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Sigma phase morphologies in cast and aged super duplex

stainless steel

Marcelo Martinsa,b,⁎ , Luiz Carlos Castelettic,1

aSULZER BRASIL S/A, BrazilbSão Paulo Salesian University Center (UNISAL), Americana Division, Av. Eng. João Fernandes G. Molina, 905 —Distrito Industrial — 13.213-080

 Jundiaí-SP — BrazilcDepartment of Materials, Aeronautical and Automotive Engineering,São Carlos School of Engineering, University of São Paulo(USP),Av. Trabalhador

São Carlense, 400 — 13.566-590 São Carlos  — SP — Brazil

A R T I C L E D A T A A B S T R A C T

 Article history:

Received 22 July 2008

Accepted 15 January 2009

Solution annealed and water quenched duplex and super duplex stainless steels are

thermodynamically metastable systems at room temperature.

These systems do not migrate spontaneously to a thermodynamically stable condition

because an energy barrier separates the metastable and stable states. However, any heat

input they receive, for example through isothermal treatment or through prolonged

exposure to a voltaic arc in the welding process, cause them to reach a condition of stable

equilibrium which, for super duplex stainless steels, means precipitation of intermetallic

and carbide phases. These phases include the sigma phase, which is easily identified from

its morphology, and its influence on the material's impact strength.

The purpose of this work was to ascertain how 2-hour isothermal heat treatments at 920 °C

and 980 °C affect the microstructure of ASTM A890/A890M GR 6A super duplex stainless

steel. The sigma phase morphologies were found to be influenced by these two aging 

temperatures, with the material showing a predominantly lacy microstructure when heat

treated at 920 °C and block-shaped when heat treated at 980 °C.

© 2009 Elsevier Inc. All rights reserved.

Keywords:

Metals

Heat treatment

Electron microscopy

Optical microscopy

1. Introduction

The sigma phase in 2205-type duplex stainless steelssolution annealed at 1080 °C and water quenched, precipi-tates through the diffusion of chromium and molybdenum

from the delta ferrite to the ferrite/austenite interfaces [1].The  δ /γ interface with high interfacial energy is consideredthe most favorable site for precipitation of this intermetallicphase. Nucleation of the sigma phase may also occur at thetwin boundaries in the austenitic phase and in stacking dislocations.

Small M23C6-type chromium carbide particles of about 0.1μmcan also be found associated with the sigma phase at   δ /γinterfaces. The interfacial precipitation of M23C6-type carbides isassociated with the partition of carbon and chromium in theaustenite and ferrite phases, respectively. M23C6 carbide particles

are sometimes found completely surrounded by rod-shapedsigma phase particles.

Research has focused on the role of preformed particles of M23C6 in the precipitation of the sigma phase in duplex stainlesssteels. It is thought these M23C6 particles maybe preferential sitesfor nucleation of the sigma phase [1].

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⁎  Corresponding author. São Paulo Salesian University Center (UNISAL), Americana Division, Av. Eng. João Fernandes G. Molina, 905  — DistritoIndustrial  — 13.213-080 Jundiaí-SP — Brazil. Tel.: +55 11 45892020; fax: +55 11 45892102.

E-mail addresses: marcelo.martins@sulzer.com (M. Martins), castelet@sc.usp.br  (L.C. Casteletti).1 Tel.: +55 16 33739580; fax: +55 16 33739590.

1044-5803/$   – see front matter © 2009 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2009.01.005

mailto:marcelo.martins@sulzer.commailto:castelet@sc.usp.brhttp://dx.doi.org/10.1016/j.matchar.2009.01.005http://dx.doi.org/10.1016/j.matchar.2009.01.005mailto:castelet@sc.usp.brmailto:marcelo.martins@sulzer.com

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The delta ferrite of the duplex structure is usually ametastable phase at temperatures below 1000 °C. Theausteniticphasegrowsspontaneously toward regions of delta ferritewhenthe material is cooled slowly after being solution annealed.M23C6  particles initially precipitated at the   δ /γ   interface canbecome “trapped” in the regions of newly grown austenite. Thepreservation of M23C6   particles in the sigma phase can beattributed to the low solubility of carbon in this phase.

A lamellar structure composed of M23C6 and γ  particles candevelop isothermally in 2205-type duplex stainless steels   [2].However, no lamellar mixture of γ and M23C6 can be found in thesame material when it is cooled continually, as in the case of 

solidification cooling, particularly if the carbon content is verylow [2]. The preferential growth of these particles in the directionof the delta ferrite is due to the fact that the ferritic phase has alarger quantity of chromium atoms and offers greater diffusivityto thiselementthanthe austeniticphase.Hence,migration of theinterface toward the ferrite is associated with growth of M23C6carbides and secondary austenite (γ2). Finally, after the lamellar precipitation is completed,thesigma phase is formed at thefrontof the lamellar precipitates as a result of the second eutectoiddecomposition.

2. Experimental Procedure

The material  – super duplex stainless steel  – was prepared in afoundry in a vacuum induction furnace with a 60 Hz networkfrequency and maximum power of 400 kW. The first stepconsisted of the foundry design of the test specimens measuring 25 mmin diameterby 300 mmin length using standard software,followed by simulation of the solidification process. The materi-al's chemical composition was identified by optical emissionspectroscopy, using a spectrometer with 47 different channels.

The solution annealing heat treatments recommended for these materials, as well as aging treatments, were applied in anelectric furnace with a capacity of up to 1300 °C. The temperatureemployed for the solution heat treatment was 1160 °C and the

aging temperatures were 920 °C and 980 °C applied for 2 h.Microstructural analysis was conducted in a light optical

microscope equipped with a 35 mm film camera, and also witha scanning electron microscope.

3. Results and Discussion

The chemical composition of the material (ASTM A890/A890MGrade 6A [3]), identified by optical emission spectrometry, isdescribed in Table 1.

Low titanium and aluminum contents are employed toavoid theformation of nitrides of these elements, whichcausesevere embrittlement of the material at both cryogenic and

room temperatures [4]. Niobium is kept at a low concentrationbecause, at concentrations exceeding 0.10% in weight, thiselement also embrittles super duplex stainless steels [5]. Fig. 1depicts the microstructures of the material in the conditionsof as cast and solution annealed heat-treated at 1160 °Cfollowed by quenching.

Fig. 1(a) shows a microstructure composed of a ferriticmatrix in the background, clearly outlined austenite andsigma phase,whichnucleated at theδ /γ interfaces andgrew inthe direction of the ferrite. The ferrite provides stabilizing elements such as chromium and molybdenum. Fig. 1(b) showsonly ferrite and austenite precipitated in the shape of an

“island”. The ferrite/austenite interfaces, as well as the ferritegrainboundaries, are devoid of precipitated secondary phases.The solution annealing heat-treatment at 1160 °C followed bywater quenching altered the morphology of the austenite,leaving it more refined and elongated.

The aging heat treatments at 920 °C and 980 °C producedsignificant changes in the microstructure. The ferrite became

Table 1 – Concentrations of the chemical elements in weight percent.

C(%) Cr(%) Mo(%) Ni(%) Si(%) Mn(%) Cu(%) W(%) N(%) P(%)

0.016 25.69 3.80 7.18 0.74 0.52 0.716 0.736 0.22 0.027Nb(%) Ti(%) Al(%) V(%) Zr(%) Co(%) Sn(%) Pb(%) S(%) Fe(%)0.014 0.005 0.016 0.049 0.065 0.055 0.007 0.002 0.008 Rest.

Fig. 1 – Optical micrographs of the 6A super duplex stainless

steel: (a) in the as cast condition, and (b) solution annealed at 

1160 °C and water quenched.

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filled with precipitates and, in the specific case of thetreatment at 920 °C, it was almost completely consumedthrough the following eutectoid reaction:

δ ⇄ σ + γ2:

Fig. 2 depicts optical micrographs of the ASTM A890 GR 6Asuper duplex stainless steel aged for 2 h at 920 °C and 980 °C.The aging heat treatment at 920 °C caused precipitation of thesigma phase with a predominantly lacy morphology, whilethemorphology of the austenitic phase, which precipitates in thesolid state of these materials, showed a predominantlydendritic formation, unlike the solution annealed condition.

On the other hand, the isothermal heat treatment at 980 °Calso promoted precipitation of sigma phase in the form of blocks, albeit in smaller quantity. Note the small quantities of ferritic matrix and secondary austenite with a more refinedmorphology. In this case, the dendritic characteristic of theaustenitic phase is less marked than in the sample treated at920 °C.

Scanning electron microscopy images, using secondaryand back-scattered electrons, depict in great detail themorphologies of sigma phase precipitated at two differenttemperatures (Fig. 3).

Back-scattered electrons provide an image that takes intoaccount the atomic weight of the chemical elements. Becauseit contains high concentrations of chromium and molybde-num, the sigma phase appears in a lighter shade of gray than

ferrite and austenite   [6]. The ferrite and austenite phases,which appear in relief, are clearly visible because of mechan-ical polishing that has worn down the softer phase (γ) morethan the harder one (δ). This is not visible in the sampletreated at 920 °C, which shows only two phases: austenite andsigma phase; the delta ferrite was completely consumedthrough the eutectoid reaction that produced the intermetallicphase (σ) and the secondary austenite (γ2) [6].

4. Conclusions

•   Solution annealing at 1160 °C led to complete dissolution of the sigma phase, which precipitated during solidificationcooling.

•  Aging at 920 °C for 2 h caused precipitation of the sigmaphase through the eutectoid dissolution of the delta ferritein secondaryaustenite andsigma phase. The morphology of the intermetallic phase was mostly lacy, with no sign of precipitations in other geometric shapes.

•   The isothermal aging treatment at 980 °C altered themorphology of the lacy sigma phase into a block-shaped

Fig. 2 –  Microstructures after isothermal heat treatments at 

(a) 920 °C, and (b) 980 °C.

Fig. 3 –

 SEM micrographs showing: (a) Sample aged at 920 °Cusing secondary electrons, and (b) Sample aged at 980 °C

using back-scattered electrons.

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morphology with nucleation at the   δ /γ   interfaces andgrowth in the direction of the delta ferrite. At this specifictemperature, the delta ferrite was not totally consumedthrough the eutectoid reaction.

•   The morphology of the austenite in the aged material,especially the material aged at 920 °C, was predominantlydendritic, while the same phase was more refined in the

solution heat-treated material.

R E F E R E N C E S

[1] Solomon HD, Devine TM. In: Lula RA, editor. Duplex stainlesssteels  —  a tale of two phases. Metals Park,Ohio: ASM; 1982. p. 693–756.

[2] Laycock NJ, Newman RC. Localized dissolution kinetics, saltfilms and pitting potentials. Corros Sci 1997;39(10–11):1771–9.

[3] American Society for Testing and Materials  —  ASTM A890/A890M-91. Standard practice for castings,iron–chromium–nickel–molybdenum corrosion resistant,duplex (austenitic/ferritic) for general application. AnnualBook of ASTM Standards. V.01.02. Ferrous Castings;Ferroalloys. p.556–569, 1999.

[4] Weber J, Schlapfer HW. Austenitic–ferritic duplex steels.Wintertur  — Switzerland: SULZER Brothers Limited; 1986. p. 1–10.

[5] Rossitti, Sergio Mazzer.  Efeito do nióbio na estrutura e nas propriedades mecânicas do aço inoxidável super duplex fundido SEW 

410 W. Nr. 1.4517. Tese (Doutorado)  —  Área Interunidades emCiência e Engenharia de Materiais, Universidade de São Paulo,São Carlos, 2000.

[6] Martins, M. Caracterização microestrutural-mecânica e resistência àcorrosão do aço inoxidável superduplex ASTM A890 / A890MGrau 6A,Tese de Doutorado, Universidade de São Paulo, InterunidadesEESC-IFSC-IQSC, 2006.

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