The strength-ductility trade-off has been a long standing dilemma in material science. With the use of laser surface treatments, effort has been made in order to obtain a heterogeneous material with a well defined architectured microstructure to optimize this trade-off. In the present study, it is investigated how the deformation behaviour of Martensite/Austenite steel microstructures in a Fe-25Ni-0.2C alloy can be tailored by the creation of patterned microstructures with localized laser treatment. Due to these treatments strong variations in microstructure are observed. Two different patterns are created (i) a dotted diagonal pattern and (ii) a dotted horizontal pattern which are both evaluated using as-quenched and tempered martensite as base material. The deformation behaviour of the patterned microstructures is investigated using both experiments and simulations. In the experimental approach characterization of the laser treated specimens is carried out using Optical Microscopy (OM) and micro-hardness measurements. The local deformation of the patterned microstructure is investigated using Digital Image Correlation (DIC). During deformation, strain partitioning is observed in the austenitic areas. In these areas mechanically induced martensitic transformation takes place, influencing the hardening of the material. The phase strength does not seem to influence the austenite stability in the analysed patterns, however it does show difference in hardness of the freshly formed martensite. For the simulation, Crystal Plasticity Finite Element Modelling (CPFEM) is used to describe the plastic deformation of the patterned material. Both the local and overall behaviour are investigated and validated with the experimentally obtained results. The simulation can serve as a tool to identify patterns which show promising deformation behaviour.

Laser surface treatment shows great potential to locally create distinct phases in martensitic steels due to its highly localized laser heat flux. Architectured materials with microstructures of metastable austenite/martensite phases exhibit advanced mechanical properties, such as a better combination of strength and ductility. In addition, the presence of laser-reverted austenite in martensitic microstructure improves the pitting corrosion behavior due to the better corrosion resistance of austenite over the martensite phase. However, the combined microstructure of different phases might lead to the emergence of galvanic corrosion, which deteriorates the general corrosion behavior of the laser-treated materials. These findings suggest that the laser surface treatment affects the general and the localized corrosion behavior differently, which is an interesting prospect that requires further investigation.
Fe-25Ni-0.2C, the material in this study, has the starting martensite formation temperature (Ms) below room temperature due to its high nickel percentage, which is thermodynamically possible to form reverted austenite and remain austenite phases during laser treatment. In this study, a high-power Nd:YAG laser system is utilized to locally create an austenitic region in a cryogenically-formed martensitic Fe-25Ni-0.2C alloy. Optical microscope (OM) and scanning electron microscope (SEM) are used to assess the microstructure prior to and after laser treatment. The observed microstructure is related to the high spatial gradients in peak temperature and the heating rate of localized laser treatment, which governs the formation mechanisms. Moreover, the effect of laser processing parameters, such as laser power (P) and scanning speed (v), on the laser-affected zone (LAZ) is also investigated.
The corrosion behavior is characterized by potentiodynamic polarization tests in a 3.5% NaCl solution. The corroded surface is examined by optical microscope (OM) and the three-dimensional depth measurement of the pit morphology. In this work, both the general and the pitting corrosion behavior are discussed. The results are influenced by the microstructure created by different heat treatments. The effect of localized laser treatment on the corrosion behavior is investigated on a combined microstructure of laser-reverted austenite/bulk martensite, which has a surface fraction of laser-reverted austenite up to nearly 53%.
 

Laser surface treatments offer interesting prospects for the creation of architectured microstructures formed by distinct phases in metastable austenitic steels. In the present study, a laser-based localised heat treatment was developed to locally create an austenitic region in a quenched Fe25Ni0.2C martensitic microstructure. The highly localised laser heat flux gives rise to high spatial gradients in peak temperature and heating rate. This results in strong variations in the microstructures observed over short distances, which are related to local changes in the martensite to austenite phase transformation temperatures and formation mechanisms, the occurrence of grain growth and recrystallization in the newly formed austenite, and the tempering of the initial martensite. Moreover, thermal stresses and surface effects influence the final microstructure. In this work, effects of heating rates and peak temperatures are studied by dilatometry whereas Electron Backscatter Diffraction (EBSD) and optical microscopy are used to assess austenite grain size and morphology. This information is linked to a Finite Element Model of the local thermal history to investigate the evolution of the local microstructure throughout the zone affected by the laser heat source. This research provides insight into localised microstructural control of steels with laser surface treatment and provides a thermal model for detailed understanding of the mechanisms controlling the microstructural changes taking place during these treatments.

Microstructural evolutions during heat treatment of steel have been generally studied employing post-processed (ex-situ) characterisation methods. However, the data obtained cannot describe the kinetics of the transformation processes (in-situ) itself. With several microstructural processes happening simultaneously during an isothermal treatment below Ms, an in-situ characterisation method which can provide information on the microstructure development is needed. In this regard, Synchrotron X-ray Diffraction (SXRD) method is found to be a suited method. This thesis aims to study the microstructural developments during isothermal treatments below the Ms temperature by in-situ SXRD measurements in a model alloy of composition Fe–0.2C–3.51Mn–1.52Si–0.25Mo (wt.%). Results from this study show that during the isothermal treatment below Ms, bainite was formed right after the isothermal treatment started, which supports previous dilatometry experiments. During the isothermal treatment below Ms, four processes are suggested to occur simultaneously: (i) carbon redistribution processes from martensite, (ii) formation of bainite at the martensite/austenite interface, (iii) formation of bainite at the austenite/austenite interface, and (iv) carbon redistribution processes associated with bainite formation.

The role of carbon in the strengthening mechanisms of martensitic steels has been studied for decades. However, uncertainties still exist regarding how the distribution of carbon to various locations inside martensite contributes to the development of its observed microstructure and high strength. The strengthening mechanisms depend on carbon content and process parameters,but are also inter-related. The current work is an attempt to address the existing uncertainties regarding the relation between martensite dislocation density, prior austenite grain size (PAGS) and the manner in which total carbon is distributed into martensite interstitial sites and segregations near dislocations.Two steel alloys with different carbon content were selected to study, having following composition in wt.%: 0.3C-3.6Mn-1.5Si and 0.6C-3.5Mn-1.5Si. Alloys were heat treated in dilatometer to obtain martensite with different prior austenite grain sizes (PAGS). Microstructure characterization was performed using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) & Electron back-scatter diffraction (EBSD) and strength was evaluated by hardness measurements. It was found that the main factor influencing the carbon distribution in martensite is the martensite dislocation density, but the magnitude of its influence depends upon PAGS and alloy composition. In 0.3 wt.% C alloy, dislocation density decreases with increasing PAGS, as a result carbon atoms migrating towards dislocations during cooling also decrease, leaving higher number of carbon atoms at interstitial sites. However in 0.6 wt.% C alloy, such trend is not observed. On increasing the total carbon content in the alloy, very small increase in interstitial carbon content of martensite is observed if the PAGs are large. An increase in hardness was observed when the samples were introduced in liquid nitrogen, even in those cases where contribution from all strengthening mechanisms remained almost the same. This was unexpected and is most probably due to the relaxation of residual stresses and formations of carbon clusters at cryogenic temperature. In the end, an extension to already existing model is proposed to connect several microstructural features together in order to explain how they interact and evolve to give rise to observed strength of martensite.