06 friction stir welding of super duplex ss

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Friction Stir Welding of Super Duplex Stainless Steel

Yutaka S. Satoa and Hiroyuki Kokawab

Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-6-02Aramaki-aza-Aoba, Aoba-ku, Sendai 980-8579, J apan

[email protected], [email protected]

Abstract. In this study, the microstructure and mechanical properties of friction stir (FS) welded

SAF 2507 super duplex stainless steel were examined. High-quality welds were successfully

produced in the super duplex stainless steel by friction stir welding (FSW) using poly crystalline

cubic boron nitride (PCBN) tool. The base material had a microstructure consisting of the ferrite

matrix with austenite islands, but FSW refined grains of the ferrite and austenite phases in the stir

zone. Ferrite content was held between 50 and 60 % throughout the weld. The smaller grain sizes of

the ferrite and austenite phases caused increases hardness and strength within the stir zone. Welded

transverse tensile specimen failed near the border between the stir zone and TMAZ at the retreating

side as the weld had the roughly same strengths as the base material.

Keywords: Duplex stainless steel, Friction Stir Welding, Microstructure, Mechanical PropertiesIntroduction

Duplex stainless steels have a mixed microstructure consisting of ferrite (bcc) and austenite (fcc)

phases [1,2]. When duplex stainless steels have the optimum phase balance, which is usually

approximately equal proportions of ferrite and austenite phases, they exhibit higher resistance to

stress corrosion cracking and higher strength than austenitic stainless steels [1]. However, the

melting and solidification associated with fusion welding processes destroy the favorable duplex

microstructure of these stainless steels. Microstructure of wrought duplex stainless steels has a

pronounced orientation of austenite islands in the ferrite matrix, but fusion welding produces amicrostructure consisting of coarse ferrite grains, and both intergranular and intragranular austenite

phases in the weld metal and heat affected zone (HAZ) [1-5]. In general, the volume fraction of

ferrite is much higher than that of austenite in the weld metal and HAZ. These changes in

microstructure cause the loss of low-temperature notch toughness and corrosion resistance in the

weld [1,2]. To alleviate these problems, careful control of the weld metal composition and

temperature are often required during welding.

Friction stir welding (FSW) is a solid-state joining process developed and patented by The

Welding Institute (TWI) in UK in 1991 [6]. Since inception, FSW had been restricted to the lower

melting temperature materials, such as aluminum (Al) and magnesium (Mg) alloys [7-10].

However, over the past five years, much progress has been made in FSW of high temperature

materials by numerous investigators [11-16]. These studies have reported that FSW achievessimilar grain refinement in the stir zone of the steels as those observed in aluminum. Additionally,

FSW does not accompany melting and solidification, alleviating the formation of porosity and

adverse phase transformations during welding process. This situation suggests that the favorable

duplex microstructure of the duplex stainless steel should be maintained, but there have been few

papers dealing with FSW of duplex stainless steel.

The present study applied FSW to SAF 2507 super duplex stainless steel using polycrystalline

cubic boron nitride (PCBN) tool. Then, the post weld microstructure and mechanical properties of

the FS weld were examined.

Experimental ProceduresThe base material used in the present study is a commercial SAF 2507 (UNS 2750) super duplex

stainless steel, 4 mm in thickness. FSW tool used in this study had a shoulder diameter of 25 mm

with the pin being 3.8 mm in length. The shoulder and pin section of the tool were manufactured


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from solid PCBN. FS welds were completed on a vertical milling machine fitted with servomotors

and control system. An argon atmosphere was introduced through a gas cup around the tool at a

flow rate of 2.8 x 105 mm3/s (1 m3/hour) to avoid surface oxidation. A 3.5 deg tilt was applied to

the tool during FSW. The welding direction (WD) was identical to the rolling direction (RD) of

the plate. The welding parameters were: rotational speed of 450 rpm, and weld travel speed of 1

mm/s.Microstructure in the weld was examined by optical microscopy (OM) and orientation imaging

microscopy (OIMTM). Sample for OM examination were electrolytically etched in a 10wt% oxalic

acid solution at 30 V for 20 s. Vickers hardness test was conducted on the cross section

perpendicular to the welding direction, using a Vickers indenter with a 9.8N load for 10s.

Transverse tensile specimens were removed perpendicular to the welding direction and prepared in

accordance with ASTM E8. Tensile tests were carried out at room temperature on a 445 kN MTS

tensile testing machine at a crosshead speed of 0.05 mm/s. A 51 mm extensometer was used to

determine the 0.2% offset yield strength.

Results and Discussion

Low-magnification overview of friction stir welded SAF 2507 duplex stainless steel is presented

in Fig. 1. In the cross section, the left- and right-hand sides of the weld center are consistent with

retreating and advancing sides of the rotating tool, respectively. The stir zone is seen around the

weld center. The border between the stir zone and the thermo-mechanically affected zone (TMAZ)

is very distinct on the advancing side, while it is more diffuse on the retreating side. It is apparent

that the weld interior exhibits a high degree of continuity and no defects.

Fig. 1 Cross section perpendicular to the welding direction offriction stir welded super duplex stainless steel 2507.

Fig. 2 Optical micrograph and phase map obtained by OIM of the base material.

An optical micrograph and a phase map of the as-received base material are shown in Fig. 2.

The base material has a typical microstructure of wrought duplex stainless steels consisting of

ferrite matrix with austenite islands. OIM analysis revealed that the austenite islands contained a

higher number of grain boundaries (mostly twin type boundaries) than the ferrite. OIM analysis

also revealed that the ferrite content was about 51 %. Average grain sizes of austenite and ferrite

phases in the base material were about 4.3 and 5.1 m, respectively.


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Fig. 3 Optical microstructures of regions A, B, C and D shown in Fig. 2.

Optical microstructures of regions A, B, C and D shown in Fig. 1 are indicated in Fig. 3.

Region B lies on the weld center, and region D is located on the border of the stir zone and TMAZ.

Regions A and C are located around 2 mm away from the weld center at the retreating and

advancing sides, respectively. Region B has the microstructure consisting of ferrite matrix with the

more elongated austenite islands. Austenite islands of region B look finer than those of the base

material. Region A has the similar microstructure to region B, while region C seems to contain

finer austenite islands than region B. Distribution of the austenite islands is finest in the stir zone

at the advancing side, as shown in micrograph of region D. In this region, D, the austenite in thestir zone exhibits an average grain size of 2.2 m lying immediately adjacent to elongated austenite

islands in the TMAZ.

Fig. 4 Hardness profile across the stir zone in the weld.


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Fig. 5 (a) Transverse tensile properties of the weld and (b)

cross section of the fractured tensile sample.

A typical transverse hardness profile of a FSW in 2507 super duplex stainless steel is indicated

in Fig. 4. Since the ferrite content in the stir zone was roughly uniform, the increase hardness in

the stir zone suggests that the hardness profile is related to the grain sizes of ferrite and austenitephases in the weld.

Transverse tensile properties of the weld are shown in Fig. 5(a). The as-FSW 2507 super

duplex exhibits roughly the same 0.2% offset yield and ultimate tensile strengths as the base

material, with the exception of the elongation. Total elongation to failure based on the standard 51

mm gauge length was roughly 34 % of the base material. However, given the amount of reduction

in area as observed in Fig. 5(b), the actual % ductility of the FSW specimens is likely much higher

than reported. All tensile failures occurred roughly 7 mm from the weld center at the retreating

side, i.e. near the border of the stir zone and TMAZ, as shown in Fig. 5(b). This is consistent with

the data presented in Fig. 5 where the failures tend to move toward the TMAZ as a result of the

higher hardness, which is proportional to strength in the metallic materials [9], of the stir zone.

It should be pointed out that the tensile specimens used in this study did not have uniformthickness (see Fig. 1) across the weld. Typically, the thinnest section of a FSW is located at the

centerline of the weld as a result of the tilt angle used during welding. This is a reason why the

transverse tensile samples consistently fractured near the border of the stir zone and TMAZ, rather

than in the base material which exhibited the lowest hardness.

Conclusions

The present study examined the microstructure and mechanical properties of FSW in SAF 2507

super duplex stainless steel. FSW using PCBN tool produced high-quality welds in the super

duplex stainless steel. FSW significantly refined the ferrite and austenite phases through dynamic

recrystallisation. The smaller ferrite and austenite grains created increase hardness and strength inthe stir zone. As a result, weld transverse tensile failures consistently occurred near the border of

the stir zone and TMAZ, exhibiting roughly the same yield and ultimate tensile strengths as the base

material.


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06 friction stir welding of super duplex ss

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