Friction Stir Welding of Stainless Steels and Nickel Base Alloys · 2020. 5. 4. · Friction Stir...
Transcript of Friction Stir Welding of Stainless Steels and Nickel Base Alloys · 2020. 5. 4. · Friction Stir...
-
Friction Stir Welding of SAF 2507 (UNS S32750) Super
Duplex Stainless Steel
Russell J. SteelProject Engineer
MegaStir Technologies275 West 2230 NorthProvo, UT 84604, [email protected]
Carl D. SorensenAssociate Professor ofMechanical EngineeringBrigham Young University
435 CTBProvo, UT 84602, [email protected]
Claes-Ove PetterssonManager, Welding R&D
Sankvik Materials TechnologyS-811 81 SANDVIKEN,
SWEDENclaes-
Yutaka S. SatoPost Doctoral ResearcherBrigham Young University
435 CTBProvo, UT 84602, [email protected]
Tracy W. NelsonAssistant Professor ofMechanical EngineeringBrigham Young University
435 CTBProvo, UT 84602, [email protected]
Colin J. SterlingGraduate Research AssociateBrigham Young University
435 CTBProvo, UT 84602, [email protected]
Scott M. PackerPresident
Advanced Metal Products Inc.2320 North 640 West
West Bountiful, UT 84087,[email protected]
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:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]
-
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