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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.

To enhance the fundamental understanding for micromechanical lath martensite deformation, the microstructure as well as macro- and microscopic tensile properties of as -quenched 15-5 PH stainless steel are systematically analysed depending on the austenitisation temperature. Based on electron backscatter diffraction (EBSD) and backscattered electron (BSE) analysis, it is noted that the martensite morphology alters from a less defined to a more clearly defined parallel arrangement of the block and lath structure with increasing temperature. For an indepth quantification of the hierarchical boundary strengthening contributions in relation to local stress/strain heterogeneity, separate high-fidelity virtual microstructures are realised for the different scales (prior austenite grains, packets and blocks). This is consistent with the materials transformation process. The virtual microstructures are simulated employing the crystal plasticity finite element method (CPFEM) adapted for handling high dislocation density and encompassing all relevant strengthening mechanisms by boundaries, dislocations and solute atoms. While accurately capturing the measured size -dependent stress-strain behaviour, the simulations reveal in line with the experiments (Hall-Petch) that blocks are the most effective dislocation motion barrier, causing increased strain hardening and stress/strain heterogeneity. Furthermore, since strain localisation is predicted strongest in the distinct block structure, the experimentally observed early plastic material yielding is thought to be favoured here., QC 20240404

This master thesis was a part of an ongoing project at Research institutes of Sweden (RISE) and Chalmers University of technology, studying the formation of white layers (WLs) upon hard machining AISI 52100 steel. With a focus on the nanocrystalline microstructure of the machined steel, X-ray diffraction (XRD), white light interferometry (WLI), optical microscopy (LOM) and scanning electron microscopy (SEM) was utilized in the analysis of gathering an in-depth understanding of the WL formation mechanism. By introducing varying cutting parameters as part of the machining process, the effect of cutting speed and tool wear could be observed to directly impact the WL formation and could be linked to the thermomechanical contribution to the formation mechanism. Both thermal and mechanical WLs were observed and could be distinguished by the occurrence of dark layers in thermal WLs. The purpose of this thesis was to observe the influence of retained austenite (RA) on WL formation and from the XRD analysis the residual stress for different RA content could not be concluded. Furthermore, SEM concluded differences in the microstructure where a higher abundance of carbides was observed in the case of lower RA, a phenomenon most likely originating in the heat treatment process. Indications of facilitated mechanical WL formation for lower RA was observed but could not be deemed conclusive. The RA content could not be concluded to have an impact on the surface roughness nor the residual stress where variations were rather linked to the cutting parameters.

Laser welding of 1.2 GPa-class cold-rolled quenching and partitioning steel with martensite + retained austenite microstructure was performed at different heat inputs ranging from 91–240 J·mm−1 using a CO2 laser. Scanning electron microscopy, electron backscatter diffraction, and uniaxial tensile testing equipped with digital image correlation were employed to characterize the microstructural and mechanical evolution of the joints. The results revealed that the fusion zone and upper-critical HAZ comprised full martensite with high microhardness. The HAZ contained a narrow softened zone with a minimum width of ∼0.6 µm and at least 48 HV microhardness drop because of the martensite tempering. Martensite decomposition and precipitation of carbides were noticeable at higher heat inputs, so that the hardness of the softened zone was declined. At the welding heat input of 91 J·mm−1, the yield and tensile strengths were 841 and 1270 MPa, respectively, which represented reductions of 5.5 and 0.78% compared to the base material properties. The joint efficiency of 99.2% was achieved at the lowest heat input. The tensile deformation was primarily concentrated in the softened zone and was small in the fusion zone. However, the participation of the base material to tensile deformation was enhanced at lower heat inputs, leading to higher elongation values up to ∼11.9% during tensile testing.

This dataset contains all raw data used for the publication of the paper "Multiferroic Microstructure Created from Invariant Line Constraint" including SEM, TEM, MFM, VSM, RXD data.

In this study, we demonstrate the significance of austenite annealing twin boundaries when calibrating laser ultrasonic measurements for gauging austenite grain size in situ during the thermomechanical processing of high-strength low-alloy steels. Simple calculations show how differences in twinning density can lead to errors in grain size measurements if twins are disregarded during calibration and the method is used for a broad range of steels. Conversely, when calibration is performed using alloys with a metastable austenite microstructure at room temperature, the same calibration is suitable for a broad range of HSLA steels, provided that annealing twins are taken into account. Since light optical microscopy does not allow the characterization of annealing twins in low-alloy steel, the verification of the laser ultrasonic results was conducted using the novel approach of comparing the twinned grain sizes obtained using the ultrasonic method in low-alloy steels with the austenite grain maps reconstructed from martensite orientation maps measured using electron backscatter diffraction. Finally, we show how differences in twinning density occur even for alloys with a roughly similar stacking fault energy, further highlighting the importance of annealing twins in the calibration of laser ultrasonic measurements for industrial use., QC 20231030

Martensites of Ni-Mn-Ga-based alloys consist of hierarchical twinning domains spanning from micro-to nanoscale. This affects the diffraction pattern and thus can decrease the accuracy of the determination of the crystal structure. We propose a method to obtain different martensitic phases in Ni-Mn-Ga-Fe alloy with simplified variant microstructures and domain sizes of more than 2 micrometers. The use of simplified variant microstructures allows the influence of nanometer-scale domains on diffraction line position to be circumvented and enabls the comparison of the lattice parameters of non-modulated (NM), five-layered modulated (10M), and seven-layered (14M) phases in the same temperature range due to the large hysteresis of the intermartensitic transformations. It is found that the short crystallo-graphic axes in NM, 14M, and 10M martensites at the same temperature have different lengths. As a result, equilibrium NM structure building blocks cannot be used to build the crystal structures of 14M and 10M martensites. Instead, we introduce a constant plane shift model with identical shift values of the nearest planes (110) along [(1) over bar 10] or [1 (1) over bar0] as a replacement for the tetragonal building blocks model. The work demonstrates that plane shift values differ dramatically between mar-tensites, which agrees with ab initio calculations. The application of the constant plane shift and hard sphere models in modulated lattices for atomic-level twinning considerations is discussed.

Outgoing

In quality control, microstructures are investigated rigorously to ensure structural integrity, exclude the presence of critical volume defects, and validate the formation of the target microstructure. For quenched, hierarchically-structured steels, the morphology of the bainitic and martensitic microstructures are of major concern to guarantee the reliability of the material under service conditions. Therefore, industries conduct small sample-size inspections of materials cross-sections through metallographers to validate the needle morphology of such microstructures. We demonstrate round-robin test results revealing that this visual grading is afflicted by pronounced subjectivity despite the thorough training of personnel. Instead, we propose a deep learning image classification approach that distinguishes steels based on their microstructure type and classifies their needle length alluding to the ISO 643 grain size assessment standard. This classification approach facilitates the reliable, objective, and automated classification of hierarchically structured steels. Specifically, an accuracy of 96% and roughly 91% is attained for the distinction of martensite/bainite subtypes and needle length, respectively. This is achieved on an image dataset that contains significant variance and labeling noise as it is acquired over more than 10 years from multiple plants, alloys, etchant applications, and light optical microscopes by many metallographers (raters). Interpretability analysis gives insights into the decision-making of these models and allows for estimating their generalization capability.

The objective, both presently and going forward, is to minimise the frequency and impact of physical errors by improving processes and techniques. By going from chemical composition to hardened steel with the use of thermodynamic computations and FEM-simulations creates the possibility of predicting the material properties far in advance before manufacturing. With this method the effects of different compositions can be analysed early, resulting in fewer physical tests and a more cost-effective process. This study focuses on the SSAB steel Hardox 450, a steel that is predominantly martensitic, with high yield- and tensile strength. Through the use of thermodynamic computation, the material properties such as heat capacity and thermal conductivity can be determined. By the use of thermal computations CCT diagrams for the corresponding chemical compositions can be established. The CCT diagrams phase transformations and cooling curves, combined with the additional material parameters are then integrated in FEM-simulations. The aim is to determine the resulting phase composition after the steel has been hardened, through calibration of a material model that is based on the calculated CCT diagram. The results from this study show that the method has potential, demonstrating that it can generate good results. In addition, the reliability of the material model's calibration is clear, as the out-data results show a strong correlation with the in-data. Because the in-data in this case consists of the data from a CCT diagram, it is of high importance that the diagram has high reliability. The choice of software impacts the structure of the CCT diagram, concluding that the choice of simulation tool to execute the thermodynamic computations is of importance., Målet, både nu och i framtiden, är att minimera mängden och omfattningen oönskade fel genom utveckling av process och teknik. Genom att gå från kemisk sammansättning till härdat stål med hjälp av termodynamiska beräkningar och FEM-simuleringar skapas möjligheten att förutspå materialegenskaper långt innan tillverkning. Med denna metod kan effekten av olika sammansättningar analyseras tidigt, vilket resulterar i färre fysiska tester och en mer kostnadseffektiv framställningsprocess. Denna studie fokuserar på SSAB-stålet Hardox 450, ett stål som är övervägande martensitiskt, med hög sträckgräns och brottgräns. Med hjälp av termodynamiska beräkningar bestäms materialegenskaper såsom värmekapacitet och värmeledningsförmåga för materialet. Genom de termodynamiska beräkningarna tas även CCT-diagram för materialet fram. CCT-diagrammets fasomvandlingar och kylhastigheter, kombinerat med de övriga materialparametrarna integreras sedan i en FEM-simulering av materialet. Syftet är att fastställa den resulterande fassammansättningen efter härdning av stålet genom kalibrering av en materialmodell baserad på det framställda CCT-diagrammet. Resultaten från detta arbete påvisar att metoden har potential och på att den kan generera goda resultat. De tyder även på att kalibreringen av materialmodellen är tillförlitlig, där resultaten visar att ut-datan korrelerar mycket väl med in-datan. Då in-datan i detta fall utgörs av ett CCT-diagram är det av stor vikt att detta har en hög tillförlitlighet. Val av programvara har inverkan på CCT-diagrammets utformning, vilket betyder att val av verktyg för att utföra de termodynamiska beräkningarna är av betydelse.

Medium carbon low alloyed tool steels are used today in various areas toshape plastics, nonferrous metals, and steels, and they are crucial in themanufacturing industry. To be effective, tool steels must be strong andtough, and have high wear resistance and temperature stability. To achievethe desired properties, materials are alloyed so that secondary phaseparticles precipitate during processing, especially during the tempering ofmartensitic steels. However, the hardening contribution relates to the size,volume fraction and number density of precipitates, controlled by alloycomposition and heat-treating parameters. It is therefore essential tounderstand how, where, and when the particles nucleate and how theprecipitation sequence and kinetics are affected by alloying additions,tempering temperature, and time.This work is aimed to study carbide precipitation in two commercial lowalloytool steel using small-angle neutron and X-ray scattering. To supportthese methods, samples were characterized with transmission electronmicroscopy (TEM) and atom probe tomography (APT). With a combinationof high-resolution techniques, it was possible to establish the precipitationsequence in these steels. It was also possible with various small anglescattering techniques to determine the evolution of volume fraction andnumber density of precipitates as a function of tempering parameters.First, small angle neutron scattering (SANS) was used, which is an excellentmethod for bulk quantification of small precipitates in steel. It was possiblewith SANS to broadly study the precipitation process depending onannealing temperature and time. However, it is difficult with regular smallangle scattering (SAS) to distinguish particle types with overlapping sizedistributions. To possibly separate the scattering signal from differentcarbide types, measurements were carried out with polarized small angleneutron scattering (SANSPOL) and anomalous small-angle X-ray scattering(ASAXS). With ASAXS it was p, Verktygsstål används idag inom en mängd olika områden för att förädlaplaster, metaller och stål, och är således viktiga inom tillverkningsindustrin.Utmärkande egenskaper för verktygsstål är kombinationen av styrka,seghet, nötningsbeständighet och värmebeständighet vid applikationen. Föratt nå dessa egenskaper legeras materialen så att partiklar av sekundära faserskiljs ut under tillverkningen, framför allt under anlöpning av martensitiskastål. Vid anlöpning genomgår materialet en isotermvärmebehandling vid500–600°C under ca 1–4 timmar. Detta är oftast tillräckligt för att nå denoptimala kombinationen av små partiklar (1–10 nm) med hög nummertäthetsom effektivt härdar materialet. Det optimala härdningsbidraget varierardock beroende på legeringssammansättning och anlöpningsparametrar. Detär således viktigt att förstå hur, var och när partiklarna kärnbildas, och hurpartikelutskiljningen påverkas av legeringstillsatser ochanlöpningstemperatur och tid. Detta är viktig information som kan användasför en mer effektiv legeringsutveckling.Målet med detta projekt har varit att studera partikelutskiljning i tvåkommersiella låglegerade verktygsstål med hjälp av lågvinkelspridning avneutroner (SANS) och röntgenstrålar (SAXS). För att underlättautvärderingen av spridningsdata har prover även utvärderats itransmissionselektronmikroskop och med atomsondstomografi. Olikavarianter av lågvinkelspridning har använts, både ex-situ och in-situ, för attutvärdera möjligheten studera utskiljning i kommersiella verkstygstål.Till en början utvärderades prover med lågvinkelspridning av neutroner(SANS), med vilka det var möjligt att i stora drag studerautskiljningsförloppet beroende på anlöpningstemperatur och tid. Dock fannsdet osäkerheter kring resultaten kopplat till överlappandestorleksfördelningar av olika karbidtyper. För att bättre kunna separeraspridningssignalen från olika karbidvarianter genomfördes mätningar med”anomal lågvinkel röntgenspridning” (ASAXS). Med vilket det var möjligtat

Hydrogen permeation technique is a widely used testing method for the determination of hydrogen diffusion coefficient (D), which is an important parameter considering hydrogen embrittlement. A palladium (Pd) or nickel (Ni) coating is often utilised on the hydrogen detection side of the test specimens. Here, we investigate the effect of Pd and Ni coatings on hydrogen diffusion in a martensitic 500 HBW hardness low-alloy steel in the thickness range of 0.5–0.8 mm using a refined successive transient method and compare against an uncoated reference specimen. Both coatings yield similar average D values (6–6.6 × 10⁻⁷ cm²/s), but the best repeatability is achieved with Pd coating. With Ni coating, D values decrease with the increasing specimen thickness, which is partly caused by a slower hydrogen diffusion in Ni, and therefore a concentration gradient at the specimen-coating interface. The uncoated specimen has a poor transient fit, and significantly lower D (2.1 × 10⁻⁷ cm²/s) due to surface oxidation. With both coatings, the steepness of the last decay transient was highly affected by specimen thickness, and therefore the density of reversible hydrogen traps is only comparable for similar thicknesses.

Medium carbon low alloyed tool steels are used today in various areas to shape plastics, nonferrous metals, and steels, and they are crucial in the manufacturing industry. To be effective, tool steels must be strong and tough, and have high wear resistance and temperature stability. To achievethe desired properties, materials are alloyed so that secondary phaseparticles precipitate during processing, especially during the tempering of martensitic steels. However, the hardening contribution relates to the size,volume fraction and number density of precipitates, controlled by alloy composition and heat-treating parameters. It is therefore essential to understand how, where, and when the particles nucleate and how the precipitation sequence and kinetics are affected by alloying additions,tempering temperature, and time. This work is aimed to study carbide precipitation in two commercial low-alloy tool steel using small-angle neutron and X-ray scattering. To support these methods, samples were characterized with transmission electronmicroscopy (TEM) and atom probe tomography (APT). With a combination of high-resolution techniques, it was possible to establish the precipitation sequence in these steels. It was also possible with various small angle scattering techniques to determine the evolution of volume fraction and number density of precipitates as a function of tempering parameters. First, small angle neutron scattering (SANS) was used, which is an excellent method for bulk quantification of small precipitates in steel. It was possible with SANS to broadly study the precipitation process depending on annealing temperature and time. However, it is difficult with regular small angle scattering (SAS) to distinguish particle types with overlapping size distributions. To possibly separate the scattering signal from different carbide types, measurements were carried out with polarized small angle neutron scattering (SANSPOL) and anomalous small-angle X-ray scattering(ASAX, Verktygsstål används idag inom en mängd olika områden för att förädla plaster, metaller och stål, och är således viktiga inom tillverkningsindustrin. Utmärkande egenskaper för verktygsstål är kombinationen av styrka,seghet, nötningsbeständighet och värmebeständighet vid applikationen. För att nå dessa egenskaper legeras materialen så att partiklar av sekundära faser skiljs ut under tillverkningen, framför allt under anlöpning av martensitiska stål. Vid anlöpning genomgår materialet en isotermvärmebehandling vid500–600°C under ca 1–4 timmar. Detta är oftast tillräckligt för att nå den optimala kombinationen av små partiklar (1–10 nm) med hög nummertäthet som effektivt härdar materialet. Det optimala härdningsbidraget varierar dock beroende på legeringssammansättning och anlöpningsparametrar. Det är således viktigt att förstå hur, var och när partiklarna kärnbildas, och hur partikelutskiljningen påverkas av legeringstillsatser och anlöpningstemperatur och tid. Detta är viktig information som kan användasför en mer effektiv legeringsutveckling. Målet med detta projekt har varit att studera partikelutskiljning i två kommersiella låglegerade verktygsstål med hjälp av lågvinkelspridning av neutroner (SANS) och röntgenstrålar (SAXS). För att underlätta utvärderingen av spridningsdata har prover även utvärderats i transmissionselektronmikroskop och med atomsondstomografi. Olika varianter av lågvinkelspridning har använts, både ex-situ och in-situ, för att utvärdera möjligheten studera utskiljning i kommersiella verkstygstål. Till en början utvärderades prover med lågvinkelspridning av neutroner(SANS), med vilka det var möjligt att i stora drag studera utskiljningsförloppet beroende på anlöpningstemperatur och tid. Dock fanns det osäkerheter kring resultaten kopplat till överlappande storleksfördelningar av olika karbidtyper. För att bättre kunna separera spridningssignalen från olika karbidvarianter genomfördes mätningar med ”anomal lågvinkel röntgenspridning” (ASAXS). Med

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., Funding Agencies|Sweden's Innovation Agency Vinnova [2017-02281]; Swedish Agency for Economic and Regional Growth [20201438]

Both 0.3C (high) Si-Al and 0.3C medium-Mn steels are considered ideal recipes for developing the third-generation advanced high strength steels using the innovative quenching and partitioning (QP) and austenite reversion transformation (ART) treatments, respectively. This study reports the effects of QP and ART processing on the evolved microstructural characteristics, focusing on phase transformation behavior and resultant mechanical properties, augmented by the presence of finely divided retained austenite (RA). While QP processing trials were carried out with (high) Si-Al steels, both QP and ART treatments were utilized to optimize the processing of medium-Mn steels to achieve high strength-ductility combinations. The processes were later scaled up for laboratory rolling simulations, involving thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP) in the case of (high) Si-Al steels, and TMR-QP and TMR-ART treatments for medium-Mn steels. The prime objective was to gain in-depth knowledge about the effects of processing on evolved microstructures, operating structural mechanisms and corresponding mechanical properties. Physical simulation experiments were conducted on a Gleeble simulator, followed by scaling up of the experiments on a laboratory rolling mill. Detailed microstructural characterization, including the differences in fractions, morphology, C-content, and stability of RA were performed by X-ray diffraction (XRD) and electron probe microanalysis (EPMA), besides using various materials characterization and electron microscopy techniques. Mechanical properties were evaluated in respect of hardness, tensile and impact toughness properties, and structure-property correlations established. The result showed that fine division of RA between martensitic laths, besides the refinement of martensitic packets and laths, notably improved the low-temperature impact toughness. Various combinations of high tensile strengths with good, Tiivistelmä 0.3C (korkea) pii- ja alumiiniseostetut sekä 0.3C keskimangaaniteräkset ovat houkuttelevia kandidaatteja uudeksi kolmannen sukupolven autoteollisuuden muovattavaksi suurlujuusteräkseksi. Tässä väitöskirjassa on tutkittu näiden teräksien mikrorakenteiden ominaisuuksia, austeniitti-martensiitti- ja martensiiti-austeniittifaasimuutosilmiöitä sekä näiden vaikutusta mekaanisiin ominaisuuksiin. Keskeytettyä sammutusta yhdistettynä matalan lämpötilan hehkutus (QP) prosessiin sekä austeniitin reversiomuutos (ART) -prosessiin on käytetty hyvien lujuus-sitkeysominaisuuksien saavuttamiseksi. Tavoite oli lisätä tietoa siitä, kuinka mikrorakenteen eri piirteet vaikuttavat termomekaanisesti valssatun suorasammutetun QP (TMR-DQP) ja termomekaanisesti valssatun ART (TMR-ART) teräksen ominaisuuksiin. Termomekaanista Gleeble-simulaattoria ja laboratoriokuumavalssainta on käytetty mikrorakenteen ominaisuuksien ja jäännösausteniitin hajaantumisen tutkimisessa. Lisäksi käytettiin erilaisia elektronimikroskooppeja ja analysaattoreita. Mekaanisten ominaisuuksien määrittämiseen käytettiin kovuusmittareita, veto- ja iskusitkeyskokeita. Tulokset osoittivat, että iskusitkeyteen vaikuttaa jäännösauteniitin määrän lisäksi myös rakenteen sälekoko. QP- ja ART-käsittelyillä voidaan saavuttaa erilaisia suuria vetolujuus-/venymäyhdistelmiä riippuen jäännösausteniitin määrästä, koosta, muodosta, jakaantumasta ja jäännösausteniitin hiilipitoisuudesta sekä martensiitin ja mahdollisen bainiitin morfologiasta. On haasteellista ottaa DQP-käsittely käytäntöön, koska keskeytetyn sammutuksen lämpötila vaikuttaa teräksen ominaisuuksiin ja sen hallinta tarkasti teollisessa mittakaavassa on haasteellista. ART-käsittelyssä haasteena on kapean prosessi-ikkunan (lämpötila ja hehkutusaika) hallinta ART-hehkutuslämpötiloissa.

Precipitation of carbides after tempering of a medium carbon low alloyed Cr-Mo-V tool steel at 600 degrees C was studied with scanning precession electron diffraction (SPED) in a transmission electron microscope (TEM). The precipitation sequence was evaluated by mapping the carbide distribution on carbon extraction replicas prepared from samples tempered for different durations of up to 24 h. The SPED results were supplemented by equilibrium calculations and energy dispersive x-ray spectroscopy (EDX) measurements in a TEM. It was found that e-Fe2C precipitates within martensite laths during quenching via auto-tempering. During reheating to tempering temperature e-Fe2C was dissolved and replaced by cementite, 0-M3C, which predominately form on martensite boundaries. It was further found that the small carbides in the early stage of tempering have predominantly a cubic MC structure, even if the V/Mo-ratio of the studied steel was only 0.12, and it is known from literature that V and Mo form cubic MC or hexagonal M2C carbides, respectively. Later during tempering more stable carbides, such as M7C3 and M23C6, also form, and it was concluded that the M7C3 form both by separate nucleation and precipitation on the cementite/matrix interface. The latter phenomenon was seen as particles with a core of cementite and a shell of M7C3 after 24 h of tempering., QC 20230706

Powder metallurgy is a branch of metal forming technology where metal powders are used to manufacture parts and components. It is a flexible and economical technique for manufacturing complicated shapes. This present work focuses on press and sinter technology and forms a part of Höganäs’s efforts of modelling hardenability through quenching. It aims to reduce the number of experimental trials for optimising heat treatment. Hardenability is a measure of how much martensite can be formed during heat treatment, thereby making steels hard, tough and impart strength. The presence of alloying elements like carbon, manganese, chromium, molybdenum, and nickel affects the hardenability of the steel and improves performance like fatigue strength and corrosion resistance. These elements influence the critical cooling rate necessary to form martensite during heat treatment. Component geometry also influences hardenability. Depending on the surface area available to cool, and volume of component, cooling rates may locally be different thereby resulting in an inhomogeneous structure. The work focuses particularly on two grades of powders manufactured by Höganäs AB - Astaloy® CrA and Astaloy® CrS which are evaluated for their hardenability. The aim of this work is to take cooling conditions observed in the actual furnace, use them to predict the amount of martensite present and the martensite start temperature and then compare it with experimental results thereby linking experiment to simulations. For the experimental part, dilatometry was used. Quenching data is obtained from the furnace along with heat capacity of the component and are used as input in Abaqus, which gives us the cooling rates for the component in the furnace. This data is then utilised as an input to dilatometry, where the samples are representative of sections of component. After dilatometry, vital information like martensite start temperature is recorded and metallography is performed, where phase fraction is, Pulvermetallurgi är en gren av metallformningsteknik där metallpulver används för att tillverka delar och komponenter. Det är en flexibel och ekonomisk teknik för att tillverka komplicerade former. Detta nuvarande arbete fokuserar på press- och sinterteknik och är en del av Höganäs arbete med att modellera härdbarhet genom härdning. Det syftar till att minska antalet experimentella försök för att optimera värmebehandlingen. Härdbarhet är ett mått på hur mycket martensit som kan bildas vid värmebehandling, vilket gör stålen hårda, sega och ger styrka. Närvaron av legeringselement som kol, mangan, krom, molybden och nickel påverkar stålets härdbarhet och förbättrar prestanda som utmattningshållfasthet och korrosionsbeständighet. Dessa element påverkar den kritiska kylningshastighet som krävs för att bilda martensit under värmebehandling. Komponentgeometrin påverkar också härdbarheten. Beroende på den yta som är tillgänglig för kylning och volymen av komponenten, kan kylningshastigheterna lokalt vara olika, vilket resulterar i en inhomogen struktur. Arbetet fokuserar särskilt på två kvaliteter av pulver tillverkade av Höganäs AB - Astaloy® CrA och Astaloy® CrS som utvärderas för sin härdbarhet. Syftet med detta arbete är att ta kylförhållanden som observerats i den faktiska ugnen, använda dem för att förutsäga mängden närvarande martensit och martensitens starttemperatur och sedan jämföra den med experimentella resultat och därigenom koppla experiment till simuleringar. För den experimentella delen användes dilatometry. Släckningsdata erhålls från ugnen tillsammans med värmekapaciteten hos komponenten och används som indata i Abaqus, vilket ger oss kylhastigheten för komponenten i ugnen. Dessa data används sedan som indata till dilatometry, där proverna är representativa för sektioner av komponenten. Efter dilatometri registreras viktig information som martensitstarttemperatur och metallografi utförs, där fasfraktion erhålls. Hårdhetsmätningar utförs också för att veri

Rapid tempering is a remarkable sustainable and energy-efficient way to modify properties of as-quenched martensite, particularly its toughness and ductility. In this work, tensile and fracture toughness properties, and microstructural evolution of a medium-carbon (0.4 wt%), low-alloy steel under different rapid tempering circumstances are discussed. The hot-rolled and direct-quenched specimens were subjected to rapid tempering treatments (heating rate of about 90 °C/s) to the different tempering temperatures (320–720 °C). As a reference material, one sample was subjected to conventional tempering at 420 °C for one hour. The results indicated that the mechanical properties and microstructural features are influenced considerably by tempering temperature rather than the holding time. Despite the short tempering duration, the strength of tempered martensite dropped (from a max. tensile strength of ∼ 2100 MPa to a min. of ∼ 1100 MPa) while ductility increased with increasing tempering temperature (from a tensile elongation of 4% to ∼ 15%). However, the rapidly tempered sample at 420 °C experienced a loss of fracture toughness as a result of coarse stick-like cementite precipitation. Microstructural observations revealed that, even with a short time frame, tempering temperature plays a significant role in the resulting cementite morphology. At lower tempering temperatures, the cementite precipitates have a stick-like structure, while at higher temperatures, they become more globular/spheroid-like. This change in cementite morphology led to an enhancement in fracture toughness. Overall, the results indicated that, by a proper design of the rapid tempering process, an excellent combination of tensile properties and fracture toughness can be obtained compared to conventional tempering while significantly saving time and energy.

Nanostructured bainite is recognized as a novel structure engineered through innovative processing thus enabling tensile strength as high as 2000 MPa in medium carbon steels. Low martensite start (MS) temperature of a typical high-carbon steel enables bainitic transformation at relatively lower temperatures, thus facilitating the formation of fine bainite platelets at the nanometre level. In this study, the target was to understand the physical metallurgy principles governing the operating structural mechanisms leading to the desired ultrafine/nanostructured bainitic microstructures and mechanical properties in medium carbon steels. The results suggest that ausforming applied above MS not only accelerated the bainite transformation kinetics but also initiated a strain-induced transformation of austenite into bainite/martensite. Ausforming below MS, however, hindered the onset of bainite transformation but subsequent transformation rates were faster, suggesting strain-induced acceleration of transformation kinetics. The influence of strain-induced phase transformation on flow stress was obvious at slow strain rates 0.01–0.5 s-1, particularly at temperatures below 400 °C. Ausforming led to significant refinement and uniformity of the bainitic plates with a concomitant increase in Vickers hardness. Low-temperature ausforming and subsequent isothermal holding and cooling schedules resulted in the decomposition of austenite into bainite of different morphologies. This was alongside the formation of small fractions of high-carbon martensite during cooling, together with the retention of desired fractions of finely divided, carbon-enriched austenite of various morphologies. A novel, multiphase bainite-martensite-austenite microstructure, achieved via low-temperature ausforming and subsequent air-cooling, was beneficial in terms of mechanical properties. The bainitic plates could be refined as fine as 20–50 nm, though with the presence of coalesced plates as wide a, Tiivistelmä Nanobainiitti tunnetaan matalan lämpötilan isotermisen pidon tuloksena saatavana uudenlaisena mikrorakenteena, joka mahdollistaa jopa 2000 MPa murtolujuuden keskihiilisissä teräksissä. Korkea hiilipitoisuus laskee martensiitin muodostumislämpötilaa, mikä mahdollistaa matalan bainiitin muodostumislämpötilan (MS), joka puolestaan johtaa hienojen, jopa nanometriluokan bainiittisäleiden muodostumiseen. Tämän tutkimuksen tavoitteena on ymmärtää fysikaalisen metallurgian mekanismeja, jotka johtavat toivottuun nanobainiittiseen mikrorakenteeseen ja mekaanisiin ominaisuuksiin. Tulokset osoittavat, että vaikka teräksen ausforming-käsittely MS-lämpötilan alapuolella viivästytti bainiitin muodostumisen alkamista, se kuitenkin kiihdytti bainiitin muodostumisnopeutta, ja sai myös aikaan venymän aiheuttaman faasimuutoksen austeniitista bainiitiksi tai martensiitiksi. Venymän aiheuttama faasimuutos voitiin nähdä muodonmuutoslujuuden huomattavana nousuna erityisesti hitailla muokkausnopeuksilla 0.01–0.5 s-1 matalissa, alle 400 °C lämpötiloissa. Ausformingin nähtiin pienentävän ja yhdenmukaistavan bainiittisäleitä, sekä nostavan terästen Vickers-kovuutta. Matalan lämpötilan ausforming, senjälkeinen isoterminen pito ja jäähdytys huoneenlämpötilaan saivat austeniitin hajaantumaan erimuotoisiin bainiitteihin ja martensiittiin, joskin pieni osa austeniitista säilyi rakenteessa jäännösausteniittina. Uudenlainen, matalan lämpötilan ausformingilla ja sopivilla pidolla ja jäähdytyksellä saavutettu bainiitti-martensiitti-austeniitti-rakenne oli mekaanisten ominaisuuksiensa puolesta edullinen mikrorakenne, jossa hienoimmat bainiittisäleet olivat jopa vain 20–50 nanometrin levyisiä. Karkeimmat, yhteenkasvaneet säleet olivat kuitenkin jopa 2000 nanometrin levyisiä. Tämä tutkimus antaa suuntaa erinomaiset mekaaniset ominaisuudet mahdollistavien nanobainiittisten keskihiilisten terästen kehittämiselle innovatiivisten prosessointiratkaisujen kautta.

Isothermal heat treatments below the martensite start temperature are carried out in a 0.25C-1.4Mn-1.4Si-0.32Mo steel (in wt%). The microstructural observation reveals a combination of tempered lath and plate martensite, accompanied by bainite and retained austenite. The temperature variation of the austenitic carbon content under different constrained equilibrium models is calculated and compared with experimental measurements. Neither of the existing models seems to predict the behavior of the measured austenitic carbon content in the temperature range studied. The results suggest that the diffusional formation of bainite governs the carbon content of austenite at temperatures nearby the MS., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Team Maria Santofimia Navarro

VDL Weweler has developed a novel heat treatment approach for their trailing arms, which eliminates the traditional austenisation step and instead capitalises on the inherent heat used for hot-forming to austenise the steel before quenching. This not only saves energy, but surprisingly also improves the lifetime of the products. The underlying reasons for this improvement in fatigue life are the central focus of the present study. Two trailing arms were used for this study, one from the traditional ‘Reference’ process, and one from the new ‘Gasloos’ process. Both were produced from the same heat of raw 51CrV4 steel bars that are inductively heated to 1200°C before hot-forming, after which Gasloos is immediately quenched to 180°C. Reference is left to cool to 600°C before being reheated in a gas oven at 900°C for one hour and is finally also quenched to 180°C. After quenching, both are tempered at 450°C for one hour, quenched to room temperature, and shot peened. Reference also underwent the usual painting procedure, Gasloos was collected before this. Both underwent fatigue bending tests, where the Gasloos trailing arm lasted about seven times longer. Fracture surfaces and unfatigued parts were collected from both trailing arms for analysis. Since it was obvious early on that the Reference trailing arm has a partially intergranular fracture, a literature survey was performed to assess the possibility that grain boundary segregation and embrittlement would occur. Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-Ray Spectroscopy (EDS) was used to perform fracture and microstructural analysis, as well as to measure grain boundary segregation. X-Ray Diffraction (XRD) analysis was used to measure the depth profiles of the residual stress to assess the effectiveness of the shot-peening. Samples for XRD were cut using Electronic Discharge Machining (EDM). In both trailing arms, cracks initiated on slag inclusions present from the steelmaking process. Ref, Materials Science and Engineering

High Manganese Austenitic Steel (HMnAS) is a novel steel exhibiting excellent mechanical properties at arctic temperatures. Defense Material Organisation (DMO) is interested in the feasibility of using HMnAS in their navy's ships alongside EH36 high-strength shipbuilding steel, using 307PF stainless steel weld filler metal. The feasibility of using HMnAS is investigated by performing mechanical tests on both the base materials and the weld. Microstructure and fractographs were analysed using optical and electron microscopy, and the chemical composition was measured using EDS, EPMA and XRD. The results of mechanical tests were compared to current standards for EH36. This study yielded three main observations. The first important result is the finding of severe anisotropy in the HMnAS. Impact toughness in the L-T direction was threefold the amount in the T-L direction. This is thought to be because of stringer MnS inclusions, elongated in the rolling direction. The second observation was the inadequate mechanical behaviour at the EH36 fusion line and the weld, these did not meet the requirements of IACS standards. Lastly, after observing a crack path along the fusion line of the CTOD fatigue pre-cracking, microhardness tests over the fusion lines were conducted and pointed out a hard band along the EH36 fusion line. This band is found to be a martensitic band (15-108 microns in width) that hampers mechanical performance. This investigation concludes that the current weld joint is not as desired. More research is required to improve the weld joint before HMnAS can be implemented on navy ships. Future research for DMO should mainly focus on the welding characteristics and the filler metal composition, to improve weld performance., Materials Science and Engineering

Localised laser treatments enable the creation of sophisticated austenite/martensite mesostructures in Fe–Ni–C steel with the potential of achieving enhanced mechanical performance. The control of phase topology is essential to modify the properties of these structures on demand and requires a profound understanding of the effect of the processing parameters on the development of the different phases upon the application of laser treatment. In this work, the microstructure evolution under exceptional gradients in temperature and heating rates is thoroughly investigated. The extent of the laser-affected zone and the heat input were tailored by varying laser parameters and specimen thickness, based on a model that considers transient material properties and the coupling between temperature and microstructure. The predicted temperature fields resulted in a complex interplay between martensite to austenite phase transformation and martensite tempering. Considering the high heating rates of up to 25000 K/s and the observed microstructures, it is suggested that austenite was formed by a pseudo-displacive mechanism and subsequently fully recrystallised in the zones most directly affected by the laser heat source. A smooth strength transition from austenite to martensite, affected by the laser parameters, could be exploited for more effective deformation mechanisms and improved material mechanical properties., Team Marcel Hermans, Team Maria Santofimia Navarro

The work presents results on the effect of prior martensite formation on bainite transformation kinetics in a 3% medium-Mn multiphase steel. The material was subjected to two isothermal holding temperatures: 400 °C (without martensite) and 350 °C (with prior martensite). According to obtained dilatometric results, the formation of prior martensite leads to the acceleration of bainite transformation kinetics. The bainite formation starts and finishes much faster, when the prior martensite was present before the isothermal holding. The microstructural investigation of the steel after heat treatment was carried out using light and scanning electron microscopy. The microstructures were composed of fine bainitic laths with retained austenite and small amount of martensitic-austenitic islands at 400 °C. At 350 °C the presence of large tempered martensite laths was detected. The bainite is composed of a mixture of fine and coarse laths. The increase of the bainitic lath thickness is attributed to the coalescence process occurring at the lower holding temperature. The differences in the steel hardness after the two heat treatments were relatively small (~ 13 HV10).

Metastability of martensite (and bainitic ferrite) as carbon super-saturated structures Metastability of retained austenite driving work-hardening behavior in TRIP (and TWIP)-assisted steels Ultra high strength material with the ability to harden under deformation allowing it to absorb energy before fracture.

"Hypereutectoid steels such as 1%C-1.5%Cr steel (52100 / 100Cr6) or 1%C-1.75%Cr-0.3%Mo (100CrMo7- 3) are largely used in the bearing industry, either after quenching and tempering to achieve a tempered martensite microstructure, or after austempering at relatively low temperature (220-230 °C) to achieve a fine lower bainite one [1-2]. Depending on austenitising conditions and tempering temperature, a significant fraction of retained austenite may be present in the microstructure. During tempering however, bainite formation may take place in the retained austenite. In the absence of at least 1% Si in the steel, this reaction will lead to the complete decomposition of the austenite [3]. In the presence of ~1%Si or more, the bainite reaction in retained austenite is incomplete. While this is well known and described for isothermal bainite formation, the presence of prior martensite introduces an additional source of carbon that may stabilise retained austenite along with the bainite reaction. In the following we investigate the influence of variable fractions of prior martensite on the subsequent bainite reaction in retained austenite, using dilatometry and X-ray diffraction. Variable fractions of retained austenite after quenching are achieved either by varying the austenitising conditions (in the austenite + cementite domain), or by changing the end quench temperature. A significant role of prior martensite is highlighted, in defining the maximum volume of retained austenite susceptible of bainitic decomposition. A simple model is proposed to account for the results. This research is funded by Research Fund for Coal and Steel, grant number RFCS-2019-899251."

"Bainitic microstructures developed by isothermal transformation at low transformations temperatures are characterized by a high fraction of nanosized bainitic ferrite plates interwoven with carbon enriched retained austenite films [1–3]. This microstructure gives the steel unrivalled mechanical properties when considering the simplicity of the chemical composition and the heat treatment applied. Although the use of such microstructures is gaining ground in the market, one of its main drawbacks is the slow rate at which transformation takes place. One approach to solve this problem that is becoming increasingly relevant in recent times is the generation of martensite prior to the bainitic transformation. The introduction of martensite/austenite interfaces and a high dislocations density by the martensitic transformation acts as catalysts for the bainitic reaction [4–6]. In this work. certain amounts of martensite have been introduced taking into account the constraints imposed by industrial treatments and the changes introduced both in the kinetics of the bainitic transformation and in the resulting microstructure has been analysed. This research is funded by Research Fund for Coal and Steel, grant number RFCS-2019-899251"

Isothermal transformation at low temperatures produces bainitic microstructures with a high fraction of nanosized bainitic ferrite plates interwoven with carbon-enriched retained austenite films. This microstructure gives the steel exceptional mechanical properties despite the simplicity of the chemical composition and heat treatment applied. While the use of microstructures is becoming more popular in the market, one of its main drawbacks is the slow rate of transformation. To address this issue, generating martensite prior to the bainitic transformation has become increasingly relevant. This introduces martensite/austenite interfaces and a high dislocation density, which act as catalysts for the bainitic reaction. This work analyses the changes in the kinetics of the bainitic transformation and resulting microstructure due to the introduction of certain amounts of martensite, while considering the constraints imposed by industrial treatments.

Both bainite and martensite reactions involve a sudden, ordered movement of iron atoms, which is accompanied by a given crystal correspondence, and a macroscopic shape strain of the transformed structure. Carbon supersaturation and tetragonality are not exclusive of martensite. A high level of carbon supersaturation in bainitic ferrite accompanied by a change in symmetry of the unit cell from cubic to tetragonal have been observed at low transformation temperatures confirming that, as martensite, bainitic ferrite grows without diffusion of carbon. It is therefore difficult to identify the morphological differences to discriminate metastable ferritic structures. In this work, comparable bainitic and martensitic structures were obtained and investigated by transmission electron microscopy (TEM) and atom probe tomography (APT) to identify all the subtle differences to discriminate both types of ferritic structures.

Needle-like microstructures are often observed in shape memory alloys near macro-interfaces that separate regions with different laminate orientation. We study their shape with a two-dimensional model based on nonlinear elasticity, that contains an explicit parametrization of the needle profiles. Energy minimization leads to specific predictions for the geometry of needle-like domains. Our simulations are based on shape optimization of the needle interfaces, using a polyconvex energy density with cubic symmetry for the elastic problem, and a numerical implementation via finite elements on a dynamically changing grid., Peer Reviewed

The article deals with phase transformations and austenitizing behavior of the X155CrMoV12 tool steel. Dilatation analyses of a series of samples were performed at various cooling rates, chosen in the range from 10 °C/s to 0,1° C/s. Acquired experimental data were used for evaluation of dilatometric curves in order to map the temperature ranges of phase transformations of the austenite to pearlite, bainite or martensite. All experimental samples from dilatometric analyses were then subjected to microstructural analyses and hardness measurements to characterize the microstructure and hardness for each tested heat treatment regime., Příspěvek se zabývá fázovými přeměnami a austenitizačním chováním nástrojové oceli X155CrMoV12. Byly provedeny dilatační analýzy řady vzorků při různých rychlostech ochlazování, zvolených v rozmezí od 10 °C/s do 0,1 °C/s. Získaná experimentální data byla použita pro vyhodnocení dilatometrických křivek s cílem zmapovat teplotní rozsahy fázových přeměn austenitu na perlit, bainit nebo martenzit. Všechny experimentální vzorky z dilatometrických analýz byly následně podrobeny mikrostrukturním analýzám a měřením tvrdosti, aby se charakterizovala mikrostruktura a tvrdost pro každý testovaný režim tepelného zpracování.

Martensitik faz dönüşümleri şekil hafızalı alaşımların ana deformasyon mekanizmasını oluşturan, çeliğin mekanik özelliklerinin belirlenmesinde çok büyük önemi olan katı-katı faz dönüşümleridir. Bu dönüşümlerin oluşumu esnasında, cismin bazı bölgelerindeki yüksek dönüşüm gerinimleri yüksek gerilmelerin oluşmasına sebep olur. Bu bölgelerde inelastik gerinimler oluşur ve faz dönüşümünün termodinamiğini ve kinetiğini etkiler. Uygulanan yük ve sıcaklık sonucunda oluşan faz dönüşümü süresince mikroyapıda meydana gelen değişikliklerin belirlenebilmesi, malzemenin mekanik özelliklerinin bu değişikliklere göre tasarlanmasına olanak sağlar. Bu amaçla, yakın zamanda elastik ve inelastik malzemelerdeki martensitik faz dönüşümlerinin mikromekanik tabanlı modelleri (Özsoy ve Levitas, 2007) geliştirilmiştir. Bu çalışmada elastik malzemelerdeki faz dönüşümleri için detaylı bir sayısal çalışma yapılmasına rağmen gerçek malzeme parametreleri kullanılmamıştır. Ayrıca, inelastik malzemelerdeki martensitik faz dönüşümlerinin sayısal çalışması tamamlanmam şt r. Bu tez çalışmasında, öncelikle Özsoy ve Levitas'ın (2007) elastik malzemeler için geliştirdikleri model şekil hafızalı bir alaşım için kullanılarak deneysel çalışmalarla karşılaştırma yapılarak doğrulamıştır. Daha sonra inelastik malzemeler için geliştirilen modelin farklı şartlardaki mikroyapı değişimleri, bu değişimlerin sebepleri, plastik deformasyonun ve uygulanan gerilme veya sıcaklığın faz dönüşümü üzerindeki etkisinin belirlenmesi için hesaplamalı bir teknik geliştirilerek nümerik çalşması yapılmıştır., Martensitic phase transformations are solid to solid phase transformations, which have an important role in determination of mechanical properties of steels and represent the main deformation mechanism of shape memory alloys. During such transformations, large transformation strain in some regions of a body results in development of large stresses. At these regions, inelastic strains develop and acts the thermodynamics and kinetics of the phase transformation. Determination of changes that occur in the microstructure during phase transformation taking place under applied load and temperature enables the design of mechanical properties of a material with respect to these changes. For this purpose, micromechanics based models for martensitic phase transformations in elastic and inelastic materials (Ozsoy and Levitas, 2007) have been developed recently. Although a detailed numerical study has been made for elastic materials in the study, real material parameters have not been used. Also, numerical study of martensitic phase transformations in inelastic materials has not been completed. In this thesis study, rstly, the model developed by Ozsoy and Levitas (2007) for elastic materials is studied numerically for a shape memory alloy and veried by comparison with experimental results. Then, the model developed for inelastic materials is studied numerically by developing a computational technique to determine the changes in microstructure under dirent conditions, the reason behind such changes, and inence of plastic deformation and applied stress or temperature on phase transformations.

[Resumo] A industria demanda cada vez máis solucións tecnolóxicas que permitan diversificar e personalizar os seus produtos. A tecnoloxía láser, dadas as súas calidades, é unha ferramenta ideal para o tratamento personalizado de superficies, sobre todo, pola flexibilidade que ofrece, grazas á posibilidade de conformar ou distribuír a enerxía do feixe de forma localizada e eficiente. Neste traballo preséntase un estudo dos procesos de endurecemento e de revenido superficial mediante un feixe láser oscilante, conformado por medio dun escáner de espellos galvanométricos de alta dinámica e xerado por un láser de diodos de onda continua. O estudo céntrase, por unha banda, en determinar a influencia do parámetro de frecuencia de oscilación durante o proceso de endurecemento superficial do aceiro 42CrMo4. Doutra banda, investígase o efecto do proceso de revenido superficial realizado sobre unha rexión do material previamente endurecida pola mesma fonte láser, sobre a temperatura, microestructura, estado tensional e dureza do aceiro tratado. Finalmente, proponse unha estratexia particular de conformado, para obter un deseño optimizado de distribución de enerxía láser equivalente, capaz de endurecer e revenir de forma simultánea a superficie do material, sen necesidade de realizar ambos os procesos por separado., [Abstract] Industry is increasingly demanding technological solutions for diversifying and customising its products. Laser technology, given its attributes, is an ideal tool for the personalised treatment of surfaces, above all, due to the flexibility it offers, thanks to the possibility of shaping or distributing the beam energy in a localised and efficient way. This work presents a study of the surface hardening and tempering processes using an oscillating laser beam, shaped by means of a high-dynamic galvanometric mirror scanner and generated by a continuous wave diode laser. The study focuses on the one hand, on determining the influence of the oscillation frequency parameter during the surface hardening process of 42CrMo4 steel. On the other hand, the effect of the surface tempering process, carried out on a region of the material previously hardened by the same laser source, on the temperature, microstructure, stress state and hardness of the treated steel, is investigated. Finally, a particular shaping strategy is proposed to obtain an optimised design of equivalent laser energy distribution, capable of simultaneously hardening and tempering the surface of the material, without the need to perform both processes separately., [Resumen] La industria demanda cada vez más soluciones tecnológicas que permitan diversificar y personalizar sus productos. La tecnología láser, dadas sus cualidades, es una herramienta ideal para el tratamiento personalizado de superficies, sobre todo, por la flexibilidad que ofrece, gracias a la posibilidad de conformar o distribuir la energía del haz de forma localizada y eficiente. En este trabajo se presenta un estudio de los procesos de endurecimiento y de revenido superficial mediante un haz láser oscilante, conformado por medio de un escáner de espejos galvanométricos de alta dinámica y generado por un láser de diodos de onda continua. El estudio se centra, por un lado, en determinar la influencia del parámetro de frecuencia de oscilación durante el proceso de endurecimiento superficial del acero 42CrMo4. Por otro lado, se investiga el efecto del proceso de revenido superficial realizado sobre una región del material previamente endurecida por la misma fuente láser, sobre la temperatura, microestructura, estado tensional y dureza del acero tratado. Finalmente, se propone una estrategia particular de conformado, para obtener un diseño optimizado de distribución de energía láser equivalente, capaz de endurecer y revenir de forma simultánea la superficie del material, sin necesidad de realizar ambos procesos por separado.

The realisation of sophisticated hierarchically patterned multiphase steels has the potential to enable unprecedented properties in engineering components. The present work explores the controlled creation of patterned multiphase steels in which the patterns are defined by two different crystal structures: face centre cubic or fcc (austenite) and body centre cubic or bcc (martensite). These austenite/martensite mesostructures are generated by solid–solid phase transformations during the application of localised laser heat treatments in a Fe-Ni-C alloy. In particular, four patterned configurations are analysed in this work consisting of one or two horizontal austenite line structures imprinted in a base of as-quenched or tempered martensite. Digital image correlation analysis during tensile testing of the developed materials showed that both the strength of the base martensite and the mesostructure at the gauge have a strong effect on the resulting properties. Clear differences were observed among the configurations in strain partitioning, hardening of the different constituents and failure. The uniform elongation and tensile strength are increased with respect to that of the reference martensite and austenite, respectively. Concepts explored in this work can be extended to more complex patterns and other base microstructures, opening novel strategies to engineer properties in steel and other alloys., Team Maria Santofimia Navarro, Team Jilt Sietsma

Three composite bronzes reinforced with steel dendrites from steels of different classes, which perform the function of a bearing surface in sliding friction, are studied. The chemical compositions of the dendrites and of the matrix are determined as a function of the heat treatment of bronze BrZhNOA 12-7-2-1. The fine structure of dendrites in bronze BrZhNKoA 9-4-1-1 is studied and the effect of the aging temperature in the intermetallic reinforcement of the dendrite with composition corresponding to maraging steel N23Yu1 is determined. The type of the crystal lattice of the dendrite is shown to affect substantially the tribological properties of bronze BrZhNKhA 12-9-3-1. © 2022, Springer Science+Business Media, LLC, part of Springer Nature.

The article deals with phase transformations and austenitizing behavior of the X155CrMoV12 tool steel. Dilatation analyses of a series of samples were performed at various cooling rates, chosen in the range from 10 °C/s to 0,1° C/s. Acquired experimental data were used for evaluation of dilatometric curves in order to map the temperature ranges of phase transformations of the austenite to pearlite, bainite or martensite. All experimental samples from dilatometric analyses were then subjected to microstructural analyses and hardness measurements to characterize the microstructure and hardness for each tested heat treatment regime., Příspěvek se zabývá fázovými přeměnami a austenitizačním chováním nástrojové oceli X155CrMoV12. Byly provedeny dilatační analýzy řady vzorků při různých rychlostech ochlazování, zvolených v rozmezí od 10 °C/s do 0,1 °C/s. Získaná experimentální data byla použita pro vyhodnocení dilatometrických křivek s cílem zmapovat teplotní rozsahy fázových přeměn austenitu na perlit, bainit nebo martenzit. Všechny experimentální vzorky z dilatometrických analýz byly následně podrobeny mikrostrukturním analýzám a měřením tvrdosti, aby se charakterizovala mikrostruktura a tvrdost pro každý testovaný režim tepelného zpracování.

Various approaches are reviewed for determining elastic anisotropy and its coupling to crystallographic texture, with special reference to ultrasonic measurements. Two new methods are described for measuring the anisotropy of P-wave velocity using laser-ultrasonics. Making measurements across the diameter of a cylindrical specimen as it is rotated makes it possible to maintain a very constant known path length. This permits extremely accurate measurements with a precision of better than 0.01%. Results on 316 stainless steel in different conditions are compared with calculated values obtained from EBSD textures together with measured densities and crystalline coefficients from the literature. Excellent agreement is obtained when applying the Hill geometrical average procedure. A similar approach is adopted to measure the variation of wave velocity in a martensitic steel, after tempering at a range of temperatures. Changes in the anisotropy associated with thermal softening are discussed. The second method uses Galvano mirrors to steer the generating laser to different positions over a sheet surface, allowing wave velocities to be determined along different directions in the anisotropic material., QC 20230213

Biomedical beta-phase Ti-Nb-Ta-Zr alloys usually exhibit low elastic modulus with inadequate strength. In the present work, a series of newly developed dual-phase Ti-xNb-yTa-2Zr (wt.%) alloys with high performance were investigated in which the stability of beta-phase was reduced under the guidelines of ab initio calculations and d-electronic theory. The effects of Nb and Ta contents on the microstructure, compressive and tensile properties were investigated. Results demonstrate that the designed Ti-xNb-yTa-2Zr alloys exhibit typical characteristics of alpha+beta dual-phase microstructure. The microstructure of the alloys is more sensitive to Nb rather than Ta. The as-cast alloys exhibit needle-like alpha' martensite at a lower Nb content of 3 wt.% and lamellar alpha' martensite at an Nb content of 5 wt.%. Among the alloys, the Ti-3Nb-13Ta-2Zr alloy shows the highest compressive strength (2270 +/- 10 MPa) and compressive strain (74.3% +/- 0.4%). This superior performance is due to the combination of alpha+beta dual-phase microstructure and stress-induced alpha '' martensite. Besides, lattice distortion caused by Ta element also contributes to the compressive properties. Nb and Ta contents of the alloys strongly affect Young's modulus and tensile properties after rolling. The as-rolled Ti-3Nb-13Ta-2Zr alloy exhibits much lower modulus due to lower Nb content as well as more alpha '' martensite and beta phase with a good combination of low modulus and high strength among all the designed alloys. Atom probe tomography analysis reveals the element partitioning between the a and beta phases in which Ta concentration is higher than Nb in the alpha phase. Also, the concentration of Ta is lower than that of Nb in the beta phase, indicating that the beta-stability of Nb is higher than that of Ta. This work proposes modern alpha+beta dual-phase Ti-xNb-yTa-2Zr alloys as a new concept to design novel biomedical Ti alloys with high performance., QC 20220810

Ultra-high strength steels are widely utilized in many applications operating in harsh abrasive wear conditions. For instance, the machineries used in mining and mineral handling or in agricultural sector require robust, but cost-effective wear-resistant materials. Steels provide excellent combination of mechanical properties and usability. This study encompasses mechanical and wear testing of an experimental medium-carbon press-hardening steel. The as-received material was austenitized at two different temperatures and quenched in water. Additionally, low-temperature tempering was applied for one variant. In total, three variants of the press-hardening steel were produced. Microstructural characterization and mechanical testing were conducted for the steel samples. The wear testing was carried out with high-stress abrasive method, in which the samples were rotated inside a crushed granite bed. A commercial 400 HB grade wear-resistant steel was included in the wear testing as a reference. The experimental steel showed very high mechanical properties reaching tensile strength up to 2600 MPa with hardness of 750 HV10. Wear testing resulted in only minimal differences between the three variants indicating that the improved impact toughness by tempering did not significantly affect the wear resistance. The reference steel had nearly two times greater mass loss compared to the higher hardness press-hardening steels. Microhardness measurements on the worn surface showed drastic increase in hardness for the deformed structure for all samples. It was concluded that even the high-hardness martensitic steels exhibit notable wear surface work-hardening. Therefore, hardness was determined to be the most significant factor affecting the wear performance of studied steels.

The effect of prior deformation on the evolution of the martensite phase in Ti-6Al-4V alloy is reported by varying the hot-rolling temperature to have different stored energies. While the morphology of the martensite phase is greatly influenced by the prior deformation, the phase fraction is primarily dependent on the quenching temperature. The compositional deviation from the equilibrium condition, emerging from the differences in the diffusion of elements at different rolling temperatures, govern the nucleation and aspect ratio of the martensite laths. Higher stored energy in the deformed samples, calculated as the dislocation density, is found to derive the formation of a twinned plate martensite. Martensitic transformation, irrespective of the prior deformation condition, induces a strong variant selection based on the minimization of the transformation strain energy. However, a strong correlation between the morphology and the character of the intervariant boundaries of the martensite phase is established. The intervariant boundary distribution in martensite showed three major angle–axis pairs associated with the Burgers orientation relationship. The hardness of the martensite phase is determined by the solid solution strengthening, martensite morphology, and microstrain present in the sample., QC 20230523

Hydrogen embrittlement (HE) is a well-recognized issue with ultrahigh-strength steels (UHSS) and an intensively studied subject to minimize the risk of sudden, catastrophic failures in structural applications. A novel tuning-fork test (TFT) is developed in this thesis to study HE of UHSS. The testing method utilizes constant displacement for stressing of the tuning-fork specimens by bending, combined with electrochemical hydrogen charging. With an isolated tensile stress region, crack initiation is controlled and can be monitored with different clamping arrangements. The main objectives were to create a simple and fast testing method, which allows ranking of UHSS, and to investigate the effects of prior austenite grain (PAG) structure. Traditional hydrogen permeation tests with a set-up built within this thesis complement the results. The first part of the TFT implementation is composed of an evaluation of the HE susceptibility of 300–600 HBW martensitic steels with relative threshold stress levels. Increasing the strength level leads to lower threshold stress levels and increased susceptibility to HE, especially for steels with over 400 HBW hardness. With the addition of a loadcell to the TFT clamping system, a more precise investigation was conducted for 500 HBW steels with various PAG structures. The effect of PAG morphology (elongated/equiaxed) on HE susceptibility was studied with steels that had different alloying compositions and with steels that had different PAG morphologies but the same alloying. An elongated PAG structure leads to enhanced resistance against HE in comparison to equiaxed PAG structures. Crack propagation rates were slower with more elongated microstructure, which is linked to transgranular quasi-cleavage crack propagation and slower hydrogen diffusion. Equiaxed microstructure exhibits partly intergranular cracking with faster crack propagation rate and hydrogen diffusion. Crack propagation was affected by the microstructure alignm, Tiivistelmä Ultralujien terästen vetyhauraus on tunnettu ilmiö, jonka tutkiminen on tärkeää koska se voi pahimmillaan johtaa äkkinäisiin, rakenteiden katastrofaalisiin vaurioihin. Tässä työssä kehitetään uutta testausmenetelmää nimeltään äänirautakoe (Tuning-Fork Test, TFT) ultralujien terästen vetyhaurauden tutkimiseksi. Äänirautakokeessa yhdistetään vakiosiirtymän kautta määritetty elastinen jännitystila ja sähkökemiallinen vetylataus. Erityyppisten geometrioiden ja puristimien avulla äänirautanäytteeseen vaikuttava jännitystila saatiin rajattua tarkasti alueelle, jossa särön ydintyminen tapahtuu, jolloin särönkasvua voidaan kontrolloida ja seurata. Väitöskirjan päätavoitteena oli kehittää nopea ja yksinkertainen testausmenetelmä, joka mahdollistaa ultralujien terästen vertailun ja mikrorakenteellisten tekijöiden vaikutuksen tutkimisen vedyn aiheuttamassa murtumisessa. Täydentävää tutkimusta tehtiin perinteisellä vedyn permeaatiokokeella. Ensimmäisessä vaiheessa 300–600 HBW kovuusluokan martensiittisten terästen taipumusta vetyhaurauteen arvioitiin määrittämällä terästen suhteellinen kynnysjännitys. Tulokset osoittavat kovuuden/lujuuden kasvun lisäävän teräksen alttiutta vetyhaurauteen, joka korostuu teräksen kovuuden ollessa yli 400 HBW. Voima-anturin lisääminen testauslaitteistoon paransi mittaustarkkuutta, ja mahdollisti perinnäisen austeniitin raerakenteen vaikutuksen tutkimisen. Vertailussa käytettiin 500 HBW teräksiä, jotka erosivat perinnäisen austeniitin muodon (venynyt/tasa-aksiaalinen) ja koon suhteen ja osa seostukseltaan. Venynyt perinnäisen austeniitin raerakenne paransi vetyhaurauden kestoa. Särön etenemisnopeus oli hitaampi venyneessä mikrorakenteessa, joka johtui rakeiden läpi etenevästä murtumasta sekä hitaammasta vedyn diffuusiosta. Tasa-aksiaalisessa rakenteessa vedyn diffuusio oli nopeampaa ja särön etenemisnopeus kasvoi ja murtuma tapahtui osittain raerajamurtumana. Mikrorakenteellinen anisotropia vaikutti särön etenemiseen venyneissä rakeissa

The residual compressive stresses and dimensional changes related to the lattice strains of retained austenite (RA) phase in a high-Si, medium-carbon steel (Fe-0.53C-1.67Si-0.72Mn-0.12Cr) are investigated for samples austenitized and quenched for isothermal bainitic transformation (Q&B) in the range 5 s to 1 h at 350 °C. Also, samples are directly quenched in water (DWQ) from the austenitization temperature for comparison with Q&B samples. Field emission scanning electron microscopy (FE-SEM) combined with electron backscatter diffraction (EBSD) analyses, and X-ray diffraction are used to investigate the microstructural evolution, phase distribution, and lattice parameters of RA phase. While the Q&B samples showed formation of bainite and high-carbon fresh martensite in conjunction with stabilization of various fractions of RA, the DWQ samples displayed nearly complete martensitic microstructure. For short holding durations (≪200 s), there was limited formation of bainite and the inadequate carbon partitioning to the adjacent untransformed austenite areas resulted in significant martensite formation and the associated c/a ratio of martensite resulted in high compressive residual stresses within the RA phase. While, at long isothermal holding times (≫ 200 s), there was a significant formation of bainite. The DWQ samples displayed maximum lattice strain in a small fraction of untransformed RA phase.

Over the last ten years, global demand for high-strength steels has grown significantly. Stricter environmental criteria and more demanding applications have accelerated the development of high-strength steels and improved the properties of these steels. The chemical composition, as well as the microalloying, of direct quenched steels, together with the manufacturing process, substantially affect the properties of the resulting steel. The use of various more special alloying elements has become more common, especially in steels made for the most demanding applications. Used correctly, these alloying elements can clearly improve the mechanical properties of steel. The aim of the dissertation was to study the effects of certain alloying elements on the microstructure and mechanical properties of direct quenched ultra-high-strength steels, and to optimize their use for maximum benefit. The compositions of the steels in the study were based on existing commercial steel grades and therefore, the achieved results can be used in the development of new steel grades. The results of the dissertation showed that phase transformations in steel can be influenced by microalloying, and especially the use of molybdenum can strongly promote the formation of lower temperature phases such as bainite and martensite. With molybdenum alloying, together with the correct process parameters, it is possible to obtain excellent mechanical properties in direct quenched steel. The results also showed that molybdenum increased tempering resistance without actual precipitation strengthening. The greatest advantage of niobium alloying with direct quenched steels was seen to be in slowing down the recrystallization process, which led to the formation of a finer martensite structure, increasing the strength as well as improving the toughness. The effect of niobium was also significant in tempering resistance, where the strong precipitation strengthening clearly increased the yield strength, Tiivistelmä Viimeisen kymmenen vuoden aikana lujien terästen kysyntä on kasvanut maailmalla merkittävästi. Tiukentuneet ympäristökriteerit sekä yhä vaativammat käyttökohteet ovat nopeuttaneet lujien terästen kehitystyötä sekä parantaneet näiden terästen ominaisuuksia. Suorasammutettujen terästen kemiallinen koostumus sekä käytettävät mikroseosaineet yhdessä valmistuspraktiikan kanssa vaikuttavat olennaisesti syntyvän teräksen ominaisuuksiin. Erilaisten seosaineiden käyttö on lisääntynyt varsinkin vaativimpien käyttökohteiden teräksissä. Näillä seosaineilla voidaan oikein käytettynä parantaa selvästi teräksen mekaanisia ominaisuuksia. Väitöstyön tavoitteena oli tutkia tiettyjen seosaineiden vaikutusta ultralujien terästen mikrorakenteeseen ja mekaanisiin ominaisuuksiin, sekä optimoida niiden käyttö maksimaalisen hyödyn aikaansaamiseksi. Tutkittavien terästen koostumukset pohjautuivat jo olemassa oleviin kaupallisiin teräksiin, joten saatuja tuloksia on mahdollista hyödyntää uusien teräslajien kehityksessä. Väitöstyön tulokset osoittivat, että mikroseostuksessa voidaan vaikuttaa teräksen faasimuutoksiin ja etenkin molybdeenin käytöllä voidaan voimakkaasti edesauttaa alempien lämpötilojen faasien, kuten bainiitin ja martensiitin muodostumista. Molybdeeniseostuksella, yhdessä oikeiden prosessiparametrien kanssa, on mahdollista saada erinomaiset mekaaniset ominaisuudet suorasammutetulle teräkselle. Tulosten perusteella nähtiin myös, että molybdeeni lisäsi päästönkestävyyttä ilman suoranaista erkautuslujittamista. Niobiseostuksen suurin etu suorasammutetuilla teräksillä nähtiin olevan rekristallisaatioprosessin hidastamisessa, mikä johti hienomman martensiittirakenteen syntyyn nostaen lujuutta sekä parantaen sitkeyttä. Niobin vaikutus oli merkittävä myös päästönkestävyydessä, missä voimakas erkautuslujittaminen nosti myötölujuutta selvästi verrattaessa ilman niobia olleisiin teräksiin. Boorivapaa niobiseostettu teräs omasi erittäin hyvät iskusitkeysominaisuudet johtuen te

This work deals with the kinetic aspects of bainite formation during isothermal holding above and below the martensite start (Mₛ~275 °C) temperature using a low-alloy, high-silicon DIN 1.5025 steel in a range suitable for achieving ultrafine/nanostructured bainite. Dilatation measurements were conducted to study transformation behaviour and kinetics, while the microstructural features were examined using laser scanning confocal microscopy and electron backscatter diffraction (EBSD) techniques combined with hardness measurements. The results showed that for isothermal holding above the Mₛ temperature, the maximum bainitic transformation rate decreased with the decrease in isothermal holding temperature between 450 and 300 °C. On the other hand, for isothermal holding below the Mₛ temperature at 250 and 200 °C, the maximum rate of transformation was achieved corresponding to region I due to the partitioning of carbon and also possibly because of the ledged growth of isothermal martensite soon after the start of isothermal holding. In addition, a second peak was obvious at about 100 and 500 s, respectively, during holding at 250 and 200 °C due to the occurrence of bainitic transformation, marking the beginning of region II.