2,521 research outputs found

    Impact toughness of an electron-beam welded 0.2C direct-quenched and partitioned steel

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    AbstractThird generation advanced high-strength steels, e.g., quenched and partitioned steels, are forthcoming structural materials, which consist of a martensitic matrix and a substantial proportion of stabilized residual austenite for improved deformability. A novel less energy-intensive processing route of direct-quenching and partitioning advances this concept by facilitating carbon partitioning to untransformed austenite directly from the quench-stop temperature. However, a major challenge also with these steels is how to maintain structural integrity in the welded end-products after additional heat-input reaching above a temperature where given microstructure is still stable. Heat-input limiting beam welding processes are a solution to this by minimizing degradation of the heat-affected zone (HAZ) and producing even-strength welded joints for S1100 and above. In this study, we report toughness properties of an electron-beam (EB) welded 0.2C-1.5Mn-0.5Si-0.8Al-1.1Cr-0.8Ni (wt.%) direct-quenched and partitioned steel (DQ&P) having a yield strength of ∌1100 MPa, and a direct-quenched (DQ) was used as a reference. Low-temperature post-weld heat treatment (PWHT) was considered, too. Weld seam, coarse-grained HAZ, and the base materials were tested for impact toughness. Both the DQ and DQ&P base materials have excellent impact toughness transition temperatures T28J below -100°C. The weld seam has very good low-temperature toughness already at this stage of optimisation with T28J of -66°C, which shows robustness of the chosen alloy. Increased residual austenite content increased upper shelf toughness but not T28J. Furthermore, both the DQ and DQP HAZs have T28J below -70°C, pointing to the weld seam as the weakest link. PWHT reduced low-temperature impact toughness in all the cases with T28J being above -40°C, clearly demanding reassessment of its feasibility.Abstract Third generation advanced high-strength steels, e.g., quenched and partitioned steels, are forthcoming structural materials, which consist of a martensitic matrix and a substantial proportion of stabilized residual austenite for improved deformability. A novel less energy-intensive processing route of direct-quenching and partitioning advances this concept by facilitating carbon partitioning to untransformed austenite directly from the quench-stop temperature. However, a major challenge also with these steels is how to maintain structural integrity in the welded end-products after additional heat-input reaching above a temperature where given microstructure is still stable. Heat-input limiting beam welding processes are a solution to this by minimizing degradation of the heat-affected zone (HAZ) and producing even-strength welded joints for S1100 and above. In this study, we report toughness properties of an electron-beam (EB) welded 0.2C-1.5Mn-0.5Si-0.8Al-1.1Cr-0.8Ni (wt.%) direct-quenched and partitioned steel (DQ&P) having a yield strength of ∌1100 MPa, and a direct-quenched (DQ) was used as a reference. Low-temperature post-weld heat treatment (PWHT) was considered, too. Weld seam, coarse-grained HAZ, and the base materials were tested for impact toughness. Both the DQ and DQ&P base materials have excellent impact toughness transition temperatures T28J below -100°C. The weld seam has very good low-temperature toughness already at this stage of optimisation with T28J of -66°C, which shows robustness of the chosen alloy. Increased residual austenite content increased upper shelf toughness but not T28J. Furthermore, both the DQ and DQP HAZs have T28J below -70°C, pointing to the weld seam as the weakest link. PWHT reduced low-temperature impact toughness in all the cases with T28J being above -40°C, clearly demanding reassessment of its feasibility

    Grain Growth after Intercritical Rolling

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    Work hardening behaviors of a low carbon Nb-microalloyed Si-Mn quenching-partitioning steel with different cooling styles after partitioning

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    In this paper, the strain hardening behaviors of a low carbon Nb-microalloyed Si–Mn quenching–partitioning (Q–P) steel were investigated. The microstructures were analyzed by the scanning electron microscope (SEM) and transmission electron microscope (TEM). Mechanical tests were used to evaluate the room temperature tensile properties of the steel. The work hardening behaviors of the tested specimens were analyzed using the Hollomon approach. The results showed that a two-stage work hardening behavior was observed during deformation processes. In the first stage, for the quenched samples, martensite deforms plastically and the hardening exponent decreased. For the air-cooled samples, however, the carbide-free ferrite deforms preferentially, and then, the carbide-free ferrite and martensite co-deform. In the second stage, due to the effect of transformation induced plasticity of retained austenite, the hardening exponent decreased slowly and plateaus were observed in the plots of ni–Δt until fracture. Variations of the work hardening behaviors were related to the martensite and the volume fraction of retained austenite in Q–P steels and the microstructural evolution during partitioning and following cooling process

    Microstructure design of third generation advanced high strength steels

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    This dissertation demonstrates that substantial ductility improvement is possible for low-manganese transformation induced plasticity steel compositions through the quenching and partitioning heat treatment approach using a Gleeble thermo-mechanical simulator. Two investigated compositions had unique microstructures and mechanical behavior from an identical applied quenching and partitioning process. Electron backscattered diffraction analyses indicate that Comp-2 and Comp-5 both contained retained austenite which resulted in enhanced ductility. The face-centered cubic phase (austenite) more efficiently mitigates strain incompatibilities when located at martensitic grain boundaries known for hot spots and damage initiation. This location effect leads to enhanced ductility and improved toughness in a lean, transformation induced plasticity steel. However, the increase in ductility in Comp-2 and Comp-5 is limited; the partitioning of carbon cannot stabilize austenite to reach strength/ductility targets set by the Department of Energy. Comp-2 and Comp-5 lack sufficient manganese to stabilize austenite to a higher degree. Chem-2A will be explored to determine if the partitioning stage can stabilize austenite closer to the martensite finish temperature. Periodic intercritical annealing will be applied to Chem-1A to see if mechanical properties can be increased further than current research values. Ultimately, through literature, Manganese is proven to be a more effective austenite stabilizer than carbon, and with tailored heat-treatment, the DOE targets can be reached

    Increasing phase transformation rate in advanced high strength steel applications

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    The bainite transformation rate has been shown to increase by starting the heat treatment with partial martensite transformation after austenitization. Based on this fact, a process very similar to “Quenching and Partitioning” (Q&P) is used to produce a fine-grained complex microstructure of martensite, bainite and retained austenite with outstanding mechanical properties in a very short time. During this process, different mechanisms including bainite transformation, carbon partitioning, carbide precipitation, grain growth may occur. All these mechanisms can affect the mechanical properties such as strength, ductility and toughness. Investigation of the different mechanisms influencing the properties and subsequent optimization of these is important. In this work, different mechanisms of the Q&P heat treatment process and its practical industrial applications have been investigated. Firstly, the implementation of a Q&P method directly after laser welding for a few seconds to substitute any post welding treatment has been studied. To investigate the feasibility, limitations, and advantages of this method for a low-carbon low-silicon high strength steel, the microstructure and mechanical properties by both modelling and experimental approach were studied. Promising results show that this method can decrease the ordinary post-welding treatment time from a few minutes to a few seconds, and in addition improve the mechanical properties of the fusion zone and the heat affected zone to the same or even higher values in comparison with the base material. In the second part of this work, the effect of quenching and partitioning on the microstructure and mechanical properties of a high carbon steel has been studied. The aim with this part was to optimize the phase transformation rate for production of ultra-high strength steel by controlling its microstructural evolution. The results show that it is possible to get good strength values also for high carbon steels by Q&P treatment. In addition, the approach with process control maps can give a good overview of which properties can be achieved by this method. Hardness value of over 700 HV, and tensile strength of up to 2.5 GPa with a relatively good ductility of 4-6% has been achieved by quenching to room temperature and partitioning for less than one minute at 400 °C resulting in a microstructure consisting of martensite and retained austenite. In a nutshell, the approach to bainite transformation with pre-existing martensite shorten the processing time for development of advanced high strength steels significantly. This method is also possible to be used in industrial processes as in welding.Die Rate, mit der Austenit zu Bainit umwandelt, wurde erhöht, wenn bereits teilweise Martensit nach der Austenitisierung gebildet wurde. Basierend darauf wurde ein Prozess entwickelt, der sehr Ă€hnlich mit dem „Quenching & Partitioning“-Prozess ist und der mit kurzer Prozesszeit ein feines und komplexes StahlgefĂŒge bestehend aus Martensit, Bainit und Restaustenit mit exzellenten mechanischen Eigenschaften produziert. WĂ€hrend dieses Prozesses treten mehrere Mechanismen auf, unter anderem die Bainitumwandlung, Kohlenstoffpartitionierung, Karbidausscheidung und Kornwachstum. All diese Mechanismen beeinflussen die mechanischen Eigenschaften wie Festigkeit, DuktilitĂ€t und ZĂ€higkeit. Die Untersuchung und anschließende Optimierung dieser Wirkweisen hinsichtlich der genannten Eigenschaften sind sehr wichtig. In der vorliegenden Arbeit wurden die verschiedenen Mechanismen des Q&P-Prozesses auf ihre industrielle Anwendbarkeit hin untersucht. Als erstes wurde der Einsatz von Q&P ĂŒber wenige Sekunden direkt nach dem Laserschweißen untersucht, um jegliche Schweißnachbehandlungen zu ersetzen. Sowohl Modellierung, als auch Charakterisierung der entstehenden Mikrostruktur und der mechanischen Eigenschaften eines niedrigkohligen siliziumarmen hochfesten Stahls wurden herangezogen, um die Anwendbarkeit, Grenzen und Vorteile dieser Methode zu untersuchen. Die Ergebnisse zeigen, dass diese Methode die Schweißnachbearbeitungszeit, die ĂŒblicherweise mehrere Minuten betrĂ€gt, auf Sekunden reduzieren kann. ZusĂ€tzlich werden die mechanischen Eigenschaften der Schweißzone und der WĂ€rmeeinflusszone derart verbessert, dass sie den Level des Ausgangsstahls erreichen oder sogar ĂŒbersteigen. Im zweiten Teil wird der Einfluss von Q&P auf die Mikrostruktur und mechanischen Eigenschaften eines hochkohligen Stahls untersucht. In diesem Fall war das Ziel die Optimierung der Umwandlungsrate zur Erzeugung eines ultra-hochfesten GefĂŒges. Es zeigte sich, dass auch fĂŒr hochkohlige StĂ€hle gute Festigkeitswerte mittels Q&P erzeugt werden können. DarĂŒber hinaus liefern Karten zur Prozesskontrolle eine gute Übersicht, welche Eigenschaften durch diese Methode erzielt werden können. HĂ€rtewerte ĂŒber HV700, und Zugfestigkeiten bis zu 2,5 GPa mit verhĂ€ltnismĂ€ĂŸig guten DuktilitĂ€ten von 4-6 % wurden erreicht, indem auf Raumtemperatur abgeschreckt („Quenching“) und fĂŒr weniger als eine Minute bei 400 °C gehalten wurde („Partitioning“). Das resultierende GefĂŒge bestand daraufhin aus Martensit und Restaustenit. Zusammengefasst lĂ€sst sich die Prozesszeit deutlich verkĂŒrzen, wenn die Bainitumwandlung in einem GefĂŒge der Austenit bereits teilweise zu Martensit umgewandelt ist. Diese Methode besitzt daher auch großes Potential bei der Anwendung in industriellen Schweißprozessen.Omvandlingen av austenit till bainit kan pĂ„skyndas genom att inleda vĂ€rmebehandlingen genom att först omvandla en del av austeniten till martensit. Baserad pĂ„ detta faktum, har en process som Ă€r mycket lik den sĂ„ kallade ”Snabbkylning och Omfördelningen” (“Quenching and Partitioning”, (Q&P)) anvĂ€nts för att skapa en finkornig komplex mikrostruktur bestĂ„ende av martensit, bainit och restaustenit med utmĂ€rkta mekaniska egenskaper pĂ„ mycket kort tid. Under denna process kan olika mekanismer innefattande bainitomvandling, kolomfördelning, karbidutskiljning och korntillvĂ€xt förekomma. Dessa kan pĂ„verka de mekaniska egenskaperna som hĂ„llfasthet, duktilitet och seghet. En undersökning av de olika mekanismerna som pĂ„verkar egenskaperna och den efterföljande optimeringen av dessa Ă€r viktig. I detta arbete har olika mekanismer som pĂ„verkar Q&P vĂ€rmebehandlingsprocessen och dess praktiska industriella tillĂ€mpningar undersökts. I första delen av detta arbete har med denna utgĂ„ngspunkt implementeringen av en Q&P metod direkt efter lasersvetsning för nĂ„gra sekunder för att ersĂ€tta andra efterbehandlingsmetoder studerats. För att studera möjligheterna, begrĂ€nsningarna och fördelarna av denna metod för ett lĂ„gkolhaltigt höghĂ„llfast stĂ„l med lĂ„g kiselhalt har mikrostrukturen och de mekaniska egenskaperna studerats genom sĂ„vĂ€l modellering som experimentellt. Resultaten Ă€r lovande och visar att den vanliga efterbehandlingstiden kan minskas frĂ„n nĂ„gra minuter till nĂ„gra fĂ„ sekunder, och dĂ€rtill kan de mekaniska egenskaperna för den smĂ€lta zonen och den vĂ€rmepĂ„verkade zonen förbĂ€ttras och kan vara lika bra eller till och med bĂ€ttre i jĂ€mförelse med grundmaterialet. I den andra delen av detta arbete har effekten av Q&P pĂ„ mikrostrukturen och de mekaniska egenskaperna för ett högkolhaltigt stĂ„l studerats. MĂ„let med denna del var att optimera omvandlingshastigheten för att tillverka ultrahöghĂ„llfast stĂ„l genom att styra mikrostrukturens utveckling. Resultaten visar att det Ă€r möjligt att erhĂ„lla goda hĂ„llfasthetsegenskaper Ă€ven för högkolhaltiga stĂ„l genom Q&P behandling. DĂ€rtill, ger ansatsen med processtyrningskartor en bra översikt över vilka egenskaper som kan erhĂ„llas med denna metod. HĂ„rdhetsvĂ€rde pĂ„ över 700 HV och brottgrĂ€ns pĂ„ upp till 2.5 GPa med relativt god duktilitet pĂ„ 4-6 % har erhĂ„llits genom snabbkylning till rumstemperatur och omfördelning vid 400 °C under mindre Ă€n en minut resulterade i en mikrostruktur bestĂ„ende av martensit och restaustenit. I ett nötskal kan ansatsen att genomföra bainitomvandling med hjĂ€lp av en viss andel martensit som skapas i förvĂ€g förkorta tiden avsevĂ€rt för processer för att utveckla avancerade höghĂ„llfasta stĂ„l. Metoden Ă€r ocksĂ„ möjlig att anvĂ€nda för industriella processer som svetsning.Erasmus mundus+ (DocMase program) funding for 3 years, LuleĂ„ university of technology funding for 1 yea

    Nanoscale austenite reversion through partitioning, segregation, and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C steel

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    Austenite reversion during tempering of a Fe-13.6Cr-0.44C (wt.%) martensite results in an ultrahigh strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite-martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e., changing tempering time and temperature tailors the desired strength-ductility profiles (e.g. tempering at 400{\deg}C for 1 min. produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30 min. at 400{\deg}C results in a UTS of ~ 1.75 GPa with an elongation of 23%). The austenite reversion process, carbide precipitation, and carbon segregation have been characterized by XRD, EBSD, TEM, and atom probe tomography (APT) in order to develop the structure-property relationships that control the material's strength and ductility.Comment: in press Acta Materialia 201

    Fracture toughness characteristics of thermo-mechanically rolled direct quenched and partitioned steels

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    AbstractThe propensity of the retained austenite to transform to martensite under external strain is known to induce plasticity (TRIP) in steels. If the retained austenite has a high volume fraction, it generally has blocky morphology and increases the tensile ductility via TRIP effect but can potentially deteriorate the yield strength. The present work demonstrates improved balance of strength and fracture toughness in a Si-containing 0.2%C steel having a refined martensitic matrix and evenly distributed and finely divided retained austenite (RA). Such a microstructure is obtained by thermo-mechanical rolling followed by direct quenching and partitioning (TMR-DQP) of the steel that results in establishing a constrained carbon equilibrium stabilising the austenite to room temperature. Transmission electron microscopy was used to characterise the nano-twinned martensite and the interlath nanometer thick austenite. An improved combination of yield strength, YS (1171 MPa), ultimate tensile strength, UTS (1419 MPa) and elastic-plastic fracture toughness, KJC (154 MPa √m) was achieved as compared to YS 1240 MPa, UTS 1654 MPa and KJC 111 MPa √m, respectively, for the fully martensitic counterpart. X-ray diffraction combined with Rietveld analysis revealed a reduction in the retained austenite from 9% away from the crack, which undergoes little strain, to 4 vol% near the crack, under higher strain, demonstrating strain induced martensitic transformation. The constrained nature of the austenite-to-martensite transformation within the rigid surrounding martensite is believed to increase the energy required to drive the crack forward, which raises the fracture toughness. Importantly, the results show that the uniformly distributed and nanoscale retained austenite is effective in imparting transformation-induced- plasticity at relatively small strength penalty.Abstract The propensity of the retained austenite to transform to martensite under external strain is known to induce plasticity (TRIP) in steels. If the retained austenite has a high volume fraction, it generally has blocky morphology and increases the tensile ductility via TRIP effect but can potentially deteriorate the yield strength. The present work demonstrates improved balance of strength and fracture toughness in a Si-containing 0.2%C steel having a refined martensitic matrix and evenly distributed and finely divided retained austenite (RA). Such a microstructure is obtained by thermo-mechanical rolling followed by direct quenching and partitioning (TMR-DQP) of the steel that results in establishing a constrained carbon equilibrium stabilising the austenite to room temperature. Transmission electron microscopy was used to characterise the nano-twinned martensite and the interlath nanometer thick austenite. An improved combination of yield strength, YS (1171 MPa), ultimate tensile strength, UTS (1419 MPa) and elastic-plastic fracture toughness, KJC (154 MPa √m) was achieved as compared to YS 1240 MPa, UTS 1654 MPa and KJC 111 MPa √m, respectively, for the fully martensitic counterpart. X-ray diffraction combined with Rietveld analysis revealed a reduction in the retained austenite from 9% away from the crack, which undergoes little strain, to 4 vol% near the crack, under higher strain, demonstrating strain induced martensitic transformation. The constrained nature of the austenite-to-martensite transformation within the rigid surrounding martensite is believed to increase the energy required to drive the crack forward, which raises the fracture toughness. Importantly, the results show that the uniformly distributed and nanoscale retained austenite is effective in imparting transformation-induced- plasticity at relatively small strength penalty

    Effect of Cooling Practice on the Mechanical Properties of Medium‐Manganese Aluminum‐Alloyed Steels after Intercritical Annealing Quench and Partition Treatment

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    Abstract This study reports the effect of different cooling practices after hot rolling on the microstructure and mechanical properties of intercritically annealed quench and partitioned low-carbon medium-manganese aluminum-alloyed steel. The outcomes show that the tensile strength and uniform elongation of medium-manganese steels can be improved by manipulating the cooling cycle after hot rolling. The starting microstructure, obtained after hot rolling and cooling, influences the fraction of austenite formed at the end of intercritical annealing, thereby impacting the fraction of martensite produced at the interrupted quenching step. The results illustrate that during intercritical annealing austenite tends to nucleate at a higher temperature from a ferritic microstructure compared to a microstructure consisting of mainly bainite or a mixture of ferrite, martensite, cementite, and retained austenite. Partition temperature of 400 °C facilitates the partition of carbon from martensite to austenite while partition temperature of 450 °C supports the formation of high carbon secondary martensite.Abstract This study reports the effect of different cooling practices after hot rolling on the microstructure and mechanical properties of intercritically annealed quench and partitioned low-carbon medium-manganese aluminum-alloyed steel. The outcomes show that the tensile strength and uniform elongation of medium-manganese steels can be improved by manipulating the cooling cycle after hot rolling. The starting microstructure, obtained after hot rolling and cooling, influences the fraction of austenite formed at the end of intercritical annealing, thereby impacting the fraction of martensite produced at the interrupted quenching step. The results illustrate that during intercritical annealing austenite tends to nucleate at a higher temperature from a ferritic microstructure compared to a microstructure consisting of mainly bainite or a mixture of ferrite, martensite, cementite, and retained austenite. Partition temperature of 400 °C facilitates the partition of carbon from martensite to austenite while partition temperature of 450 °C supports the formation of high carbon secondary martensite
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