Over the last decades, several studies have been dedicated to the development of new high silicon bainitic steels, since their exceptional combination of tensile strength, ductility, and impact toughness. In fact, as shown in the literature these new steels can reach UTS levels of around 2 GPa, with total elongation around 20%, and fracture toughness level of around 100MPa√m.These steel grades are characterized by a multiphase microstructure consisting of bainitic ferrite supersaturated in carbon and carbon-enriched retained austenite, obtained after the so-called austempering treatment. In this thesis work a novel composition of a medium carbon bainitic steel was developed and characterized. This manuscript is in detail organized into ten different chapters. In the first chapter of the thesis, bainite will be generally introduced and described, while in Chapter 2 is then dedicated to the production and description of the investigated developed alloy. In chapter 3 discusses the effect of the isothermal transformation temperature on the bainitic transformation, the microstructural evolution and the mechanical properties during tensile tsets. Then the effect of multistep isothermal treatments on the bainitic transformation and on the microstructural evolution will be discussed in Chapter 4.This work was performed during a research period spent at UCLouvain, under the supervision of prof. Pascal Jacques. Chapter 5 is dedicated to the investigation of the wear resistance of the carbide-free bainitic microstructure under dry sliding conditions since the possibility to adopt these steel grades in the railway industry. In particular, the effect of different isothermal transformation temperatures and therefore microstructures will be investigated by means ball on disc tests. It will be discussed the wear mechanism and the microstructural evolution induced by the contact and the relative movement between bainite and the counter body. Furthermore, a pearlitic microstructure will be used as a standard reference. This work was performed with the University of Ferrara. Chapter 6 concerns themomechanical treatments. The impact of the deformation temperature bainite morphology was analyzed, accompanied by the use of advanced characterization techniques. Moreover, the nature of the occurring transformation will be discussed also through the adoption of a thermodynamical approach. This research activity was carried out in the frame of a research project at the University of Padova (BIRD 2021 Luca Pezzato). Chapters 7 and 8 are dedicated to the investigation performed during the research period performed at CENIM centre in Madrid in collaboration with dr. Carlos Garcia-Mateo. The first part of the collaboration aims to investigate the effect of prior austenite grain size on both bainite morphology and the kinetic of the transformation in order to find an explanation for the controversial results in the literature. Then, a systematic study of martensite and bainite tempering was performed by dilatometric experiments, microstructural investigations and theoretical calculations. Chapter 9 concerns the weldability of carbide-free bainitic steels. The research was performed in collaboration with INE S.p.A. The effect of different welding techniques (GMAW, SMAW and GTAW), consumables (low- and medium-carbon steels) and post-welding heat treatments will be discussed. Finally, chapter 10 is devoted to a preliminary study on the additive manufacturing fabrication of carbide-free bainitic steels,("Progetto di grande rilevanza Italia-Russia" entitled " Investigation on 3d-printing of Advanced High Strength Steels with TRIP effect for realization of self-adapting aerospace structural elements", financed by the MAECI)”.
In questi due ultimi decenni numerose ricerche sono state dedicate allo studio degli acciai bainitici carbide-free grazie alle loro eccezionali combinazioni di proprietà meccaniche, in termini di resistenza meccanica, duttilità e resilienza. Questi acciai sono infatti in grado di raggiungere carichi di rottura attorno ai 2 GPa, allungamenti a rottura del 20% e resilienza pari a 100 MPa m1/2. La tesi è suddivisa in dieci diversi capitoli. Il primo capitolo consiste in una introduzione riguardante la microstruttura bainitica, che parte dalla sua scoperta e continua trattando i principali aspetti della trasformazione di fase., invece nel nel secondo capitolo è descritto il materiale utilizzato in questa attività di ricerca e il processo produttivo necessario per la sua produzioneIl terzo e il quarto capitolo espongono i risultati ottenuti durante il periodo di ricerca presso UCLouvain sotto la supervisione del professor Pascal Jacques. Si è affrontato lo studio dell’effetto di diverse temperature di trasformazione isoterma e di trattamenti multistep sulla microstruttura bainitica e sulle caratteristiche meccaniche Successivamente, nel capitolo 5, sono presentati i risultati di uno studio sulle proprietà tribologiche dell’acciaio bainitico oggetto di questa tesi. In particolare, è stato studiato l’effetto di temperature di diverse temperature di trasformazione bainitica sulla resistenza ad usura, misurata attraverso test ball-on disc. Inoltre la resistenza ad usura è stata confrontata anche con una microstruttura perlitica. Particolare attenzione è stata dedicata allo studio del meccanismo di usura e alle trasformazioni di fase che sono conseguenza dell’azione del movimento reciproco tra materiale e antagonista. L’attività di ricerca è stata svolta in collaborazione con l’Università di Ferrara. Nel capitolo 6 sono raccolti i risultati dell’attività di ricerca svolta nell’ambito del progetto finanziato dall’Università di Padova BIRD2021 (responsabile Luca Pezzato), riguardante i trattamenti di termomeccanici di ausforming. In particolare si è studiato l’effetto della deformazione e della temperatura di deformazione sulla trasformazione bainitica per questo acciaio, attraverso l’utilizzo di un simulatore termomeccanico accompagnato dalle tipiche tecniche di caratterizzazione e calcoli termodinamici. I capitoli 7 e 8 sono dedicati all’esposizione dei risultati ottenuti durante il periodo di ricerca svolto presso il centro CENIM-CSIC a Madrid, sotto la supervisione di Carlos Garcia-Mateo. Il primo capitolo tratta lo studio dell’effetto dei grani austenitici di partenza sulla cinetica di trasformazione e la morfologia della microstruttura bainitica, mentre il secondo riguarda uno studio sistematico effettuato mediante analisi dilatometriche, caratterizzazioni microstrutturali e calcoli termodinamici.Nel capitolo 9 si è studiata la saldabilità dell’acciaio bainitico oggetto di questa tesi in collaborazione con l’azienda INE S.p.A., che ha fornito strumenti, consumabili e operatori qualificati. Si è studiato l’effetto di tecniche (GTAW, SMAW, GMAW), consumabili (acciai a basso e medio tenore di carbonio) e trattamenti post saldatura (austemperign) su difetti, tensioni residue, microstruttura e caratteristiche meccaniche del giunto saldato. Il capitrolo 10 riguarda il progetto "Progetto di grande rilevanza Italia-Russia" dal titolo " Investigation on 3d-printing of Advanced High Strength Steels with TRIP effect for realization of self adapting aerospace structural elements", finanziato da MAECI. Il lavoro include uno studio preliminare per l’ottimizzazione dei parametri e lo studio microstrutturale prima e dopo trattamento termico anche tramite analisi di tipo dilatometrico.
Sviluppo di un acciaio bainitico ad alto silicio assistito da effetto TRIP
FRANCESCHI, MATTIA
2023
Abstract
Over the last decades, several studies have been dedicated to the development of new high silicon bainitic steels, since their exceptional combination of tensile strength, ductility, and impact toughness. In fact, as shown in the literature these new steels can reach UTS levels of around 2 GPa, with total elongation around 20%, and fracture toughness level of around 100MPa√m.These steel grades are characterized by a multiphase microstructure consisting of bainitic ferrite supersaturated in carbon and carbon-enriched retained austenite, obtained after the so-called austempering treatment. In this thesis work a novel composition of a medium carbon bainitic steel was developed and characterized. This manuscript is in detail organized into ten different chapters. In the first chapter of the thesis, bainite will be generally introduced and described, while in Chapter 2 is then dedicated to the production and description of the investigated developed alloy. In chapter 3 discusses the effect of the isothermal transformation temperature on the bainitic transformation, the microstructural evolution and the mechanical properties during tensile tsets. Then the effect of multistep isothermal treatments on the bainitic transformation and on the microstructural evolution will be discussed in Chapter 4.This work was performed during a research period spent at UCLouvain, under the supervision of prof. Pascal Jacques. Chapter 5 is dedicated to the investigation of the wear resistance of the carbide-free bainitic microstructure under dry sliding conditions since the possibility to adopt these steel grades in the railway industry. In particular, the effect of different isothermal transformation temperatures and therefore microstructures will be investigated by means ball on disc tests. It will be discussed the wear mechanism and the microstructural evolution induced by the contact and the relative movement between bainite and the counter body. Furthermore, a pearlitic microstructure will be used as a standard reference. This work was performed with the University of Ferrara. Chapter 6 concerns themomechanical treatments. The impact of the deformation temperature bainite morphology was analyzed, accompanied by the use of advanced characterization techniques. Moreover, the nature of the occurring transformation will be discussed also through the adoption of a thermodynamical approach. This research activity was carried out in the frame of a research project at the University of Padova (BIRD 2021 Luca Pezzato). Chapters 7 and 8 are dedicated to the investigation performed during the research period performed at CENIM centre in Madrid in collaboration with dr. Carlos Garcia-Mateo. The first part of the collaboration aims to investigate the effect of prior austenite grain size on both bainite morphology and the kinetic of the transformation in order to find an explanation for the controversial results in the literature. Then, a systematic study of martensite and bainite tempering was performed by dilatometric experiments, microstructural investigations and theoretical calculations. Chapter 9 concerns the weldability of carbide-free bainitic steels. The research was performed in collaboration with INE S.p.A. The effect of different welding techniques (GMAW, SMAW and GTAW), consumables (low- and medium-carbon steels) and post-welding heat treatments will be discussed. Finally, chapter 10 is devoted to a preliminary study on the additive manufacturing fabrication of carbide-free bainitic steels,("Progetto di grande rilevanza Italia-Russia" entitled " Investigation on 3d-printing of Advanced High Strength Steels with TRIP effect for realization of self-adapting aerospace structural elements", financed by the MAECI)”.| File | Dimensione | Formato | |
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revised thesis Mattia Franceschi.pdf
embargo fino al 10/05/2026
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https://hdl.handle.net/20.500.14242/80566
URN:NBN:IT:UNIPD-80566