| 1. |
- Bi, Yanyan, et al.
(författare)
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Investigation of cold-forming properties of Sanicro 25-a potential candidate for superheater and reheaters in high efficiency A-USC fossil power plants
- 2017
-
Ingår i: PROCEEDINGS OF THE ASME POWER CONFERENCE JOINT WITH ICOPE-17, 2017, VOL 1. - : AMER SOC MECHANICAL ENGINEERS. - 9780791857601
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Konferensbidrag (refereegranskat)abstract
- Sanicro 25 material is approved for use in pressure vessels and boilers according AMSE code case 2752, 2753 and VdTuN blatt 555. It shows good resistance to steam oxidation and flue gas corrosion, and has higher creep rupture strength than any other austenitic stainless steels available today. It is a candidate material for superheater and reheaters, enabling higher steam parameters of up to about 650 degrees C steam (ie about max 700 degrees C metal) without the need for expensive nickel based alloys. The effect of cold-forming on time and temperature-dependent deformation and strength behavior has been examined in a comprehensive study. The objective was to determine the maximum allowable degree of cold-forming to be used without additional heat treatment. The findings of these investigations indicate that the maximum allowed cold deformation could be possible to increase from todays maximum 20 % (VdTuV 555), 15 % (540-675 degrees C) and 10 % (higher than 675 degrees C) respectively (ASME 2011a Sect I PG19). A solution annealing after the cold bending will recover creep ductility but will also at the same time increase manufacturing costs. Higher allowed degree of cold-forming without the need for post bend heat treatments, would allow for more narrow bending radii and thereby a more compact construction that would result in a significant decrease in production costs. This paper presents the findings in the mentioned study and is to be a background for possible coming discussions with involved entities on a revision of the max allowed deformation of this material without the need for solution annealing.
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| 2. |
- Calmunger, Mattias, et al.
(författare)
-
Advanced Microstructure Studies of an Austenitic Material Using EBSD in Elevated Temperature In-Situ Tensile Testing in SEM
- 2014
-
Konferensbidrag (refereegranskat)abstract
- In this study an advanced method for investigation of the microstructure such as electron backscatter diffraction (EBSD) together with in-situ tensile test in a scanning electron microscope (SEM) has been used at room temperature and 300°C. EBSD analyses provide information about crystallographic orientation in the microstructure and dislocation structures caused by deformation. The in-situ tensile tests enabled the same area to be investigated at different strain levels. For the same macroscopic strain values a lower average misorientation in individual grains at elevated temperature indicates that less residual strain at grain level are developed compared to room temperature. For both temperatures, while large scatters in grain average misorientation are observed for grains of similar size, there seems to be a tendency showing that larger grains may accumulate somewhat more strains.
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| 3. |
- Calmunger, Mattias, et al.
(författare)
-
Characterisation of creep deformation during slow strain rate tensile testing
- 2015
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The strain-rate dependent deformation of the superalloy Haynes 282 during slow strain-rate tensile testing (SSRT) at 700 C has been investigated. The stress-strain response is remarkably well described by a simple constitutive model over a wide range of different strain-rates. The microstructure development is characterised and related to the influence of both strainrate dependent and independent deformation. Damage and cracking similar to what has been observed previously during conventional creep testing of Haynes 282 was found and explained. The model and the microstructure investigations show that the deformation and damage mechanisms during SSRT are essentially the same as under creep.
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| 4. |
- Calmunger, Mattias, et al.
(författare)
-
Characterization of austenitic stainless steels deformed at elevated temperature
- 2017
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer-Verlag New York. - 1073-5623 .- 1543-1940. ; 48A:10, s. 4525-4538
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Tidskriftsartikel (refereegranskat)abstract
- Highly alloyed austenitic stainless steels are promising candidates to replace more expansive nickel-based alloys within the energy-producing industry. The present study investigates the deformation mechanisms by microstructural characterisation, 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 400 C up to 700 C. The materials showed different influence of temperature on ductility, where the ductility at elevated temperatures increased with increasing nickel and solid solution hardening element content. The investigated materials showed planar dislocation driven deformation at elevated temperature. Scanning electron microscopy showed that deformation twins were an active deformation mechanism in austenitic stainless steels during tensile deformation at elevated temperatures up to 700 C.
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| 5. |
- Calmunger, Mattias, et al.
(författare)
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Creep and Fatigue Interaction Behavior in Sanicro 25 Heat Resistant Austenitic Stainless Steel
- 2016
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Ingår i: Transactions of the Indian Institute of Metals. - : Springer. - 0972-2815 .- 0975-1645. ; 69:2, s. 337-342
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Tidskriftsartikel (refereegranskat)abstract
- Sanicro 25 is a newly developed advanced high strength heat resistant austenitic stainless steel. The material shows good resistance to steam oxidation and flue gas corrosion, and has higher creep rupture strength than other austenitic stainless steels available today. It is thus an excellent candidate for superheaters and reheaters for advanced ultra-super critical power plants with efficiency higher than 50 %. This paper provides a study on the creep–fatigue interaction behavior of Sanicro 25 at 700 °C. Two strain ranges, 1 and 2 %, and two dwell times, 10 and 30 min, were used. The influences of dwell time on the cyclic deformation behavior and life has been evaluated. Due to stress relaxation the dwell time causes a larger plastic strain range compared to the tests without dwell time. The results also show that the dwell time leads to a shorter fatigue life for the lower strain range, but has no or small effect on the life for the higher strain range. Fracture investigations show that dwell times result in more intergranular cracking. With the use of the electron channeling contrast imaging technique, the influences of dwell time on the cyclic plastic deformation, precipitation behavior, recovery phenomena and local plasticity exhaustion have also been studied.
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| 6. |
- Calmunger, Mattias, et al.
(författare)
-
Damage and Fracture Behaviours in Advanced Heat Resistant Materials During Slow Strain Rate Test at High Temperature
- 2013
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- As a renewable energy resource, biomass or biomass co-firing in coal-fired power plants with high efficiency are desired which corresponding to elevated temperature and high pressure. An upgrade of the material performance to austenitic stainless steels is therefore required in order to meet the increased demands due to the higher temperature and the more corrosive environment. These materials suffer from creep and fatigue damage during the service. In this study, these behaviours are evaluated using slow strain rate testing (SSRT) with strain rate down to 1*10-6/s at temperature up to 700°C. The influence of temperature and strain rate on strength and ductility in one austenitic stainless steel and one nickel base alloys are investigated. The damage and fracture due to the interaction between moving dislocations and precipitates are studied using electron channelling contrast imaging (ECCI) and electron backscattering diffraction (EBSD). The deformation and damage mechanisms active during SSRT are essentially the same as under creep. The influence of dynamic strain ageing (DSA) phenomena that appears in the tested temperature and strain rate regime is also discussed, DSA is intensified by increased temperature and decreased strain rate.
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| 7. |
- Calmunger, Mattias, et al.
(författare)
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Damage and Fracture Behaviours in Aged Austentic Materials During High-Temperature Slow Strain Rate Testing
- 2014
-
Konferensbidrag (refereegranskat)abstract
- Biomass power plants with high efficiency are desired as a renewable energy resource. High efficiency can be obtained by increasing temperature and pressure. An upgrade of the material performance to high temperature material is therefore required in order to meet the increased demands due to the higher temperature and the more corrosive environment. In this study, the material’s high-temperature behaviours of AISI 304 and Alloy617 under slow deformation rate are evaluated using high-temperature long-term aged specimens subjected to slow strain rate tensile testing (SSRT) with strain rates down to 10-6/s at 700°C. Both materials show decreasing stress levels and elongation to fracture when tensile deformed using low strain rate and elevated temperature. At high-temperature and low strain rates cracking in grain boundaries due to larger precipitates formed during deformation is the most common fracture mechanism.
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| 8. |
- Calmunger, Mattias, et al.
(författare)
-
Deformation and damage behaviours of austenitic alloys in the dynamic strain ageing regime
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Deformation and damage behaviours influenced by dynamic strain ageing (DSA) in three austenitic stainless steels and two nickel-base alloys have been investigated using tensile tests at elevated temperatures. The deformation and damage behaviours have been analysed using electron channeling contrast imaging and electron backscatter diffraction. The results from this study show that DSA not always reduce ductility, in fact for some materials the ductility can increase in the DSA regime. This is attributed to the formation of nano twins by DSA stimulated twinning induced plasticity. Damage mechanisms due to DSA were also investigated and discussed.
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| 9. |
- Calmunger, Mattias, et al.
(författare)
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Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature
- 2014
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Ingår i: Theoretical and Applied Mechanics Letters. - : American Institute of Physics (AIP). - 2095-0349. ; 4:041004
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Tidskriftsartikel (refereegranskat)abstract
- In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel (AISI 316L) and one nickel-base alloy (Alloy 617) have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromechanisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650°C.
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| 10. |
- Calmunger, Mattias
(författare)
-
High-Temperature Behaviour of Austenitic Alloys : Influence of Temperature and Strain Rate on Mechanical Properties and Microstructural Development
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The global increase in energy consumption and the global warming from greenhouse gas emission creates the need for more environmental friendly energy production processes. Biomass power plants with higher efficiency could generate more energy but also reduce the emission of greenhouse gases, e.g. CO2. Biomass is the largest global contributor to renewable energy and offers no net contribution of CO2 to the atmosphere. One way to increase the efficiency of the power plants is to increase temperature and pressure in the boiler parts of the power plant.The materials used for the future biomass power plants, with higher temperature and pressure, require improved properties, such as higher yield strength, creep strength and high-temperature corrosion resistance. Austenitic stainless steels and nickel-base alloys have shown good mechanical and chemical properties at the operation temperatures of today’s biomass power plants. However, the performance of austenitic stainless steels at the future elevated temperatures is not fully understood.The aim of this licentiate thesis is to increase our knowledge about the mechanical performance of austenitic stainless steels at the demanding conditions of the new generation power plants. This is done by using slow strain rate tensile deformation at elevated temperature and long term hightemperature ageing together with impact toughness testing. Microscopy is used to investigate deformation, damage and fracture behaviours during slow deformation and the long term influence of temperature on toughness in the microstructure of these austenitic alloys. Results show that the main deformation mechanisms are planar dislocation deformations, such as planar slip and slip bands. Intergranular fracture may occur due to precipitation in grain boundaries both in tensile deformed and impact toughness tested alloys. The shape and amount of σ-phase precipitates have been found to strongly influence the fracture behaviour of some of the austenitic stainless steels. In addition, ductility is affected differently by temperature depending on alloy tested and dynamic strain ageing may not always lead to a lower ductility.
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| 11. |
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| 12. |
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| 13. |
- Calmunger, Mattias, et al.
(författare)
-
Influence of deformation rate on mechanical response of an AISI 316L austenitic stainless steel
- 2014
-
Ingår i: Advanced Materials Research. - : Trans Tech Publications Inc.. - 1022-6680 .- 1662-8985. ; 922, s. 49-54
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Tidskriftsartikel (refereegranskat)abstract
- Austenitic stainless steels are often used for components in demanding environment. These materials can withstand elevated temperatures and corrosive atmosphere like in energy producing power plants. They can be plastically deformed at slow strain rates and high alternating or constant tensile loads such as fatigue and creep at elevated temperatures. This study investigates how deformation rates influence mechanical properties of an austenitic stainless steel. The investigation includes tensile testing using strain rates of 2*10-3/ and 10-6/s at elevated temperatures up to 700°C. The material used in this study is AISI 316L. When the temperature is increasing the strength decreases. At a slow strain rate and elevated temperature the stress level decreases gradually with increasing plastic deformation probably due to dynamic recovery and dynamic recrystallization. However, with increasing strain rate elongation to failure is decreasing. AISI 316L show larger elongation to failure when using a strain rate of 10-6/s compared with 2*10-3/s at each temperature. Electron channelling contrast imaging is used to characterize the microstructure and discuss features in the microstructure related to changes in mechanical properties. Dynamic recrystallization has been observed and is related to damage and cavity initiation and propagation.
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| 14. |
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| 15. |
- Calmunger, Mattias, et al.
(författare)
-
Influence of High Temperature Ageing on the Toughness of Advanced Heat Resistant Materials
- 2013
-
Konferensbidrag (refereegranskat)abstract
- Advanced biomass, biomass co-firing in coal-fired and future advanced USC coal-fired power plants with high efficiency require the materials to be used at even higher temperature under higher pressure. The reliability and integrity of the material used are therefore of concern. In this study, the influence of ageing at temperatures up to 700°C for up to 3 000 hours on the toughness of two advanced heat resistant austenitic steels and one nickel alloy are investigated. The influence on toughness due to differences in the chemical composition as well as the combined effect of precipitation and growth of the precipitates has been analysed by using SEM techniques. The fracture mechanisms that are active for the different ageing treatments are identified as a function of temperature and time. Local approach methods are used to discuss the influence of the precipitation and growth of precipitates on the toughness or fracture in the different aged materials.
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| 16. |
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| 17. |
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| 18. |
- Calmunger, Mattias, et al.
(författare)
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Mechanical Behaviours of Alloy 617 with Varied Strain Rate at High Temperatures
- 2014
-
Ingår i: Materials Science Forum. - : Trans Tech Publications Ltd. - 0255-5476 .- 1662-9752. ; 783-786, s. 1182-1187, s. 1182-1187
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Tidskriftsartikel (refereegranskat)abstract
- Nickel-base alloys due to their high performances have been widely used in biomass and coal fired power plants. They can undertake plastic deformation with different strain rates such as those typically seen during creep and fatigue at elevated temperatures. In this study, the mechanical behaviours of Alloy 617 with strain rates from 10-2/s down to 10-6/s at temperatures of 650°C and 700°C have been studied using tensile tests. Furthermore, the microstructures have been investigated using electron backscatter detection and electron channeling contrast imaging. At relatively high strain rate, the alloy shows higher fracture strains at these temperatures. The microstructure investigation shows that it is caused by twinning induced plasticity due to DSA. The fracture strain reaches the highest value at a strain rate of 10-4/s and then it decreases dramatically. At strain rate of 10-6/s, the fracture strain at high temperature is now smaller than that at room temperature, and the strength also decreases with further decreasing strain rate. Dynamic recrystallization can also be observed usually combined with crack initiation and propagation. This is a new type of observation and the mechanisms involved are discussed.
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| 19. |
- 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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| 20. |
- Calmunger, Mattias, et al.
(författare)
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Surface Phase Transformation in Austenitic Stainless Steel Induced by Cyclic Oxidation in Humidified Air
- 2015
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Ingår i: Corrosion Science. - : Pergamon Press. - 0010-938X .- 1879-0496. ; 100, s. 524-534
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Tidskriftsartikel (refereegranskat)abstract
- The formation of α’ martensite at the surface of an AISI 304 stainless steel subjected to cyclic heating in humidified air is reported. The α’ martensite formed during the cooling part of the cyclic tests due to local depletion of Cr and Mn and transformed back to austenite when the temperature again rose to 650 °C. The size of the α’ martensite region increased with increasing number of cycles. Thermodynamical simulations were used as basis for discussing the formation of α’ martensite. The effect of the α’ martensite on corrosion is also discussed.
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| 21. |
- 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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| 22. |
- Chai, Guocai
(författare)
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A study on fatigue damage and crack initiation in austenitic steel matrix during very high cycle fatigue
- 2024
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Ingår i: International Journal of Fatigue. - : ELSEVIER SCI LTD. - 0142-1123 .- 1879-3452. ; 179
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Tidskriftsartikel (refereegranskat)abstract
- Fatigue damage and crack initiation behavior has been studied in an austenitic stainless steel using a novel progressive stepwise load increasing method with a cycle step greater than 108 cycles and investigated with a FIB-SEM. Subsurface crack origin with a fine granular area in the matrix was formed. Heterogeneous plastic deformation causes strain localization and grain fragmentation. Localized plasticity exhaustion induces crack initiation. A crack origin is completed once a short crack reaches the stress intensity factor threshold for stage II crack propagation. This study provides a fundamental discussion how damage and crack initiation in matrix occur during very high cycle fatigue.
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| 23. |
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| 24. |
- Chai, Guocai
(författare)
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Analysis of microdamage in a nickel-base alloy during very high cycle fatigue
- 2016
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Ingår i: Fatigue & Fracture of Engineering Materials & Structures. - : WILEY-BLACKWELL. - 8756-758X .- 1460-2695. ; 39:6, s. 712-721
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Tidskriftsartikel (refereegranskat)abstract
- Fatigue damage in a metallic material during very high cycle fatigue can strongly be correlated to the microstructure. This paper provides a review and a discussion on the micro damage behaviours in a nickel-base alloy during very high cycle fatigue using microplasticity and material mechanics. The results show that cyclic plastic deformation in this material can occur very locally even with an applied stress that is much lower than the yield strength. The fatigue damage occurs mainly at grain or twin boundaries because of local impingement and interaction of slip bands and these boundaries. The crystallographic properties, Schmid factors and orientations of grain and boundaries play very important roles to the fatigue damage. Subsurface fatigue crack initiation in the matrix is one of very high cycle fatigue mechanisms. Twinning and detwinning can also occur during the very high cycle fatigue process.
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| 25. |
- Chai, Guocai, 1956-, et al.
(författare)
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Crack Initiation in Bulk Matrix of Austenitic Stainless Steel during Very High Cycle Fatigue
- 2023
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Ingår i: Materials Performance and Characterization. - : American Society for Testing Materials. - 2379-1365 .- 2165-3992. ; 12:2
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Tidskriftsartikel (refereegranskat)abstract
- In the very high cycle fatigue regime, fatigue crack initiation in high-strength steels is usually correlated to a subsurface inclusion with a fine granular area (FGA). Localized stress-strain concentration at the subsurface inclusion is a critical factor. Fatigue crack initiation with an FGA in the bulk matrix without any defect has rarely been reported. In this paper, a fundamental study on the formation of FGAs in the bulk matrix of an austenitic stainless steel has been carried out using a progressive stepwise load-increasing test with a cycle step of about 108 cycles. FGA formation in the subsurface bulk matrix has been observed. The micro structural damage in the fatigue-tested specimens has been studied using the electron channeling contrast imaging electron microscopy technique. Strain localization and grain fragmentation are the main processes for the formation of FGAs. Local plasticity exhaustion leads to crack initiation due to local stress concentrations. This method can also be used to predict the fatigue damage process, especially the damage rate in individual specimens.
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| 26. |
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| 27. |
- Chai, Guocai, et al.
(författare)
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Creep and LCF Behaviors of Newly Developed Advanced Heat Resistant Austenitic Stainless Steel for A-USC
- 2013
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Ingår i: Procedia Engineering. - : Elsevier. - 1877-7058. ; 55, s. 232-239
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Tidskriftsartikel (refereegranskat)abstract
- Austenitic stainless steel grade UNS S31035 (Sandvik Sanicro® 25) has been developed for use in super-heaters and reheaters in the next generation of A-USC power plants. This new grade shows very good resistances to steam oxidation and hot corrosion, and higher creep rupture strength than other austenitic stainless steels available today. This makes it an interesting alternative for super-heaters and reheaters in future high-efficient coal fired boilers. This paper will mainly focus on the study of the creep and LCF behavior of the material at temperatures from 600 °C to 750 °C by using TEM and ECCI. The mechanisms at different temperatures and loading conditions have been identified. The interactions between dislocations and precipitates and their contribution to the creep rupture strength have been discussed. In this paper, different models have been used to evaluate the long-term creep behavior of the grade. A creep rupture strength near 100 MPa at 700 °C for 100 000 h has been predicted.
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| 28. |
- Chai, Guocai, et al.
(författare)
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Creep Behavior in A Newly Developed Heat Resistant Austenitic Stainless Steel
- 2015
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Ingår i: Berg- und Huttenmännische Monatshefte (BHM). - : Springer. - 0005-8912 .- 1613-7531. ; 160:9, s. 400-405
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Tidskriftsartikel (refereegranskat)abstract
- UNS S31035 austenitic stainless steel grade is a newly developed advanced heat resistant material for use in coal fired boilers at metal temperatures up to 700 °C. This new grade that has recently got two AMSE code cases shows good resistance to steam oxidation and flue gas corrosion and high creep rupture strength. This paper will mainly focus on the characterization of long term structure stability and performances such as the creep behaviors at different temperatures for up to 86,000 h at high temperatures. The creep damage mechanisms were studied using electron transmission microscopy, electron backscatter diffraction, and electron channeling contrast image analysis. The results show that the creep strength is related to the intragranular nano particles that act as obstacles for dislocation movements. Plastic deformation and transgranular fracture is the main creep fracture mechanism in the creep test samples of UNS S31035. The material has good creep ductility by formation of twins during the creep test. This material has been installed and tested in several European power plants, and has shown good performance. The material is an excellent alternative for superheaters and reheaters in future high-efficient coal fired boilers with design material temperatures up to 700 °C, instead of more costly nickel based alloy.
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| 29. |
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| 30. |
- Chai, Guocai
(författare)
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Damage Mechanism of Low Cycle Fatigue in an Advanced Heat Resistant Austenitic Stainless Steel at High Temperature
- 2014
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Ingår i: Procedia Materials Science. - : Elsevier. - 2211-8128. ; 3, s. 1754-1759
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Tidskriftsartikel (refereegranskat)abstract
- Sandvik Sanicro 25 is a newly developed heat resistant austenitic stainless steel grade for the next generation of coal fired advanced ultra-super critical (AUSC) power plants. In this paper, low cycle fatigue behavior and damage mechanisms of the material were studied. The low cycle fatigue test was performed in air at room temperature, 600 °C to 700 °C. The microstructures were studied using electron back scatter diffraction and electron channeling contrast image techniques. At room temperature, the material shows a conventional hardening and softening behavior as most metal materials. At high temperatures, however, it shows only a cyclic hardening behavior. Dynamic strain ageing is found to be one of the mechanisms. The damage and fatigue crack initiation mechanisms due to cyclic loading at different temperatures and loading conditions have been identified. The interactions between dislocations or slip bands with grain boundary or twin boundary are the main damage mechanism at low temperature or at high temperature with large strain amplitudes. Strain localization due to dislocation slipping is the main mechanism for the fatigue damage in grains.
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| 31. |
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| 32. |
- Chai, Guocai, et al.
(författare)
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Development of Advance Heat Resistant materials for IGCC and AUSC Power Plants
- 2014
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Ingår i: Proceedings of the 2014 Energy Materials Conference (CD-ROM). - Hoboken, NJ, USA : Wiley-TMS. - 9781119027942 ; , s. 227-234
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Konferensbidrag (refereegranskat)abstract
- Integrated gasification combined cycle (IGCC) power plants and advanced ultra-supercritical (AUSC) thermal power plants are believed to be used as future power plants for high efficient and clean energy production. Increase in the efficiency of these plants is mainly attributed to the increase in temperature and pressure, and the consequent environments become much tougher. This will give a great challenge to the materials used in these plants. The new materials with even higher creep strength combined with better corrosion resistance need to be developed. This paper will provide an overview on the newly developed advanced heat resistant materials for these applications. It will mainly focus the following two types of materials. One is a newly developed advanced heat resistant austenitic stainless steels for AUSC boilers. The material has been tested in several boilers in Europe. Another is one type of composite tube material for convective syngas cooler in the coal gasification process, reverse composite tubes for the fire-tube boiler. A 15 years' application experience of this type of composite tube material will be discussed.
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| 33. |
- Chai, Guocai, 1956-, et al.
(författare)
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Extra-long creep rupture life of Alleima 3R60™ (316/316L) stainless steel
- 2024
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Ingår i: Materials at High Temperature. - : TAYLOR & FRANCIS LTD. - 0960-3409 .- 1878-6413. ; 41:1, s. 169-176
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Tidskriftsartikel (refereegranskat)abstract
- Alleima 3R60 (TM) is an AISI 316/316 L type of stainless steel. This alloy shows extra-long creep lives. At 700 degrees C with an applied stress of 45 MPa, the specimen broke first after 240 131 hours, a 140% longer creep life than the predicted. The reasons have been studied using SEM/EDS, EBSD, ECCI and TEM. Two unexpected phenomena have been observed. One is the presence of homogeneously dispersed small Mo2Fe4CrSi(Ni-0.5)f063type of precipitates in the matrix. According to the thermodynamic calculation, this phase should not form in this alloy at 700 degrees C. The other is the fine grains formed in the matrix during the creep-testing at 700 degrees C for such an extra-long time. It is believed that both small precipitates and fine grain size contribute to this extra-long creep life of the steel. With the microstructural study and thermodynamic simulation, the mechanisms to form precipitates and fine grains have been discussed.
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| 34. |
- Chai, Guocai, et al.
(författare)
-
Fatigue Behaviors in Duplex Stainless Steel Studied Using In-Situ SEM-EBSD Method
- 2014
-
Ingår i: Procedia Materials Science. - : Elsevier. - 2211-8128. ; 3, s. 1748-1753
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Tidskriftsartikel (refereegranskat)abstract
- Austenite and ferrite in duplex stainless steels have different physical and mechanical properties. They can behave different during cyclic loading. To understand the fatigue behaviors of these two phases, an in-situ SEM/EBSD fatigue test has been performed. Flat specimens made from the specimens of pre-fatigue tested with three point bending were cyclically loaded in a scanning electron microscope via a compact test rig. By in situ/ex situ SEM/EBSD examination, slip activities and propagation of the fatigue cracks have been studied. Microstructures along the path of the fatigue crack were characterized. The different phase properties seem to lead to certain difference in the slip activity and formation of PSBs. Inhomogeneous slip activities and local strain concentrations were also found, which developed with increasing number of load cycles. Crack propagation behaviors in grain and cross the grain or phase boundaries have been discussed. Crack deflection occurs at the phase boundaries, but crack branching occurs mainly in the grains due to the dislocation slip. In-situ SEM/EBSD fatigue test confirms that crack propagation deflection and formation of crack branches can significantly reduce the crack propagation rate.
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| 35. |
- Chai, Guocai, et al.
(författare)
-
Fatigue Crack Branching Behavior in Dual Phase Material
- 2009
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Fatigue crack branching behaviour in a dual phase steel has been investigated using an in-situ SEM/EBSD fatigue test and a conventional da/dN test. Crack branching results mainly from the extrusions and intrusions of slip bands developed in the grains. The number of crack branches formed depends strongly on the loading condition and the microstructure of the material. The in-situ observation confirms that the formation of crack branches can significantly reduce the crack propagation rate that leads to crack growth retardation in the main mode I crack path. The crack branches formed are usually not ideal. They can propagate almost transversely to the main crack direction with a mode II stress intensity factor, SIF, and a rate that is much higher than that of the main crack.
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| 36. |
- Chai, Guocai
(författare)
-
Fatigue Crack Initiation in Metallic Matrix
- 2014
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Ingår i: TMS 2014 SUPPLEMENTAL PROCEEDINGS. - : John Wiley & Sons. - 9781118889855 - 9781118889725 ; , s. 647-654
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Konferensbidrag (refereegranskat)abstract
- This paper will provide a study on the behaviors of fatigue crack initiation in metal matrix in five metallic materials using very high cycle fatigue testing and electron microscopy with electron backscatter diffraction and electron channeling contrast technique. The damage and crack initiation mechanisms in metal matrix have been focused. It shows that straining in these materials during the fatigue process was highly localized. This strain localization has led to the damage or fatigue crack initiation at grain boundaries or twin boundaries by impingement cracking. High strain localization causes dislocation accumulation during each cyclic straining and consequently the formation of local "fine grain zone" that increases the local damage by plasticity exhaustion. This can cause the formation of fatigue crack origin in the matrix. Strain localization and local plasticity exhaustion can be used to explain this phenomenon.
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| 37. |
- Chai, Guocai, et al.
(författare)
-
Formation of fine grained area in martensitic steel during very high cycle fatigue
- 2015
-
Ingår i: Fatigue & Fracture of Engineering Materials & Structures. - : WILEY-BLACKWELL. - 8756-758X .- 1460-2695. ; 38:11, s. 1315-1323
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Tidskriftsartikel (refereegranskat)abstract
- A fine grained area around a subsurface fatigue crack origin can usually be observed on fracture surface of a metallic material after very high cycle fatigue. This paper provides a fundamental study on the mechanisms to form this fine grained area using a martensitic stainless steel and advanced analysis instruments. The results show that the formation of a fine grained zone is a local behaviour. It is only a few micrometres in the transversal direction (cross section) and one micrometre in the longitudinal direction (crack propagation direction). High plastic deformation such as localized dislocation slip bands can be observed in this fine grained area. They interact with grain boundaries and cause the formation of damage by impingement cracking. The results indicate that occurrence of cyclic localized plastic deformation during very high cycle fatigue near the subsurface defect leads to the formation of fine grained area.
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| 38. |
- Chai, Guocai, et al.
(författare)
-
Formation of fine granular area in a non-defect matrix of austenitic stainless steel during very high cycle fatigue
- 2023
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Ingår i: Fatigue & Fracture of Engineering Materials & Structures. - : John Wiley & Sons. - 8756-758X .- 1460-2695. ; 46:6, s. 2364-2373
-
Tidskriftsartikel (refereegranskat)abstract
- A fine granular area, FGA, is a typical phenomenon observed at the very high cycle fatigue fracture crack origin with a subsurface defect in the material. The FGA has been widely investigated, and different mechanisms have been proposed. In this paper, the formation of FGA in a non-defect matrix of one austenitic steel during very high cycle fatigue was studied using a progressive stepwise load-increasing method and electron scanning microscopy/electron channeling contrast imaging (ECCI) technique. A nano rough surface area or FGA at the fatigue crack origin has been observed in the subsurface matrix without any defect. It is a new phenomenon. A mechanism was proposed using the dislocation plasticity theory. The formation of FGA in a non-defect matrix is a localized plasticity exhausting process by strain localization, grain fragmentation, stress concentration and nano crack initiation and propagation along low-angle grain boundaries.
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| 39. |
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| 40. |
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| 41. |
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| 42. |
- Chai, Guocai, et al.
(författare)
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Influence of Dynamic Strain Ageing and Long Term Ageing on Deformation and Fracture Behaviors of Alloy 617
- 2016
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Ingår i: THERMEC 2016. - : Trans Tech Publications. ; , s. 306-311
-
Konferensbidrag (refereegranskat)abstract
- 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, plenty of micro or nanotwins can be observed. The formation of these twins leads to a high ductility and toughness which is a new observation or a new concept for this type of material.
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| 43. |
- Chai, Guocai, et al.
(författare)
-
Local Plasticity Exhaustion in a Very High Cycle Fatigue Regime
- 2012
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Ingår i: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 66:10, s. 769-772
-
Tidskriftsartikel (refereegranskat)abstract
- Very high cycle fatigue behaviors of four metal materials with different microstructures have been studied focusing on their damage mechanisms. It was found that the cyclic plastic deformation in the materials was highly localized in the very high cycle regime or the elastic deformation regime. This high strain localization can cause local plasticity exhaustion, which leads to a stress concentration and consequently fatigue crack initation, and finally the formation of a subsurface non-defect fatigue crack origin.
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| 44. |
- Chai, Guocai, et al.
(författare)
-
Low and High Cycle Fatigue Behavior of Nickel-base Alloy att High Temperatures
- 2013
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Ingår i: Procedia Engineering. - : Elsevier. - 1877-7058.
-
Konferensbidrag (refereegranskat)abstract
- Low and high cycle fatigue behaviors of Alloy 690 have been investigated at temperatures up to 330 °C and number of cycles up to 2 × 109. Two interesting phenomena were observed. At high temperature, the alloy shows a secondary strain hardening in the cyclic stress-strain response. Formation of nano-twins and interactions between moving dislocations and stacking faults or interstitial atoms could contribute to this secondary strain hardening. For very high cycle fatigue, subsurface fatigue crack initiation at grain boundaries has been observed. EBSD investigation shows that strain accumulation is much localized. The fatigue damage is a localised plasticity exhaustion process.
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| 45. |
- Chai, Guocai
(författare)
-
Low Cycle Fatigue Behavior and Mechanism of Newly Developed Advanced Heat Resistant Austenitic Stainless Steels at High Temperature
- 2014
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Ingår i: 11th International Fatigue Congress. - : Trans Tech Publications Inc.. ; , s. 377-382
-
Konferensbidrag (refereegranskat)abstract
- Austenitic stainless steel grade UNS S31035 (Sandvik Sanicro® 25) has been developed for the next generation of 700°C A-USC power plant. This paper will mainly focus on the study of low cycle fatigue behavior and damage mechanisms of the material at room temperature, 600C to 700C by using electron back scatter diffraction and electron channeling contrast image techniques. At room temperature, the material shows a hardening and softening behavior as usual. At high temperature, however, it shows only a cyclic hardening behavior. Dynamic strain ageing can be one of the mechanisms. The damage and fatigue crack initiation mechanisms due to cyclic loading at different temperatures and loading conditions have been identified. The interactions between dislocations or slip bands with grain boundary or twin boundary are the main damage mechanism at low temperature or at high temperature with large strain amplitudes. Strain localization due to dislocation slipping is the main mechanism for the damage in grain.
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| 46. |
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| 47. |
- Chai, Guocai, et al.
(författare)
-
Mechanisms of Hydrogen Induced Stress Crack Initiation and Propagation in Super Duplex Stainless Steels
- 2009
-
Ingår i: STEEL RESEARCH INTERNATIONAL. - 1611-3683. ; 80:7, s. 482-487
-
Tidskriftsartikel (refereegranskat)abstract
- Austenitic and ferritic duplex stainless steels, DSS, have recently suffered from hydrogen stress induced cracking, HISC, in subsea components with a cathodic protection. This paper provides discussions on possible HISC mechanisms. HISC initiation can occur at the ferritic grain boundaries and phase boundaries at a stress lower than the yield strength, but dominantly at phase boundaries at a stress higher than the yield strength. EBSD analysis shows that HISC in DSS results from the interaction between the dynamic plasticity by creep and hydrogen diffusion. A model on the formation of microstresses in these two phases under creep conditions is proposed, which explains why HISC occurs mainly in the ferritic phase. Discontinuous two-dimensional HISC paths were observed. The austenitic phase acts as obstacles for crack propagation. The fracture covers "valleys" and "peaks" with the cleavage ferrite and the austenite with microfacets or striations due to the hydrogen-enhanced localized-plasticity.
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| 48. |
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| 49. |
- Chai, Guocai, et al.
(författare)
-
Micro Fatigue Crack Propagation Behavior in a Duplex Stainless Steel Studied Using In Situ SEM/EBSD Method
- 2014
-
Ingår i: 11TH INTERNATIONAL FATIGUE CONGRESS, PTS 1 AND 2. - : Trans Tech Publications Inc.. ; , s. 313-318
-
Konferensbidrag (refereegranskat)abstract
- Fatigue crack propagation behaviors in a duplex stainless steel have been studied using an in-situ SEM/EBSD fatigue test and a conventional da/dN test. Crack propagation behaviors in grain, effect of Schmid factor, propagation cross the grain or phase boundaries have been discussed. Crack propagation occurs mainly in the grains with a high Schmid factor, but with very small Schmid factor. Crack deflection occurs mainly at the phase boundaries, but crack branching occurs mainly in the grains due to the dislocation slip. In-situ SEM/EBSD fatigue test confirms that crack propagation deflection can lead to a decrease in crack propagation rate. Formation of crack branches can significantly reduce the crack propagation rate, which can cause crack growth retardation in the main crack path in the worst case. The crack branches formed are usually not ideal. They can propagate almost transversely to the main crack direction with a mode II stress intensity factor, SIF, and a rate that is much higher than that of the main crack.
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| 50. |
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