Friction Stir Welding of SAF 2507 (UNS S32750) Super

Duplex Stainless Steel

Russell J. SteelProject Engineer

MegaStir Technologies275 West 2230 NorthProvo, UT 84604, USArsteel@smith.com

Carl D. SorensenAssociate Professor ofMechanical EngineeringBrigham Young University

435 CTBProvo, UT 84602, USAcarl_sorensen@byu.edu

Claes-Ove PetterssonManager, Welding R&D

Sankvik Materials TechnologyS-811 81 SANDVIKEN,

SWEDENclaes-

ove.pettersson@sandvik.com

Yutaka S. SatoPost Doctoral ResearcherBrigham Young University

435 CTBProvo, UT 84602, USAytksato@byu.edu

Tracy W. NelsonAssistant Professor ofMechanical EngineeringBrigham Young University

435 CTBProvo, UT 84602, USAnelsontw@byu.edu

Colin J. SterlingGraduate Research AssociateBrigham Young University

435 CTBProvo, UT 84602, USAsterling@byu.edu

Scott M. PackerPresident

Advanced Metal Products Inc.2320 North 640 West

West Bountiful, UT 84087,USAscott@advmp.com

Key Words

SAF 2507, super duplex stainless steel, friction stir welding, welding, polycrystalline

cubic boron nitride, corrosion.

Abstract

SAF 2507 (UNS S32750) is a super duplex stainless steel used for its high strength

and corrosion resistance. Friction stir welding (FSW) is a solid state joining process

that joins material at temperatures below the melting temperature of the material.

Traditionally FSW has been limited to joining lower melting temperature materials

such as aluminum or copper. This study explores the feasibility of using

polycrystalline cubic boron nitride as a FSW tool material for the FSW of SAF 2507

mailto:rsteel@smith.commailto:carl_sorensen@byu.edumailto:claes-ove.pettersson@sandvik.commailto:claes-ove.pettersson@sandvik.commailto:ytksato@byu.edumailto:nelsontw@byu.edumailto:cjs@et.byu.edumailto:scott@advmp.com

super duplex stainless steel. Microstructure, mechanical properties, and pitting

corrosion resistance data are presented.

Introduction

Duplex stainless steels (DSS) are a special class of steels that are produced

with both ferrite and austenite within the grain structure. Because of the duplex

microstructure, DSS have an excellent combination of mechanical and corrosion

properties. Hardness and strength are generally attributed to the ferrite phase, while

ductility is generally attributed to the austenite phase. In addition, due to the duplex

nature of the material, the grain size is at least half that of austenitic stainless steels.

Grain size is the primary strengthening mechanism of DSS as it restricts dislocation

movement.1

DSS are produced with specific levels of alloying elements that control the

phase distribution, as well as the mechanical and physical properties. Chromium and

molybdenum provide corrosion resistance and act as ferrite formers. While

chromium and molybdenum add corrosion resistance, they also promote the

formation of detrimental metallic phases upon cooling from elevated temperatures (i.e.

sigma, chi, carbides, and nitrides). Nitrogen, while also providing increased

corrosion resistance, acts as an austenite former. Nickel acts as the primary austenite

stabilizer, providing the correct balance of austenite in the material. Nitrogen and

nickel also act as inhibitors in delaying the formation of these detrimental

intermetallic phases.2

The weldability of modern DSS is considered equal to austenitic stainless

steel, largely due to the addition of nitrogen.3 Most problems related to welding DSS

occur in the heat affected zone (HAZ). DSS have a maximum time within the

temperature range of 705 to 980C (1300 to 1800F). High heat inputs from welding

along with multiple welding passes and slow cooling rates produce detrimental

intermetallic precipitates from overexposure at these elevated temperatures.2

SAF 2507 (UNS 32750) falls in the category of a super duplex stainless steel

(SDSS). This group of alloys are classified as having a pitting resistance equivalent

(PRE) above 40, using the equation PRE = Cr + 3.3 x Mo + 16 x N, compared to

medium DSS such as SAF 2205 (UNS 31803) which has a PRE of approximately 35.

This alloy is often used as an alternate to the 6% Mo sea water austenitic stainless

steels, due to its higher yield strength and improved resistance to hot cracking.4-5

SAF 2507 has a high alloy content providing increased corrosion resistance and

higher strength.1

Friction stir welding was developed and patented by The Welding

Institute (TWI) in England in 1991.6 FSW is a solid state welding process in which a

rotating nonconsumable tool is translated along the interface between two materials to

be joined. The tool consists of a protruding pin, which is plunged into the work

pieces, and a larger shoulder section, which is maintained on the surface of the joint.

The shoulder consists of a concave surface, which produces a mixture of frictional

heating and forging pressure. Welding parameters of FSW consist of the travel speed

of the tool through the base material and the rotational speed of the tool. These

parameters are governed by the tool geometry (i.e. shoulder and pin diameter),

mechanical properties of material to be joined (i.e. flow stress), and material

thickness.

Frictional heating produced by shoulder and pin rotation in contact with base

material produces a local plasticized region around the tool. As the tool is traversed

along the weld joint, plasticized material is displaced around the tool. Because of the

high amounts of deformation and the large forging pressures produced by the tool

shoulder, a full metallurgical bond is produced.

Friction stir welding is a solid state welding process. Unlike typical arc

welding processes, FSW is not susceptible to solidification defects such as porosity

and hot cracking because there is no liquid phase present. Distinct regions are

characterized much like those in arc welds. These include: 1) the dynamic

recrystallized zone (DXZ), 2) thermal mechanically affected zone (TMAZ), 3) heat

affected zone (HAZ), and 4) unaffected base material. The DXZ consists of fine

equiaxed grains. Recrystallization has occurred in order to relieve the high amount of

plastic strain introduced by the FSW process. Peak temperatures during welding are

above the solvus and as a result, this region is in some state of solid solution.

Adjacent to the DXZ are the TMAZ and HAZ regions. The TMAZ is distinguished

by an elongated plastically deformed grain structure. Grains in this region have been

deformed at an elevated temperature by the formation of the DXZ and have a higher

dislocation density.7-9 The HAZ, just as in an arc weld, is the region unaffected by

the mechanical process but has undergone an elevated thermal cycle.

In the past, FSW was restricted to lower melting temperature materials (i.e.

aluminum, copper, and lead) due to inadequate tool materials able to withstand the

harsh environment of FSW in higher melting temperature materials. Recently,

polycrystalline cubic boron nitride (PCBN) has shown promise as a FSW tool

material. PCBN has the ability to withstand high temperatures while maintaining

high hardness. In addition, PCBN is relatively inert to iron and nickel base alloys at

high temperatures. PCBN has proven viable as a FSW tool material in joining a

variety of metals.10

This paper presents the feasibility of FSW in SAF 2507 SDSS using PCBN

FSW tools. Microstructure, corrosion, and mechanical properties are also discussed.

Experimental Procedure

SAF 2507 (UNS 2750) having the nominal composition in wt pct 0.03 max C,

system to broadcast tool temperature was used for weld trials. Initial welds were

produced at partial penetration using a tool with a 3mm pin length. An argon

atmosphere was introduced through a gas cup around the tool at a flow rate of 1

m3/hour (40 ft3/hour). A 3.5 degree tilt was applied to the tool during welding. A

basic parameter study was used in which spindle speed and travel speed were varied

until fully consolidated welds were produced in the DXZ region. Full penetration

welds were then created using a tool with a pin length of 3.8 mm. Final welding

parameters of 450 revolutions/min. spindle speed and 6 cm/min (2.5 in./min.) travel

speed were chosen for mechanical and metallurgical evaluation.

Mechanical testing consisted of transverse tensile tests in accordance with

ASTM E8. Tensile coupons were prepared transverse to the weld and perpendicular

to the rolling direction. A 445 KN (100 Kip) MTS tensile testing machine was used

with a crosshead speed of 0.05 mm/sec (0.002 in./sec). A 51 mm (2 in.) extensometer

was used to determine the 0.2 percent yield strength.

Transverse metallurgical samples were polished and etched to analyze

microstructure and weld quality. Further analysis using Orientation Imaging

Microscopy (OIMTM) was used to determine phase distribution and grain size.

Corrosion tests were completed in accordance to ASTM G-48C. This test

determined the critical pitting temperature (CPT) for the FSW joints. Due to time

constraints only the partial penetration welds using a tool of 3 mm pin length were

corrosion tested.

Results and Discussion

A. FSW Process Results

Partial penetration welds were initially produced to explore the feasibility of FSW

in SDSS materials. Welding parameters of 450 revolutions/min. provided fully

consolidated welds while also providing a smooth surface finish, as shown in figure 2.

Variations in spindle speed exhibited an effect on the surface finish of the weld.

Higher spindle speeds, over 600 revolutions/ min., produced a layer of fine flakes of

material on the surface. Lower spindle speeds, below 350 revolutions/min., provided

a clean surface finish but did not produce a consolidated weld in the DXZ. This

phenomenon is also seen in the FSW of other stainless and nickel base alloys and is

due to the surface speed of the tool, correlated with the shoulder diameter. Full

penetration welds exhibited similar characteristics.

From the telemetry system of the FSW machine, a thermocouple was placed

in the locking collar on the edge of the PCBN shoulder to monitor temperature during

welding. The temperature was broadcast to a data acquisition system and monitored

during the welding cycle. The thermocouple reading did not provide the actual

temperature of the tool within the center of the weld, but due to the high thermal

conductivity of PCBN, did give a comparative value for different welding parameters

and final microstructure. Welds were performed on a dynamometer in which X, Y,

and Z loads are measured during the process. The Z axis load was also used as a

welding parameter in which a constant load on the tool was obtained through

modulation of the Z axis servo motor. Figure 3 shows the tool temperature and X and

Z axis load data for a particular weld produced using parameters of 450

revolution/min., 6 cm/min., and a Z axis load of 33 KN (7400 lbf). The tool

temperature reached a steady state temperature of approximately 740˚C (1364˚F)

during the welding thermal cycle. Proper tool depth was controlled by Z axis load. A

33 KN load was used for tools of 3 mm pin length and, for tools of 3.8 mm pin length,

a 36 KN (8000 lbf) Z axis load was used.

B. Microstructure

The microstructure exhibited a fine equiaxed grain structure. Figure 4 shows a

transverse cross section of the weld illustrating the various regions (figure 4a) of the

weld and the microstructure found in each region (figure 4b). Significant grain

refinement was exhibited through the DXZ of the weld. Both austenite and ferrite

were equally distributed throughout the weld and detrimental intermetallic phases

were not observed. It appears that from the microstructure that the welding

temperature did not exceed 1000˚C and that the ferrite present in the weld did not

form upon cooling from the austenitic temperature range due to the equally

distributed austenite and ferrite along with the extremely fine grain size.

Further examination of the microstructure was performed using OIMTM. OIM

scans were made at 1.5 mm (0.06 in.) intervals along the centerline of a transverse

section of the weld. Regions of 200 µm by 200 µm were scanned with a Philips

XL30 S-FEG scanning electron microscope with a 0.8 µm step size. Scans were

made of the base metal, HAZ, TMAZ, and DXZ regions. The average grain size

across the weld regions are shown in figure 5. The austenitic phase exhibited a

smaller grain size in both the base material and in the weld regions with the grain size

being approximately 60 percent smaller in the weld. The phase distribution was also

examined by OIM. Figure 6 is a plot of the average percentage of ferrite at 1.5 mm

intervals across the centerline of the weld. The ferrite distribution varied between 40

and 52 percent across the joint.

C. Mechanical Properties

Transverse tensile specimens were taken from each weld. The ultimate tensile

strength, 0.2 percent yield strength, and total elongation was calculated and compared

to the base metal properties. Values are shown in table 1. Mechanical fracture of the

tensile samples was located within the DXZ at 45 degrees from the axis of load.

Grain size is a major strengthening mechanism for SDSS. The significant reduction

in grain size produced mechanical properties above that of the original base metal.

An important factor to highlight is the reduced area caused by the FSW process. The

shoulder of the tool is below the surface of the material during the welding process,

which reduces thickness (and effectively the cross sectional area) at the centerline of

the weld. This allows for a measurable increase in mechanical properties over the

surrounding base material while exhibiting mechanical failure in the weld region

rather than in the HAZ or base metal.

Microhardness data was also taken transverse across the weld along the

centerline. Figure 7 shows a slight decline in hardness in the HAZ with an increase in

hardness through the DXZ, likely due to the reduction in grain size through the DXZ.

D. Corrosion Testing

A commonly used corrosion test for SDSS is ASTM G-48C. This test

determines the critical pitting temperature (CPT). CPT values for FSW joints in SAF

2507 yielded a CPT value of 65˚C. Typical arc welding processes yield CPT values

between 40 to 55˚C depending upon the welding process.3-4 The high CPT value is

due to the lower peak temperature and less time at temperature in FSW as compared

to traditional arc welding processes.

Summary

FSW of SAF 2507 using a PCBN tool produced fully consolidated welds

exhibiting a wrought microstructure having a fine equiaxed grain structure. The

tensile and yield strength was increased due to the smaller grain size present and the

ferrite phase distribution was maintained between 40 and 52 percent. Corrosion tests

of SAF 2507 yielded a CPT value of 65˚C, which is 15 to 20˚C higher than typical arc

welding processes.

References

1. Frodigh, J; Nicholls, JM; “ Mechanical Properties of Sandvik Duplex

Stainless Steels”; Lecture; AB Sandvik Steel, S-32-30-ENG, 1994.

2. International Molybdenum Association; Practical Guidelines for the

Fabrication of Duplex Stainless Steels; Revised Edition, London, UK, 2001.

3. Pettersson, C; Fager, S; “ Welding Practice for the Sandvik Duplex Stainless

Steels SAF 2304, SAF 2205 and SAF 2507”; Lecture; AB Sandvik Steel, S-

91-57-ENG, 1994, Revised 1995.

4. Fager, S; Odegard, L; “Welding of the Super Duplex Stainless Steel Sandvik

SAF 2507TM (UNS S32750)”; Stainless Steel Europe; 5, (10), 40-45, 1993.

5. Fager, S; Odegard, L; Ekstrom, U; “ Welding of SAF 2507”; Welding

Reporter; AB Sandvik Steel, S-WR291, 1992.

6. Thomas, WM et al; “Friction Stir Butt Welding”; International Patent No.

PtCT/GB92702203, June 1993.

7. Ditzel, PJ; “Microstructure / Property Relationships in Aluminum Friction Stir

Welds”; Thesis, The Ohio State University, 1997.

8. Mahoney, MW; Rhodes, CG; Flintoff, JG; Spurling, RA; Bingel, WH;

“Properties of Friction-Stir-Welded 7075 T651 Aluminum”; Metallurgical and

Materials Transactions A, 29A, July 1998, 1955-1964.

9. Rhodes, CG; Mahoney, MW; Bingel, WH; Spurling, RA; Bampton, CC;

“Effects of Friction Stir Welding on Microstructure of 7075 Aluminum”;

Scripta Materialia, 1997, 1, (36), 69-75.

10. Sterling, CJ; Nelson, TW; Sorensen, CD; Steel, RJ; Packer, SM; “Friction Stir

Welding of Quenched and Tempered C-Mn Steel”; Friction Stir Welding and

Processing II, The Minerals, Metals, and Materials Society, 2003, 165-171.

Figures

Figure 2. SAF 2507 friction stir weld.

Figure 1. PCBN friction stir welding tool assembly.

Figure 3. Tool temperature and load data for SAF 2507 friction stirweld.

Figure 4. Microstructure of SAF 2507 friction stir welds. A) Transversemacrograph of DXZ, TMAZ, and base material, B) Base metal microstructure(500X), C) TMAZ and DXZ interface microstructure (500X), D) DXZmicrostructure (500X).

Figure 5. Average grain size across SAF 2507 friction stir weld.

Figure 6. Percentage of ferrite across SAF 2507 friction stir weld.

Figure 7. Microhardness traverse across SAF 2507 friction stir weld.

Table 1. Mechanical properties of SAF 2507 friction stir welds.

Friction Stir Welding of SAF 2507 (UNS S32750) SupRussell J. Steel Project Engineer435 CTBClaes-Ove PetterssonManager, Welding R&D

Yutaka S. SatoPost Doctoral Researcher

Brigham Young University435 CTB435 CTBProvo, UT 84602, USAColin J. Sterling

AbstractIntroductionExperimental ProcedureFigures