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Microstructure and phase analysis of duplex stainless steel after heat treatment

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Question: How can one determine the effect of Sigma and Chi phases on the mechanical properties of duplex stainless steel samples?

App.7 - Mic ros t ruc ture and phase ana lys i s o f dup lex s ta in less s tee l a f te r heat t reatment © HKL Techno logy - 2005

IntroductionDuplex stainless steels (DSS) can contain ferrite and austenite phases and have a wide range of applications where it is required both to have excellent resistance to corrosive environments and maintain good mechanical properties, such as in refinery pipes and off-shore platforms. The resistance to corrosion, intergranular corrosion and pitting corrosion is increased by increasing the Nickel (6-8 wt.%), Chromium (18-20 wt.%) and Molybdenum (2-4 wt.%) contents in the DSS. Corrosion along grain boundaries can be a serious problem, particularly after a high temperature treatment such as welding. This type of intergranular corrosion is sometimes referred to as weld-decay [1]. Duplex stainless steels when heated can develop a type of embrittlement. The effect of embrittlement can be pronounced with increasing Chromium and Molybdenum content. The deleterious Chromium and Molybdenum rich intermetallic Sigma and Chi phases form between 650°C and 1000°C and usually have a catastrophic influence on the mechanical properties of DSS, often associated with a reduction in both impact properties and corrosion resistance [2]. Figure 1 shows a graph of the Vickers hardness of a 2205 DSS alloy (Cr 22 wt.%, Ni 5 wt.%, Mo 3 wt.%, N 0.2 wt%) heated at 850°C up to 8 hours, where the increase in hardness is an indication of the increasing content of the hard embrittling intermetallic precipitates [3]. The presence of ferrite greatly accelerates the formation of the intermetallic phases, which

is known to nucleate at the austenite and ferrite boundaries. The ferrite, being richer in Chromium, tends to be preferentially absorbed during the growth of the intermetallic phases. Elements such as Molybdenum lead to further acceleration of the formation of the intermetallic phases. Therefore it is very useful to be able to identify the quantity of Sigma and Chi phases in the DSS. Electron back scatter diffraction (EBSD) can be used to easily distinguish the various phases within the DSS.

EBSD AnalysisTable 1 shows the EBSD analysis conditions used to investigate samples of the 2205 DSS alloy, which were heated treated at 850C for 0.1, 1 and 8 hours prior to the investigation.

Table 1: Details of EBSD analysis

Sample Preparation: Mechanical polish with OPS

SEM Type: FEG-SEM

EBSD System: HKL CHANNEL5 with Nordlys II detector

Accelerating voltage: 20 kV

Probe Current: 2 nA

Figure 1: Vickers hardness measurements for the 2205 DSS samples heat-treated at 850°C as a function of time [3].

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Table 2:List of phases investigated in the DSS alloy using EBSD.

Phase Crystal structure Lattice parameters (Å)Ferrite Body Centred Cubic (BCC) a= 2.87

Austenite Face Centred Cubic (FCC) a= 3.66

Sigma Tetragonal a= 8.80, c= 4.56

Chi Body Centred Cubic (BCC) a= 8.92

Results

App.7 - Mic ros t ruc ture and phase ana lys i s o f dup lex s ta in less s tee l a f te r heat t reatment © HKL Techno logy - 2005

In Table 2 the list of phases and their crystallographic information is shown, which were found to exist in the 2205 DSS alloy. Figure 2 shows example indexed EBSD patterns from both Sigma and Chi intermetallic phases. Figure 3 shows a fore-scatter SEM image from the alloy heated at 850C for 1 hour and the EBSD phase map from this region is shown in Figure 4. The map clearly shows the distribution of the different phases in this region, where the intermetallic phases are formed at the ferrite phase boundaries. The Chi phase was found to be have higher Molybdenum content compared to the Sigma phase as shown in Figure 5.

The orientation relationships between the phases are clearly seen in the inverse pole figures shown in Figure 6. The austenite and ferrite phases were found to have the Kurdjumov-Sachs relationship, i.e. (110)Ferrite||(111)Austenite. The Sigma and Chi phases can be seen to have an orientation relationship with the ferrite phase, (001)Sigma||(110)Ferrite, (110)Chi||(110)Ferrite. The intermetallic phases were formed at the expense of the ferrite phase with the increase in time of the heat-treatment, where the area fractions measured from EBSD results are shown in Figure 7. The area fractions of the Sigma phase increased from 0.01% to 19% and the Ferrite phase decreased from 41% to 1%, as the heat-treatment time increased from 0.1 to 8 hours.

Figure 2:Example EBSD patterns from the intermetallic phases found in the 2205 DSS alloy, which were identified as Sigma and Chi.

Sigma Chi

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App.7 - Mic ros t ruc ture and phase ana lys i s o f dup lex s ta in less s tee l a f te r heat t reatment © HKL Techno logy - 2005

Figure 3:Fore-scatter SEM image from the duplex stainless steel sample heat treated at 850°C for 1 hour.

Figure 4:EBSD phase map from the same region shown in Figure 3. Ferrite (Blue), Austenite (Red), Sigma(Yellow), Chi(Green).

EBSD phase map EDS map

Figure 5:EBSD phase map and EDS map for Mo (Lα) from the region highlighted in Figure 4.

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Conclusion

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App.7 - Mic ros t ruc ture and phase ana lys i s o f dup lex s ta in less s tee l a f te r heat t reatment © HKL Techno logy - 2005

Ferrite Austenite Sigma Chi

Figure 6:Inverse pole figures for the phases in the EBSD map shown in Figure 4.

Figure 7:Area fraction of phases measured by EBSD in the samples heat treated at 850C for 0.1, 1 and 8 hours.

EBSD has been shown to be a useful tool for phase identification in duplex stainless steels, which is particularly evident in the analysis of the Sigma and Chi intermetallic phases. The intermetallic phases have very little difference in chemical composition, however they have very different crystal structures.

The EBSD results clearly show that the time of heating should be minimised in order to reduce the formation of the intermetallic phases which are detrimental to the mechanical and corrosion properties of duplex stainless steels.

Answer: EBSD can be used to easily identify and measure the quantity of the Sigma and Chi phases and help to determine their effects on the mechanical and corrosion properties of duplex stainless steels.References1. R .W.K. Honeycombe and H.K .D.H. Bhadesh ia , S tee l s – Mic ros t ruc ture and proper t ies, But te rwor th-He inemann, Oxford , 2003.2 . R .N. Gunn, Dup lex s ta in less s tee l s – Mic ros t ruc ture, p roper t ies and app l i ca t ions, Woodhead pub l i sh ing L td . , Cambr idge, 1997.3 . J.F. A lmagro, ACERINOX S.A. Los Bar r ios, Spa in . P r i va te communicat ion .

AcknowledgementTh is app l i ca t ion note has been wr i t ten in co l laborat ion wi th J.F. A lmagro, ACERINOX S.A. Los Bar r ios, Spa in .