Precipitation phenomenon study in austenoferritic steel of 22%
chromium and 5% nickel during aging treatment.
Naima Ouali1, Mabrouk Bouabdallah2 , Brahim Belkessa1
1 Welding and NDT Research Center (CSC) B.P 64, Cheraga, Algiers.
2 Department of Metallurgy, National polytechnic school of Algiers.
Keywords: Duplex stainless steel, Heat treatment, Microstructure, grain refinement
Abstract: The aim of this study is to refine the duplex microstructure without using the conventional processes, as the thermo mechanical treatments. The adopted way consists in carrying out and optimizing aging and annealing heat treatments. A preliminary treatment of hardening since 1250°C was applied to increase the proportion of ferrite in the matrix. The treatments of aging were carried out at the temperature of 850°C during variable duration periods from 02h, 10h and 30 hours. The refinement of the grains had mainly on the level of ferrite, this being with the simultaneous germination of ferrite and austenite during the dissolution of the precipitates
1. Introduction
Duplex stainless steel (DSS) with a microstructure comprised of nearly equal proportions of δ- ferrite and austenite γ, combine the attractive properties of ferritic and austenitic stainless steels.
Thanks to these characteristics DSS are widely used in such industries as petrochemical, pharmacy, marine and many other fields. [1-3].
However, a number of undesirable phases such as carbides, nitrides and intermetallic compounds may appear in δ ferrite areas and δ/γ interfaces if the manufacturing processes are not carefully respected. Among these secondary precipitates, σ-phase and carbides with fast formation kinetics have been particularly noticed because they can cause a dramatic deterioration of the toughness and the corrosion resistance of duplex stainless steels [4].
The aim of this present study is to refine the 2205 DSS microstructure without using conventional processes, as the thermo mechanical treatments. The adopted procedure consists in carrying out and optimizing aging and annealing heat treatments.
2. Experimental procedure
The material study is a SAF 2205 duplex stainless steel (UNS31803). The material was received as tube of 170-mm diameter and 7-mm in wall thickness. The chemical composition of the material used in this study is given in Table 1.
The stainless steel was initially solution treated at 1250°C for 60min. After the annealing, and to search for the optimum heat treatment parameters, small size’s samples (10x10x7) were aged at 850°C for different time intervals : 02, 10 and 30 hours.
At last, an annealing treatment was carried out, for all specimens, at 1080°C for 10min., 30min. and 60min. in order to redissolve any precipitates and to restore the - phases balance of the 2205 duplex stainless steel. All these heat treatments were followed by quenching in water.
After the metallographic samples preparation, different etchings were used to characterize microstructure after annealing and aging heat treatments.
Samples were etched with a solution containing 10 ml HNO3, 20 ml glycerol and 30 ml HCl. In order to reveal the σ-phase and measure its volume fraction for each aging treatment, an electrolytic etching was used with KOH solution at a potential of 12 V for 8 s. σ-phase and δ-ferrite volume fractions were estimated by automatic image analysis using an ATLAS® computer program attached to an optical microscope. The microstructures were analysed by using optical and SEM microscopy. X- Rays diffraction method and EDX attached to the scanning electron microscope were used to identify phase transformation.
Microhardness measurements were made on each specimen to assess the age hardening effect.
3. Results and Discussions
Fig.1. shows an optical and SEM microstructure of the as received hot rolled 2205 duplex stainless steel. It shows clearly the banded structure of the DSS. No other secondary precipitate appears on the microstructure.
Element C Si Mn Ni Mo Cr P S V Cu N
2205 steel (Wt. %) 0.03 0.36 1.77 5.70 2.58 22.05 0.018 0.015 0.10 0.20 0.13
As shown in fig. 2 a change of the microstructure morphology of the steel annealed at 1250°C to cellular form and a significant increase of δ ferrite grain size, causing a hardening of the duplex stainless steel (Figure 3).
Figure 2. Optical microstructure of SAF 2205 after annealing at 1250°C
Figure 3. Hardness of SAF 2205 as received and after annealing at 1250°C Figure 1. Optical and SEM microstructure of SAF 2205 as-received condition Table 1. Chemical composition of material used in this study
230 235 240 245 250
As received Annealed at 1250°C
Hardness Hv
3.1 Microstructural evolution during aging at 850°C
Fig. 4 shows the microstructures of the 2205 duplex stainless steel respectively, after 850°C aging at 02, 10 and 30 hours. The figure shows clearly the presence of sigma phase at the ferrite austenite interfaces, which are considered as preferential nucleation sites for the heterogeneous precipitation of intermetallic compounds. The formation of sigma phase in duplex stainless steels is described by the decomposition of δ ferrite through an eutectoid transformation. After the nucleation process, sigma phase particles grow into the adjacent δ ferrite grains.
The precipitation of M23C6 carbides occurs first at the δ/γ interfaces, and grows with austenite into the δ ferrite grains. The nucleation and growth phenomenon of M23C6 carbides is accompanied with a migration of initial δ/γ interface boundaries into the δ ferrite phase [5-6].
Figure 4. Optical microstructure of SAF 2205 aged at 850 °C for: (a) 02 h, (b) 10h and (c) 30h
Figure 5. Evolution of sigma phase volume Figure 6. Evolution of Hardness with aging fraction with aging time. time.
Fig. 5, 6 shows respectively the evolution of the sigma phase volume fraction and hardness with aging time; the amount of sigma phase increases rapidly between 2 and 10 hours of treatment at 850 °C. After 10 hours, precipitation becomes less rapid. During the sigma phase precipitation in duplex stainless steel, as the amount of sigma phase increases the amount of ferrite decreases, until its total consumption. And since sigma is a hard phase, the hardness of the aged steel increases slightly with aging time. (Fig. 6) [7].
3.2 Heat treatment restoration: Annealing treatment at 1080°C:
After aging treatments, all samples were annealed at 1080°C for 10 min., 30min., and 60min. and quenched in water. Only the treatment carried out for 60 min led to completely redissolve the sigma phase and precipitates and thus restore the δ/γ balance of the duplex stainless steel
Micrographs below (Fig. 7) show a redistribution of δ-ferrite and austenite γ phases and a located refinement of ferrite and austenite grains. We also observe the presence of austenite fine grains within the bands of the δ-ferrite phase.
Figure 7. Optical microstructure after annealing at 1080°C-60min. of sample aged at 850°C -02 h (a) Etching: Oxalic acid (b) Electrolytic etching with KOH
The phase repartitioning in the different samples was measured using a quantitative metallographic technique. The corresponding amounts of δ ferrite and austenite are given in Table 2.
( a) ( b)
δ
γ
0 50 100 150 200 250 300 350
0 1000 2000
Hardness
Aging time (min) 0
10 20 30 40
0 500 1000 1500 2000 Sigma phase volume fraction (%)
Aging time (min)
It’s noted that δ/γ balance of the duplex stainless steel was restored after annealing after1080°C for 60min.
Table 2. Ferrite and austenite volume fraction measured in SAF 2205 after 1080°C annealing treatment for 60min.
4. Conclusion
Heat treatments carried out at 850 °C, modified the structure of the 2205 duplex stainless steel by causing the appearance of a precipitation phenomenon. The precipitates were identified by X-rays diffraction as an intermetallic phase, and M23C6 chromium carbides.
This precipitation occurs in the interfaces ferrite/ferrite and ferrite/austenite and is propagated inside the ferritic grains.
The phase equilibrium of the duplex steel was restored after treatment at 1080 ° C, and a grain refining was mainly observed at the ferrite, caused by the simultaneous germination of δ-ferrite and austenite γ at the dissolution of precipitates and intermetallic phases.
5. References
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[7] D. Y. Kobayashi, S. Wolynec. Evaluation of the low corrosion resistant phase formed during the sigma phase precipitation in duplex stainless steel
Phase (%) δ γ δ/γ
Sample aged at 850°C -2h 49.8 50.2 0.99
Sample aged at 850°C -10h 50.4 49.6 1.01
Sample aged at 850°C -30h 47.1 52.9 0.9
