Olaf Keßler

Das kontinuierliche Zeit-Temperatur-Ausscheidungs-Diagramm einer Aluminiumlegierung EN AW-6005A*

Kurzfassung Der Einfluss der Abschreckgeschwindigkeit auf das Ausscheidungsverhalten von Aluminiumlegierungen wird, in Anlehnung an ZTU-Diagramme von Stählen, in kontinuierlichen Zeit-Temperatur-Ausscheidungs-Diagrammen dargestellt. Solche Diagramme sind bisher kaum verfügbar, können jetzt aber mit Hilfe der Differential Scanning Calorimetry (DSC) erfasst werden. Wird eine Charge der Aluminiumlegierung EN AW-6005A in einem Kühlgeschwindigkeitsbereich von 0,1–30 K/min von Lösungsglühbedingungen in einem DSC abgekühlt, sind in den Abkühlkurven zwei exotherme Ausscheidungsreaktionen zu erkennen, eine Hoch- sowie eine Niedertemperaturreaktion. Um Aufschluss darüber zu erhalten, welche Phasen dabei ausgeschieden werden, wurden umfangreiche Gefügeanalysen mittels Licht- und Rasterelektronenmikroskopie, energiedispersiver Röntgen-Mikroanalyse (EDX), Röntgendiffraktometrie (XRD) und Elektronen-Rückstreubeugung (EBSD) sowie Härteprüfungen an unterschiedlich abgekühlten Proben durchgeführt. Es konnte gezeigt werden, dass bei der Hochtemperaturreaktion Mg2Si ausgeschieden wird. Die Keimbildung der Mg2Si-Partikel erfolgt überwiegend heterogen an Primärausscheidungen.

The Ideal Quenching Medium? – Characterisation of Ionic Liquids for Heat Treatment of Metallic Components

Abstract Quenching as a part of heat treatment is an important process in the manufacturing chain of metallic components. One of the most common processes is immersion quenching in vaporising liquids. Unfortunately, this process is affected by the Leidenfrost-phenomenon. This effect can generate inhomogeneous quenching and thus asymmetrical residual stresses and avoidable distortion. In this work, ionic liquids have been investigated as new quenching media, which can be used as baths near room temperature. Aluminium cylinders have been quenched in baths of different ionic liquids with varying water contents and bath temperatures. The time-temperature curves have been recorded and the heat transfer coefficients were determined and compared to quenching in water. Addition of a few percents water to the ionic liquids increases the cooling rate significantly. Even at higher water contents, bubble boiling and convection are predominant and almost no Leidenfrost-effect occurs. This offers a huge potential to accelerate and homogenise immersion quenching.

In Situ Techniques for the Investigation of the Kinetics of Austenitization of Supermartensitic Stainless Steel

The austenitization and inter-critical annealing of X4CrNiMo16-5-1 (1.4418) supermartensitic stainless steel were investigated in-situ with synchrotron X-ray diffraction (XRD), dilatometry and differential scanning calorimetry (DSC) under isochronal heating conditions. Austenitization occurred in two stages: the austenitization started at approx. 600 °C, decelerated at approx. 700 °C at 60 to 75 v.% of transformed austenite, and first resumed after heating for approx. 100 °C. This plateau in the transformation curve was more dominant for faster heating rates. Inter-critical annealing at 675 and 700 °C revealed, that austenite can to a certain extent be stabilized to room-temperature. There was good agreement for the transformation curves yielded by dilatometry and XRD. Some deviation occurred due to the different applied heating principles, different temperature monitoring and the impact of surface martensite formation on the XRD measurement. The applicable temperature range for DSC as well as the close proximity of the Ac1- and the Curie-temperature limited the usage of the technique in the present case.

Room temperature ionic liquids in a heat treatment process for metals

Ionic liquids were found to be useful media for quenching of aluminum and steel alloys. In mixtures containing up to 10% w/w water or ethanol they prevent film boiling (Leidenfrost effect) and result in very high and constant cooling rates. The ionic liquids [EMIm][NTf2], [EMIm][EtSO4] and Ammoeng®110 exhibit remarkable stability when metals at temperatures well above the ionic liquid decomposition temperature were quenched.

Reliable Absolute Vapor Pressures of Extremely Low Volatile Compounds from Fast Scanning Calorimetry

Vapor pressure determination of extremely low volatile compounds, e.g., ionic liquids, is challenging and time-consuming using conventional techniques. Particularly, ionic liquids tend to decompose already at temperatures where the vapor pressure is still very low. Conventional methods for the determination of evaporation rates are thus limited to temperatures below the decomposition temperature where evaporation proceeds very slowly. A new method for the vapor pressure determination of low-volatile compounds, presented here, is able to overcome this limitation using differential fast scanning calorimetry on very short time scales in inert atmospheres. The method is based on the relatively fast evaporation of nanogram samples, exhibiting a significantly enhanced (up to a factor of 104) surface-to-volume ratio compared to conventional thermogravimetric samples. Due to extremely high heating rates, the sample is exposed to the thermal stress only for milliseconds. In these conditions the evaporation dominates in the mass loss even at temperatures above the possible onset of the decomposition process. In addition, since the method allows very high heating and cooling rates (up to 106 K s−1) evaporation of the samples on the way to and from the evaporation temperature is avoided and thus much higher temperatures can be reached in the measurement of the mass loss rate as compared to conventional methods. This method was tested using the diffusion pump oil Santovac® 5 and the ionic liquid [EMIm][NTf2] at temperatures up to 780 K and in atmospheres of different inert gases. The absolute vapor pressures of several aprotic ionic liquids: [EMIm][NTf2], [BMIm][Br], [BMIm][BF4], [BMIm][PF6], [EMIm][Cl], [BMIm][Cl], [EMIm][NO3], and [BMIm][NO3] were measured. The vapor pressures were fitted to the Clarke–Glew equation. The vaporization enthalpies and boiling temperatures of the ionic liquids were estimated. The advantages and limitations of this new method of absolute vapor pressure determination were discussed and the results are compared with the data available in the literature.

Stability studies of ionic liquid [EMIm][NTf2] under short-term thermal exposure

Ionic liquids (ILs) as new media for synthesis and as functional fluids in technical applications are still of high interest. Cooling a steel component from an annealing temperature of nearly 850 °C down to room temperature in a liquid bath is a technically important process. The use of ionic liquids offers advantages avoiding film boiling of the quenching medium. However, such a high immersion temperature exceeds the thermal stability of the IL, for example such as [EMIm][NTf2]. To obtain information about formation of potential toxic decomposition products, potential fragments at varied states of decomposition of [EMIm][NTf2] were studied by various spectroscopic and gravimetric methods. For the first time it was possible to quantify fluorine-containing products via mass spectrometry coupled directly with thermogravimetric (TG) measurements. While chemical and spectroscopic analysis of thermally stressed ILs revealed no hints concerning changes of composition after quenching hot steel for several times, the mass-spectrometer (MS) coupled TG analysis gives information by comparing the decomposition behaviour of fresh and used ILs. A number of fragments were detected in low amounts confirming the proposed decomposition mechanism.

Kalorimetrische und dilatometrische Analyse des Anlassverhaltens in der Wärmeeinflusszone von Schweißnähten des Stahls T24

Kurzfassung Der Werkstoff 7CrMoVTiB10-10 (T24) wurde als warmfester Stahl für den Einsatz in der Membranwand neuer Kohlekraftwerke der 600/620°C-Klasse entwickelt. Trotz seiner umfangreichen Qualifizierung kam es zu Schäden bei der Inbetriebnahme durch Spannungsrisskorrosion. Zur Reduzierung der Spannungsrisskorrosionsempfindlichkeit wurden verschiedene Anlassbehandlungen durchgeführt. Grundlegende Untersuchungen sollen nun die Prozesse beleuchten, die bei diesen Glühbehandlungen ablaufen. Für die geplanten Untersuchungen wurde Probenmaterial aus zwei nahtlosen Rohren (aus unterschiedlichen Schmelzen) des Stahls T24 entnommen. In den Proben wurde anhand von Wärmebehandlungen in einem Dilatometer ein Schweißgefüge eingestellt, wie es in der Wärmeeinflusszone (WEZ) der Schweißverbindungen auftritt. An diesem Probenmaterial erfolgten anschließend unterschiedliche Anlassbehandlungen. Die sich einstellenden Gefügeveränderungen wurden in Abhängigkeit von der Erwärmrate bzw. Anlasstemperatur und -dauer durch kalorimetrische und dilatometrische Analysen charakterisiert. Hierzu wurden Proben kontinuierlich in verschiedenen DSC-Geräten und in einem Dilatometer mit Heizraten zwischen 0,02 K/s und 100 K/s erwärmt. Zusätzlich erfolgten isotherme Glühbehandlungen dieser Proben mit Temperaturen zwischen 450 °C und 700 °C im Dilatometer. Die Ergebnisse der Dilatometrie und Kalorimetrie lassen auf mehrere Anlassreaktionen schließen. Die Ergebnisse erlauben eine Zuordnung der beobachteten Reaktionen zu den in der Literatur beschriebenen Anlassstufen und somit eine gezielte Gestaltung der Anlassbehandlung der geschweißten T24-Verbindungen zur Reduzierung ihrer Rissanfälligkeit im laufenden Betrieb.