Duplex Stainless Steel
Ⅰ. Duplex in general
Ⅱ. Corrosion & embrittlement
Ⅲ. Mechanical properties
Ⅳ. Welding
V. Summary & discussion
IL Yoo / PLANT S-ENG’G TEAM
Jan. 06-07, 2011 at #801 Meeting room
Ⅰ. DUPLEX IN GENERAL
1. Introduction
2. Development
3. DSS Family
4. Alloy elements
5. Metallurgy
6. Intermetallic formation
7. detrimental phases
Ⅰ. DUPLEX IN GENERAL
1. Introduction
 Dual phases(Austenitic-ferritic) stainless steel
–
Allowed ferrite contents are 30-65% by API582, 35-65% by NACE
Outokumpu stainless paper :
 1% consumption of the total stainless volume 80From
Years with duplex steels, a historic review and prospects for the future
 More uses in industries : economical combination of strength + corrosion resistance
–
–
Prices would be between 300 series SS and super-austenitic SS(or nickel alloys)
Often replacing austenitic SS in services where would have problems with Cl- pitting or SCC
Stainless Steel
ASMT and ASTM Specifications for DSS
Martensite
Fe-Cr
Ferrite
Austenite
Fe-Cr-Ni
Precipitation
Duplex
Ⅰ. DUPLEX IN GENERAL
2. Development
1st generation
2nd generation
PRE = %Cr + 3.3 x %Mo + 16 x %N
60
S32707
50
S32750
40
30
S32404
S31500
S32205
S31803
S32304
S32900
S32906
S32003
S32101
20
Hyper-duplex
Super-duplex
Duplex
Lean Duplex
10
0
1950
1960
1970
1980
Year
1990
2000
2010
Ⅰ. DUPLEX IN GENERAL
Hyper DSS
27Cr/6.5Ni/4.8Mo/0.4N
UNS S32707
Super DSS
25Cr/7Ni/4Mo/0.3N
UNS S32750, S32760
3. DSS family
Strength
Corrosion resistance
+
+
Mo, N ↑
> 45 PRE
N↑
+
Mo, N↑
(Cr, W, Cu)
> 40 PRE
Mo, Ni ↓
> 30 PRE
Standard DSS
22Cr/5Ni/3Mo/0.17N
UNS S31803, S32205
Lean DSS
-
22Cr/1.5Ni/0.3Mo/0.22N
UNS S32101, S32304
PRE = %Cr + 3.3 x %Mo + 16 x %N
Ⅰ. DUPLEX IN GENERAL
4. Alloy elements in DSS
 Cr (BCC former)
–
–
Essential alloy element for SS to form passive film
Promoting the formation of Intermetallic phases with higher Cr
 Mo (BCC former)
–
–
Increasing pitting corrosion resistance
Promoting the formation of Intermetallic phases(restricted to less than 7.5% in γ-steel, 4% in α+γ-steel)
 N (FCC former)
–
–
Increasing pitting & crevice corrosion resistance and substantially increasing strengthening
Reducing Intermetallic content(can’t prevent but delay the formation)
 Ni (FCC former)
–
–
Improving toughness
Delaying the formation of detrimental intermetallic phases(only volume fraction, accelerating precipitation kinetic)
Add
Nickel
BCC(Ferrite)
FCC(Austenite)
Ⅰ. DUPLEX IN GENERAL
5. Metallurgy
Ferrite
Austenit
e
*
*T = Annealing temp. 1050~1150℃
Ⅰ. DUPLEX IN GENERAL
6. Intermetallic formation
 Numerous structural changes during heat treatment
–
–
Mostly affects only ferrite due to much faster diffusion rate of alloying elements in ferrite(100 times)
Precipitation of intermetallic phases will increase with higher alloying content of the steel
•
–
Probably a limit in PRE value of 50 due the increased risk of intermetallic phase formation
Most intermetallic compounds reduces corrosion resistance and mechanical properties
Cr
Mo
Fe
Mo
Ⅰ. DUPLEX IN GENERAL
7. detrimental phases
 Precipitation in the temp. range 600-1000℃
–
σ(Fe-Cr-Mo) : very brittle(Tetragonally close-packed)
•
–
–
–
Hardness can’t be determined the presence of σ-phase: no significant effect until σ-phase exceeds 4%
CrN, Cr2N : most deleterious phases with σ(often coexisting with Cr2N)
γ2 : lower Cr, Mo, N than γ(lower PRE)
χ(Fe36Cr12Mo10), R(Fe-Cr-Mo), π(Fe7Mo13,N4)
•
Affects in a similar way as the σ-phase, but present in smaller quantities than the σ-phase
 Precipitation in the temp. range 300-525℃(475℃ embrittlement)
–
Cr-rich(α’) and Fe-rich(α) phases forming at prolonged heat exposures
Time
Ⅱ. CORROSION & EMBRITTLEMENT
1. General corrosion in reducing acid
2. General corrosion in oxidizing acid
3. General corrosion in organic acid
4. Pitting corrosion
5. Crevice corrosion
6. Intergranular corrosion
7. SCC by Cl8. Hydrogen embrittlement
9. Ammonium bisulfide
Ⅱ. CORROSION & EMBRITTLEMENT 1. General corrosion in reducing acids
 Good corrosion resistant to mild reducing acid
 But much less resistant to strong reducing acid due to insufficient Ni
Reducing area
Ⅱ. CORROSION & EMBRITTLEMENT 2. General corrosion in oxidizing acids
 Good corrosion resistant to oxidizing acids due to high Cr content
Oxidizing area
(24Cr/20.5Ni/0.1Mo)
Ⅱ. CORROSION & EMBRITTLEMENT 3. General corrosion in organic acids
 No data for Naphthenic acid
–
DSS would be acceptable due to high Mo content by API938
 Good candidate for strong organic acids
Ⅱ. CORROSION & EMBRITTLEMENT 4. Pitting corrosion
 Very good resistance to chloride-induced localized corrosion due to high Cr, Mo, N contents
316L
Ⅱ. CORROSION & EMBRITTLEMENT 5. Crevice corrosion
 Same mechanism as pitting, but CCT is lower 15~20℃ than CPT
Ⅱ. CORROSION & EMBRITTLEMENT 6. Intergranular corrosion
 Good intergranular corrosion resistance due to its microstructure
–
–
Carbide grows much faster in ferrite due to much faster Cr diffusion rate
Very wide, shallow Cr-depleted zone on the ferrite / very narrow, deep Cr-depleted zone on the austenite side
•
•
–
Austenite-carbide interface : can be quickly eliminated by minor rediffusion of Cr due to very narrow zone
Ferrite -carbide interface : does not reach a sufficiently low level for Cr
ASTM A262 C, E are often used for checking intergranular corrosion
Austenitic SS
Huey
test
DSS
Strauss
test
Ⅱ. CORROSION & EMBRITTLEMENT 7. SCC by Cl Acc. to Copson’s curve, 8~12% Ni containing austenite is very susceptible to SCC
 The earliest uses of DSS were based on their resistance to chloride SCC
Ⅱ. CORROSION & EMBRITTLEMENT 8. Hydrogen embrittlement
 HE susceptibility : Austenite < Duplex < Ferrite
Ⅱ. CORROSION & EMBRITTLEMENT 9. Ammonium bisulfide
NH4Cl deposits on a HDS
kerosine heat exchanger
Reference: R.J Hovath,M.S.Cayard,R.D.Kane –
Prediction and Assessment of NH4HS corrosion
under refinery sour water service conditions –
Paper No.06576 -2006
Ⅲ. MECHANICAL PROPERTIES
1. Strength & elongation
2-1. Strengthening mechanism
2-2. Strengthening mechanism
3. Hardness
4. Hardness conversion
5. Toughness
6. Thermal expansion
Ⅲ. MECHANICAL PROPERTIES
1. Strength & elongation
 High strength & moderate elongation
–
2-3 times higher for YS and 1.5 times higher for TS compare to the austenitic SS
Min. YS : only valid for thickness ≤ 4mm
Grade
S32750, Th’k max. 20mm
S32205, Th’k max. 20mm
S31254, Th’k ≤5mm
S31603 < 10mm
Proof Strength
Tensile Strength
Rp0.2MPa(min.)
Rp1.0MPa(min.)
MPa
550
485
310
220
640
500
340
250
800 – 1000
680 – 880
675 – 850
515 – 690
Ⅲ. MECHANICAL PROPERTIES
2-1. Strengthening mechanism
 Smaller grain sizes(less than half compare to austenitic SS)
 Cold deformation
Austenitic SS
Hall-Petch relation
X600
Duplex SS
X600
Ⅲ. MECHANICAL PROPERTIES
2-2. Strengthening mechanism
 Substitutionally solution with Cr, Mo, Ni
 Interstitial solution with N
–
–
–
Stronger effect on strength than substitutionally solution hardening
Austenite is preferentially strengthened by N due to its FCC structure
The austenite become stronger than the ferrite at a bulk nitrogen concentration higher than 0.2% for S32205
Ferrite
Austenite
Different interstitial space
SAF2205,
SAF2507,
Influence of nitrogen on TS & YS
Ⅲ. MECHANICAL PROPERTIES
3. Hardness
 Relating to the strength of DSS
 Favourable influence on the resistance to erosion & wear due to high hardness
 Normally higher hardness on weldments
–
–
Increasing hardness in both weld metal and HAZ(particularly in the root region) due to strain induced by the
heating and cooling cycle(compression)
Multipass welds in thicker material produce higher hardness values
Ⅲ. MECHANICAL PROPERTIES
4. Hardness conversion
 Conversion between hardness scales
–
–
Conversions between Rockwell C and Vickers are different from CS and other low-alloy, ferritic steels
NACE MR0103 limits hardness (No requirement for MR0175, for more detail please see NACE code)
•
•
28HRC max. for wrought & casting in the solution-annealed and liquide-quenched condition
310HV 10 Avg., 320HV 10 Max. for weldments
10 kgf
From “Duplex Stainless Steels, Microstructure, Properties and Applications”, Abington Publishing, 1997
No limitation from
MR0175-2003
Ⅲ. MECHANICAL PROPERTIES
5. Toughness
 Attributed to the presence of high amounts of austenite & fine grain structure
 Brittle behaviour at a temperature around -50℃(-51℃ by ASME B31.3 Code)
1. Austenitic
2. Duplex
3. Ferritic
4. Martensitic
Ⅲ. MECHANICAL PROPERTIES
5. Thermal expansion
Ⅳ. WELDING
1. Welding metallurgy
2. Wrong welding result
3. Too slow cooling
4. Too fast cooling
5-1,2. Welding parameters
6. Joint preparation
7. Dissimilar metal welding
8. Others
Ⅳ. WELDING
1. Welding metallurgy
 Differences between duplex & austenitic SS
–
–
–
Austenitic SS : Hot cracking in weld metal itself
DSS has high ferrite content and less local thermal stress(higher thermal conductivity/lower thermal expansion)
DSS : Loss of corrosion resistance, toughness, or post-weld cracking in HAZ
 Welding physical metallurgy
–
α+γ
Fully α
γ in GB
Cr2N or σ-phase
γ2 by mulitpass
Austenite
Base Metal
Ferrite
Weld Metal

Ferrite
Χ
Cr-nitride
Phase fractions of ferrite, austenite, σ-phase, χ-phase
and Cr2N as a function of temperature in SAF 2507.
Computed using Thermocalc.
Ⅳ. WELDING
2. Wrong welding result
 Two primary problems to be avoided when welding DSS
–
–
Excessive ferrite or nitrides in the HAZ or weld deposit
The formation of harmful intermetallic phases in the HAZ
High Ferrite
(Nitrogen loss)
Surface oxidation
(Oxygen contamination)
Ferrite + Nitrides
(Fast cooling)
Sigma phase
(Slow cooling)
Secondary Austenite
(Reheating)
Ferrite + Nitrides
(Nitrogen loss)
Surface oxidation
(Oxygen contamination)
Ⅳ. WELDING
3. Too slow cooling
 Harmful intermetallic phases from excessively cumulative time at high temp.
–
Only can be removed by a full anneal with sufficient time to dissolve intermetallic compound
475° C embrittlement curve
To the left of the curves the impact strength is 27J or more
Ⅳ. WELDING
4. Too fast cooling
 Forming chromium nitrides by rapid cooling
–
–
The solubility of nitrogen ferrite is decreased at lower temp.
‘N’ needs time to escape from ferrite and stabilize austenite
 Actually, required quite rapid cooling rate
–
–
–
Excessive ferrite content can result from extremely low heat input welding or rapid quenching
When welding widely differing section sizes or heavy sections with very low heat input
The risk can be overcome by preheat or higher heat input
Excessive ferrite
Schematic representation of the mechanism of cooperative precipitation of nitrides and 2 on the /1
interfaces showing evolution of the microstructural constituents at the /1 interface during reheating
Chromium nitrides
Ⅳ. WELDING
5-1. Welding parameters
 Heat input
–
Too low : excessively ferritic, Too high : formation of intermetallic phases
 Interpass temp.
–
Low interpass temp. is desirable for minimizing the formation of intermetallic phase

+
Cr
Nitrides
-Phase
Time
Ⅳ. WELDING
5-2. Welding parameters ??
Interpass Temp 0C
300
Sigma phase
Distortion
LDX2101
SAF2205
SAF2304
200
Slow
cooling
IGC
Distortion
Grain Growth
150
IGC
SAF2507
409, 430
100
Rapid
cooling
High Ferrite
Chrome Nitrides
304 & 316
5CR12(Ferritic 11.5Cr-0.7Ni)
1
Arc Energy kJ/mm
2
Volts x Amps x 60
T/Speed x 1000
3
Table from API 582
Ⅳ. WELDING
6. Joint preparation
 Providing for full penetration & avoiding undiluted base metal in the solidifying weld metal
–
–
–
–
Machining is the best rather than grind which can cause deficiencies
The deficiencies can be removed by torch for austenite SS, but the technique can cause a longer exposure in the
harmful temp. range for DSS
As a rule, the root gap should be wider & joint angle slightly wider than for austenitic SS to ensure good
penetration
TIG is strongly recommended for root passes
Ⅳ. WELDING
7. Dissimilar metal welding
 DSS-Austenitic SS
–
Low C & intermediate Mo between two steels are suggested : AWS E309LMo, E309L
 DSS-CS or low alloy steels
–
–
Low carbon content of DSS should be preserved
Preheating/PWHT : forming intermetallic phases due to slow cooling or high temp.
•
Austenite filler metal(E309L) buttering → PWHT → DSS filler metal is one solution
 DSS-Nickel alloy
–
Nickel filler metals suggested
Table from API 582
Ⅳ. WELDING
8. Others
 Preheating
–
–
Not recommended generally because it can cause harmful precipitation
Can be beneficial to eliminate moisture with heating up 100℃ uniformly
 PWHT
–
Normally, no needed it which can cause 475℃ embrittlement
Minimum Solution Annealing Temperatures
For Duplex Stainless Steels
(Source: Producer DataSheets and ASTM A 480)
 Shielding & root gases
–
GTAW/TIG
•
–
Shielding gases : Ar+1-2% N2, Backing gases : Ar+N2(at least 5%) or pure N2
GMAW/MIG shielding gases
•
Shielding gases : Ar+CO2(1-3%), Ar+1-3%O2/Spray arc, Ar or Ar-He-O2/ Short-arc, Backing gases : Ar-N2, pure N2
Ar 99,99%
Ar + 5% N2
Different structures of the weld due to nitrogen additions in the shielding gas
V. SUMMARY & DISCUSSION
1. Duplex in general
2. Corrosion resistance
3. Mechanical properties
4. Welding
5. Economical combination
6. Any comment or question
V. SUMMARY & DISCUSSION




1. Duplex in general
Austenite + Ferrite phase
Ferrite limitation : 30-65% by API582, 35-65% by NACE
Duplex SS family : Lean, Standard(> 30 PRE), Super(>40 PRE), Hyper(>45 PRE)
Temp. limitation due to intermetallic forming
Temp. limitation for DSS for max. allowable stress value in pressure vessel design code
V. SUMMARY & DISCUSSION
2. Corrosion resistance
 Good resistance to Cl- induced local corrosion & SCC
 Moderate HE resistance
V. SUMMARY & DISCUSSION
3. Mechanical properties
 High strength due to small grain size & ‘N’
Min. YS
(only valid for thickness ≤ 4mm)
 Hardness limitation
–
NACE MR0175
•
–
No limitation for wrought and cast product in solution-annealed and rapid-cooled
NACE MR0103
•
•
•
PREN ≤ 40 : Max. 28HRC / PREN > 40 : Max. 32HRC for wrought and cast product in solution-annealed and rapid-cooled
Welding : Not exceed 310HV(Avg.) and 320HV(Individual) by 10kgf or less
Hardness shouldn’t be converted by ASTM E140 table
*Please see more detail on NACE MR0103-2010
V. SUMMARY & DISCUSSION
4. Welding
 Avoiding phase imbalance
–
–
High ferrite(>70%) : low ductility, loss of corrosion resistance, HE susceptability
High austenite(>80%) : low SCC resistance, low strength
 Avoiding intermetallic formation
–
Loss of corrosion resistance and impact strength
5. Economical combination
V. SUMMARY & DISCUSSION
Corrosion resistance
Mechanical strength
(PRE)
Rp0.2 [MPa]
40
254SMO
SAF2507
750
904L
2205
SAF2507
500
30
LDX
2101
20
SAF
2304
316
LDX
2101
2205
SAF
2304
254SMO
250
304
1
3
2
”Relative Price/Kg”
304
316
1
2
904L
3
”Relative Price/Kg”
HX tube price rate compare to 304L
160%
120%
80%
40%
0%
304L
316L
316Ti
321H
317L
904L
254SMO SAN28 Alloy825 SAF2304 SAF2205 SAF2507
V. SUMMARY & DISCUSSION
6. Any comment or question?
 Discussion
–
–
–
 Question
–
–
–

Comment
–
–
–
 References suggestion
–
–
–
–
Use of Duplex Stainless Steels in the Oil Refining Industry / API Technical report 938-C, 2005
Practical Guidelines for the fabrication of Duplex Stainless Steels / IMOA, 2009
The physical metallurgy of duplex stainless steels / Sandvik SMT
Welding practice for the Sandvik duplex stainless steel SAF2304, SAF2205 and SAF2507 / Sandvik SMT, 1995
REFERENCES
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Use of Duplex Stainless Steels in the Oil Refining Industry / API Technical report 938-C, 2005
Welding guidelines for the chemical, oil, and gas industries / API RP 582, 2009
Welding practice for the Sandvik duplex stainless steel SAF2304, SAF2205 and SAF2507/Sandvik SMT, 1995
Physical metallurgy and some characteristic properties of the Sandvik duplex stainless steel / Sandvik SMT, 1994
Mechanical properties if Sandvik duplex stainless steels / Sandvik SMT, 1994
Mechanical properties of Sandvik SAF2205 and Sandvik SAF2507 / Sandvik SMT, 2004
Duplex Stainless Steels, Microstructure, Properties and Applications / Abington Publishing, 1997
Practical Guidelines for the fabrication of Duplex Stainless Steels / IMOA, 2009
Prediction and assessment of ammonium bisulfide corrosion under refinery sour water service conditions / NACE
Paper No.06576
DETERMINATION OF SUSCEPTIBILITY TO INTERGRANULAR / MEHMET EMIN ARIKAN, 2008
Corrosion handbook stainless steel / Sandvik SMT
NACE MR0103-2010, 0175-2007, SP0472-2010, ASTM E140
What does the pitting resistance equivalent really tell us? / J.H. CLELAND
The Relationship between Chromium Nitride and Secondary Austenite Precipitation in Duplex Stainless Steels /
A.J. RAMIREZ, J.C. LIPPOLD, and S.D. BRANDI, 2003
The physical metallurgy of duplex stainless steels / Sandvik SMT
Phase transformations in duplex steels and the relation between continuous cooling and isothermal heat
treatment / Sandvik SMT, 1991
Chloride-induced stress corrosion cracking of duplex stainless steels… / Sandvik SMT, 1994
Hydrogen embrittlement of duplex grades UNSS32750 and UNS S31803 in connection with cathodic protection in
chloride solutions / Sandvik SMT, 1997
이상 스테인리스강의 내식성 및 기계적 성질 / 한국과학기술원, 1999
Prediction and assessment of Ammonium bisulfide corrosion… / NACE paper 06576