Alessio Malandruccolo

This study investigates the surface characteristics and corrosion behavior of a high-C martensitic stainless steel (AISI 440C) at different stages of its manufacturing process. As a class, these steels prioritize high mechanical properties and wear resistance over superior corrosion resistance. Hot working operations, such as rolling, create a surface oxide scale that must be removed via pickling to restore the material’s inherent corrosion resistance. This process also eliminates the underlying Cr-depleted layer, allowing for the re-establishment of a protective passive film. Using potentiodynamic polarization curves and micrographic analysis, the material’s behavior in different conditions, as-rolled, with a post-heat treatment oxide scale, and in a bare, oxide-free state, has been assessed. The results showed that the material lacks stable passive behavior under all conditions. The as-rolled and heat-treated conditions both exhibited active behavior and formed thick, non-adherent corrosion products. The oxide layer formed after heat treatment performed the worst, showing a significant increase in corrosion current density. These findings confirm the material’s susceptibility to corrosion in Cl⁻ ion-rich environments, highlighting the need for limited storage in such conditions and rapid pickling after thermal processing to mitigate surface damage.

Superaustenitic stainless steels (SASSs) are one of the families of high-performance stainless steels, the so-called “super” grades. While sharing the face-centered cubic lattice structure typical of standard austenitic stainless steels, their chemical composition is significantly more complex. This enables them to offer an exceptional balance of superior corrosion resistance and high mechanical strength. However, the intricate chemical makeup of SASSs brings challenges, such as the phenomenon of segregation and precipitation of deleterious intermetallics. Consequently, this leads to several challenges in their processing and use. This work aims to present SASSs in detail, starting from their chemistry and metallurgy and ending with processing and applications. Hence, the first part will be dedicated to the analysis of chemistry, resulting grades, microstructure and secondary phases along with the conditions determining their formation. Afterwards, physical, mechanical and corrosion resistance characteristics will be set forth in such a way as to understand their origin and implications for processing and possible uses, with a focus on processability limitations. In fact, manufacturing and processing options significantly affect the types of products that can be developed, and, when considered alongside material attributes and costs, they help define the target markets for these alloys.

The reliability of a prosthetic implant needs durability, biocompatibility, and osseointegration capability. Accomplishing these characteristics, Ti-6Al-4V alloy is the main used material for implant fabrication. Moreover, it can be processed by additive manufacturing technique, permitting to meet the needs of patience-tailored, often complex shaped, prosthesis topologies. Once an implant is realized, it is finished by machining operations and its osseointegration capability is heavily influenced by the resulting surface roughness. Consequently, the assessment of this latter is mandatory to evaluate the prosthesis durability. This paper presents the analysis of surface roughness of Ti-6Al-4V micro-milled specimens produced by plastic deformation, selective laser melting, and electron beam melting processes. A central composite design was employed for planning the cutting tests. The comparison between surface roughness results and its values for enhancing osseointegration, firstly permitted to individuate the range of micro-milling suitable applications, which have been individuated as ball joints, bone plates, and screws. Next, the statistical analysis of the experimental measurements allowed the identification of the most influential micro-milling parameters together with the determination of the mathematical models of surface roughness by response surface methodology. The good comparison among calculated and experimental results revealed the reliability of the model, allowing the prediction of achievable surface roughness once micro-machining parameters are selected, or their optimization as a function of a desired surface roughness value.