Sergey Guk

Influence of Deformation Conditions on the Microstructure and Formability of Sintered Mg-PSZ Reinforced TRIP-Matrix-Composites

The present technical work reports on the formability and related behaviour of components during hot rolling of metal matrix composites (MMC) produced from powder. A new metal matrix composite based on TRIP (Transformation Induced Plasticity) austenitic steel AISI304 with varying amounts (10% and 20%) of homogeneously embedded zirconium dioxide (partially stabilized with Mg (Mg-PSZ)) was investigated by means of compression tests and rolling of wedge shaped samples at temperature of 900°C and 1100°C. Two different particle size distributions of Mg-PSZ powder were investigated: fine and coarse. Based on the experimental results, the deformation behaviour (arc of contact length, roll gap ratio, strain, strain rate), the microstructure evaluation (pore amount and their cross sectional area) and the formability (equivalent fracture strain) under different stress state conditions had been analyzed.

Walzkonzept für die Kalt- und Warmumformung neuer metastabiler Stähle auf der Flachbahn

Die vorgestellten Untersuchungen beschaftigen sich mit der Formgebung metastabiler, hochlegierter CrMnNi Stahle mit TRIP/TWIP-Effekt mittels Kaltund Warmwalzen. Infolge einer Kaltumformung konnen diese Stahle sowohl eine deformationsinduzierte martensitische Phasenumwandlung, als auch eine verformungsinduzierte Zwillingbildung aufweisen. Diese Martensitbildung ist gleichzeitig Ursache eines sehr guten Verfestigungsvermogens. Um die damit einhergehende Dehnungsanomalie ausnutzen zu konnen, sind neue Walzstrategien erforderlich. Wird im elastischen Umformbereich bereits spannungsinduziert Umformmartensit gebildet, so setzt dieser sogar das Umformvermogen herab., Konventionelle Walzstrategien konnen deshalb nur begrenzt angewendet werden. Ziel war es demnach, Walzkonzepte fur die Kalt- und Warmumformung fur diese Werkstoffgruppe zu entwickeln. Unter Beachtung der werkstoffspezifischen Kennwerte, wie Auslosespannung, Stapelfehlerenergie sowie Martensitstart- und -umwandlungstemperatur, konnten sowohl fur die Kalt- als auch fur die Warmumformung mit dem Freiberger Stichplanmodell Stichfolgen entwickelt werden. Die Anwendbarkeit der Walzkonzepte konnte experimentell nachgewiesen werden.

Understanding of Processing, Microstructure and Property Correlations during Spheroidizing Heat Treatment of 100Cr6 Steel

In the present study an investigation has been carried out for spheroidization of 100Cr6 bearing steel used in forging industry. Three different spheroidization processes were considered. The first one was the annealing of normalized steel under Ac1 temperature for a long time. The second one was the annealing of normalized steel above Ar1 temperature after heating between Ac1 and Acm for one hour. The third one was the annealing of hardened steel under Ac1 temperature for a long time. For evaluation of cold workability with different spheroidization annealing periods, the yield strength and percentage of reduction in area in uniaxial tension were recorded. The present results indicate that low alloyed carbon bearing steels can be easily processed to achieve unique microstructures and properties.

Metal Formability Interactions in Laser Marking for Creating of Grid Patterns for Forming Strain Analysis of High Strength Steels

A commercially available laser marking system based on diode-pumped Nd:YVO4 laser was used for creating grid patterns for forming strain analysis of three different multiphase steels. The aim was to determine and analyze the influence of laser working parameters on the formability of the investigated sheet materials by means of an in-depth characterization of this induced microstructural and geometric inhomogeneity. The electrochemical etching served as the reference method without the negative effect of generating inhomogeneity. The formability was evaluated using the cupping test according to Erichsen. While the quantification of geometric inhomogeneity was based on the determination of the notch factor, microhardness measurement was used for the evaluation of micro-structural inhomogeneity. The results showed that multiphase steels exhibit similar values of the mark depth under the same creating parameters by means of laser. Furthermore, only the induced geometric inhomogeneity had a marked influence on the material formability. Finally, a method for the prediction of the optimal values of the grid pattern mark depth was developed from the perspective of its good visual recognizability and associated with the microstructure based material sensitivity to stress concentrators.

Material Flow in Mg-PSZ Particle Reinforced TRIP-Matrix-Composites due to Hot-Rolling

The material flow in particle reinforced metal-matrix-composites (MMC) had been investigated. The composite consisted of TRIP steel and magnesium stabilized ZrO2 particles (Mg‑PSZ) in volume fractions of 0 %, 5 % and 20 %. The basic materials were produced by hot-pressing and showed a very homogeneous particle distribution and a almost full density. Then the samples were cut to wedge shape and hot-rolled with a constant roll gap. Caused by the shape, the true strain increased over the length and reached a maximum of true strain of 0.6. The strain rate was set to be higher than 0.1 and lower than 10/ s. After rolling, it was possible to combine rolling force, true strain and the material flow due to the grid on the surface. With an increase in volume fraction of Mg‑PSZ the rolling force increases as well. Metallographic examinations were performed to determine and document the flow of particles within the composite due to true strain conditions. It was found that the particles flow with the base material and turn parallel to the rolling direction. This effect was measured using the degree of orientation of partially oriented linear structure elements Ω12, according to ASTM E 1268-01. The index was increasing with increasing true strain value. Further microscopic examination showed debonding of the interface between particles and matrix-material. For MMC’s having a volume fraction of 20 % Mg-PSZ a true strain at fracture of 0.5 to 0.6 was determined.