Academic literature on the topic 'Phase transformation, Duplex Stainless Steels, Dual Phase Steels'

Abstract The ferritic-austenitic duplex-steels have a very complex precipitation and transformation behaviour, which requires professional treatment. Especially the precipitation of the σ-phase causes considerable changes with regard to the mechanical as well as the corrosive properties, which are to be considered during the treatment of the duplex-steels.

During processing or use, duplex stainless steels are subject to a great number of significant phase transformations, such as solidification, partial ferrite transformation to austenite, ferrite eutectoid decomposition to sigma phase plus austenite, chi phase precipitation, chromium carbide precipitation, chromium nitride precipitation, ferrite spinodal decomposition, phase dissolution during solution annealing, forming of two types (epsilon and alpha prime) of strain induced martensite, martensite reversion to austenite, ferrite and austenite recrystallization. This paper summarizes the phase transformations that occur (individually or combined) in duplex stainless steels and presents some new results.

This work presents a study on the duplex stainless steels UNS S32101 and UNS S32205 when subjected to cathodic hydrogenation, to ascertain their behavior under the action of hydrogen. It was evaluated for embrittlement and phase transformations induced by hydrogen, in order to check whether nickel and molybdenum contents would improve resistance to the harmful effects of hydrogen. With the aid of optical (MO) and scanning electron microscopy (SEM), both hydrogen embrittlement in both steels after hydrogenation and degassing was evidenced, as well as pitting corrosion on UNS S32101 duplex stainless steel. It appears that hydrogen can induce the transformation of the austenitic phase (g) into the martensitic phase (α') in the two duplex stainless steels analyzed and it is verified that hydrogen can lead to the formation of sigma phase at room temperature in duplex stainless steel UNS type S32101

Duplex stainless steel has significantly broadened the range of applications of stainless steel. They have a dual-phase microstructure containing ferrite and austenite at approximately a 50–50% phase ratio. Their corrosion resistance is much better compared to the traditional austenitic stainless steel, especially in surroundings containing chloride ion. Moreover, the large stress yield of duplex steels offers significant advantages in structural applications. The ferrite phase in some duplex stainless steels is metastable due to its composition. Consequently, the ferrite can decompose to a secondary austenite and sigma phase due to heat input. The sigma phase is a hard and brittle intermetallic compound phase that significantly deteriorates the mechanical and corrosion-resistant properties of duplex stainless steel. The embrittlement can cause a safety risk in industrial applications. This paper is a preliminary study to investigate what physical properties can be used to obtain information on sigma-phase-induced embrittlement. In this work, the effect of plastic deformation and heat treatment was studied in the appearance of the sigma phase in 2507 duplex stainless steel. Magnetic saturation polarization and thermoelectric power measurements were used to monitor the microstructural changes due to cold rolling and heat treatment. It was found that the magnetic saturation polarization and thermoelectric power measurements can be effective tools for monitoring the sigma-phase formation in duplex stainless steels due to heat input. Their application helps to prevent the embrittlement problems caused by the sigma-phase formation in duplex stainless steel structures.

AbstractDuplex stainless steels (DSS) may be defined as a category of steels with a two-phase ferritic–austenitic microstructure, which combines good mechanical and corrosion properties. However, these steels can undergo significant microstructural modification as a consequence of either thermo-mechanical treatments (ferrite decomposition, which causes σ- and χ-phase formation and nitride precipitation) or plastic deformation at room temperature [austenite transformation into strain-induced martensite (SIM)]. These secondary phases noticeably affect the properties of DSS, and therefore are of huge industrial interest. In the present work, SIM formation was investigated in a 2101 lean DSS. The material was subjected to cold rolling at various degrees of deformation (from 10 to 80% thickness reduction) and the microstructure developed after plastic deformation was investigated by electron backscattered diffraction, X-ray diffraction measurements, and hardness and magnetic tests. It was observed that SIM formed as a consequence of deformations higher than ~20% and residual austenite was still observed at 80% of thickness reduction. Furthermore, a direct relationship was found between microstructure and magnetic properties.

Samples of hyperduplex stainless steels were produced experimentally and exposed to different conventional annealing heat treatments in order to obtain the microstructural balance of 50% ferrite and 50% austenite. To differentiate the ferrite and austenite from any secondary phase, selective etching was used and quantitative metallography was performed to measure the percentage of phases. Results showed that conventional annealing heat treatments promote the transformation from ferrite to sigma phase and secondary austenite, suggesting a higher occurrence of sigma phase in the experimental hyperduplex alloys compared to other duplex alloys due to the superior content of chromium and molybdenum. On the other hand, a balanced microstructure free of secondary phases was accomplished increasing the temperature of the annealing heat treatment, which allowed the transformation of ferrite into austenite during cooling.

Duplex and Super Duplex Stainless Steels are very prone to secondary phases formation related to ferrite decomposition at high temperatures. In the present paper the results on secondary phase precipitation in a 2510 Duplex Stainless Steel, heat-treated in the temperature range 850–1050 °C for 3–30 min are presented. The precipitation starts at grain boundaries with a consistent ferrite transformation for very short times. The noses of the Time–Temperature–Precipitation (TTP) curves are at 1000 °C for σ-phase and at 900 °C for χ-phase, respectively. The precipitation sequence involves a partial transformation of χ into σ, as previously evidenced in 2205 and 2507 grades. Furthermore, the experimental data were compared to the results of Thermo-Calc calculations. Understanding and ability to predict phase stability in 2510 duplex stainless steel is a key factor to design optimal welding processes that avoid any secondary phase precipitation in the weld bead as well as in the heat-affected zone.

This paper concerns the phase transformation induced by heat treatment and cold rolling in four duplex stainless steel. In 2205 and 2507 , during the isothermal heat treatments, chi-phase precipitates as small particles at the ferrite/austenite boundaries, followed by sigma precipitation. At the lowest temperature the formation kinetic of chi-phase is favoured, with the increasing of time and temperature a progressive transformation of chi to sigma occurs and the kinetic of sigma is favoured. During continuous cooling, the chi -phase appears at low cooling rates. In low Ni grades the grain boundaries precipitation of chromium nitrides were detected , but no sigma and chi. In 2101 the austenite transforms to martensite both after cold rolling and quenching

Abstract Duplex stainless steels offer a high strength alternative to stainless steel, while providing excellent corrosion resistance, due to their dual-phase microstructure. This microstructure can be significantly influenced during welding, thus the maximum recommended heat input is usually 2.5 kJ/mm. In this research, we inspected the high heat input (3 kJ/mm) weldability of NSSC 2120 lean duplex stainless steel, which is designed and developed specifically for this purpose. The welds were evaluated by metallographic techniques and corrosion tests. It was found the NSSC 2120 grade can be welded with high heat input without deterioration in the phase balance and microstructure.

In order to achieve progress in Advanced Steels development came more emphasis in solid state phase transformations are received. For achieving the desired mechanical and corrosion resistance properties in Duplex Stainless Steels (DSS), a precise knowledge of the precipitation kinetics of secondary phases, the morphology of the precipitates and effects of the alloying elements on different properties is needed. A complicated chemical composition and the production technology route make each grade of DSS a unique object for a study. Besides, when the market needs to reduce weight and increase product durability by utilizing Advance Strength Steels, a deeper understanding of their transformations is required. The aim of the present work was to study the main features of phase precipitation in diverse Duplex Stainless Steels grades, including Lean Duplex, Standard and Superduplex. Beside analyze the effects of metallurgical features on the properties of DSS and Advanced High Strength Dual Phase (DP) steels. One of the tasks was to study the effects plastic deformation after heat treatment in diverse duplex grades.
Con lo scopo di ottenere progressi industriali nello sviluppo di Advanced Steels, specie quando le necessità di mercato richiedono una riduzione di peso e un aumento della durabilità è fondamentale una più profonda comprensione delle loro trasformazioni di fase allo stato solido. Nel caso di acciai Inossidabili Duplex (DSS), per raggiungere le proprietà meccaniche desiderate e le proprietà di resistenza alla corrosione, è necessaria la precisa conoscenza della cinetica di precipitazione di fasi secondarie, la morfologia dei precipitati e gli effetti degli elementi alleganti su diverse proprietà. La complessa composizione chimica e la tecnologia di produzione rendono ciascuna tipologia di DSS come un caso di studio unico. L’obbiettivo del presente lavoro è stato quello di studiare le principali caratteristiche delle precipitazioni di fasi secondarie in diversi tipi di acciai inossidabili duplex, comprendendo i Lean Duplex, Standard e altamente legati duplex, ed inoltre di analizzare gli effetti delle caratteristiche metallurgiche sulle proprietà degli Acciai Duplex e Advanced High Strength Steels.

Duplex Stainless Steels (DSS) are used extensively in various industrial applications where the properties of both austenite and ferrite steels are required. Higher mechanical strength and superior corrosion resistance are the advantages of DSS. One of the main drawbacks for Duplex steels is precipitation of sigma phase and other intermetallic phases adversely affecting the mechanical strength and the corrosion behavior of the steels. The precipitation of these secondary phases and the associated brittleness can be due to improper heat treatment. The instability in the microstructure of Duplex stainless steels can be studied by understanding the phase transformations especially the ones involving sigma phase. To reduce the time and effort to be put in for experimental work, computational simulations are used to get an initial understanding on the phase transformations. The present thesis work is on the phase transformations involving sigma phase for Fe-Cr system and Fe-Cr-Ni system using theoretical approach in 1D and 2D geometries. A phase field model is implemented for the microstructural evolution in DSS in combination with thermodynamic data collected from the Thermo-Calc software. The Wheeler Boettinger McFadden (WBM) model is used for Gibbs energy interpolation of the system. FiPy- Finite volume PDE solver written in python is used to simulate the phase transformation conditions first in Fe-Cr system for ferrite-austenite and ferrite-sigma phase transformations. It is then repeated for Fe-Cr-Ni ternary system. In the present study a model was developed for deriving Gibbs energy expression for sigma phase based on the common tangent condition. This model can be used to describe composition constrained phases and stoichiometric phases using the WBM model in phase field modeling. Cogswell’s theory of using phase order variable instead of an interpolating polynomial in the expression for Gibbs energy of whole system is also tried.

The attention towards innovative steels at limited cost increased significantly in the last years. The research focused mainly in the development of new high strength steels where a combination of elevated mechanical properties, good formability and weldability is required and of duplex stainless steels if high corrosion resistance and mechanical properties are demanded. The possibility to design new light components thanks to the higher strength of such steels, the substitution of expensive raw elements and the new specific production processing have permitted to achieve a global costs saving. However a deep knowledge about the critical aspects of these two classes of steels is of fundamental importance to avoid problems in service and eventually catastrophic failures. The aim of this study was to analyze the effects of metallurgical features and phase transformations on the properties of duplex stainless steels (DSS), high strength low alloy (HSLA) steels and advanced high strength dual phase (DP) steels. A detailed review on the state of art of the innovative steels considered has been carried out. The experimental work has been organized into two sections dealing with the critical aspects of duplex stainless steels and high strength steels. In the section concerning DSS, an overall study about secondary phase precipitation occurring during heat treatments of different DSS grades was performed. Then a deeper investigation on lower alloyed DSS, the so called Lean DSS, and their behavior was analyzed. In particular a relation between the morphology of intermetallics precipitation and the fracture toughness was found and compared for two Lean DSS. To reduce the costs, strong austenite phase stabilizers such as Ni are substituted with less stabilizing element as Mn, leading to a certain austenite phase (γ) instability which eventually transforms into ferromagnetic lath martensite (α') during plastic deformation. This phase transformation can potentially affect the properties of the material. Therefore the possible γ→α' evolution during cold rolling was evaluated mainly through magnetic and X-ray diffraction techniques. The second section focused on the influence of microstructure on the mechanical properties and weldability of high strength and advanced high strength steels. Fatigue behavior and weldability are of extreme importance in these two types of steels, especially if designed for structural automotive applications. Hence the role of microllaoying elements and thermo-mechanical processing on fatigue properties and fracture was revealed for different micro-alloyied HSLA steels, whereas the influence of braze-welding parameters on microstructural and mechanical properties was highlighted in a DP steel.
Nel corso degli ultimi anni la ricerca si è focalizzata sulla messa a punto di acciai innovativi a costo contenuto. Grande interesse è stato posto sullo sviluppo di nuovi acciai alto resistenziali in grado di avere una buona combinazione di elevate proprietà meccaniche, formabilità e saldabilità, e sullo sviluppo di acciai inossidabili bifasici nelle applicazioni richiedenti alta resistenza a corrosione e proprietà meccaniche. Inoltre la possibilità di progettare con materiali più leggeri, grazie all’elevata resistenza meccanica che presentano, il risparmio dovuto alla sostituzione di elementi costosi e all’utilizzo di nuovi processi produttivi hanno permesso una riduzione globale dei costi. Al fine di evitare problemi in esercizio con eventuali rotture catastrofiche si è resa necessaria una approfondita conoscenza degli aspetti critici di queste tipologie di acciai di ultima generazione, considerato il potenziale ampio utilizzo di tali materiali anche in applicazioni di uso comune. L’obiettivo di questa tesi è di analizzare gli effetti da un punto di vista metallurgico sulle proprietà principali caratterizzanti gli acciai inossidabili Duplex (DSS), gli acciai basso legati ad alto limite di snervamento (HSLA) e gli acciai alto resistenziali avanzati Dual Phase. Per una maggiore completezza della ricerca e una migliore interpretazione dei risultati ottenuti nello studio sperimentale è stata condotta una dettagliata ricerca bibliografica sullo stato dell’arte delle categorie di acciai considerati. Il lavoro sperimentale è stato diviso in due parti nelle quali sono stati messi in luce gli aspetti critici degli acciai inossidabili duplex e degli acciai alto resistenziali. La sezione riguardante gli acciai inossidabili duplex comprende uno studio completo sui fenomeni di precipitazione di fasi secondarie che hanno luogo durante trattamento termico di diverse tipologie di tali acciai. In una fase successiva lo studio si è concentrato sui cosiddetti “Lean” Duplex, caratterizzati da un minore contenuto di elementi in lega. In particolare è stata rilevata una certa relazione tra la presenza e distribuzione di fasi infragilenti e le proprietà di tenacità di due acciai inossidabili “Lean” Duplex. Negli acciai “Lean” Duplex il contenuto di elementi costosi e volatili quali il Ni è ridotto per mantenere contenuto il loro costo. Il Ni viene sostituito principalmente dal Mn, avente tuttavia un minore potere stabilizzante nei confronti della fase austenitica (γ), che potenzialmente può evolvere in “lath” martensite ferromagnetica (α') con la deformazione a freddo. L’introduzione di questa nuova fase nel materiale può indurre cambiamenti nelle proprietà del materiale stesso. Pertanto la possibile trasformazione γ→α' in seguito a laminazione a freddo è stata valutata mediante misure magnetica e diffrazione a raggi X. La seconda parte del lavoro è stata incentrata sull’influenza della microstruttura sulle proprietà meccaniche e di saldabilità di acciai alto resistenziali. Le proprietà a fatica e la saldabilità sono di estrema importanza in questa classe di acciai, specialmente se destinati ad applicazioni nel campo auto motive. È stato quindi analizzato il ruolo che gli elementi microalliganti e gli specifici trattamenti termo meccanici rivestono sulle proprietà a fatica e sul relativo meccanismo di frattura in diverse tipologie di acciai HSLA. Inoltre sono stati valutati gli effetti della variazione dei parametri di saldobrasatura sulle proprietà microstrutturali e meccaniche di un acciaio DP.

Metastable austenitic and duplex stainless steels are widely used materials in industrial anddomestic applications, owing to their attractive characteristics such as good corrosion resistanceand favorable mechanical properties. Both types of steel experience enhanced mechanicalproperties during plastic deformation due to the formation of the martensite phase from theparent austenite phase, this is called deformation-induced martensitic transformation (DIMT).It is therefore of technical interest to study the transformation mechanism and its impact onmechanical properties for a better understanding and ultimately for developing new materialswith improved performance in certain applications. In the present thesis, two austenitic stainless steels (201Cu, HyTens® 301) and two duplexstainless steels (FDX25®, FDX27®) were investigated. Samples were tensile tested during insitusynchrotron radiation experiments performed at the Cornell High Energy SynchrotronSource (CHESS), Ithaca, USA. Tests were performed at both room temperature and at elevatedtemperatures. The collected diffraction data were then processed by software such as Fit2D andMATLAB. Quantitative phase fraction analysis based on the direct comparison method wasperformed successfully. Microstructural analysis of samples before deformation and after thefull tensile testing was also performed using electron microscopy. The deformation induced martensitic transformation took place in HyTens 301, FDX25 andFDX27, but in 201Cu the austenite was stable during the tensile tests conducted here. The a’-martensite formed in a significantly higher fraction than the ε-martensite in all alloys. At roomtemperature, the critical stress levels for martensitic transformation were 490 MPa, 700 MPaand 700MPa for HyTens 301, FDX25 and FDX27, respectively.

Abstract Hyper Duplex Stainless Steels (HDSS) are dual phase, ferritic-austenitic materials with a remarkable yield strength (≥ 700 MPa) and corrosion resistance (PREN/W > 49). It has been developed as an alternative to super-duplex stainless steels, where higher mechanical and corrosion performance is required. Unfortunately, such highly alloyed materials are susceptible to brittle intermetallic phase formation, such as the sigma phase. Understanding the intermetallic formation is essential to obtain its optimal properties and define advantages and limitations for a welding specification. Overlay experiments on three-layered HDSS deposited over a carbon steel plate using 1.1kJ/mm and 1.65kJ/mm shown a clear austenite/ferrite phase ratio difference between the first and last layer. The last deposited layer has a larger ferrite volume fraction and chromium nitride presence. However, no sigma phase was found on the overlay conditions. A sigma phase kinetic model was developed using Thermocalc Prisma, experimentally adjusted, and validated by physical simulation in a series of isothermal heat treatment tests. The additivity rule was used to calculate continuous-cooling-transformation (CCT) curves from the adjusted temperature-time-transformation (TTT) curves. The kinetic model predicts no sigma precipitation for cooling rates faster than 4°C/s. Physical simulation with controlled cooling rates validated the model. Also, the thermal history analysis of the overlay experiments has shown no sigma was expected due to the total time and temperature transformation and cooling rates not reaching the calculated CCT.

The effects of cooling rate ω8/5 and ω12/8 on the simulated HAZ microstructure transformation in 2205 duplex stainless steel are studied in this paper. The results indicate that 1200°C ∼ 800°C is the temperature range in which the microstructure transits the most violently for 2205 steel, and is also the cooling interval, that affects the phase proportion and microstructure morphology the most distinctly. Accordingly, It is more efficient to use ω12/8 as the parameter to investigate the microstructure transformation of welding HAZ microstructure of this material. The cooling rate in this interval will affect the microstructure transformation of HAZ microstructure of 2205 steel remarkably.