M. Bambach

Homogenization Strategy and Material Characterization of High-Manganese TRIP and TWIP Steels

In this work a hot forming strategy, consisting of forging and hot rolling, to homogenize casted blocks of high-manganese steels with 0.3 % carbon and 22 % manganese is introduced. The resulting distribution of carbon and manganese is evaluated by microprobe scans. The micro-segregation of manganese could be reduced from 7 weight percent to 2. To create the obtained hot forming strategy hot compression tests have been carried out. The deformation behavior has been characterized for two steels with 22 % manganese and between 0.3 and 0.7 % carbon content in the temperature range between 700 and 1200°C and strain rates between 0.1 and 10 s-1.

Error Analysis In Explicit Finite Element Analysis Of Incremental Sheet Forming

Asymmetric incremental sheet forming (AISF) is a relatively new manufacturing process for the production of low volumes of sheet metal parts. Forming is accomplished by the CNC controlled movements of a simple ball‐headed tool that follows a 3D trajectory to gradually shape a sheet metal blank. The local plastic deformation under the tool leads to a number of challenges for the Finite Element Modeling. Previous work indicates that implicit finite element methods are at present not efficient enough to allow for the simulation of AISF for industrially relevant parts, mostly due to the fact that the moving contact requires a very small time step. Explicit Finite Element methods can be speeded up by means of mass or load scaling to enable the simulation of large scale sheet metal forming problems, even for AISF. However, it is well known that the methods used to speed up the FE calculations can entail poor results when dynamic effects start to dominate the solution. Typically, the ratio of kinetic to internal...

Experimental Uncertainties affecting the Accuracy of Stress‐Strain Equations by the Example of a Hensel‐Spittel Approach

The accuracy of numerical simulations in metal forming highly depends on the description of the plastic flow behavior. Due to experimental uncertainties flow curves recorded at equal testing conditions (combinations of temperature and strain rate) show scatter. This scatter influences the fit of material models and the resulting fit parameters. In this paper, factors causing uncertainties and systematic errors as well as ways to statistically describe uncertainties in the flow stress are analyzed by means of finite element simulations and experimental analyses of compression tests. To this end, compression tests were conducted for a 25MoCr4 steel to record flow curves for various temperatures and strain rates. To grasp experimental uncertainties, each experiment was repeated five times. The well known Bootstrap method was applied to characterize the uncertainties in fitting a Hensel‐Spittel flow curve model to the experimental data. This method is compared with an alternative strategy of resampling the ex...

Entwicklung von schadenstoleranten hochfesten Stählen

Die mechanische Beanspruchung von Bauteilen der Anlagenund Verkehrstechnik, wie Zahnräder und Wälzlager, nimmt in den letzten Jahren immer mehr zu. Dies liegt hauptsächlich daran, dass bei der Auslegung der Bauteile angestrebt wird, möglichst effizient die Belastbarkeit des Werkstoffs auszunutzen, dadurch Ressourcen zu schonen und Gewicht zu sparen. Die verwendeten Werkstoffe müssen dabei nicht nur hochbeanspruchbar sondern auch zuverlässig und schadenstolerant sein. Für Letzteres sind hauptsächlich mikroskopische Vorgänge im Gefüge des verwendeten Werkstoffs verantwortlich. Unter anderem gehört hierzu die Rissbildung an nichtmetallischen Einschlüssen, die infolge des unterschiedlichen Last-Verformungs-Verhaltens von Einschluss und Matrix Spannungskonzentrationen bewirken, Bild 1. Aus diesem Grund wurde in der Vergangenheit vielfach allein der Reinheitsgrad des Werkstoffs als zu optimierende Größe bei der Weiterentwicklung von Stählen für die Anlagenund Verkehrstechnik herangezogen. Die Verbesserung des Reinheitsgrads stößt aber mittlerweile an technische Grenzen; darüber hinaus bedeutet das Auftreten eines Schadensfalls an einem Einschluss zumeist ein örtlich isoliertes Einzelereignis, das nicht von der Einschlussanzahl, sondern auch von der Größe und Form sowie der Position des Einschlusses im Bauteil abhängt. Die hohe Reinheit moderner Stähle reduziert zwar die statistische Wahrscheinlichkeit für Versagen; frühzeitige Bauteilausfälle vor dem berechneten Lebensdauerende können jedoch trotzdem auftreten, wenn beispielsweise einzelne nicht-metallische Einschlüsse an kritischen Bauteilstellen vorliegen. Bei Windkraftanlagen zum Beispiel erhöhen solche Bauteilausfälle innerhalb der Garantiefristen drastisch die Gewährleistungskosten und können zum Stillstand der Gesamtanlage führen. Alternativen für die Erhöhung der Schadenstoleranz der Stähle werden gesucht. Aus den oben aufgeführten Gründen sind weitere Lösungen für die Erhöhung der Zuverlässigkeit und Schadenstoleranz der eingesetzten Wälzlagerund Einsatzstähle erforderlich. Eine Möglichkeit ist es, Stähle zu entwickeln, die trotz der hohen Grundfestigkeit über ein hoch verformbares und stark verfestigendes Matrixgefüge verfügen. Dadurch kann es gelingen, die an den Einschlüssen auftretenden Spannungsspitzen durch plastische Verformung und die damit verbundene Verfestigung abzubauen und eine Rissinitiierung zu verhindern. Das Institut für Eisenhüttenkunde (IEHK) der RWTH Aachen entwickelt und untersucht in Zusammenarbeit mit vier weiteren Forschungsstellen im Rahmen des DFG-AiF Clusters „High Performance Components (HiPerComp)“ neue Werkstoffe mit schadenstoleranten Gefügen als Ersatz für den Einsatzstahl 18CrNiMo7-6 und den Wälzlagerstahl 100Cr6. 1 8 T I T E L T H E M A

Microalloyed Engineering Steels with Improved Performance - an Overview

Abstract Microalloying elements are added to a wide range of steels for improving microstructure, properties, processing or general performance. A survey is given on the various reasons for microalloying of engineering steels. In case hardening steels microalloying improves toughness and fatigue properties mainly due to a smaller and more homogeneous prior austenite grain size. In forging steels, microalloying controls the phase transformation during cooling after forging enabling shorter process routes. Furthermore, microallying contributes to the microstructural refinement improving the balance of fatigue, cyclic behavior and strength. Recently, microalloying supports the interface engineering in air hardening medium Mn steels and prevents embrittlement.

18CrNiMo7-6 with TRIP-Effect for Increasing the Damage Tolerance of Gear Components — Part I: Alloy Design

High performance components such as gear wheels shall be resistant to rolling-contactfatigue. This type of failure is usually caused by effects occurring on a microscopic scale, such ascrack initiation at non-metallic inclusions. Much effort has been invested so far in improving thesteel cleanliness. However, these high performance components often do not reach the desiredservice life. Preliminary failure within the guarantee terms still occurs which leads to high warrantycosts. Alternative to improving steel cleanliness, the damage tolerance of high performancecomponents could be increased by inducing the TRIP-effect around the crack tip. Due to high localstrain hardening, martensite transformation occurs. The high compressive stresses related to it coulddelay or stop crack propagation by reducing stress concentrations via plastic deformation. As aresult, rolling-contact fatigue resistance of carburized steels may be increased and preliminaryfailure may be avoided. Part I of this study focuses on modifying the chemical composition ofconventional 18CrNiMo7-6 steel with Al to develop a high-strength, yet ductile matrix with a highwork hardening potential. Dilatometric tests on laboratory melts analyze the possibility of adjustinga microstructure able to produce a TRIP-effect. Both isothermal annealing and Quenching andPartitioning (Q&P) are used to stabilize residual austenite and optimum process routes areidentified.

18CrNiMo7-6 with TRIP-Effect for Increasing the Damage Tolerance of Gear Components — Part II: Microstructure and Mechanical Properties

High performance components such as gear wheels shall be resistant to rolling-contact fatigue. This type of failure is usually caused by effects occurring on a microscopic scale, such as crack initiation at non-metallic inclusions. Much effort has been invested so far in improving the steel cleanliness. However, these high performance components often do not reach the desired service life. Preliminary failure within the guarantee terms still occurs which leads to high warranty costs. Alternative to improving steel cleanliness, the damage tolerance of high performance components could be increased by inducing the TRIP-effect around the crack tip. Due to high local strain hardening, martensite transformation occurs. The high compressive stresses related to it could delay or stop crack propagation by reducing stress concentrations via plastic deformation. In part II of this study, the microstructures and mechanical properties of the steels modified via Al-alloying and heat treated in process routes according to part I are compared to conventional 18CrNiMo7-6. Special interest is paid to the stability of the residual austenite as well as to the change in strain hardening rate under tension.