title: Microstructure, crystallographic orientation, and electrochemical response of 420 martensitic stainless steel
Creators: name: Salahi, Salar
Publisher: name: Memorial University of Newfoundland
Year: 2022

description: Although the corrosion performance of the 420 martensitic stainless steel has been investigated from the electrochemical perspective, the impact of process-induced metallurgical factors such as secondary phase formation, grain distribution, and crystallographic orientation on the corrosion performance of the alloy is not thoroughly disclosed. This thesis aims to understand the correlation between the microstructure, crystallographic orientation, and electrochemical response of the 420 martensitic stainless steel (420MSS) fabricated by various manufacturing techniques. The corrosion resistance of 420 MSS decreased with increasing the deformation level in both uniaxial tension and cold rolling processes. Increasing the level of uniaxial tension in 420 martensitic stainless steel enlarged the size of sensitized regions along the matrix-coarse chromium carbide precipitates interface, providing more susceptible areas to pitting initiation and propagation. As the cold rolling level increased, higher fraction of fragmented precipitates formed in the microstructure, provoking the micro-galvanic coupling, and deteriorating the corrosion resistance. On the other hand, the microstructural characterization of the wire arc additive manufactured 420 MSS revealed the formation of a martensitic matrix with residual delta ferrite and retained austenite as a result of complex thermal history. Localized corrosion attacks were primarily detected adjacent to the delta ferrite phase, regardless of the implemented interlayer temperature during the fabrication process. Through the post-fabrication heat treatment, the corrosion morphology of the annealed sample was characterized by severe pitting due to the high susceptibility of the ferritic matrix-carbide interface to pitting. The electrochemical performance of the quench and tempered (Q&T) sample was significantly improved, ascribed to the elimination of the chromium-depleted regions adjacent to the delta ferrite phase and enhanced protectiveness of the passive film on the alloy’s surface. Following the characterization of the interfacial bonding region between the wire arc additive manufactured 420 MSS part and the wrought 420 MSS, the formation of four distinct microstructural regions, including (i) far heat-affected zone (HAZ), (ii) close HAZ, (iii) partially melted zone (PMZ), and (iv) fusion zone was disclosed. The electrochemical response of the interfacial bonding region revealed intense degradation close to the PMZ and fusion zone, potentially ascribed to the formation of several distinctive microstructural regions with high susceptibility to localized corrosion attacks. The onset of localized corrosion attack in the interfacial bonding region mainly was initiated along the primary austenite grain boundaries (PAGBs), where segregation of chromium carbides was detected. Similarly, the interface of 420 MSS substrate and a repairing track of PH 13-8Mo stainless steel, fabricated by a wire arc additive manufacturing process, revealed severe deterioration on the electrochemical response possibly attributed to the formation of highly populated fine chromium-rich carbides. The corrosion onset was initiated at the carbide/matrix interface in the close HAZ and PMZ regions, where chromium depleted regions adjacent to the carbides intensified the micro-galvanic coupling effect.