Oskar Pawelski

Development of deterministic-stochastic surface structures to improve the tribological conditions of sheet forming processes

Abstract In sheet-forming processes with mixed frictional state the tribological system is characterized to a great extent by the sheet-surface structure. This structure has the functions of storage, transport, and distribution of the lubricant, and take-up and transport of surface-abrasion particles. These functional characteristics are achieved by different of surface structure, the so-called stochastic and deterministic structures. The presence of micro channels connecting the surface pockets in stochastic structures allows the distribution of the lubricant in all directions but it is not possible to detain this lubricant in areas under high pressure, it simply flows aside. This ability is given by the isolated surface pockets (hydrostatic lubrication pockets) in deterministic structures. From this micro reservoirs the lubricant can be transported directly into the deformation zone by means of building up a hydrostatic pressure. However, because of the absence of micro channels it is not possible to transport the lubricant to all areas of micro contact between tool and sheet, the risk of cold-welding effects increases. Therefore, a new type of surface structure is developed by the combination of a deterministic and a stochastic sheet-surface structure. The results of strip-drawing experiments point out that the combination between isolated lubrication pockets and a network of micro channels to transport the lubricant in the space between these surface pockets leads to a improvement of the forming behaviour.

Development of a FEM-model for the simulation of the transfer of surface structure in cold-rolling processes

Abstract In the temper rolling process the effect of transferring the roll-surface structure to the steel sheet is purposively used to adjust a defined sheet-surface structure required for further processing. The objective of the investigation presented in this paper is to analyze the transfer behaviour specific to the electron-beam texturing technique (EBT) and, in addition to that, to develop a model for the simulation of these transfer characteristics. For this purpose in the first part the results of a study on the basis of rolling tests on an industrial temper mill are presented. These results do not only point out the basic transfer mechanisms - penetration processes and reverse extrusion phenomena - but beyond that also lead to an explanation of the transfer behaviour specific to the rolling conditions by the application of similarity numbers (substitute profile model). On the basis of these results in the second part a model is developed to simulate the transfer characteristics by the application of the finite element method (FEM).

A mathematical model for computing the distribution of loads and thickness in the width direction of a strip rolled in fourhigh cold-rolling mills

Abstract In order to predict the loads and profile of strip in a cold rolling process, a mathematical model was developed. This model combines well-known classical and analytical solutions for rolling direction with a numerical approach to width direction. Up to 10 passes on 2- or 4-high mills can be simulated. The program supports a wide range of parameters. Computed results have been validated by experiments.

Measurement and Evaluation of the Anisothermal Softening of Austenite after Hot Deformation

A modified double-strain procedure is used to evaluate the anisothermal softening occurring in austenite during delay times after hot working. Both the rise in temperature due to deformation heating and the drop in temperature due to radiation during the rest time prior to reloading are taken into account. The technique requires prior determination of yield stresses in the unstrengthened test material over an adequate temperature range. By employing this method for anisothermal multi-pass hot deformation schedules, the state of austenite strengthening can be evaluated at any intermediate stage of working. Results obtained from plane-strain compression tests as a laboratory simulation of hot rolling under anisothermal conditions are presented for typical cases.

Analysis of the asymmetric upsetting test with extremely high strain rate as tool for friction measurement

Abstract The upsetting of flat plates offers a way to analyse friction conditions in the interface area between tool and workpiece with regard to surface microstructure, lubrication, and material properties. If a plate is compressed between two punches and the friction conditions at both interfaces are different, the billet bends over at both sides of the punches. The final angle is a measure for the friction difference. In traditional friction tests the forming speed is too low or the specimen is deformed only at the surface. In most metal forming processes, e.g. rolling, friction conditions cannot be described precisely because of the unknown behaviour of the lubricant at high pressure or high deformation speed. The advantages of the test proposed here are simplicity, full deformation, and feasibility with high speed. The process is cost effective and does not require large scale equipment.

Neuere Vorstellungen über den Einfluß der Geschwindigkeit auf die Schmierung beim Kaltwalzen

ZusammenfassungAusgehend von einigen neueren Angaben im Schrifttum wird gezeigt, daß beim Kaltwalzen von Bändern der Reibungsbeiwert im Walzspalt mit wachsender Walzgeschwindigkeit abnimmt. Zur Erklärung dieses Verhaltens müssen rheologische Vorgänge im Walzspalt angenommen werden. Die besondere Kinematik des Walzens läßt vermuten, daß sich der Schmierstoff dabei nicht reinviskos, sondern viskoelastisch verhält. An Hand eines Kraft-Weg-Schaubildes für die Federkraft des Walzgerüstes und die zur plastischen Umformung des Bandes nötige Walzkraft werden die Rückwirkungen der mit der Geschwindigkeit veränderten Schmierung auf die Walzkraft und die Enddicke des Bandes erläutert. Neben der Walzspaltschmierung ist dabei auch die Lagerschmierung zu berücksichtigen, wenn die Walzen in Ölflutlagern laufen. Ihr Zusammenspiel wird für das Anfahren und Abbremsen einer Kaltwalzstraße veranschaulicht.

Zieh- und Einstoßversuche an Rundstäben aus Stahl

Als Versuchswerkstoff wurde ein beruhigt vergossener, alterungsbestandiger Siemens-Martin-Stahl gewahlt. Um den Einflus unterschiedlicher chemischer Zusammensetzung zu vermeiden, wurden samtliche Ziehstabe aus einund derselben Schmelze hergestellt. Die fur verschiedene Nenn-Enddurchmesser in Tabelle 1 zusammengestellten Ergebnisse der chemischen Analysen bestatigen die erzielte Gleichmasigkeit des Ziehgutes. In der gleichen Tabelle sind auch die mechanischen Kennwerte des Werkstoffs im Anlieferungszustand eingetragen, die allerdings etwas mehr streuen.

Experimental Verification of Dieless-Drawing Theories

The dieless-drawing method is described briefly. Because of the strong thermo-mechanical coupling it is difficult to solve the underlying differential equations. Therefore several assumptions have been made to obtain simplified but nevertheless realistic models of the process. For an axi-symmetric cylindrical model the heat-conduction equation can be solved analytically and the radial dependency can be studied. For thin specimens a one-dimensional model is capable of predicting the most important parameters like axial temperature distribution and the final shape of the deformation zone. In contrast to conventional drawing the dieless-drawing method provides the possibility of producing variable cross-sections. The necessary control function is given for an example of a linearly varying final shape.

Friction Measurement by Upsetting with Asymmetric Interface Friction

The upsetting of flat plates offers several ways to analyse the influences of surface microstructure, lubrication and material on friction. If the friction conditions at the two interface areas are different, the inflection of the billet gives a measure for the friction difference. The advantage of this test is that friction measurements with high speeds and full deformation are possible, e.g. drop forging. In traditional friction tests the forming speed is too low or the specimen is deformed only at the surface. In several metal forming processes, e.g. rolling, friction conditions cannot be described precisely because of the unknown behaviour of the lubricant at high pressure and at a high deformation speed. In addition, asymmetric friction conditions can be transferred to asymmetric conditions in rolling. Friction measurement with upsetting of flat specimens is cost effective and does not require large-scale equipment.

Kantenanschärfung beim Kaltwalzen von Band

Zur Untersuchung der Profilausbildung beim Kaltwalzen unter besonderer Berucksichtigung des kantennahen Bereichs wurde ein mathematisches Modell entwickelt. Durch die Kombination eines analytischen Ansatzes in Walzrichtung mit einem numerischen Ansatz in Breitenrichtung konnen bei kurzer Rechenzeit alle relevanten Geometrie- und Werkstoffparameter in das Modell einbezogen werden. Der Ansatz berucksichtigt die Wechselwirkung von elastischer Verformung des Walzensystems und plastischer Verformung des Bandes. Das aus dem Modell entwickelte Computerprogramm ist in der Lage, Stichfolgen auf Duo- oder Quartogerusten zu simulieren. Zielgrosen der Rechnung sind das Dickenprofil des auslaufenden Bandes und der ortliche Umformgrad, der Ruckschlusse auf die Planlage erlaubt.