Improved corrosion behavior of a novel Fe85Cr4Mo8V2C1 tool steel processed by laser powder bed fusion
Laser powder bed fusion (LPBF) of tool steels offers a great potential for tailoring material properties besides technical advantages like great freedom of design or function integration. In this study, a novel high-strength Fe85Cr4Mo8V2C1 tool steel processed by LPBF and by casting was examined in a comparative study. By means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) for both conditions a microstructure composed of martensite, austenite and complex carbides was detected. Very high cooling rates of LPBF yield a significant microstructural refinement across the building direction with much more homogenous distribution of the principal phases compared with the cast counterparts. The corrosion behavior was investigated in NaCl solutions by means of potentiodynamic polarization measurements as well as surface analysis with SEM and X-ray photo electron spectroscopy (XPS) after immersion testing. Particulary in more aggressive solutions where active dissolution dominates, i.e. in 3.5% NaCl (pH 7) and in 0.035% NaCl (pH 3), the LPBF samples exhibit a retarded corrosion activity according to more positive corrosion potentials and significantly lower corrosion current densities as well as reduced anodic current densities. A wave-like corrosion pattern was detected reflecting melt track-related microstructural features, i.e. local grain coarsening and preferred crystal orientation. In milder solution, 0.035% NaCl (pH 7), Fe85Cr4Mo8V2C1 exhibits for both processing conditions relatively low free corrosion rates. For LPBF samples a complex, fine grain boundary network supports the formation of thicker but slightly more permeable layers composed mainly of iron oxides which retard the corrosion activity especially at higher anodic polarization.