| 1. |
- Kohne, Thomas, 1990-, et al.
(författare)
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Evolution of Martensite Tetragonality in High-Carbon Steels Revealed by In Situ High-Energy X-Ray Diffraction
- 2023
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 54:4, s. 1083-1100
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Tidskriftsartikel (refereegranskat)abstract
- The martensitic transformation was studied by in situ and ex situ experiments in two high-carbon, 0.54 and 0.74 wt pct C, steels applying three different cooling rates, 15 °C/s, 5 °C/s, and 0.5 °C/s, in the temperature range around Ms, to improve the understanding of the evolution of martensite tetragonality c/a and phase fraction formed during the transformation. The combination of in situ high-energy X-ray diffraction during controlled cooling and spatially resolved tetragonality c/a determination by electron backscatter diffraction pattern matching was used to study the transformation behavior. The cooling rate and the different Ms for the steels had a clear impact on the martensitic transformation with a decrease in average tetragonality due to stronger autotempering for a decreasing cooling rate and higher Ms. A slower cooling rate also resulted in a lower fraction of martensite at room temperature, but with an increase in fraction of autotempered martensite. Additionally, a heterogeneous distribution of martensite tetragonality was observed for all cooling rates. © 2023, The Author(s).
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| 2. |
- Lin, Sen, 1993-, et al.
(författare)
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Effect of Mo addition on bainite formation in steels - A high-energy X-ray diffraction study
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The influence of Mo addition (0, 0.1, 0.4 wt%) on the bainitic transformation in a steel with base composition Fe-0.2C-1.7Mn-0.4Si (in wt%) was studied by means of in-situ high-energy X-ray diffraction (HEXRD) and electron microscopy. The phase evolution of bainitic ferrite, cementite and retained austenite during isothermal heat treatment at 673, 723, and 773 K was quantitatively analyzed. The evolution of the C content in the retained austenite and the dislocation density in the bainitic ferrite were also investigated. The results show that the addition of 0.1 wt% Mo barely affects the transformation kinetics, while the addition of 0.4 wt% Mo leads to a bay in the time-temperature-transformation diagram, possibly due to a solute drag effect. The incomplete transformation occurs at the highest isothermal heat treatment temperature, i.e., 773 K. At the lower isothermal heat treatment temperature 673 K, the bainitic transformation is accelerated by adding 0.4 wt% Mo due to an increased tendency of cementite formation, which decreases the overall C content in retained austenite and C and Mo accumulation at the phase interface, thus reducing the solute drag effect.
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| 3. |
- Lin, Sen, 1993-, et al.
(författare)
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Formation of cementite and transition carbides during bainitic transformation in steels with and without incomplete transformation stage
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The kinetics of carbide precipitation during isothermal bainitic transformation in a Fe-0.448C-1.78Mn-1.20Si (in wt%) steel is investigated using in-situ high-energy X-ray diffraction and electron microscopy. For the sample heat treated at 623 K for 1 h, the bainitic transformation is incomplete and transition carbides are observed. For the sample heat treated at 673 K, the initial formation of bainitic ferrite is accompanied by the formation of transition carbide, and at a later transformation stage, cementite starts to form until the bainitic transformation reaches completion, whereas the amount of transition carbides decreases.
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| 4. |
- Lin, Sen, et al.
(författare)
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Effect of Si on bainitic transformation kinetics in steels explained by carbon partitioning, carbide formation, dislocation densities, and thermodynamic conditions
- 2022
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Ingår i: Materials Characterization. - : Elsevier BV. - 1044-5803 .- 1873-4189. ; 185
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Tidskriftsartikel (refereegranskat)abstract
- The effect of Si addition on the evolution of bainitic transformation, carbon diffusion, carbide formation, and dislocation density in steel was investigated using in-situ high-energy X-ray diffraction (HEXRD). Alloys Fe-0.4C-1.7Mn (in wt%) with 1-4 wt% Si were austenitized at 1273 K and then isothermally heat treated at 573, 623, and 673 K. According to the HEXRD results, increasing Si content reduces the bainitic transformation kinetics and causes the incompleteness of the bainitic transformation to occur at lower bainite volume fraction. This is because i) Si retards carbide formation, impeding the eutectoid bainitic transformation, and leads to the accumulation of carbon at the migrating interface; ii) Si leads to higher strain energy and more dislocations in the austenite that also hinders the migration of the interface. Carbide formation was observed to occur prior to the incomplete transformation stage. During further isothermal holding, the decrease in dislocation density due to dislocation annihilation had little effect on carbide formation or carbon diffusion. Finally, the Si content has a minor effect on the calculated T-0, T-0', and WBs lines. The measured carbon content in carbon enriched austenite agrees well with WBs and T-0 but not with T-0'.
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| 5. |
- Lin, Sen, 1993-
(författare)
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Formation of Bainite Studied by In-situ High-energy X-ray Diffraction
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Bainitic steels have attracted great attentions in recent years due to their excellent combination of properties to accommodate a wide range of applications. A deep and comprehensive understanding of how bainite forms is required to better design the production process and optimize the properties of bainitic steels. Extensive experimental investigations, mostly using the post-mortem techniques, have been conducted to shed light on the bainitic transformation. Unfortunately, the nature of bainitic transformation is still a subject of debate, which hinders further development. The bainitic transformation involves multiple events that occur concurrently, such as formation of bainitic ferrite and cementite, dislocation generation and annihilation, and carbon diffusion, etc. These events may also affect each other and, in turn, affect the overall bainitic transformation kinetics. It is difficult to quantify the evolution of these events in a large sample volume and reveal their interrelationship during bainitic transformation using conventional experimental methods. Moreover, some bainitic steel grades, such as high-strength low-alloy (HSLA) steels used in the automotive industry, possess a very rapid phase transformation kinetics and a complex final microstructure. So, it is challenging to understand the transformation progress and correlate it with the final microstructure by the sole assistance of post-mortem techniques. In this circumstance, in-situ techniques, such as high-energy X-ray diffraction (HEXRD), appear to be a better solution to these challenges. Synchrotron sources provide extremely brilliant X-rays. It enables the detection of minor phases, such as carbides, and facilitates the rapid data acquisition to resolve the rapid transformation progress. Therefore, this thesis is dedicated to utilizing of HEXRD with state-of-the-art instrumentation to study bainite formation. One objective is to explore the feasibility of HEXRD for industrially relevant questions, e.g., rapid bainitic transformation in HSLA steels. At the same time, Si and Mo are two important elements and their content is often tuned in HSLA steels. So, another objective is to systematically study the influence of Si, Mo, and temperature on the bainitic transformation and other events that occur concurrently. The thesis thus brings industrial applications, fundamental transformation mechanisms, and HEXRD methodology development together. Two commercial HSLA steels with different hardenabilities were austenitized and fast cooled to different isothermal temperatures. Austenite decomposition occurred during cooling with high transformation rates. Several transformation products, i.e. polygonal ferrite, bainitic ferrite, degenerate pearlite, and martensite, were separated by combining HEXRD and electron backscatter diffraction analyzes. The steel with higher hardenability was found to have a smaller fraction of polygonal ferrite and a higher amount of bainite, which was speculated to be caused by the larger addition of Mo. On the other hand, the low-hardenability steel with a higher addition of Si show a higher carbon content in the retained austenite, probably because of suppressed carbide formation. Following the study of HSLA steels, the effects of Si and Mo were investigated using a series of Fe-C-Mn-Si and Fe-C-Mn-Si-Mo alloys with various of heat treatment conditions. These investigations aimed to reveal the influences of the alloying elements with a focus on the correlation between the formation of bainitic ferrite, carbon diffusion, carbide formation, and dislocation density evolution during the bainitic transformation. In general, bainite formation is retarded by increasing the Si content and the isothermal temperature, and the carbon contents in retained austenite at transformation stasis were close to the Widmanstätten/bainitic ferrite start temperatures (WBs). A minor addition of Mo had a negligible effect, but a larger addition introduced a bay area in the time-temperature-transformation that can be a result of solute drag effect. Transition carbides were only found in Si-added alloys, whereas cementite was found in both Si-added and Mo-added alloys. The carbide formation had similar kinetics as bainitic ferrite but no correlation to the dislocation density evolution was found for Si-added alloys. Furthermore, an ongoing work is introduced. A HEXRD study using the state-of-the-art PILATUS area detector was performed to shed more light on the transformation mechanism of bainite in comparison with martensite. The result shows that at the same range of diffraction angle, for bainitic ferrite, a single symmetric diffraction peak was found in Fe-3.0Mn-0.4 and 0.6C (in weight percent) alloys; for martensite, the diffraction peak of the Fe-3.0Mn-0.4C alloy was similar to that of bainitic ferrite, whereas two peaks adjacent to each other were found for martensite in the Fe-3Mn-0.6C alloy. This thesis demonstrates the versatility of HEXRD in phase transformation studies. Crystallographic, chemical, volume fraction, and stress/strain information extracted from the data is of particular interest for scientific and industrial studies.
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| 6. |
- Chou, Chia-Ying, 1990-
(författare)
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Integrated Computational and Experimental Study of Additively Manufactured Steels
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The design freedom Additive Manufacturing (AM) offers provides new solutions for improving functionality in industrial applications. It also offers unique opportunities when it comes to materials design.Powder Bed Fusion – Laser Beam (PBF-LB) is currently one of the most popular commercial AM techniques for metallic materials partly due to the relatively low surface roughness and the large design flexibility. However, the number of materials suitable for the PBF-LB process is still rather low and to accelerate the development of grades tailored for this AM process, dedicated computational tools for alloy design are needed. Of importance for materials design, is computational thermodynamics and kinetics coupled with CALPHAD materials descriptions since it enables calculations for multicomponent materials making it possible to predict the effect of varying composition.In this thesis, computational thermodynamic and kinetics coupled with materials characterization are applied to study the microstructure evolution during PBF-LB. Two material classes are in focus – hot-work tool steels and ferritic stainless steels. For the hot-work tool steel, the cooling rates during PBF-LB processing are high enough to induce martensite transformation and in the as-built microstructure a martensitic matrix is observed and some fraction of retained austenite. A solidification sub-structure due to micro-segregation during printing is also observed. Solidification calculations are performed to predict the micro-segregation showing agreement with experimental measurements. The segregation results are then used as input to a semi-empirical martensite start temperature model making it possible to explain the location and amount of retained austenite.In addition to compositional optimization, a computational framework for AM alloy design needs to include the possibility to tailor the AM post heat treatments. An alternative to the conventional hardening treatment is thus studied in the current work. A model for precipitation kinetics is combined with experimental characterization to explore the effect of tempering on the as-built microstructure in comparison to the tempering effect on an austenitized microstructure. The results show that the precipitation kinetics is strongly dependent on the starting structure and that direct tempering of the as-built microstructure changes the precipitation sequence compared to the conventional heat treatment route. The calculations reproduce this result suggesting that it is a thermodynamic effect stemming from different matrix compositions.The other material class, the ferritic stainless steels, is studied in terms of its response to post-heat treatments. The as-built microstructure is characterized by high dislocation density and a fine grain structure in some cases as well as a solidification sub-structure. The mechanical properties of the as-built material are in general good for these steels, however, stress relieving is most often a required post process for the as-built components which may decrease the mechanical properties. To maximize the gained benefits from the unique process condition of PBF-LB, simulations are applied to study the possibility of post heat treatment optimization.To construct a computational framework for AM materials design, multiscale modeling capabilities are needed. This work shows the value of computational thermodynamics and kinetics for understanding the materials behavior on the microscale and hence, contributes to the construction of such a framework. By understanding the physical metallurgy, and enable modeling of the AM processes, the industrialization of AM can be accelerated.
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| 7. |
- Hedström, Peter, et al.
(författare)
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On the Three-Dimensional Microstructure of Martensite in Carbon Steels
- 2012
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Ingår i: Proceedings Of The 1st International Conference On 3D Materials Science. - Hoboken, NJ, USA : John Wiley & Sons. - 9781118470398 - 9781627489331 ; , s. 19-24
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Konferensbidrag (refereegranskat)abstract
- The mechanical properties of high-performance steels are often reliant on the hard martensitic structure. It can either be the sole constituent e. g. in tool steels, or it can be part of a multi-phase structure as e. g. in dual-phase steels. It is well-known that the morphology of martensite changes from lath to plate martensite with increasing carbon content. The transition from lath to plate is however less known and in particular the three-dimensional (3D) aspects in the mixed lath and plate region require more work. Here the current view of the 3D microstructure of martensite in carbon steels is briefly reviewed and complemented by serial sectioning experiments using a focused ion beam scanning electron microscope (FIB-SEM). The large martensite units in the Fe-1.2 mass% C steel investigated here are found to have one dominant growth direction, less transverse growth and very limited thickening. There is also evident transformation twinning parallel to the transverse direction. It is concluded that more 3D analysis is required to understand the 3D microstructure of martensite in the mixed lath and plate region and to verify the recently proposed 3D phase field models of martensite in steels.
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| 8. |
- Kohne, Thomas, 1990-, et al.
(författare)
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Impact of Cooling Rate during High-Pressure Gas Quenching on Fatigue Performance of Low Pressure Carburized Gears
- 2022
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Ingår i: Metals. - : MDPI AG. - 2075-4701. ; 12:11, s. 1917-1917
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Tidskriftsartikel (refereegranskat)abstract
- The impact of cooling rate during high-pressure gas quenching on the fatigue performance of low-pressure carburized spur gears was studied for steel grades 20MnCr5 and 17NiCrMo6-4. The results show an increased fatigue limit by 10 to 11% when applying a slower cooling rate for both steel grades. Moreover, for 20MnCr5 the slower cooled gears show an increase in compressive residual stresses by 130 MPa compared to the faster cooling, although no significant difference was observed for 17NiCrMo6-4. It is also seen that the cooling rate affects the core hardness for both steel grades, while other properties like surface hardness, case-hardness depth and martensite variant pairing were unaffected. The results for the retained austenite content and average martensite unit size show no clear effect of the cooling rate. The possible influence of different carbon distributions after quenching for the two used cooling rates on the carbide precipitation and fatigue limit is discussed. View Full-Text
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| 9. |
- Stormvinter, Albin, et al.
(författare)
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Quantitative metallography for industrial use on martensitic steels
- 2015
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Ingår i: PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015. - : International Conference on Solid-Solid Phase Transformations in Inorganic Materials. - 9780692437360 ; , s. 539-546
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Konferensbidrag (refereegranskat)abstract
- The performance of powertrain components and rock tools relies on the inherent strength and hardness of ferrous martensite. Currently the industry uses experimental measurements of surface hardness and case depth to qualify their hardening processes. Often there are additional requirements on microstructure constituents, although there are no quantitative methods available to characterize ferrous martensite. Here such methodology is discussed in relation to EBSD measurements on the full practical range of Fe-C alloys. The orientation relationships between austenite and martensite along with the variant pairing tendency of martensite are determined from the EBSD data. These results are related to the well-known morphological transition from lath to plate martensite in Fe-C alloys. Quantitative metallography using EBSD has the potential to complement hardness- and residual-stress measurements when qualifying new steel grades and hardening processes in industry. It may also prove important when investigating the coupling between material properties and fatigue performance.
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| 10. |
- Yeddu, Hemantha Kumar, 1980-
(författare)
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Martensitic Transformations in Steels : A 3D Phase-field Study
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Martensite is considered to be the backbone of the high strength of many commercial steels. Martensite is formed by a rapid diffusionless phase transformation, which has been the subject of extensive research studies for more than a century. Despite such extensive studies, martensitic transformation is still considered to be intriguing due to its complex nature. Phase-field method, a computational technique used to simulate phase transformations, could be an aid in understanding the transformation. Moreover, due to the growing interest in the field of “Integrated computational materials engineering (ICME)”, the possibilities to couple the phase-field method with other computational techniques need to be explored. In the present work a three dimensional elastoplastic phase-field model, based on the works of Khachaturyan et al. and Yamanaka et al., is developed to study the athermal and the stress-assisted martensitic transformations occurring in single crystal and polycrystalline steels. The material parameters corresponding to the carbon steels and stainless steels are considered as input data for the simulations. The input data for the simulations is acquired from computational as well as from experimental works. Thus an attempt is made to create a multi-length scale model by coupling the ab-initio method, phase-field method, CALPHAD method, as well as experimental works. The model is used to simulate the microstructure evolution as well as to study various physical concepts associated with the martensitic transformation. The simulation results depict several experimentally observed aspects associated with the martensitic transformation, such as twinned microstructure and autocatalysis. The results indicate that plastic deformation and autocatalysis play a significant role in the martensitic microstructure evolution. The results indicate that the phase-field simulations can be used as tools to study some of the physical concepts associated with martensitic transformation, e.g. embryo potency, driving forces, plastic deformation as well as some aspects of crystallography. The results obtained are in agreement with the experimental results. The effect of stress-states on the stress-assisted martensitic microstructure evolution is studied by performing different simulations under different loading conditions. The results indicate that the microstructure is significantly affected by the loading conditions. The simulations are also used to study several important aspects, such as TRIP effect and Magee effect. The model is also used to predict some of the practically important parameters such as Ms temperature as well as the volume fraction of martensite formed. The results also indicate that it is feasible to build physically based multi-length scale model to study the martensitic transformation. Finally, it is concluded that the phase-field method can be used as a qualitative aid in understanding the complex, yet intriguing, martensitic transformations.
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