Super Duplex Stainless Steels (SDSSs) are characterized by the compresence of austenitic and ferritic grains. They display very high corrosion resistance, especially in chloride-rich environments and have better mechanical properties than single-phase counterparts. Their unique combination of properties makes them the material of choice for many industries and the ever growing demand of better performing components for industrial and consumer applications ensure that the market for Super Duplex Stainless Steels will grow in the next future. Many studies have been performed to enhance the mechanical properties of SDSSs without detrimental effects on corrosion resistance. The main purpose of this work is to document the effects of different heat treatments on the mechanical properties of a cold rolled UNS S32760 steel. The solution annealed material was cold rolled up to 88% of reduction. The rolled material was heat treated at five different temperatures, 550° C, 620° C, 700° C, 800° C and 1080° C with different holding times, ranging from 1,5 minutes to 60 minutes. Much attention was devoted to the optimization of the heat treatment performed on the plastically deformed material, in order to obtain a suitable compromise between yield strength and ductility, and avoid the formation of embrittling phases, deleterious both for mechanical and corrosion resistance. Tensile tests were performed to assess the mechanical resistance of the material. The heat treatment at 620° C was considered the best compromise, combining 1539 MPa of tensile strength with 8,25 % of fracture elongation after 12,5 minutes of holding time. A detailed microstructural analysis has been carried out to understand the strengthening mechanisms acting on the material and to explain the evolution of the microstructure that leads to the superior mechanical properties of the treated steel. Beside the enhanced mechanical properties, the corrosion resistance of the material has to be carefully checked to ensure a very limited loss of performance, if any. For that reason, the corrosion resistance of the heat-treated material has been checked for the most promising thermo-mechanical treatments.
Gli acciai inossidabili super duplex sono caratterizzati da una struttura bifasica con circa la medesima frazione in volume di austenite e ferrite, ed offrono caratteristiche meccaniche e resistenza a corrosione superiore rispetto alle leghe monofasiche. La sempre crescente richiesta di specifiche tecniche superiori nella produzione e nell’utilizzo di leghe super duplex alimentano una continua ottimizzazione ed una intensa ricerca rivolta ad affinare i processi di produzione al fine di massimizzare le caratteristiche meccaniche. Questo lavoro si focalizza sulla determinazione degli effetti di differenti trattamenti termici sulle proprietà meccaniche di un acciaio UNS S32760 laminato a freddo. Il materiale allo stato ricotto è stato laminato a freddo fino a raggiungere una riduzione di sezione pari a 88 % dello spessore iniziale. Successivamente è stato trattato a cinque temperature, 550° C, 620° C, 700° C, 800° C e 1080° C con differenti tempi di trattamento, da un minimo di 1,5 minuti a un massimo di 60 minuti. Molta attenzione è stata posta nell’ottimizzazione del trattamento termico, al fine di ottenere un compromesso ottimale tra resistenza meccanica e duttilità, evitando la precipitazione di fasi infragilenti, in grado di compromettere le proprietà meccaniche e la resistenza a corrosione del materiale. Il materiale è stato testato attraverso prove di trazione uniassiali. I risultati migliori sono stati ottenuti a 620° C, dove è stata ottenuta una resistenza meccanica pari a 1539 MPa combinata ad un allungamento a rottura del 8,25% dopo 12,5 minuti di trattamento. Una attenta analisi microstrutturale è stata portata a termine al fine di comprendere i fenomeni di precipitazione indotti dai diversi trattamenti termici e spiegare l’evoluzione della microstruttura che porta ad ottenere caratteristiche meccaniche superiori nel materiale trattato. I campioni dai risultati più promettenti sono stati sottoposti ad analisi di polarizzazione potenziodinamica per verificare che la resistenza a corrosione del materiale non venga meno a seguito del trattamento termomeccanico.
Mechanical properties evolution on cold rolled and heat treated UNS S32760 super duplex stainless steel
TROMELLINI, CARLO MARIA
2015/2016
Abstract
Super Duplex Stainless Steels (SDSSs) are characterized by the compresence of austenitic and ferritic grains. They display very high corrosion resistance, especially in chloride-rich environments and have better mechanical properties than single-phase counterparts. Their unique combination of properties makes them the material of choice for many industries and the ever growing demand of better performing components for industrial and consumer applications ensure that the market for Super Duplex Stainless Steels will grow in the next future. Many studies have been performed to enhance the mechanical properties of SDSSs without detrimental effects on corrosion resistance. The main purpose of this work is to document the effects of different heat treatments on the mechanical properties of a cold rolled UNS S32760 steel. The solution annealed material was cold rolled up to 88% of reduction. The rolled material was heat treated at five different temperatures, 550° C, 620° C, 700° C, 800° C and 1080° C with different holding times, ranging from 1,5 minutes to 60 minutes. Much attention was devoted to the optimization of the heat treatment performed on the plastically deformed material, in order to obtain a suitable compromise between yield strength and ductility, and avoid the formation of embrittling phases, deleterious both for mechanical and corrosion resistance. Tensile tests were performed to assess the mechanical resistance of the material. The heat treatment at 620° C was considered the best compromise, combining 1539 MPa of tensile strength with 8,25 % of fracture elongation after 12,5 minutes of holding time. A detailed microstructural analysis has been carried out to understand the strengthening mechanisms acting on the material and to explain the evolution of the microstructure that leads to the superior mechanical properties of the treated steel. Beside the enhanced mechanical properties, the corrosion resistance of the material has to be carefully checked to ensure a very limited loss of performance, if any. For that reason, the corrosion resistance of the heat-treated material has been checked for the most promising thermo-mechanical treatments.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/134459