High Temperature Laser Scanning Confocal Microscopy is used to observe surfaces of samples at elevated temperatures up to 1750°C. The set up consists of a He-Ne Laser as a light source and an infrared heating chamber. The He-Ne laser in combination with a pinhole blocks out thermal radiation of the sample enabling the observation of dynamic processes at different temperatures. Further, the special geometrical design of the infrared furnace enables heating and cooling rates of several 100K/min. Throughout the investigation a video is recorded with a maximum frame rate of 60 frames per second. Events like solidification, formation and solution of NMI, phase transformations, oxidation- and reduction processes can therefore be observed in-situ, recorded and analysed.
Characteristics and advantages of HT-LSCM:
- The He-Ne Laser with a wavelength of 408 nm enables good lateral resolution at all sample temperatures.
- The small volume of sample holder and sample in connection with the design of the infrared furnace offers heating rates of up to several 100 K/min.
- Experiments can be executed using a defined furnace atmosphere or vacuum.
- No chemical etching is required as thermal etching is revealing dynamic processes at elevated temperatures.
- The high framerate of up to 60 pictures per second enables recording of fast processes.
The HT-LSCM provides a unique technique for the in-situ observation of phase transformations in the solid state. The special set up of the microscope blocks thermal radiation of the sample and therefore enables the observation of the surface up to the liquid state. Due to the thermal etching effect phase transformations become visible in real time on the polished sample surfaces. While the sample is undergoing a defined thermal treatment, a video is recorded with a frame rate of up to 60 frames per seconds which enables a subsequent display and analysation of the microstructural changes.
As part of the development of new casting processes for new steel grades the HT-LSCM is used as a powerful tool to predict the transformation behavior after the finishing mill. Due to the significant reduced time for completion of the phase transformations compared to conventional AHSS production routes the knowledge of the transformation kinetics is a crucial factor for the success of the implementation of AHSS production in a TSDR plant. The HT-LSCM allows a quick and easy optical evaluation of:
- Phase transformation temperatures,
- Occurring phases,
- Influence of grain size on dynamics of phase transformation.
Grain size and grain growth kinetics are essential metallurgical factors to define process parameters and to ensure that desired quality of the final product is met. HT-LSCM features an ideal, time saving method for the in-situ observation of grain growth. Thermal etching reveals grain boundaries and their real time movement on the polished sample surface without further chemical etching. Whereas conventional approaches for the determination of grain size evolution compromise numerous annealing experiments, rapid quenching and subsequent metallographic sample preparation, HT-LSCM enables capturing of a grain size evolution curve for a given thermal cycle in just one experiment. The method is applicable to wide range of alloying concepts also including ULC-steels.
HT-LSCM provides the possibility of:
- Real time observation of grain growth and recrystallization processes,
- Evaluation of effectiveness and thermal stability of microalloying elements,
- In-situ observation of the evolution of grain size distributions (normal, abnormal grain growth).
- Fuchs, N., Bernhard, C., Michelic, S.; Dippenaar, R., HT-LSCM as a Tool for Indirect Determination of Precipitates by Real Time Grain Growth Observations, 2020 TMS Annual Meeting & Exhibition, San Diego.
- Fuchs, N., Krajewski, P. & Bernhard, C. In-situ Observation of Austenite Grain Growth in Plain Carbon Steels by Means of High-temperature Laser Scanning Confocal Microscopy. Berg Huettenmaenn Monatsh 160, 214–220 (2015).