantaneous stress increase. Based on these assumptions, the proposed method is used to assess the creep strain from SSRT in good agreement with conventional creep test results.
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| 20. |
- Romanov, Pavel
(författare)
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Hardening of Carbon Steel by Water Impinging Jet Quenching Technique : Differential Cooling of Steel Sheets and Quenching of Cylindrical Bars
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Austenitization followed by quenching is a well-known conventional heat-treating procedure which is widely used on carbon steels with the aim to obtain high strength in as-quenched condition. Such quenching is usually done by immersing a steel product into the cooling medium which provides a uniform cooling of the surface. The cooling rate can be adjusted to a certain degree on a “component” length-scale by using different cooling mediums such as water, oil, polymer solution, etc. However, certain steel products such as beams, pillars in automobile industry or different machinery parts in agriculture require a proper and controllable cooling gradient and thus mechanical property gradient within the product. It is difficult to control the cooling rates locally on the length-scale smaller than the product only by replacing the quenching medium. In addition, quenching by immersing the product into the cooling medium is accompanied by thermal stresses due to the different cooling rates of the surface and the core, and also accompanied by transformation stresses due to the volume change during phase transformations. These stresses may lead to negative effects such as undesired residual stresses or even cracks. Therefore, cooling must be properly optimized and controlled to eliminate these drawbacks. Such a controllable cooling can be performed by several impingements of the water jets onto a hot austenitized surface at certain locations. By controlling the water flow, number of jets, their locations and other parameters, the global and the local cooling rates can be optimized for a specific industrial application later on. This thesis demonstrates the potential and capability of the water Impinging Jet Quenching Technique (IJQT) to provide a flexible and controllable cooling for both differential and for uniform quenching cases. The test rig of IJQT was developed in the University of Gävle and was used to perform quenching experiments in this study: differential cooling of thick sheets and uniform quenching of bars to different depths. Differential cooling was performed on square-shaped carbon steel sheets with thickness of 15 mm, and the uniform quenching with different flow rates was performed on carbon steel cylindrical bars with 100 mm in diameter. Along with the physical experiments, Comsol Multiphysics 5.6 software was used to solve a 1D heat transfer problem to estimate the cooling rate profile along the radius of the bar. The experiments were verified by observations and characterization of the microstructure using light optical microscopy (LOM), and by examining the mechanical properties through tensile tests and hardness measurements. The results of the quenching experiments and verifications showed a high potential and flexibility of the IJQT in differential cooling case as well as in the uniform quenching case.
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| 21. |
- Romanov, Pavel, et al.
(författare)
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Hardening of Cylindrical Bars with Water Impinging Jet Quenching Technique
- 2024
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Ingår i: Steel Research International. - : Wiley. - 1611-3683 .- 1869-344X. ; 95:6
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Tidskriftsartikel (refereegranskat)abstract
- Hardening of carbon steel products by austenitization and immersion in a quenching medium is a widely used heat treatment to obtain a hard and strong martensitic structure. To avoid the undesired consequences, such as residual stresses or insufficient hardening depth, the cooling rates must be accurately measured and controlled. This can be achieved using the impinging water jet quenching technique. The aim of this work is to perform hardening of four low-alloyed 70 mm cylindrical carbon steel bars, using impinging water jet quenching technique with different jet flow rates, and to analyze its effect on thermal evolution and residual stresses. The temperature evolution during quenching experiments is recorded and used as input to a comprehensive quenching model to predict phase transformations, final hardness, and residual stresses of cylindrical bars. All four quenching experiments result in a fully hardened martensitic state. Furthermore, a decrease in jets’ flow rate, within a certain interval, results in different thermal histories and in lower compressive residual stresses on the surface. The results from quenching simulations show promising hardness, microstructure, and residual stress predictions that are validated by hardness measurements, optical microscopy, and residual stress analysis using X-Ray diffraction method.
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| 22. |
- Romanov, Pavel, et al.
(författare)
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Quenching of Carbon Steel Plates with Water Impinging Jets : Differential Properties and Fractography
- 2023
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Ingår i: Procedia Structural Integrity. - : Elsevier. - 2452-3216. ; 43, s. 154-159, s. 154-159
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Tidskriftsartikel (refereegranskat)abstract
- The demand for steel components with tailored properties is constantly growing. To obtain a specific variation of microstructures and mechanical properties along the component it must undergo a controllable cooling. One way to control the cooling rates along the component is by using different simultaneous water jet impingements on a hot austenitized surface. This can be done by a newly developed test rig for water Impinging Jet Quenching Technique (IJQT). This work discusses the effect of IJQT on mechanical properties and fracture behavior of 15 mm steel plates containing 0.27 and 0.38 mass-% carbon. The samples were cooled in a specifically designed setup of the technique to obtain simultaneous water and air cooling resulting in diverse microstructures. The mechanical property gradients of both steels were analyzed through hardness measurements and tensile tests. The fracture surfaces and the near fracture regions were observed using scanning electron microscope and light optical microscope respectively. The results from tensile tests showed that the larger part of the sample with higher carbon content was fully hardened, however smoothly transitioning to a more ductile region. The sample with lower carbon content combined various degrees of hardening and transitioned from higher to lower ultimate tensile strength values. Fracture behavior of higher carbon steel was predominantly brittle transitioning to a ductile, while the lower carbon steel had a small region showing brittle fracture transitioning to a larger region of predominant ductile fracture behavior.
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| 23. |
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| 24. |
- Wärner, Hugo, 1988-, et al.
(författare)
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Creep-Fatigue Interaction in Heat Resistant Austenitic Alloys
- 2018
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Ingår i: MATEC Web of Conferences 165 , 05001 (2018). - : EDP Sciences.
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Konferensbidrag (refereegranskat)abstract
- This work includes an investigation of two commercial austenitic steels: UNS S21500 (Esshete 1250) and UNS S31035 (Sandvik Sanicro (TM) 25). The materials were exposed to isothermal strain controlled fatigue with load controlled dwell time at maximum strain. The testing temperature used was 700 degrees C and the test cycles were performed in tension. Mechanical test data were obtained and analysed in order to define creep-fatigue damage diagrams at failure for the investigated austenitic alloys. During the given conditions, Sanicro 25 showed superior creep-fatigue life, suffered less amount of creep elongation for the same amount of strain amplitude and dwell times compared to Esshete 1250. Both alloys showed creep-fatigue interaction damage for specific test configurations.
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| 25. |
- Wärner, Hugo, 1988-
(författare)
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High-Temperature Fatigue Behaviour of Austenitic Stainless Steel : Influence of Ageing on Thermomechanical Fatigue and Creep-Fatigue Interaction
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The global energy consumption is increasing and together with global warming from greenhouse gas emission, create the need for more environmental friendly energy production processes. Higher efficiency of biomass power plants can be achieved by increasing temperature and pressure in the boiler section and this would increase the generation of electricity along with the reduction in emission of greenhouse gases e.g. CO2. The power generation must also be flexible to be able to follow the demands of the energy market, this results in a need for cyclic operating conditions with alternating output and multiple start-ups and shut-downs.Because of the demands of flexibility, higher temperature and higher pressure in the boiler section of future biomass power plants, the demands on improved mechanical properties of the materials of these components are also increased. Properties like creep strength, thermomechanical fatigue resistance and high temperature corrosion resistance are critical for materials used in the next generation biomass power plants. Austenitic stainless steels are known to possess such good high temperature properties and are relatively cheap compared to the nickel-base alloys, which are already operating at high temperature cyclic conditions in other applications. The behaviour of austenitic stainless steels during these widened operating conditions are not yet fully understood.The aim of this licentiate thesis is to increase the knowledge of the mechanical behaviour at high temperature cyclic conditions for austenitic stainless steels. This is done by the use of thermomechanical fatigue- and creepfatigue testing at elevated temperatures. For safety reasons, the effect of prolonged service degradation is investigated by pre-ageing before mechanical testing. Microscopy is used to investigate the microstructural development and resulting damage behaviour of the austenitic stainless steels after testing. The results show that creep-fatigue interaction damage, creep damage and oxidation assisted cracking are present at high temperature cyclic conditions. In addition, simulated service degradation resulted in a detrimental embrittling effect due to the deterioration by the microstructural evolution.
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| 26. |
- Wärner, Hugo, 1988-
(författare)
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High Temperature Fatigue Behaviour of Austenitic Stainless Steel : Microstructural Evolution during Dwell-Fatigue and Thermomechanical Fatigue
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The global energy consumption is increasing and together with global warming from greenhouse gas emission, a need for more environmentally friendly energy production processes is created. Higher efficiency of biomass power plants can be achieved by increasing temperature and pressure in the boiler section, this would increase the generation of electricity along with the reduction in emission of greenhouse gases e.g. CO2. The generation of power must also be flexible to be able to follow the demands of the energy market and this results in a need for cyclic operating conditions with alternating output and multiple start-ups and shut-downs.Because of the need for flexibility, higher temperature and higher pressure of future biomass power plants, the demands of improved mechanical properties of the materials used for the components are also increased. Properties like creep strength, maintained structural integrity, thermomechanical fatigue resistance and high temperature corrosion resistance are critical for materials used in the next generation biomass power plants. Highly alloyed austenitic stainless steels are known to possess such good high temperature properties and are relatively cheap compared to the nickel-base alloys, which are already used in high temperature cyclic conditions for other applications. The behaviour of austenitic stainless steels subjected to future biomass power plants operating conditions are not yet fully investigated.This thesis presents research that includes investigations of the mechanical and microstructural behaviour during high temperature cyclic conditions of austenitic stainless steels. This is done using thermomechanical fatigue testing, dwell-fatigue testing and impact toughness testing at elevated temperatures. Material service degradation as an effect of microstructural evolution is investigated by ageing of some test specimens before testing. Microscopy is used to investigate the connection between the mechanical behaviour and the microstructural deformation- and damage mechanisms of the austenitic stainless steels after testing.The results show that creep-fatigue interaction damage, creep damage and oxidation assisted cracking are present during high temperature cyclic conditions. In addition, ageing results in a less favourable microstructural configuration which negatively affects the resistance to high temperature damage mechanisms. An example of this is the lowering of impact toughness due to precipitation and coarsening of detrimental phases of some aged austenitic stainless steels. Moreover, TMF testing of aged austenitic stainless steels induce oxidation assisted cracking and an embrittling effect that cause significant cyclic life decrease. The creep-fatigue interaction behaviour during dwell-fatigue testing of two austenitic stainless steels generates various crack propagation characteristics. The higher alloyed material shows interchanging intra- and intergranular propagation with dynamic recrystallization, while the lower alloyed material shows propagation exclusively along the grain boundaries by the assistance of fatigue induced slip bands interaction with grain boundary precipitates.The research of this thesis provides a deeper understanding of the structural integrity, deformation mechanisms, damage mechanisms and fracture mechanisms during high temperature cyclic conditions of austenitic stainless steels. Long term, this is believed to contribute to development of suitable materials used as components of future biomass-fired power plants to achieve sustainable power generation.
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| 27. |
- Wärner, Hugo, 1988-, et al.
(författare)
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High Temperature Fatigue of Aged Heavy Section Austenitic Stainless Steels
- 2022
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Ingår i: Materials. - Basel, Switzerland : MDPI. - 1996-1944. ; 15:1
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Tidskriftsartikel (refereegranskat)abstract
- This work investigates two austenitic stainless steels, Sanicro 25 which is a candidate for high temperature heavy section components of future power plants and Esshete 1250 which is used as a reference material. The alloys were subjected to out-of-phase (OP) thermomechanical fatigue (TMF) testing under strain-control in the temperature range of 100 ∘C to 650 ∘C. Both unaged and aged (650 ∘C, 3000 h) TMF specimens were tested to simulate service degradation resulting from long-term usage. The scanning electron microscopy methods electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) were used to analyse and discuss active failure and deformation mechanisms. The Sanicro 25 results show that the aged specimens suffered increased plastic straining and shorter TMF-life compared to the unaged specimens. The difference in TMF-life of the two test conditions was attributed to an accelerated microstructural evolution that provided decreased the effectiveness for impeding dislocation motion. Ageing did not affect the OP-TMF life of the reference material, Esshete 1250. However, the structural stability and its resistance for cyclic deformation was greatly reduced due to coarsening and cracking of the strengthening niobium carbide precipitates. Sanicro 25 showed the higher structural stability during OP-TMF testing compare with the reference material.
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| 28. |
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| 29. |
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| 30. |
- Wärner, Hugo, 1988-, et al.
(författare)
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Microstructural Evolution During High Temperature Dwell-fatigue of Austenitic Stainless Steels
- 2021
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Ingår i: International Journal of Fatigue. - : Elsevier. - 0142-1123 .- 1879-3452. ; 143
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Tidskriftsartikel (refereegranskat)abstract
- Microstructural evolution related to the mechanical response from isothermal dwell-fatigue testing at 700 °C of two austenitic steels, Esshete 1250 and Sanicro 25, is reported. Coherent Cu-precipitates and incoherent Nb-carbides were found to impede dislocation motion, increase hardening and improving the high temperature properties of Sanicro 25. Sparsely placed intergranular Cr- and Nb-carbides made Esshete 1250 susceptible to creep damage and intergranular crack propagation, mainly from interaction of the carbides and fatigue induced slip bands. Dynamic recrystallization of the plastic zone at the crack tip appeared to affect crack propagation of Sanicro 25 by providing an energetically privileged path.
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| 31. |
- Wärner, Hugo, 1988-, et al.
(författare)
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Structural Integrity and Impact Toughness of Austenitic Stainless Steels
- 2019
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Ingår i: Proceedings of the 13th International Conference on the Mechanical Behaviour of Materials. - : International Congress on Mechanical Behavior of Materials. - 9781922016652 - 9781713805946 ; , s. 270-275
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Konferensbidrag (refereegranskat)
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| 32. |
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| 33. |
- Wärner, Hugo, 1988-, et al.
(författare)
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Thermomechanical Fatigue Behaviour of Aged Heat Resistant Austenitic Alloys
- 2019
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Ingår i: International Journal of Fatigue. - : Elsevier. - 0142-1123 .- 1879-3452. ; :127, s. 509-521
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Tidskriftsartikel (refereegranskat)abstract
- The increasing demands for efficiency and flexibility result in more severe operating conditions for the materials used in critical components of biomass power plants. These operating conditions involve higher temperature ranges, more pronounced environmental effects and cyclic operations. Austenitic stainless steels have shown to possess promising high temperature properties which makes them suitable as candidates for critical components in biomass power plant. However, their behaviour under such conditions is not yet fully understood. This work investigates three commercial austenitic alloys: Esshete 1250, Sanicro 25 and Sanicro 31HT. The alloys were subjected to in-phase (IP) thermomechanical fatigue (TMF) testing under strain-control in the temperature range of 100–800 °C. Both virgin and pre-aged TMF specimens were tested in order to simulate service degradation resulting from long-term usage. The results show that the pre-aged specimens suffered shorter TMF-life compared to the virgin specimens. The scanning electron microscopy methods electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) were used to analyse and discuss active failure and deformation mechanisms. The difference in TMF-life produced by the two testing conditions was attributed to an embrittling effect by precipitation, reduced creep properties and oxidation assisted cracking.
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| 34. |
- Yu, Cheng-Han, 1992-, et al.
(författare)
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Anisotropic Deformation and Fracture Mechanisms of an Additively Manufactured Ni-Based Superalloy
- 2020
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Ingår i: Superalloys 2020. - Cham : Springer International Publishing. ; , s. 1003-1013
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Konferensbidrag (refereegranskat)abstract
- This study investigates the anisotropic mechanical and microstructural behavior of the laser powder bed fusionLaser powder bed fusion (LPBF) manufactured Ni-based superalloy Hastelloy X (HX) by using slow strain rate (10−5 and 10−6s−1) tensile testing (SSRT) at 700 °C. LPBF HX typically exhibits an elongated grain structure along the building direction (BD) and the texture analysis from the combination of neutron diffractionNeutron diffraction and EBSD discloses a major texture component <011> and a minor texture component <001> along BD, and a texture component <001> in the other two sample directions perpendicular to BD. Two types of tests have been performed, the horizontal tests where the loading direction (LD) is applied perpendicular to BD, and the vertical tests where LD is applied parallel to BD. The vertical tests exhibit lower strength but better ductility, which is explained by the texture effect and the elongated grain structure. A comparison of the mechanical behavior to the wrought HX shows that LPBF HX has better yield strength due to the high dislocation density as proved by TEM images. Creep voids are observed at grain boundaries in SSRT for both directions and are responsible for the poor ductility of the horizontal tests. The vertical ductility in SSRT maintains the same level as the reference tensile test at the strain rate of 10−3s−1, due to the extra deformation capacity contributed by the discovered deformation twinningDeformation twinning and lattice rotation. The deformation twinningDeformation twinning, which is only observed in the vertical tests and has not been found in the conventionally manufactured HX, is beneficial to maintain the ductility but does not strengthen the material.
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| 35. |
- Yu, Cheng-Han, 1992-, et al.
(författare)
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Thin-wall Effects and Anisotropic Deformation Mechanisms of an Additively Manufactured Ni-based Superalloy
- 2020
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Ingår i: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 36
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Tidskriftsartikel (refereegranskat)abstract
- Laser powder bed fusion (LPBF) of Ni-based superalloys shows great potential for high temperature applications, for example, as a burner repair application for gas turbines where the thin-walled structure is important. It motivates this work to investigate the evolution of microstructure and the anisotropic mechanical behavior when plate-like specimens are built with a thickness from 4 mm down to 1 mm. By performing texture analysis using neutron diffraction, a clear transition in fiber texture from <011> to <001> is indicated when the specimen becomes thinner. The residual stress shows no thickness dependence, and at the subsurface the residual stress reaches the same level as the yield strength. Due to the rough as-built surface, a roughness compensation method for mechanical properties of thin-walled structures is outlined and demonstrated. Tensile tests from room temperature up to 700 °C have been carried out. Anisotropic mechanical behavior is found at all temperatures, which is strongly related to the anisotropic texture evolution. Stronger texture evolution and grain rotations are discovered when the tensile loading is applied along the building direction. The mechanical behavior has been compared to a wrought material, where the high dislocation density and the subgrain structure of the LPBF material result in a higher yield strength. Combining the statistical texture analysis by neutron diffraction with mechanical testing, EBSD grain orientation mapping and the investigation of dislocation structures using transmission electron microscopy, this work illustrates the significance of texture for the thin-wall effect and anisotropic mechanical behavior of LPBF materials.
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