Overlapping regions of laser surface treatment are necessary features when processing large surface areas or cylindrical specimens. However, complex microstructural changes that appear in the regions with multiple heat treatment can affect their mechanical properties. Therefore, this study focuses on examining thermal cycle characteristics and resulting microstructures, particularly martensite and retained ferrite structures, to better understand the correlation between experienced thermal cycles and resulting microstructures. Laser surface hardening experiments on 44MnSiVS6 microalloyed steels together with thermal diffusion simulations were conducted to relate microstructures after the secondary pass of the laser treatment to the local thermal cycles experienced during the process. The amount of retained ferrite was calculated and compared to the respective thermal cycle characteristics. Regions which experienced thermal cycles below Ac3 temperature showed microstructures similar to those after tempering. The sizes of retained ferrite structures were found to decrease as the total holding time increases regardless of how the holding time is distributed in multiple laser treatments. However, the size of retained ferrite structures were constant in the region where tempering effect occurred. This shows that the amount of retained ferrite can be tailored by modifying the experienced total holding time and a reduction of retained ferrite structure happens only if the secondary thermal cycle is above Ac3 temperature.

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.

Laser hardening is a very efficient technique for local surface treatment, however, in case of complex and large components robust processing is highly challenging due to limitations in terms of the absolute size of the overall heat-affected zone. As is shown in the present work, an increased in-depth effect can be achieved by tailoring the laser parameters without melting the surface layer. Optimization of process parameters leads to an elaborate test design demanding numerous verification measurements to determine essential material properties. In this context, the evaluation of compressive residual stress values in the surface layer is very important, e.g. in case of fatigue loaded components. However, residual stress profile measurements obtained by X-ray diffraction are very time-consuming and, thus, can significantly impair the laser parameter development cycle. For this reason, the present study introduces a novel pragmatic approach allowing for qualitative evaluation of laser-induced compressive residual stress states, in particular for multiple laser pass processes based on a Gaussian-like intensity profile. Based on straightforward analytical evaluation, several characteristic features of the affected surface layer, e.g. the position of the residual stress transition zone, can be correlated to a change of the local energy input. A novel parameter referred to as modified area energy is established in present work for this purpose. This novel energy approach provides for an essential contribution to the field of laser hardening to considerably shorten the experimental effort within the laser parameter search.

Mill scale formed on the surface of hot rolled steels consists of magnetite (Fe3O4), hematite (Fe2O3) and wustite (FeO) layers, which can protect the steels from corrosion and other atmospheric effects. Existence of mill scale on the specimens' surface has shown to be able to decrease the cut edge quality. Since the mechanism behind influence of mill scale on the laser cutting process is unknown, this work performs direct observation of oxygen laser cutting processes on specimens with and without removed mill scale layers. Oxygen laser cutting processes were carried out using Ytterbium fibre laser 1070 nm along the edge of 20-mm-thick-steel specimens which were attached to a borosilicate glass. Focal point of the laser beam was positioned to be 0.7 mm below the specimens' surface. A high speed imaging system was arranged to face the glass, recording the cut front and kerf dynamics during cutting processes. It was found that cut front inclination angle increase when the mill scale was removed from the specimens' surface. This implies that mill scale on the specimens' surface seem to contribute in increasing the exothermal energy during laser cutting processes.

The depth homogeneity of laser-treated zones is one possible factor to define the quality and efficacy of altered mechanical properties in materials. For instance, half-rounded cross-sectional shapes of laser hardened zones using Gaussian beams provide dissimilar hardened depth in the edges and center of the treated area. This means that the in-depth distribution of compressive residual stress varies between the edges and the center of the hardened area. Nonhomogeneity of compressive residual stress distributions can inhibit fatigue properties and can lead to product failure. The utilization of oscillated laser beams has been proven to improve the welding efficiency and energy input distribution to the material, which promises achieving a homogeneous depth of laser-treated zones in hardening applications. Therefore, this work examines the influence of triangular, square, and circular beam oscillation strategies on the energy input distribution during the process and the geometry of the laser-treated zones on microalloyed steel. Laser beam pathways were assembled using a vector graphic editor to visualize the energy distribution from each oscillation strategy. Cross section images of the hardened tracks were taken and related to the thermal energy input profiles. It was revealed that each oscillation strategy demonstrates characteristic temporal and spatial thermal energy input distribution, influencing the geometry of the hardened zone. The circular oscillation strategy produced a widely constant depth in contrary to the triangular and square beam oscillation due to its characteristic energy distribution that allows homogeneous heat dissemination in the material. This confirms that the laser beam oscillation strategy can tailor the energy input distribution to optimize the processing outcome.

Laser surface hardening provides for many advantages in terms of flexible production due to very localized and controlled energy input into the material. Laser processing offers the possibility to treat surfaces in order to locally strengthen the areas that are prone to fatigue cracking. It is well known that laser energy absorption depends on many parameters, e.g., the surface structure and the surface orientation. The incident angle of the laser beam plays a key role in this regard. When complex geometries like crankshaft fillets are treated, the surface cannot be considered a series of flat surfaces. Obviously, this leads to locally varying degrees of energy absorption. In the present work, curved surface structures were chosen in order to analyze the impact of the geometrical characteristics on surface and subsurface material properties after laser treatment. Microstructure evolution generally was found to be similar for flat and curved geometries. However, even if higher absorption in the groove due to the illumination at larger incident angles was expected, the outer parts of the curved geometry were not fully hardened. Thus, the increased effective length of the complex geometry-treated and the larger heat-affected volume are expected to have a more dominant influence on the final appearance of the subsurface microstructure. Eventually, for austenitization of the complete illuminated surface volume, the energy density needs to be increased.

Fatigue property improvement for automotive components such as crankshafts can be achieved through material selection and tailored surface design. Microalloyed steels are of high interest for automotive applications due to their balanced properties, excellent hardenability and good machinability. Lasers facilitate efficient and precise surface processing and understanding the laser-material-property interrelationships is the key to process optimisation. This work examines microstructural development during laser surface treatment of 44MnSiVS6 microalloyed steel and the resulting mechanical properties. Laser beam shaping techniques are employed to evaluate the impact of beam shaping on the process. It revealed that ferrite structures remain in the treated area surrounded by martensite due to insufficient heating and dwell time of carbon diffusion.

Improvement of crankshaft fatigue properties can be approached by altering its mechanical properties in the surface, such as laser surface treatment. Laser beam treatment offers efficient and precise surface hardening processing with possibility of reducing the production cost compared to the conventional hardening techniques. However, its characteristic of having short thermal cycle can be a challenge for the development of laser surface hardening techniques, such as inadequacy of literatures in phase transformation and resulting mechanical properties under rapid heating and cooling rate. Therefore, this work investigated the impact of short thermal cycles induced by the laser beam on the resulting microstructure and hardness properties in the surface of 38MnSiVS5 and 44MnSiVS6 microalloyed steels. Temperature cycles during the process were recorded and examined with the resulting microstructure along with microhardness values. 44MnSiVS6 microalloyed steel, which contains ca. double the amount of vanadium compared to 38MnSiVS5 steel, produces finer ferrite grains in the treated area for all investigated short thermal cycles. This fine-grained microstructure leads to steady hardness distributions in the treated area. The short thermal cycle was assumed to be unable to dissolve the vanadium precipitates that reside in the ferrite grains, which then initiate precipitation hardening.