Microscopic Observation of Precipitation Behavior at Friction Stirring Zone of SuperDuplex Stainless Steel

Itto Sugimoto1,+, Seung Hwan C. Park1, Satoshi Hirano1, Hikaru Saito2 and Satoshi Hata2

1Research and Development Group, Hitachi Ltd., Hitachi 319-1221, Japan2Kyushu University, Kasuga 816-8580, Japan

To investigate the precipitation behavior of super duplex stainless steel in its weld by friction stir welding (FSW) at a low welding speed,we carried out microstructural observation and analysis. An intermetallic compound phase, ·, was observed in the heat affected zone (HAZ). The· phase precipitated at the interfaces between ¤-Fe (ferrite) and £ -Fe (austenite) grains 1­2mm away from the stir zone (SZ)/HAZ boundary.On the other hand, the Cr2N was observed together with the belt-like £ -Fe grain aggregates in the vicinity of the advancing side (AS) of the SZ.The other intermetallic phase, », was also observed at a triple junction of £ -Fe grains. This demonstrates that the precipitation of the Cr2N and »phases correlates with the transformation of ¤-Fe to £ A-Fe (secondary austenite). [doi:10.2320/matertrans.M2019069]

(Received March 11, 2019; Accepted June 3, 2019; Published August 25, 2019)

Keywords: super duplex stainless steel, friction stir welding, sigma phase, chromium nitride, chi phase

1. Introduction

Super duplex stainless steels are one of the next-generationhigh durability structural materials boasting both highstrength and high corrosion resistance. The microstructureof duplex stainless steels consists of ferrite (¤-Fe) andaustenite (£-Fe). The duplex stainless steels are mainly Fe­Ni­Cr ternary systems containing other elements such asCr, Mo, W, and N to improve corrosion resistance. Althoughsuper duplex stainless steels have excellent material proper-ties compared to other duplex stainless steels, their corrosionresistance is significantly impaired by various precipitationsin the welding process, such as · phase and Cr2N.1,2)

Therefore, it is important to investigate the precipitationbehavior during welding.

The present study focuses on friction stir welding (FSW)of super duplex stainless steel. The FSW is a bonding methodinvented at The Welding Institute (TWI) in 1991, and itsprocess is a kind of solid phase bonding.3) Its joining partfaces very little welding trouble such as grain coarsening,residual stress, deformation, porosity, and solidification crackafter construction. Figure 1 shows (a) a photo of the FSWtool (Sialon), (b) a bird’s-eye view of the FSW joining, and(c) a schematic cross-sectional image of the weld. The FSWdevice consists of a rotation tool with a probe and a shoulder.The rotation tool is inserted into the material and is movedalong a joining line with keeping a loaded state. The softenedareas by frictional heat are stirred and plastically flowed, andthen joined by recombining at the back of the rotation tool.The cross-sectional structure after FSW consists of a stir zone(SZ), a heat affected zone (HAZ), and a base material. TheHAZ formed by the FSW can be divided into a thermo-mechanically affected zone (TMAZ), which is affected byplastic deformation, and a HAZ with no effect of the plasticflow. Note that the left and right sides of the cross-sectionalimage of the welds do not show a symmetrical structure inmany cases. The left and right sides are distinguished as anadvancing side (AS) where the directions of the tool rotation

and the material joining coincide with each other (left) and aretreating side (RS) where these two directions are oppositeto each other (right).

While it can be challenging to join super duplex stainlesssteels with FSW, there have been several promising reportson the development of various high durability tools.According to these reports,4­7) the welds of super duplexstainless steel consist of fine grains of ¤-Fe and £-Fe.However, to the best of our knowledge, there has been littlereport on precipitates in the FSW welds of super duplexstainless steels. In this study, the FSW is conducted on asuper duplex stainless steel at a lower welding speed thanusual, and its weld part is observed in detail by electronmicroscopy. On the basis of the electron microscopyexperiment, we clarify the precipitation behavior at the weldof the super duplex stainless steel, which is indispensableknowledge for using the welded material safely.

2. Experimental Procedure

The super duplex stainless steel prescribed in UNS-S32750was used for the present study. Table 1 lists the chemicalcomposition of the super duplex stainless steel used in thepresent study. The nitrogen concentration, 0.24mass% N, ishigher than that of other duplex stainless steels. A rolledmaterial 300mm long, 100mm wide, and 12mm thick wasprepared, and a bead-on-plate test of the FSW with a weldinglength of 200mm was carried out at the center of the adjacentmaterials to join.

In the welding test, the rotation speed was 400 rpm, therotation tool tilt angle was 3°, and the setting depth was5.1mm. The tool was made of a commercially availableceramic material, Sialon (Si­Al­O­N), with a probe lengthof 4.5mm and a shoulder diameter of 18.0mm. The weldingspeed was 0.42mm/s, which is 1/4 of the proper speedof 1.7mm/s. The welding speed is an important parameterto obtain good weld joints by FSW. For the presentexperimental settings and materials, no welding defect wasformed by setting the welding speed to 1.7mm/s or less.Cross-sectional specimens of the weld from the central part+Corresponding author, E-mail: itto.sugimoto.nu@hitachi.com

Materials Transactions, Vol. 60, No. 9 (2019) pp. 2003 to 2007©2019 The Japan Institute of Metals and Materials

of the weld bead were prepared for microstructuralobservations. First, the sample was electrolytically etchedwith a 10 kmol/m3 KOH solution for macroscopic character-ization using optical microscopy. Second, micrometer-scalemicrostructural characterizations were carried out usingscanning electron microscope (SEM) combined with anelectron back-scatter diffraction (EBSD) technique. Third,nanometer-scale microstructural characterizations were per-formed using transmission electron microscope (TEM) andscanning transmission electron microscope (STEM). Afocused ion beam (FIB) sampling technique was utilizedfor the TEM/STEM specimen preparation. Compositionwas analyzed with energy dispersive X-ray spectrometry(EDX) attached to STEM.

3. Results and Discussion

3.1 Micrometer-scale observationsFigure 2 shows cross-sectional views of the weld. By

setting the low welding speed, 0.42mm/s, no welding defect

occurred. However, this figure shows an unknown imagecontrast in the SZ near the AS. This unknown image contrastshowing a belt-like morphology is recognized in the opticalmicroscopy and is notably different from the uniform imagecontrast of the (¤-Fe + £-Fe) two-phase matrix. In thefollowing EBSD analysis, areas A, B, C, and D denoted inFig. 2 were selected for the fields of view. Area A is locatedat center of SZ, area B is in the belt-like structure of the SZ,area C is located at the SZ/TMAZ interface, and area D islocated 1.5mm away from the interface in the HAZ outsidethe AS.

Figure 3 shows image quality maps and phase maps ineach area by the SEM-EBSD analysis. No precipitate wasrecognized in area A at center of SZ and area C at theSZ/TMAZ interface. However, the · phase was observed inthe HAZ that was 1.5mm away from the SZ/TMAZinterface, as shown in area D. · phase is the tetragonalintermetallic phase described as Fe­Cr(­Mo).8­11) The ·

phase precipitates from the ¤-Fe/£-Fe interface. This factcoincides with the precipitation behavior of · phase during

Table 1 Chemical composition of super duplex stainless steel used in the present study.

Fig. 1 (a) Photo of FSW tool (Sialon), (b) bird’s-eye view of the FSW joining, and (c) schematic cross-sectional image of the weld.

Fig. 2 Optical microscopic cross-sectional image of the weld.

I. Sugimoto, S.H.C. Park, S. Hirano, H. Saito and S. Hata2004

heat treatments reported previously.4,5) The precipitation ofthe · phase in the HAZ was also observed at a location about1mm away from the SZ/TMAZ interface at the RS.

The SEM-EBSD analysis of B, in which the belt-likestructure in SZ was observed, shows a precipitate differentfrom the · phase. Specifically, the SZ shows a microstructurein which the crystalline grains are small: a few micrometersas a whole. The phase map revealed that the belt-shapedstructure is an aggregate of £-Fe in which ¤-Fe does notexist, even though the surrounding matrix of the belt-shapedstructure consists of the (£-Fe + ¤-Fe) two-phase structure.Furthermore, fine Cr2N precipitates are formed in the grainboundaries of £-Fe as shown in the phase map of Fig. 3.Cr2N is a hexagonal nitride phase commonly observed insuper duplex stainless steels containing significant amountsof nitrogen.12,13) It should be noted that there is a micro-structure composed of Cr2N and £-Fe phases that has a highersolid solubility of nitrogen than that of ¤-Fe.

3.2 Nanometer-scale observationsFigure 4 shows STEM high-angle annular dark-field

(HAADF) images of an area depicting Cr2N precipitates atwhich the specimen was prepared by FIB. The HAADFimage in Fig. 4(a) indicates that Cr2N showing a darkerimage intensity precipitates at the grain boundaries of the£-Fe matrix. Figure 4(b) is a magnified view of the square

area depicted in (a). At the triple junction of the £-Fe grains,precipitate A showing a brighter image intensity is formedwith the two adjacent Cr2N particles. Although dark contrastparticles can also be seen in the £-Fe grain (as shown inFig. 4(c)), they were wear debris of the Sialon tool usedfor the composition analysis by EDX. Figure 5 showscomposition profiles of the main elements (Fe, Cr, Ni) at aCr2N grain analyzed by EDX. While the profile of Cr has ahigh concentration in a Cr2N grain, the composition gradientin which Cr is depleted in the close area of the grainboundary is confirmed in £-Fe grains. In the compositiongradient, an increase in Fe and Ni is observed at the sametime as chromium deficiency. Also, Cr2N grains does notcontain Sialon elements such as Al and Si, the wear debrisdoes not affect the precipitation.

EDX analysis was also performed for unknown precip-itates at triple junctions, and the composition was found tobe slightly more Mo than the £-Fe matrix. Figure 6 showsselected area diffraction patterns by TEM of the same fieldof view as the STEM observation in Fig. 6(b). A selectedarea diffraction pattern in Fig. 6(a) was acquired from the£-Fe matrix, and that in Fig. 6(b) was acquired from theprecipitate A at the triple junction. Indexing the diffractionpattern in Fig. 6(b) revealed that the precipitate at the triplejunction is of the » phase. » phase is a cubic intermetallicphase having a composition of Fe36Cr12Mo10.14,15)

Fig. 3 Image quality maps and phase maps analyzed by SEM-EBSD.

Fig. 4 STEM high-angle annular dark-field (HAADF) images of area showing Cr2N precipitates.

Microscopic Observation of Precipitation Behavior at Friction Stirring Zone of Super Duplex Stainless Steel 2005

The STEM/TEM analysis indicates the existence ofaggregates of the £-Fe matrix containing the grain boundaryprecipitates of Cr2N and » phases in the SZ. This suggeststhat the Cr2N phase in the form of sheets precipitated at grainboundaries of the £-Fe matrix and the » phase precipitated attriple junctions of the £-Fe matrix.

3.3 DiscussionKinetic behaviors of the precipitates under heating at

constant temperature were calculated using JmatPro software.Figure 7 shows calculated time-temperature-transformation(TTT) diagrams for the possible precipitates in the presentalloy system. The same chemical composition as the materialused in the experiment (Table 1) was assumed for thecalculations. While no precipitation is expected in the matrixphases of ¤-Fe and £-Fe at higher than 1100°C, multipletypes of precipitations appear at 1000°C or lower. Thecalculated TTT diagram indicate that the · phase is mostlikely to precipitate around 950°C and that the alloy shouldbe kept at that temperature for more than 1min to promotethe · precipitation. The maximum temperature of the SZunder welding is considered to be about 80% of the meltingpoint of the materials to be joined,3) and is considered to

reach about 1100°C in this study. The · phase precipitated inthe HAZ that was not influenced by plastic flow during theFSW process. In the weld, the region 1­2mm away from SZcould be kept just in that temperature region close to 950°C,and the · phase be precipitated.

The calculated results also indicate that Cr2N and » phaseprecipitate in a short duration compared with the · phase.Ramirez et al.16) investigated the precipitation behavior ofCr2N under heat treatment and reported the followingrelationship between Cr2N and £ A-Fe (secondary austenite)phases: Cr2N nucleates from the ¤-Fe phase side at ¤-Fe/£-Feinterfaces. Because the Cr2N precipitation consumes nitro-gen, the grain boundaries containing the Cr2N precipitatesreduce nitrogen concentrations. As the nitrogen enriched inthe £-Fe matrix diffuses toward the ¤-Fe/£-Fe interfaces, the¤-Fe matrix partially transforms to austenite, which is calledsecondary austenite (£ A-Fe). The phase transition processesdescribed above is summarized as

¤-Feþ £-Fe ! ð¤-Feþ Cr2Nþ £ 0-FeÞ þ £-Fe

In the present super duplex stainless steel, Cr2N wasobserved in the aggregates of the austenite not containing¤-Fe phase. If the phase evolution proceeds along the

Fig. 7 Calculated time-temperature-transformation (TTT) diagrams forvarious precipitations.

Fig. 6 Selected area diffraction patterns observed by TEM.

Fig. 5 Composition profiles of main elements (Fe, Cr,Ni) at a Cr2N grainanalyzed by EDX.

I. Sugimoto, S.H.C. Park, S. Hirano, H. Saito and S. Hata2006

reactions described above, it can be assumed that Cr2Nnucleated at the grain boundary of the ¤ phase. At the SZ,the grain sizes of the (¤-Fe + £-Fe) matrix became smaller (tothe level of micrometer) during the friction stirring process.The ¤-Fe phase around the Cr2N phase could be rapidlytransformed to £ A-Fe because nitrogen diffusion from the£-Fe phase stabilizes £ A-Fe more than ¤-Fe. As a result,aggregates consisting of the £-Fe and £ A-Fe grains wereformed around the Cr2N precipitates. As shown in Fig. 5, Crdepletion layers would be formed around Cr2N grains. Feand Mo were released into the matrix where Cr is depletedwhen the ¤-Fe phase transformed to Cr2N and £ A-Fe, and the» phase having a composition of Fe36Cr12Mo10 was thenformed at the triple junctions. The Cr depletion layer itself isconsidered to cause localized corrosion in a corrosion studyon precipitation of · phase.17,18)

The Cr2N and the » phase precipitated in the vicinity of theAS in the SZ, and aggregates of £-Fe and £ A-Fe formed withthe Cr2N and » precipitates. Park et al.18) investigated theweld of 304 stainless steel and also reported the result offormation of · phase and Cr depletion layer in the AS of SZ.The vicinity of the AS is a place where plastic flowcirculating with the rotating tool accumulates, as shown inFig. 1(a).19,20) The belt-shaped structure (as shown in Fig. 2)is interpreted as a trace of the stirred flow. It seems that thebelt-like fluid was circulated with the tool multiple times, wasexposed in a heating environment for a sufficient duration,and precipitated Cr2N and £ A-Fe. On the other hand, the Cr2Nand the » phase didn’t precipitate in HAZ in which the ·

phase was precipitated. It is reported that the precipitationof · phase is accompanied by £ A-Fe same manner as theprecipitation of Cr2N.11,17,21) Therefore, Cr2N is consideredto be difficult to precipitate in the temperature range wherethe · phase is stable. Also, the » phase is known to transformto the · phase after heat treatment for enough time.15)

Therefore, the » phase was not confirmed simultaneouslywith · phase in this study.

4. Conclusion

The precipitation behavior of super duplex stainless steelin a weld formed by FSW at a low welding speed was

investigated using optical and electron microscopy tech-niques. The results are shown below.(1) The · phase precipitated in the HAZ that was 1­2mm

away from the interface with the SZ. On the other hand,the Cr2N and the » phase precipitated in the SZ nearthe AS.

(2) The Cr2N precipitated in the austenite aggregates witha belt-like morphology. It is suggested that the phasetransition from ¤-Fe to £ A-Fe occurred simultaneouslywith the precipitation of Cr2N. Also, the » phaseprecipitated at a triple junction of the austenite grains.

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Microscopic Observation of Precipitation Behavior at Friction Stirring Zone of Super Duplex Stainless Steel 2007