Qualitative and Quantitative Assessment of γ and δ Phases in Duplex Stainless Steel Weldments by the X-Ray Diffraction Technique

Qualitative and Quantitative Assessment of γ and δ Phases in Duplex Stainless Steel Weldments

by the X-Ray Diffraction Technique

A. Kellai, S. Dehimi, M.F. Benlamnouar, S. Kahla, S. Ouallam, Z. Boutaghou Research Center in Industrial Technologies, CRTI, P.O. Box 64, 16014 Cheraga, Algiers, Algeria

a.kellai@crti.dz

Abstract—This paper is focused on the quantitative and qualitative characterization of austenitic-ferritic stainless steel welds by the X-ray diffraction technique. The first weldment realized by gas tungsten arc welding process GTAW with ER2209 electrode, the second weldment by shielded metal arc welding process SMAW with E2209-15 electrode. The results show that the presence of two phases, austenite γ and ferrite δ without any precipitation of secondary phases either in the base metal BM or in the two welded zones. Moreover, there is an increase in the ferrite content and the existence of non-uniform compressive stress in the GTAW weld zone.

Keywords: welding; X-ray diffraction technique; duplex stainless steel; ferrite and austenite.

I.INTRODUCTION

The surprising success of duplex stainless steels over austenitic stainless steels results from the volatility of nickel prices on the market. Another problem with austenitic steels is the lower durability in several aggressive environments. The chemical composition of duplex stainless steel allows good proportion of ferrite (δ)/austenite (γ) in the vicinity of 1:1 offers high mechanical strength and noticeable corrosion resistance in several aggressive environments [1-3].

Since a large part of their applications uses welding as a manufacturing or maintenance process, various welding processes are applied to the constructions made by duplex stainless steels, among these GTAW and SMAW processes.

Each welding process has various advantages and disadvantages that determine the choice of the method to be used for given job. Welding is an assembly process which aims to create a physical continuity between two metallic elements.

The thermal cycle of welding causes changes either at the structural or mechanical level, for this reason several researchers working in this field who find results and propose solutions and perspectives [4, 5].

The X-ray diffraction technique is one of the methods used to study the microstructural evolution of steels in the different area of weldment, base metal BM, heat affected zone HAZ and weld zone WZ, and also to calculate the volume fraction of existing phases. It also employed to identify presence of

intermetallic precipitates in the weld zone by detecting peaks appeared at different angles and compared to values obtained from literature. XRD analysis of the welded specimens was used to confirm the results obtained with quantitative metallography [6].

In this study, a qualitative and quantitative characterization of 2205 duplex stainless steel welds realized by two electric arc welding processes, with tungsten refractory electrode GTAW and coated electrode SMAW. First an overview is given of stainless steel and filler metals, as well as the welding techniques used. After that, the metallography is presented under an optical microscope. The results obtained are

carefully interpreted and compared to the literature.

II. MATERIALS AND METHODS

The material that is the subject of this work is a duplex stainless steel type 2205. It is in the form of tubes, the first is 50.8 mm in diameter and 5.54 mm thick and the second is 32 inches in diameter and 23.83 mm thick. For the realization of the different welds, we used filler metals of grade ER2209 and E2209-15 according to the AWS classification (American Welding Society). A radiographic test (RT) was performed on the welds, and it shows that the two joints are totally within whole types of welding defects. The welding parameters used are listed in Table I. The chemical composition of the base metal and the filler metals are shown in Table II.

TABLE I. WELD PARAMETERS

Process GTAW SMAW

Wire diameter Φ (mm) 2.4 2.5 – 3.2

Intensity (A) 80 – 100 90 – 110

Voltage (V) 10 – 12 22 – 25

Welding speed (cm/min) 4 – 7 5 – 12 Welding energy (kj/cm) 10 – 12 14 – 24 Temperature between passes °C 150 150

Protection gas Ar 99.999% -

Gas flow (l/min) 12 -

Microscopic observations in the welded joints are executed on cross sections to the welding direction, using standard techniques for mechanical polishing to obtain a mirror state.

Then, the specimens were electrolytic etched in 10% KOH solution at 3 V for 20 s. The optical observations were performed using a light microscope (Nikon Eclipse LV100ND).

X-ray diffraction analysis was used for characterization of the phases and for confirmation of the volume fractions of ferrite and austenite, as determined by quantitative metallography. The specimens are exposed to 2θ scan series on a microstructure range of the steel (in the base metal and the weld zone that was performed on a BRUKER D2 bench- top X-ray diffractometer PHASER 2G using a cobalt anticathode delivering an X-ray wavelength of 1.79 nm, an acquisition step of 0.02°, a voltage V= 30kV and an intensity I= 10mA.

To calculate the volume fraction of phases, the following formulas are usied:

1

1 .100%

1 . .

V

= I I

R

+

V

= 100% − V

Vγ and Vδ are the volume fractions of austenitic and ferritic phases respectively, Iγ and Iδ are the intensities of peaks (111)γ and (110)δ respectively and R is a constant of 0.85 [7].

III. RESULTS AND DISCUSSIONS A.Microstructural Observations

Fig. 1.a shows that the micrograph of the base metal has a two-phase structure, the austenitic and ferritic phases being in the form of elongated slats in the rolling direction, so that the austenitic γ phase is immersed in the ferrite matrix δ.

The microstructure shown in Fig. 1.b assist a solidification structure produced by the GTAW process, characterized by coarse ferrite grains, in which the austenite is fine and appears in different morphologies: allotriomorphic, Widmanstätten structure and intra-granular. On the other hand, the microstructure of the SMAW welded zone (Fig. 1.c) shows a medium grain size structure with a high content of austenite [8, 9].

Fig. 1. Optical micrograph of the base metal BM (a) and the two welds zone: GTAW (b), SMAW (c)

Element C Mn Si S P Cr Ni Mo Cu Nb N

2205 0,015 1,13 0,49 0,0005 0,026 22,15 5,33 3,18 - - 0,17

ER2209 0,02 1,57 0,46 0,010 0,01 22,9 8,6 3,1 0,1 0,01 0,16 E2209-15 0,04 0,90 0,57 0,004 0,018 23,30 9,10 3,20 0,11 - 0,15

TABLE II. CHEMICAL COMPOSITION OF DUPLEX STAINLESS STEEL AISI2205 AND FILLER METALS ER2209 ET E2209-15

(a)

(b)

(c)

B.X-Ray Diffracion Analysis

Fig. 2 shows the x-ray diffractograms of base metal and of specimens welded with GTAW and SMAW processes. The presence of two phases, the austenite γ and ferrite δ without any precipitation of secondary phases. Note also that the intensity of the peak (110)δ of the base metal and the weld metal is greater than that of (111)γ, so the phase balance is in favor of the ferritic phase, and the opposite for the weld metal SMAW [10, 11].

The difference between the intensities of the peaks (111)γ and (110)δ for the two welds metal is not as great as that recorded in the base metal, this can be related to the distribution of the δ and γ phases in the weld metal and its chemical composition. It should also be noted that this difference in intensity in the case of the weld metal SMAW is lower compared to that of the weld metal GTAW, due to the weakening of the texture effects with the increase in the number of welding passes [10].

Fig. 2. X-ray diffractograms obtained in the base metal and the two weld zones SMAW and GTAW

It is known that if the peak does not leave its initial position, the material undergoes no constraint, but if the peak moves towards the small angles, we will have a uniform stress (expansion of the stitch), if it is positioned towards the large angles we can say that there is a non-uniform constraint (compression of the stitch) [11, 12].

In our case, the peaks of the austenitic phase for all samples show no displacement, hence lack of constraint. On the other hand, the peaks of the ferritic phase move towards large values relative to that of the base metal, hence the existence of a non- uniform compressive stress or insertion precipitates, in particular in the GTAW welded zone (Table III) [11, 12].

The half-height width of the peaks can give a lot of information about grains size, or the increase or decrease of FWHM values (Full-Width Half-Maximum) can indicate that there is a refinement or grain enlargement (Fig. 3). We are going to speak only about the two intense peaks (111)γ and (110)δ, where the majority of the grains are oriented towards the same direction.

Fig. 3. Diffractogram of the lines (111) γ and (110) δ for the base metal

The value of the peak FWHM (110)δ for all samples is lower than that of the peak (111)γ, which proves that the size of the austenitic grains is smaller than that of the ferritic phase.

The value of the peak FWHM (110)δ for the two welded zones is lower than that of the base metal, that is, they have larger ferritic grains.

The comparison between the two welded zones shows that the value of FWHM peak (110)δ is decreased in the GTAW welded zone, thus an increase in the size of ferritic grains and refinement of the austenitic grains [11].

Measurements of the volume fraction of two phases presented in Table III show that the SMAW welded zone is characterized by low ferrite content with respect to the GTAW welded zone and the base metal BM.

TABLE III.THE CHARACTERISTICS OF DRX AND THE PHASE RATE

Base metal Weld zone GTAW

Weld zone SMAW (110)δ (111)γ (110)δ (111)γ (110)δ (111)γ 2θ (°) 52,22 51,00 52,85 51,00 52,26 51,00 FWHM (°2θ) 0,3898 0,4010 0,3254 0,3821 0,3788 0,4943

% Phase 58,48 41,52 54,94 45,06 38,10 61,90

IV. CONCLUSION

• The microstructure of the welded zone reveals three different forms of the austenitic phase:

allotriomorphic to grain boundaries, widmanstätten structure and intragranular austenite particles.

• The slow cooling regime of SMAW welding results in low ferrite content, coarse morphology of austenite, low austenite widmanstätten and allotriomorphs.

• The increase of the diffraction peak intensity for a phase shows that this phase is more stable, its volume proportion is greater and its grains are wider.

• The XRD analysis proves that the size of the ferritic grains in the welded zone GTAW is greater than that of the SMAW welded zone.

• The chemical elements of the filler metal play a key role in determining the phase rate and stability.

• The performance of the XRD technique shows its ability to study qualitatively and quantitatively duplex stainless steel welds.

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