1. CHARACTERIZATION AND ULTRASONIC EVALUATION IN DUPLEX STAINLESS STEEL
HEAT TREATED AT 750 °C
Jhoana Paola Trocoli1, Belkis Sulbaran2, Edda Rodríguez1.
1Simón Bolívar University, Department of Materials Science, Caracas-Venezuela.
2Guadalajara University, Engineering Department, Jalisco-México.
.
Introduction
The duplex stainless steels (DSS) are an intermediate class between ferritic and austenitic stainless
steels. Simultaneous presence of ferrite and austenite phases provides a combination of the best
properties of these two phases, like good corrosion resistance and improved mechanical properties.
However, heat treatment leads to a series of metallurgical transformations, which take place in the
ferrite or at its grain boundaries, such as the formation of chi (χ) and sigma phase (σ). Ultrasonic non-
destructive testing is a sensitive tool to characterize micro structural features and for evaluation of
mechanical properties. The wave propagation speed and energy losses through interactions with the
microstructure are the fundamental factors of the material’s ultrasonic characterization
Abstract
The effect of isothermal treatment at 750 °C on the microstructure of SAF 2205 duplex stainless steels has been characterized by ultrasonic pulse-echo technique. The specimens were heat treated at 750 ◦C for different
aging times (2 h, 3 h, 4 h, 6 h and 8 h) , in order to investigate the effect of aging treatment on precipitation of intermetallic phases. Two kinds of Cr- and Mo-enriched intermetallic phases, sigma (σ) and chi (χ), were found to
precipitate preferentially at ferrite–austenite interface and within the ferrite phase. The volume fraction of σ phase increased with the time of aging and developed into coarse particles. Microanalysis undertaken across the
particles of σ and χ indicates that the time of aging has a meaningful effect on the chemical composition, especially on the Mo content. Ultrasonic velocities and attenuation co-efficient of ultrasonic waves produced with 3,5;
5; 10 and 22 MHz transducers were evaluated for these heat-treated samples by pulse-echo method. The microstructural changes are correlated to the ultrasonic characteristics of the alloy.
Experimental Procedure
Specimens of a commercial duplex stainless steel type SAF 2205, supplied by Sandvik of Venezuela
were heat treated at 750 °C for 1, 2, 4, 6 and 8 h followed by quenching in water. Afterwards, they were
examined by optical microscopy and by scanning electron microscopy (SEM) after electrolytic etching in
20% NaOH solution. Chemical microanalysis of individual phases was analyzed with an energy dispersive
X-ray system (EDX) coupled to the SEM. A commercial software was used to estimate the fractions of α, γ,
χ and σ phases as a function of aging time. Ultrasonic evaluation was carried out by the pulse-eco
technique using a Krautkramer UNS-58L flaw detector and ¼” diameter normal beam transducers of 3,5;
5; 10 and 22 MHz central frequencies.
Conclusions
Precipitation of  and σ phase and secondary austenite formation within the ferrite was observed
and correlated with the ultrasonic parameters. Microstructural modifications produced by  and σ
precipitation may be satisfactorily evaluated by means of 5 MHz transducers because increasing
velocity values with low experimental deviation were observed in the measurements. However,
For aging times up to 8 h, results obtained at all frequencies showed that the attenuation
coefficient distinguish the microstructural changes that takes place in DSS for prolonged aging
times.Figure 2 shows the phase fractions in samples aged at different times. It is observed that at 8 h of
treatment, the original α phase fraction was replaced by 13,56% of  and . Theses phases
preferentially nucleates at the interface α/γ and its growth extends along ferrite producing a gradual
consume of this phase, until reaching 34,33%. However, the precipitation of σ promotes the partial
transformation of remaining ferrita into secondary austenite, reaching 52,08% of austenite phase.
On the other hand, Figure 3 shows the microstructural evolution of specimens treated at different
times.  and  phase proportion increases with treatment time. The EDS results in Figure 4 revealed
the white brilliant phase to be Mo-rich χ. Compared to the chemical composition obtained from the
adjacent ferrite, intermetallic particles are evidently enriched in Mo.
Fig. 5. Scanning electron microscopy micrographs of the DSS in the aged condition: 750 °C for 8 h.
Figure 5 shows the SEM micrograph of a sample aged at 750 °C for 8 h. Widmanstätten-type
austenite (γ2) appears in the form of islands formed within the ferrite.
Fig. 4. Chemical composition obtained by energy-dispersive X-ray spectroscopy (wt.%) of the DSS in
the aged condition: 750 °C for 6 h
γ
γ2
α
σ, χ
Fig. 3. Microstructure of 2205 duplex stainless steel after aging at 750 °C (20% NaOH: 6 V, 18 s).
α
γ
χ
σ
Cr K 25,50
Fe K 66,75
Ni K 4,05
Mo L 3,71
(α)
(σ) Cr K 27,80
Fe K 65,47
Ni K 4,51
Mo L 4,19
(χ) Cr K 24,85
Fe K 58,53
Ni K 4,14
Mo L 12,47
Cr K 21,95
Fe K 68,02
Ni K 7,11
Mo L 2,92
(γ)
Figure 6 shows a rising tendency of the velocity values when aging time increases up to 8 h, this
increase may be associated with the higher Young modulus of the χ and σ phase, in comparison
with those exhibited by the austenitic and ferritic phases. Results also show a clear rise of the
attenuation as the intermetallic phases fraction is increased, followed by a deceanse caused by
the estabilization of the phases in the microstructure.
σ
Fig. 6. Variation in ultrasonic longitudinal velocity and coefficient attenuation.
σ
α
α
γ
γ
σ, χ
χ, σ
2 h 3 h
χ ,σ
χ
α
γ
8 h
σ, χ
α
γ
6 h
σ, χ
σ, χ
γ
4 h
α
0
0,1
0,2
0,3
5700
5740
5780
5820
5860
5900
0 2 4 6 8 10
Attenuationcoefficient(dB/mm)
Longitudinalwavevelocity(m/s)
Aging time (h)
3,5 MHz
0
0,1
0,2
5700
5740
5780
5820
5860
5900
0 2 4 6 8 10
Attenuationcoefficient(dB/mm)
Longitudinalwavevelocity(m/s)
Aging time (h)
10 MHz
0
0,1
0,2
0,3
5700
5740
5780
5820
5860
5900
0 2 4 6 8 10
Attenuationcoefficient(dB/mm)
Longitudinalwavevelocity(m/s)
Aging time (h)
5 MHz
Results and Discussion
A general view of the microstructure of the duplex stainless steel before precipitation aging is shown in
Figure 1. A banded structure of elongated austenite islands in a ferrite matrix is observed in the
longitudinal section.
Fig. 1. Microstructural aspect of DSS before
precipitation
Fig. 2. Evolution of α, γ, χ and σ phase fractions as a
function of aging time.
α
γ
0
10
20
30
40
50
60
70
0 2 4 6 8 10
PhaseFraction(%)
Aging time (h)
%ferrite
%Austenite
%Secondary
phases
References
• Escriba D.M., Materna-Morris E., Plauta R.L, Padilha A.F. (2009) Chi-phase precipitation in a
duplex stainless steel. Materials Characterization. Vol. 60, 2009; pp.1214-1219.
• Michalska J y Sozańska M. Qualitative and quantitative analysis of σ and χ phases in 2205
duplex stainless steel. Materials Characterization. Vol 56, 2006; pp.355-362.
• Ghosh SK, Mondal S. High temperature ageing behaviour of a duplex stainless steel. Mater
Charact Vol 59, 2008; pp. 1776–83
• Kumar, A., Jayakumar, T., Raj, B. Ultrasonic spectral analysis for microstructural
characterization of austenitic and ferritic steels, Philosophical Magazine, Vol.80, 2000;
pp.2469-2487
0
0,1
0,2
0,3
5700
5740
5780
5820
5860
5900
0 2 4 6 8 10
Attenuationcoefficient(dB/mm)
VelocidaddeOnda(m/s)
Aging time (h)
22 MHz