Thermal spray coatings for corrosion and wear protection of naval Diesel engines components

Hard chrome plating has been widely used as the standard solution for valves stem, but its limited wear-corrosionresistance and the high toxicity of its galvanic baths suggest to look for suitable alternatives, such as thermalsprayed ceramic-metallic (cermet) and self-fluxing alloys.The present study aims to compare different solutions in terms of corrosion resistance under a selected acidenvironment and of wear resistance both for as-sprayed and post-corrosion samples.Coatings have been deposited by HVOF both on martensitic steel and Ni-based superalloy. A Design of Experimentprocedure has been used for spray parameters optimization, based on porosity and deposition efficiency ofthe coatings. Corrosion tests have shown the effect of porosity of the coatings, while wear tests confirmed thefundamental role of the dispersed hard phases.All the solutions here proposed have shown a significant improvement in terms of corrosion and wear resistancecompared to hard chrome plating

Rivestimenti Thermal Spray per la Protezione da Corrosione ed Usura di Componenti di Motori Diesel Marini

Le valvole di aspirazione e scarico dei motori diesel marini soffrono di diversi problemi causati dagli effetti combinati e sinergici di sollecitazioni meccaniche cicliche e di un ambiente aggressivo. Rivestimenti galvanici in cromo duro sono stati largamente impiegati quali protezione standard per gli steli valvola, ma la loro limitata resistenza a corrosione-usura e l’elevata tossicità dei bagni galvanici utilizzati per applicare la cromatura hanno suggerito di ricercare soluzioni più efficaci. Nella letteratura recente sono stati proposti quali alternative possibili vari rivestimenti thermal spray, tra cui diversi compositi metallo-ceramici e le leghe self-fluxing. Nello studio presente diverse soluzioni sono state confrontate analizzandone la resistenza alla corrosione nello specifico ambiente acido e la resistenza all’usura, prima e dopo l’attacco corrosivo. I rivestimenti sono stati depositati mediante HVOF (high velocity oxy-fuel) su substrati in acciaio martensitico o in superlega a base nichel. L’ottimizzazione dei parametri di spruzzatura è stata condotta attraverso procedure di “design of experiment”, sulla base di porosità e durezza dei rivestimenti e dell’efficienza della loro deposizione. I test di corrosione, eseguiti in soluzioni bollenti al 5% di acido solforico hanno evidenziato l’effetto importante del substrato selezionato e della porosità del rivestimento. I test di usura-strisciamento, effettuati a secco in configurazione “block on ring” sia su rivestimenti tal quali che pre-corrosi, hanno confermato il ruolo fondamentale delle caratteristiche delle fasi dure disperse (composizione, distribuzione dimensionale, possibili ossidazione/decarburazione, coerenza con la matrice metallica). Tutte le soluzioni analizzate hanno mostrato resistenza alla tribocorrosione fortemente migliorata rispetto alla convenzionale cromatura

Ceramic-metal thermal spray coatings for the protection of marine diesel engines components

The present study is focused on the development and testing of thermally sprayed coatings for the protection of exhaust and intake valves of marine diesel engines. Different solutions were tested and compared for both corrosion resistance to the specific acidic environment and abrasive wear resistance, before and after the corrosion attack. Coatings were deposited by high velocity oxy-fuel (HVOF) thermal spraying on both martensitic steel and Nimonic 80A substrates. Optimization of thermal spray parameters were carried out by design of experiment procedures, on the basis of coatings cohesion, adhesion, hardness and thickness. Corrosion experiments were carried out in a boiling solution of 5% sulfuric acid, while the abrasive wear tests were conducted adopting the block on ring configuration on both as sprayed and corroded coatings. The most promising coating was selected for an “on field” test: exhaust and intake valves were coated and put in service on a ship engine in order to evaluate the lifetime of the coatings working in operating conditions

Thermal spray coatings for corrosion and wear protection of naval Diesel engines components

Intake and exhaust valves of naval Diesel engines suffer from several problems due to the combined and synergistic effects of mechanical stresses and a chemically aggressive environment. Hard chrome plating has been widely used as the standard solution for valves stem, but its limited wear-orrosion resistance and the high toxicity of its galvanic baths suggest to look for suitable alternatives, such as thermal sprayed ceramic-metallic (cermet) and self-fluxing alloys. The present study aims to compare different solutions in terms of corrosion resistance under a selected acid environment and of wear resistance both for as-sprayed and post-corrosion samples. Coatings have been deposited by HVOF both on martensitic steel and Ni-based superalloy. A Design of Experiment procedure has been used for spray parameters optimization, based on porosity and deposition efficiency of the coatings. Corrosion tests have shown the effect of porosity of the coatings, while wear tests confirmed the fundamental role of the dispersed hard phases. All the solutions here proposed have shown a significant improvement in terms of corrosion and wear resistance compared to hard chrome plating

Uniaxial fatigue properties of closed die hot forged 42CrMo4 steel: Effect of flash and mechanical surface treatments

Uniaxial fatigue properties of 42CrMo4 steel, produced by closed die hot forging and then heat treated by conventional quenching and tempering treatment, are studied in the present work. The steel specimens underwent sand blasting. Successively, a set of samples was subjected to shot peening treatment, using 2 different intensities. Surface morphology and chemical composition as well as microstructure were investigated by scanning electron microscopy and stylus profilometry. Residual stresses were evaluated by X-ray Diffraction. The mechanical properties were studied by tensile tests and Vickers micro-hardness profiles. The fatigue limit was calculated with the Dixon Mood. In order to assess the causes of crack initiation, a simple model based on the local fatigue strength concept was adopted, considering the influence of residual stresses. The experimental findings showed that the surface treatments affect the fatigue properties. The crack nucleation site for the sand blasted specimens is close to the surface, while for the shot peened specimens the crack nucleation is linked to the activation of surface defects or to the presence of inclusions locally reducing the fatigue limit. The most probable crack nucleation region is in proximity of the flash