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%.