High Alloyed Duplex Stainless Steels

High Alloyed Duplex Stainless Steels
Aspects on Welding and Fabrication
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Contents
Safety first
• Materials
• Properties
• Welding generally
• Welding duplex stainless steel plate
4
Classification of stainless steels
Safety first
• Martensitic
• Martensitic-Austenitic
• Ferritic
• Austenitic
• Austenitic-Ferritic
Development of new steels is ongoing for,
in principle, all five groups
5
Materials: Chemical Composition
•This presentation has focus on Austenitic-Ferritic Stainless Steels
GRADE
SAF 2304
SAF 2205
SAF 2507
SAF 2707HD
SAF 2906
SAF 3207HD
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C
<0,03
<0,03
<0,03
<0,025
<0,03
<0,03
Si
0,5
<0,10
<0,08
0,25
0,40
< 0,8
Mn
1,2
1,2
<1,2
1,0
1,0
<1,5
Cr
23
22,5
25
27
29
32
Ni
4
5,5
7
6,5
7
7
Mo
3
4
4,8
2,4
3,5
N
0,12
0,15
0,25
0,39
0,36
0,5
Structure stability
Safety first
• Structure stability decreases with increasing alloy content (Cr, Mo)
• Sensitive temperature range
 Austenitic, 900 – 1050°C
 Austenitic-Ferritic, 600 – 1000°C
• Austenitic-Ferritic steels are also prone to 475°C embrittlement
• Influences welding
 Weld metal more sensitive than base metal (wrought materials)
 Good control of the heat input
7

+
-Phase
g
l i ng
coo
coolin
w
slo
Too
pid
Too ra
HAZ: very rapid
Temperature
Structure stability: Sigma phase and CrN2
Time
8
Structure stability
Safety first
•Pitting resistance
PRE= Cr + 3.3Mo + 16N (%)
•Cr, Mo and N import for
increased pitting resistance
•High Cr,Mo,W, Si also promote
intermetallic phases, wich
•Reduced impact strength
•Reduced corrosion resistance
9
Structure stability
Stress reliving?
No
Stress reliving? YES
10
Structure stability
Safety
first
Structure stability decreases with increasing alloy content (Cr, Mo)
10 5 0
Temperature, °C
10 0 0
950
900
S32750
850
S32707
800
750
700
650
600
1
10
10 0
Tim e , m in
11
10 0 0
10 0 0 0
Precipitation of Intermetallic Phases
`
G
2


R
300 – 525°C
(G-fas)
300 – 400°C
(Gamma)
<650 to 900°C
(Sigma)
600 – 1000°C
(Chi)
700 – 900°C
(Laves)
550 – 650°C
Cr2N, CrN
700 - 950°C


(Pi)
550 – 600°C
(Tau)
550 – 650°C
650 – 950°C
M7C3, M23C6
12
Structure stability
• Sandvik metallurgy is based on
Cr, Ni, Mo and N
because
• W increases the formation of
intermetallic phases as
chi,  och sigma, .
13
”Structural stability of super duplex Stainless Weld metals and its dependence on Tungsten and Copper”
Metallurgical and Materials Transactions A, Volume 27A, August 1996
Structure stability
• Secondary austenite (2) formation
• give less Cr and N I the duplex structure, which give bad
corrosion properties
• W and Cu, give more
secondary austenite (2) in
the weld metal
 Give a lower CPTvalue
14
• No W and Cu, give less
secondary austenite (2) in
the weld metal
 Give a higher CPTvalue
Cold working
Safety first
• More force needed with increasing yield strength/alloy content
• Austenitic-Ferritic steels give more ”spring back” than austenitic steels
at cold forming
• A cold work up to approximately 10% can be done without stress
relieving
• The steels have good resistance to stress corrosion cracking
• Local heat treatment of the cold worked area is very difficult and not
recommended
• If heat treatment is necessary, a full quench annealing cycle has to be
done (Often difficult in practice)
15
Cold working and Machinability
The
machinabilty
decreases with increasing alloy content
Safety
first
•
• Austenitic-Ferritic steels somewhat better compared to the austenitic, eg.
N08904 in the same area and even lower than S32750
• Excessive wear on the rake face of the tungsten carbide bit
Machinability
Austenitic
Duplex
AISI 304
AISI 316
SAF 2205
SAF 2507
SAF2707HD
Corrosion res.
16
Safety first = f (alloy content)
Properties
• Mechanical properties, Yield strength
Proof strength, 0,2% offset as function of alloy
content, MPa
Austenitic
100ksi=690MPa
Proof strength,
0,2%,MPa
•
Austenitic-Ferritic
800
600
400
200
0
S32205
S32750
S32707
N08904
S31254
S32654
Alloy
17
Corrosion - pitting
Safety first
• Caused by failure in oxide film, impurities at surface & inclusions
• Proceeds quickly after initiated - unpredictable
• Use Molybdenum alloyed grades
Cathode
Anode
18
Pitting corrosion
Safety first
19
Austenitic stainless steel
PRE (Pitting Resistance Equivalent)
PRE = Cr + 3.3(Mo+0,5W) + 16N
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GRADE
Cr
304L
316L
SAF 2304
SAF 2205
SAF 2507
SAF 2707HD
SAF 2906
SAF 3207HD
18
17
23
22,5
25
27
29
32
Mo
2,5
3
4
4,8
2,4
3,5
N
PRE
0,12
0,15
0,25
0,39
0,36
0,5
18
25
25
35
42
49
42,7
51,5
SCC (Stress Corrosion cracking)
Alloy
Comparisons
Base Metal
Safety
first
21
Welding - Joint preparation
Safety first
Good control of the heat input
Compared to 300-series stainless
steels, high alloyed duplex and
austenitic steels need
• A wider gap
• A more open angle
due to the poorer fluidity of the weld
pool.
22
Welding - Heat input
Safety first
• Excess heat input shall always be avoided when welding stainless steels
• Austenitic-ferritic steels is not so sensitive to excess heat input as Austenitic steels
• Very important that the interpass temperature is kept
Grade
UNS no. Heat input
[kj/mm]
SAF 2205
S32205 0,5 – 2,5
SAF 2507
S32570 0,2 – 1,5
SAF 2707HD S32707 0,2 – 1,0
SAF 2906
S32906 0,2 – 1,5
SAF 3207HD S33207 0,2 – 1,0
23
Interpass
[°C]
150
100-150
100
100-150
100
Welding - Shielding gas and Purging gas
Safety first
• Nitrogen is an important element due to
 Austenite formation
 Corrosion resistance
• GMAW (MIG/MAG, 131/135)
 welding with pulse arc modus use Ar + 30%He + 1-2,5% CO2
• GTAW (TIG, 141)
 Welding with:
Shielding gas
Purging gas
Ar + 2-3% N2
100% N2
Welding with pure argon as shielding and root gas  nitrogen is lost
and must be compensated for in order to maintain the corrosion resistance
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Welding - shielding gas
Safety first
Shielding gas: Ar
Excessive ferrite & nitrids
25
Shielding gas: Ar + 2-3% N2
Good ferrite / austenite balance
Welding - Filler metal
• Austenitic-ferritic stainless steels are welded with austenitic-ferritic filler
metal.
• Dissimilar joints of duplex and highly alloyed austenitic steels, nickel-based
fillers have to be used.
• Welding austenitic-ferritic steel with nickel-based filler metal gives
phenomenons like
 Un-mixed zones (UMZ)
 Nitrogen depleted fusion line
26
Welding - Nitrogen in weld metal
Safety first % Nitrogen in position (o’clock)
Welder
3
5
6
7
11
12
1
0,17
0,18
0,15
0,16
0,19
0,19
2
0,18
0,20
0,19
0,21
0,19
0,20
Top
Root
3
0,22
0,21
0,21
0,21
0,20
0,20
Top
Root
- In all cases the filler metal had a nitrogen content of 0,25%
- TIG weld
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Top
Safety first
Welding - Filler metal
GTAW/TIG/141 GMAW/MAG/135 SMAW/MMA/111
SAW/12
LDX 2101®*
23.7.L
23.7.L
23.7.L
SAF 2304
SAF 2205
SAF 2507
SAF 2707HD
23.7.L
22.8.3.L
25.10.4.L
27.9.5.L
23.7.L
22.8.3.LSi
25.10.4.L
27.9.5.L
23.7.L
22.8.3.L
25.10.4.L
27.9.5.L
SAF 2906
SAF 3207HD
29.8.2.L
27.9.5.L
29.8.2.L
27.9.5.L
28
22.9.3.LR
25.10.4.LR
29.8.2.LR
29.8.2.L
27.9.5.L
Welding - Filler metal
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Welding - Filler metal
• Duplex welded with Ni-base alloy filler material
UMZ in root area with sigma
Ferritic area in fusion line
UMZ in top of weld
• Duplex welded with duplex filler material
30
Root with normal duplex
structure. Filler 27.9.5.L
Top of weld, normal duplex
structure
Welding - Processes
• The highly alloyed stainless steels are welded by the common arc welding
processes
 GTAW/TIG/141
 GMAW/MAG/135
 SMAW/MMA/111
 SAW/12
• Welding processes giving high heat inputs,
eg SAW shall be used with care for the high alloyed steels
31
Welding - Properties of Weldments
•The corrosion resistance of weldments can still be more improved by
removal of oxides and chrome depleted layer
32
Welding - Properties of Weldments
To ensure good weldments
•Higher alloying content of a stainless steel => improved properties
• corrosion resistance
• mechanical strength
• Can problem solvers in severe corrosive environments
•Higher alloyed steels => more knowledge needed in fabrication
• Welding
• Cold working
• Heat treatment
33
Close contact with the material supplier recommended
Welding duplex stainless steel plate
Zhiliang Zhou, Ph.D, IWE presented at an internal NACE group 2011
Contents
• Embrittlement
• Precipitation of intermetallic phases
• 475°C embrittlement
• Welding defects
• Crack in SAW
• Lack of penetration
• Duplex plate application in chemical tank
35
How to get balanced microstructure in weldment
S32205 (SAF 2205)
36
API 938–C:
NACE MR0175:
DNV-OS-F101:
Ferrite content:
30-65% Base
40-65% HAZ
25-60% Weld
Ferrite content
30-70 vol.%
Ferrite content:
35-65% HAZ + Weld
How to get balanced microstructure in weldment
SAF 2205
Steel grade
Heat input [kJ/mm]
Interpass temp. [°C]
SAF 2205
0,5 - 2,5
<150
t = 12–25 mm
1,0 – 2,0
<150
• Nitrogen improves austenite reformation in duplex steel HAZ
Nitrogen in S31803 0,08 – 0,20 wt%
Nitrogen in S32205 0,14 – 0,20 wt% (preferred)
• N2 in shielding gas and purge gas improves the balanced weld microstructure
37
Welding defects – hot cracking
Steel grade: Sandvik SAF 2205
Thickness: t = 25 mm
Joint angle: 60
Welding method: SAW
W/D: 0,6
Cause:
Too narrow joint  80 -90
Too low W/D ratio
 Small land & thin pass
Too high welding speed could
also be a reason
38
Solidification
cracking
Welding defect – Lack of penetration
Steel grade: Sandvik SAF 2205
Thickness: t = 12mm
Joint angle: 40
Welding method: MIG/MAG
Root gap: 1,5 mm
Cause:
Too narrow joint  60 - 70
Too small root gap  2 - 3 mm
Too low current
Too high voltage
Defect: Incomplete penetration
39
Welding defect – slag inclusion
Steel grade: Sandvik SAF 2205
Thicknss: t=15mm
Welding method: FCAW
Joint design: K type
Bevel angle: 30
Root gap: 2 mm
Land: 3 mm
Cause:
Too narrow joint  50 -55
Too small root gap  min. 3 mm
Too low current
Defect: Slag inclusion
40
One application of duplex steel plate
41
Chemical tank
9
8
1 or 3
5 and 6
1 or 4
7
1
2
42
1 or 3
1
Welding of deck
Steel grade
•Sandvik SAF 2205
•t = 12 - 25 mm
Joint design
•Bevel angle: 60 - 70
•Land: 1,5 - 2 mm
•Root gap: 3 - 5 mm
43
Welding method:
•FCAW+SAW
FCAW for the first 2 layer
SAW fill up
A
B
C
Summary
• To get balanced microstructure and minimize the
risk of embrittlement in duplex weldment
 Control of heat input and interpass temperature
 Nitrogen in the plate (prefer S32205)
 Use nitrogen in shielding gas and purging gas
• To avoid welding defects
 Joint design very important – open grooves up
 Welding parameters
44
www.sandvik.com
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