This research work investigates nickel-base superalloys and biphasic stainless steels because of their excellent capacity to withstand a wide range of severe operating conditions. These alloys have a great importance in several engineering fields, especially the oil and gas industry, for high-demanding applications which typically involve combination of high loads, low temperatures and aggressive environments. For this reason, good mechanical strength should be combined with an excellent corrosion resistance. Moreover, especially for biphasic stainless steels, low-temperature toughness is necessary to prevent catastrophic brittle failures in such complex applications. This class of materials represents not only the strongest, but also the most economically sustainable choice thanks to its unique properties. The nickel-base alloy 625 provides an excellent combination of mechanical and corrosion properties enabling its exploitation in several industrial fields from cryogenic temperatures up to 1000 °C and in severely corrosive atmospheres. However, during high-temperature deformation processes and heat treatments, grain growth can irremediably deteriorate the mechanical strength also considering the absence of phase transformation temperatures. The presence of insufficient mechanical properties cannot be improved by the standard heat treatments. Therefore, to prevent waste of material and resources, tailored non-standard age-hardening treatments can be exploited to recover the loss in mechanical strength. Single- and double-aging treatments were investigated and optimized by hardness, tensile and corrosion tests considering different time-temperature conditions. A performance chart was built to identify the conditions corresponding to the best compromise between the tensile properties and the corrosion resistance. Additionally, the influence of the grain size on the age-hardening response was analysed varying the solution temperature. The aging treatment was also investigated and optimized in the selective laser melted alloy 625 by hardness, tensile and corrosion tests. In this case, also the stress-relieving treatment was improved to limit the loss in corrosion resistance induced by the standard temperature recommended for the conventional wrought material that is less precipitation prone. The 2507 biphasic stainless steel provides an excellent combination of mechanical strength, corrosion resistance and low-temperature toughness. However, these exceptional potentialities are obtained at the expense of a great complexity regarding tuning of chemical composition, control of high-temperature deformation and heat-treating parameters. In fact, upon thermal exposure above 300 °C, formation of detrimental phases can be obtained. Among these, spinodal decomposition of ferrite is one of the most severe embrittling phenomena. It can be obtained not only during heat treatment and service, but also during the manufacturing cycle, for instance in the welding process. When its occurrence becomes unavoidable, the investigation of recovery methods is useful to prevent waste of material and resources, also considering the associated economic and environmental impact. The spinodal decomposition phenomenon upon aging at 475 °C was analysed by macro- and micro-hardness tests on the constituent phases and by Charpy impact tests. Finally, reversion heat treatments at 550 °C and 600 °C were investigated to evaluate the possibility of recovering the detrimental effects of the α′ embrittlement.
Questo attività di ricerca ha interessato il campo delle superleghe base nichel e degli acciai inossidabili bifasici per la loro eccellente resistenza ad un’ampia gamma di condizioni operative particolarmente severe. Queste leghe possiedono una grande importanza in diversi campi ingegneristici, in particolare nel settore oil and gas, per applicazioni critiche che tipicamente coinvolgono sollecitazioni meccaniche elevate, basse temperature ed ambienti aggressivi. Per questo motivo, è opportuno combinare una buona resistenza meccanica ad un’ottima resistenza alla corrosione. Inoltre, soprattutto per gli acciai inossidabili bifasici, la tenacità alle basse temperature è di fondamentale importanza per prevenire catastrofici cedimenti in esercizio a carattere fragile. Questa famiglia di materiali rappresenta non solo la scelta ottimale per la combinazione di proprietà meccaniche e a corrosione, ma anche quella più sostenibile dal punto di vista economico. La superlega base nickel 625 fornisce un’eccellente combinazione di proprietà meccaniche ed a corrosione che ne consente l’utilizzo in diverse applicazioni industriali da temperature criogeniche fino a 1000 °C ed in atmosfere fortemente corrosive. Tuttavia, durante i processi di deformazione ad alta temperatura e il trattamento termico, l’accrescimento del grano può deteriorare in modo irreversibile la resistenza meccanica anche considerando l’assenza di temperature di trasformazione di fase. La presenza di proprietà meccaniche insufficienti non può essere migliorata impiegando i trattamenti termici standard. Pertanto, per evitare sprechi di materiale e risorse, è possibile utilizzare trattamenti ad hoc di invecchiamento per recuperare la perdita di resistenza meccanica. I trattamenti di invecchiamento singolo e doppio sono stati studiati ed ottimizzati mediante prove di durezza, trazione e corrosione considerando diverse condizioni tempo-temperatura. Utilizzando i risultati sperimentali, è stata costruita una mappa riassuntiva per identificare le condizioni corrispondenti al miglior compromesso tra proprietà a trazione e resistenza alla corrosione. Inoltre, è stata investigata l’influenza della dimensione del grano sulla risposta all’indurimento per precipitazione variando la temperatura di solubilizzazione. Il trattamento di invecchiamento è stato ottimizzato anche nella superlega base nickel 625 ottenuta via fusione selettiva a letto di polvere. In questo caso, il trattamento termico di distensione è stato migliorato per risolvere la perdita di resistenza a corrosione associata alla temperatura standard di trattamento consigliata per il materiale convenzionale da deformazione plastica. L’acciaio inossidabile bifasico 2507 offre un’eccellente combinazione di resistenza meccanica, alla corrosione e tenacità a bassa temperatura. Tuttavia, queste potenzialità sono ottenute a discapito di una particolare complessità in termini di composizione chimica, deformazione plastica ad alta temperatura e trattamento termico. Infatti, in caso di mantenimento eccessivo al di sopra di 300 °C, è possibile attivare la precipitazione di fasi secondarie dannose. Tra queste, la decomposizione spinodale della ferrite è uno dei fenomeni di infragilimento di maggior importanza. Questo può essere ottenuto non solo durante il trattamento termico ed in servizio, ma anche durante il ciclo produttivo, ad esempio nel processo di saldatura. Tuttavia, quando la sua comparsa diventa inevitabile, la disponibilità di tecniche di recupero delle proprietà è utile per prevenire sprechi di materiale e risorse, considerando anche l’impatto economico ed ambientale. Il fenomeno della decomposizione spinodale a seguito di invecchiamento a 475 °C è stato investigato mediante prove di micro-durezza sulla fase ferritica e attraverso prove di resilienza. Infine, sono stati testati trattamenti termici di recupero a 550 °C e 600 °C per valutare la possibilità di recupero dell’infragilimento da decomposizione spinodale.
Metallurgical optimization and modeling of advanced metallic alloys for demanding applications
Panzeri, Davide
2024/2025
Abstract
This research work investigates nickel-base superalloys and biphasic stainless steels because of their excellent capacity to withstand a wide range of severe operating conditions. These alloys have a great importance in several engineering fields, especially the oil and gas industry, for high-demanding applications which typically involve combination of high loads, low temperatures and aggressive environments. For this reason, good mechanical strength should be combined with an excellent corrosion resistance. Moreover, especially for biphasic stainless steels, low-temperature toughness is necessary to prevent catastrophic brittle failures in such complex applications. This class of materials represents not only the strongest, but also the most economically sustainable choice thanks to its unique properties. The nickel-base alloy 625 provides an excellent combination of mechanical and corrosion properties enabling its exploitation in several industrial fields from cryogenic temperatures up to 1000 °C and in severely corrosive atmospheres. However, during high-temperature deformation processes and heat treatments, grain growth can irremediably deteriorate the mechanical strength also considering the absence of phase transformation temperatures. The presence of insufficient mechanical properties cannot be improved by the standard heat treatments. Therefore, to prevent waste of material and resources, tailored non-standard age-hardening treatments can be exploited to recover the loss in mechanical strength. Single- and double-aging treatments were investigated and optimized by hardness, tensile and corrosion tests considering different time-temperature conditions. A performance chart was built to identify the conditions corresponding to the best compromise between the tensile properties and the corrosion resistance. Additionally, the influence of the grain size on the age-hardening response was analysed varying the solution temperature. The aging treatment was also investigated and optimized in the selective laser melted alloy 625 by hardness, tensile and corrosion tests. In this case, also the stress-relieving treatment was improved to limit the loss in corrosion resistance induced by the standard temperature recommended for the conventional wrought material that is less precipitation prone. The 2507 biphasic stainless steel provides an excellent combination of mechanical strength, corrosion resistance and low-temperature toughness. However, these exceptional potentialities are obtained at the expense of a great complexity regarding tuning of chemical composition, control of high-temperature deformation and heat-treating parameters. In fact, upon thermal exposure above 300 °C, formation of detrimental phases can be obtained. Among these, spinodal decomposition of ferrite is one of the most severe embrittling phenomena. It can be obtained not only during heat treatment and service, but also during the manufacturing cycle, for instance in the welding process. When its occurrence becomes unavoidable, the investigation of recovery methods is useful to prevent waste of material and resources, also considering the associated economic and environmental impact. The spinodal decomposition phenomenon upon aging at 475 °C was analysed by macro- and micro-hardness tests on the constituent phases and by Charpy impact tests. Finally, reversion heat treatments at 550 °C and 600 °C were investigated to evaluate the possibility of recovering the detrimental effects of the α′ embrittlement.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/236633