The application of thermal cycles below melting temperature can induce solid-to-solid phase transformation in steels, which is the transition between different crystalline structures of the same compound. There are many types of crystalline structures in steels produced, depending on the characteristics of the applied thermal cycle. For instance, rapid cooling can generate martensite structure that tends to increase the hardness of the steels, while slow cooling will more likely produce ferrite structure, which is less hard than the martensite structure. Laser heat treatment is one example where the laser becomes a thermal energy source, inducing thermal cycles below melting point and an extremely rapid cooling rate, which results in unexpected microstructures upon cooling. The mechanism of such phase transformations is still widely unknown, although the knowledge can be beneficial for many laser processes. Accordingly, studies on laser induced phase transformation are necessary.

The purpose of my work is explaining underlying mechanisms of solid-to-solid phase transformation in microalloyed steels due to short thermal cycles of the laser heat treatment. My work aims to (1) find the correlation between energy input distributions from the laser beam and temperature history during the laser heat treatment process and (2) describe how changes in the thermal cycle induced by laser illumination influence the phase transformation dynamics. This work focuses on martensitic transformation and infrared laser (1070 nm).

To explain martensitic transformation during laser heat treatment, this work involved ex-situ observations of the laser heat treated specimens. The study consists of varying the laser parameters, measuring the surface temperature of the specimens and simulating the in-depth temperature. Consecutively, characteristics (i.e., holding time, peak temperature, and cooling rate) of the measured and/or calculated thermal cycles were extracted, and the microstructures of the specimens were observed using microscopes. Finally, the thermal cycle characteristics and the microstructure of the specimens were related.

The results show that the energy input distributions from the laser beam (e.g., laser beam profile) determine the geometry of the treated area, while processing speed and laser power influence the cooling rate and peak temperature of the thermal cycle respectively. The short thermal cycles induced by the laser beam are able to induce martensitic structure in the specimen. However, ferrite structure unexpectedly remains in the treated area. The holding time, which is the duration of temperature staying above austenisation temperature, has an inverse correlation to the appearance of ferrite structure in the treated area. This relates to the carbon diffusion occurring during the process, in which the carbon atoms have to diffuse from rich-carbon-austenite into low-carbon-austenite before cooling. Accordingly, the amount of martensite structure in the treated area depends on the holding time value of the process. There are indications that the rapid cooling induced by the laser beam can abruptly stop the diffusion process. It is clear that the laser provides an opportunity to control martensite structure.