Effect of Inter Metallic Precipitations on the Properties Of

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Effect of intermetallic precipitations on the properties of

duplex stainless steel☆

Michael Pohl, Oliver Storz ⁎, Thomas Glogowski ⁎

Institute for Materials/Material Testing, Ruhr University Bochum, Germany

Received 31 October 2005; received in revised form 30 March 2006; accepted 30 March 2006

Abstract

The corrosion resistant group of ferritic austenitic duplex steels shows a rather complex precipitation and transformation

behavior that affects the mechanical and corrosive properties. Most critical, concerning the change of properties, are the

precipitations in the temperature field of 650–950 °C.

© 2006 Elsevier Inc. All rights reserved.

Keywords: Duplex; Stainless steel; Sigma phase; Chi phase

1. Introduction

The ferritic austenitic duplex steels belong to the

corrosion resistant steels. In contrast to the purely fer-

ritic steels, they offer better corrosion resistance, as well

as a higher strength compared to the pure austenitic

grades. Hence, the duplex steels are widely used in the

chemical industry or in offshore technologies, where a

combination of high corrosion resistance and good ten-

sile strength is required.

Besides, the widely used standard duplex grades, the

super duplex steels, with a Pitting Resistance Equivalent

of N35, have come into service (Table 1). The duplex

steels are based on the ternary Fe–Cr – Ni phase diagram

(Fig. 1). The section at 70% iron shows the quasi-binary

phase diagram (Fig. 2), which represents the duplex

stainless steels. They solidify primarily as ferritic alloys

and transform at lower temperatures by a solid state

reaction partially to austenite. Hence, the austenite ferrite

ratio is adjusted in a temperature upside 1000 °C.

Preferable a ration of 60–50% austenite is achieved.

Due to the high amount of alloying elements, the

duplex stainless steels show a rather complex precipita-

tion behavior. The effect on the mechanical and corrosive

properties of several precipitations might be extensive.

This is enhanced by a differential distribution of the

alloying elements in the ferritic and austenitic phase.

Materials Characterization 58 (2007) 65 –71

☆ Presented at the Microscopy and Microanalysis Society/Interna-

tional Metallographic Society Technical Meeting, Honolulu, July 31-

August 4th, 2005.⁎ Corresponding authors.

E-mail addresses: [email protected] (O. Storz),

[email protected] (T. Glogowski).

Table 1

Standard duplex steel grades

Steel no. Name UNS

Duplex 1.4462 X2CrNiMo 22-5-3 S 31803

Super-duplex 1.4501 X2CrNiMoCuWN 25-7-4 S 32760

1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2006.03.015

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Because of the higher diffusion rate of the ferritic phase,

all technically relevant precipitations can be found here.

Most hazardous concerning a possible embrittlement is

a precipitation of the hexagonal nitrides (Cr 2 N) in a tempe-

rature range of 700–900 °C, the α′ precipitation (475 °C

embrittlement) and the intermetallic sigma and χ phase.

The intermetallic precipitations are of greater inter-

est, because besides their influence on the mechanical

Fig. 2. Pseudo-binary Fe–Cr – Ni phase diagram at a 70% Fe section [1].

Fig. 1. Ternary Fe–Cr – Ni phase diagram.

66 M. Pohl et al. / Materials Characterization 58 (2007) 65 – 71



properties, the corrosive properties are influenced se-

verely, too.

2. Results and discussion

2.1. Precipitation of the σ phase

The σ phase is non-magnetic and intermetallic. Its

origin composition is based in the system of iron and

chromium [2]. The phase has a tetragonal crystallo-

graphic structure [3–5], with an elemental cell of 32

atoms and 5 crystallographically different atom sites.

These are occupied by different atoms, whereas the

lattice occupation of atoms itself depends on the

concentration. Concerning the ternary iron chromium

nickel system, the sigma phase is a thermodynamically

stable phase that forms itself on the chromium rich siteof the pseudo-binary phase diagram Fe–Cr – Ni (Fig. 1).

Typically, the sigma phase precipitates between 600 and

1000 °C. The mechanism of precipitation is a eutectoid

transformation of ferrite into austenite and sigma phase.

The denomination of the obtained austenite as tertiary

austenite (γ3) enables an explicit separation from other

austenitic phases with a different generation mechanism

[6]. Generally, the denomination of the austenite is

based on its forming mechanism (Table 2).

As the time temperature transformation diagram shows

(Fig. 3), the fastest precipitation rate for sigma phase can be

found between 850 and 900 °C [5,8]. According to the precipitation temperature, the morphology of the sigma

phase precipitation changes (Fig. 4). At lower precipitation

temperatures of 750 °C, a coral-like structure of sigma

phase can be found (Fig. 4c). The amount of single sigma

nuclei at the beginning of the precipitation is rather high,

depending on the shorter diffusion distances at lower

precipitation temperatures. Hence,lower diffusion velocity

causes higher local supersaturation and leads to a higher

density of precipitations. A different precipitation behavior

can be observed at higher temperatures of 950 °C (Fig. 4a).

The sigma phase is bigger and more compact at thesetemperatures and the linking between single sigma crystals

is marginal, resulting from a lower nucleation formation

force but a high diffusion rate at elevated temperatures.

The transition in linking and particle size is found

after a precipitation at 850 °C (Fig. 4 b).

This change in morphology is numerically acquired

by a form factor f . This factor describes the roundness of

precipitations by means of the formula:

f ¼ 4parea of the precipitation

perimeter 2 of the precipitation

The factor varies from 1, a completely round particle,

to 0, corresponding to an idealized, line-shaped particle

with a perimeter considerably greater than the surfacearea of this particle. Fig. 5 shows the measurements of

the form factor for the standard duplex grade

X2CrNiMo 22-5-3 with a sigma phase fractions of

15 vol.% each.

2.2. Precipitation of the χ phase

In contrast to the sigma phase, the precipitation of the

χ phase in duplex stainless steels is thermodynamically

not stable. Concerning the examined alloys, the preci-

pitation of the χ phase at 750 and 850 °C was always prior to the sigma phase. With the beginning of the

sigma precipitation, the χ phase vanishes in favor of the

sigma phase, as can be seen in Fig. 6. Here, the χ phase

residues were found partial soluted in sigma phase,

located at the former grain boundaries.

2.3. Influence on the mechanical properties

The formation of intermetallic phases between 750

and 950 °C leads to a disastrous loss of toughness.

Hence, the precipitation of intermetallic phases causes a

detrimental loss of notch bar impact values. This is due

Table 2

Types of austenite formation in duplex steels

Formation mechanism

Primary Segregation of austenite stabilizing elements towards the

eutectic rim (L→α+γ1)

Secondary Solid state transformation out of the ferrite (α→γ2)

Tertiary Eutectoid transformation with consumption of ferrite

(α→γ3+σ)

Fig. 3. Isothermal TTT diagram of the standard duplex steel 1.4501

and the standard grade 1.4462 [7].




to the bad deformability of the phases because of their

low fraction or metallic binding.

Regarding the change in morphology between the

different precipitation temperatures, notch bar impact

tests were carried out with samples, heat treated at 750,850 and 950 °C. The experimental results of those tests

show the above-mentioned detrimental loss of tough-

ness at all temperatures (Fig. 7). One percentage of

precipitated intermetallic phases leads to a loss of the

notch bar impact value down to one third of the solution

annealed state. Despite the strong decrement of

toughness, distinctions were measured between the

several precipitation temperatures.

The lower the precipitation temperature had been, the

more brittle was the behavior. This incidence primarily

depends on the morphology of the sigma phase. Even

little deformations of the material cause transcrystalline,

finely structured brittle fractures of sigma phase par-ticles, due to the TCP structure of the phase. In the more

net-like morphology precipitated at lower temperatures,

cracks are enabled to run through sigma phase particles

over long distances. The narrow fields of surrounding

ferrite are forced to cleave, as the small austenitic phases

show ductile failure.

Fig. 4. Morphology of the sigma phase with respect to the isothermal annealing temperature; (a) 950 °C, (b) 850 °C, (c) 750 °C.

Fig. 5. Results of the form factor measurement for the standard duplexgrade X2CrNiMo 22-5-3.

Fig. 6. Precipitation of χ phase and growing of the σ phase in a cast duplex steel grade.




Whereas in the bigger and more bulk sigma phase at

higher precipitation temperatures has a bigger surround-

ing matrix of ferrite and austenite. This enhances the

surrounding ferrite to a more ductile failure mode.

Hence, the examination of the fractured samples showed

less cleavage in the ferrite of samples annealed at 950 °C

than samples annealed at 750 °C.

The influence of the morphology of intermetallic

phases on the tensile strength has a different character.

The tensile and yield strength generally grows with the

amount of precipitated intermetallic phases (Fig. 8). But

as the samples annealed at 950 °C barely show an in-crease of the tensile strength, the effect ambiguously is

presented in the samples annealed at 750 and 850 °C.

The reason for this behavior can be traced back to the

formation of a sigma phase network at lower temper-

atures. Hence, it was proved, that the yield strength

grows with linking of sigma phase network. Addition-

ally, the 850 °C samples showed low stress fractures. In

mechanical testing, “low stress fractures” occur beyond

the general yield fracture level, because of internal brittle

microcracking. Those fractures will appear, too, in the

750 °C samples, as well, but equivalent amounts of

sigma phase would take too long to precipitate. Since

these tests would be technical irrelevant, they were not

carried out.

To observe the behavior of the sigma phase in situ,

tensile tests were carried out in the SEM. The sigma phase showed first cracks at tensile stresses of 600 MPa,

impartial of the phase morphology (Fig. 9). Cracking of

χ phase particles was not observed during the whole

testing.

Fig. 7. Influence of σ precipitation and morphology on the notch bar impact value.

Fig. 8. Influence of σ precipitation and morphology on tensile strength.






and structural studyProc. Conf. Duplex Stainless Steels '91, Beaune;

1991. p. 118–26.

[4] Karlsson L. In: Stainless Steel World, editor. Conf. transcript.

Duplex stainless steelKCL Publishing; 1997. p. 43.

[5] Roscoe CV, Gradwell KJ, Lorimer GW. In: Institute of Metals

book, editor. Conf. transcript, Stainless Steels; 1984. p. 563.

[6] Pohl M, Storz O. Sigma phase in duplex stainless steels. Zeit Met

2004;95:7.

[7] M. Mola Diploma thesis. Ruhr Universität Bochum 2000.

[8] Strutt AJ,Lorimer GW. Conf. Proc.,Int. Conf.on Duplex Stainless

Steel '86, The Hague; 1986. p. 310.


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Effect of Inter Metallic Precipitations on the Properties Of