mechanical properties, chemical analysis, physical properties and weldability of duplex stainless steel

1. CHALLENGING PROJECTS. IT’S WHAT WE DO.
Guide Lines for (Duplex) Stainless
Steel Welding
Zhuang Xu
Welding Engieer
McDermott Wuchuan Offshore Engineer Co., Ltd

2. Presentation Overview
 Duplex Stainless Steel
 Alloying Properties
 Piping Welding
2

3. Duplex Stainless Steel
 Lean duplex stainless steel
 Standard duplex stainless steel
 Superduplex stainless steel
 Hyperduplex stainless steel
3

4. Lean duplex stainless steel
 Lean duplex such as 2304, which contain no deliberate
Mo addition.
 The lean alloy 2304 was developed to compete primarily
with the austenitic AISI 316 grade, but with twice the
yield strength and significantly better resistance to SCC.
 The weldability of 2304 duplex stainless is generally
good when using slightly over-alloyed filler metal.
 The newly developed lean duplex garde LDX2101 has
such improved weldability that also autogenous welding
is possible and this material has contributed to the boom
in the lean duplex market.
4

5. Standard Duplex Stainless Steel
 Standard duplex stainless steel is the dominant
commercial duplex stainless steel which was developed
in the 1970s, but was later optimised with higher nitrogen
levels for improved weldability.
 The PRE of 2205 is about 33-35 resulting in a resistance
to localized corrosion intermediate between the
austenitic grade AISI 317 and the 5-6% Mo super
austenitic alloys.
 The weldability of this grade is good, but overmatching
filler with increased nickel content, e.g. 2209, is normally
required for optimum weld metal properties.
5

6. Superduplex stainless steel
 The superduplex grades were developed to withstand very
aggressive environments to compete with super-austenitics
and nickel base alloys.
 2507 has, due to high molybdenum and nitrogen contents, a
PRE of 42-43, and offers high mechanical strength and
corrosion resistance in extremely aggressive environments
such as chloride-containning acides.
 A consequence of the high alloy content, there is a risk of
precipitation of intermetallic phases, limiting the heat input
and interpass temperatures when multipass welding.
 Overmatching filler with increased nickel content is required,
e.g. 2509, to compensate element partitioning for optimum
corrosion resistance.
6

7. Hyperduplex stainless steel
 Hyperduplex stainless steel was developed as a
complement to 2507 with increased strength for use in
even more aggressive conditions, such as in hot
seawater, acidic chloride solutions and organic acides.
 SAF2707 HD can be welded with a matching filler wire of
ISO2795L type.
 Due to the high alloying content, the hyperduplex alloys
are somewhat more sensitive to secondary phase
precipitation than the superduplex grades.
7

8. Alloying Properties
 Elements content of different types of duplex stainless
steel
 Chromium
 Nickel
 Molybdenum
 Nitrogen
 Manganese
 Copper
 Tungsten
 Carbon
 Mechancial Properties
 Physical Properties
8

9. Duplex Stainless Steel Types
Type Cr Ni Mo N PRE
Lean 20-24% 1-5% 0.1-0.3% 0.1-0.22% 24-25
Standard 21-23% 4.5-6% 2.5-3.5% 0.1-0.22% 33-35
Superduplex 24-29% 4.5-8% 2.7-4.5% 0.1-0.35% >40
Hyperduplex 27% 6.5% 5% 0.4% 49
9

10. Chromium
 There is a maximum limit to the chromium content of
approximately 30%, where intermetallic phase precipitation
can markedly reduce the ductility, toughness and corrosion
resistance of these alloys.
 Chromium increases the pitting potential, the critical pitting
temperature(CPT) and the critical crevice
temperature(CCT), and improves the passive film stability
in acidic environments.
10

11. Nickel
 Nickel is a strong austenite stablizer and is a principal
addition to austenitic stainless steels.
 Nickel alloying is generally detrimental to crevice corrosion
resistance in sodium chloride, and beneficial or without
effect in pitting tests.
 In the duplex stainless steel, however, the main role of
nickel is to maintain the ferrite-austenite balance, rather
than modifying the corrosion resistance.
 Low nickel levels can result in formation of a high
proportion of ferrite, thereby lowering toughness.
Consequently most consumables for welding duplex
stainless steels are over-alloyed to contain 7-10% of nickel.
11

12. Molybdenum
 Molybdenum is a ferrite stabilizing alloying element with a
strong beneficial influence on general and pitting corrosion
resistance and on the passivation properties.
 Molybdenum is favourable in most environments, but in
strongly oxidising environments, such as warm concentrated
nitric acid, grades containing molybdenum are less resistant
than stainless steels without molybdenum.
 The addition of molybdenum should not exceed
approximately 4% since it makes the material more
susceptible to intermetallic phase precipitation by widening
the sigma phase field.
12

13. Nitrogen
 Nitrogen is an interstitial element that stabilizes the austenite
and has strong influence on several properties such as
pitting corrosion, presence of molybdenum.
 The duplex grades consequently contain up to 0.4% nitrogen
to give improved austenite formation when welding.
 Nitrogen significantly increases the strength of the duplex
stainless steels, but also improves the ductility and
toughtness of the alloy.
 Nitrogen delays the formation of intermetallic phases in
duplex stainless steels in a similar way as in austenitic
grades.
13

14. Manganese
 Manganese stabilises austenite and can partly replace
nickel.
 Additions to stainless steel have been used to increase the
solubility of nitrogen, which have a strong beneficial influence
on the pitting resistance. It has been reported that
manganese itself has a negative effect on the pitting
resistance, but combined additions of nitrogen and
molybdenum override this effect.
 Replacing nickel with manganese and nitrogen makes the
price of the material more stable since the nickel price has
fluctuated significantly.
14

15. Copper
 Copper is added to highly corrosion resistant austenitics and
duplex grades to further improve the corrosion resistance in,
for instance reducing acids such as dilute sulphuric acid.
15

16. Tungsten
 Tungsten has become more commonly used as an alloying
element in commercial stainless steels where it is used as a
compliment to molybdenum for improved corrosion
resistance.
 When used in the PRE expression, the factor for tungsten is
approximately half of that for molybdenum.
 Tungsten has, howerver, also been reported to promote
formation of intermetallic phases and cause a more rapid
embrittlement thatn molybdenum.
16

17. Carbon
 In most modern duplex alloys carbon is limited to levels of
0.03wt% to minimize the risk of formation of chromium
carbides and thereby reduce the susceptibility of the duplex
stainless steels to intergranular corrosion.
17

18. Mechanical Properties
Type Yield Tensile Elong.
ASTM A572. 50ksi (345MPa) 70ksi (485MPa) 21%
Gr.50
316L 25ksi (170MPa) 70ksi (485MPa) 40%
S32304 58ksi (400MPa) 87ksi (600MPa) 25%
S32205 65ksi (450MPa) 90ksi (620MPa) 25%
S32750 80ksi (550MPa) 116ksi (800MPa) 15%
18

19. Physics Properties
Average
Specific Heat Coefficient of Thermal
Type Density Resistivity Magnetism Capacity Linear Expansion Conductivity
/g.cm-3 /μΏ.cm / J / KG K /10-6˚C-1(0-100) /W(mK)-1
A572. Gr.50 7.64 0.10 YES 447 12.1 (100.C) 51(100。C)
316L 7.98 0.75 No 502 17.3(100.C) 16.3(100。C)
S32304 7.75 0.80 ≤YES 482 13.0(100.C) 17.0(100。C)
S32205 7.80 0.80 ≤YES 500 13.0(100.C) 17.0(100。C)
S32750 7.79 0.80 ≤YES 485 13.0(100.C) 17.0(100。C)
19

20. Pipe Welding
 General Welding Guidelines
 Welding Procedure Qualification
 Welding Methods
 Post Fabrication Clean-up
20

21. General Welding Guidelines
 Differences Between Duplex and Austenitic Stainless Steels
 Selection of Base Metal
 Material Receiving
 Handling & Storage
 Facilities
 Tools
 Clean Build Philosophy
 Cutting Duplex Stainless Steel
 Joint Design
 Preheating
 Heat Input and Interpass Temperature
 Postweld Heat Treatment
 Desired Phase Balance
21

22. Diffences Between ASS & DSS
 Problem of ASS
 Hot cracking
 Adusting the composition of the filler metal to provide a significant
ferrite content minimizes these problems, for the more highly
alloyed austenitic SS where the use of a nickel-base filler metal is
necessary and austenitic solidification is unavoidable.
 The problem is managed by low heat input, often requiring many
pases to build up the weld.
 Methodes to improve the hot cracking resistance
 Limit the content of sulfur, phosphorus and carbon
 Produce duplex microstructure, ferrtie content is 3 ~ 8%
 Add proper content of manganese (4 ~ 6%)
 Properly welding parameters (short arc, low heat input and narrow
gap)
22

23. Diffences Between ASS & DSS
 Problem of DSS
 HAZ problem
 DSS have very good hot cracking resistance due to the high ferrite content.
 The HAZ problems are loss of corrosion resistance, toughness, or post-weld
cracking.
 To avoid these problems, the welding procedure should focus on minimizing
total time at temperature in the “red hot” range rather than managing the
heat input for any one pass.
 Advantage of DSS Summrized Properties
 Much higher yield strength and tensile strength
 Stress corrosion cracking resistant
 Pitting/crevice corrosion resistant
 Erosion resistant
 Fatigue resistant
 Cost effective (lower nickle contents)
23

24. Selection of Base Metal
 Sufficient Nitrogen
 The important of the base metal containing sufficient
nitrogen has been repeatedly emphasized.
 If the starting material cooled slowly through the 700 to
1000 deg. Range, or if it is allowed to air cool into this
range for a minute or so prior to water quenching then
these actions have used up some of the “time on the
clock”
 Purchasing Guidelines for Duplex 22% Cr Stainless Steel Process
Tubing & Piping
24

25. Selection of Base Metal
 Purchasing Guidelines for Duplex 22% Cr Stainless Steel Process
Tubing & Piping
Product Form Duplex 22% Cr Stainless Steel Duplex 22% Cr Stainless Steel
Process Tubing Seamless Pipe
Material Process Tubing Seamless Pipe
Scope This material technical sheet is intended to supplement existing material standards and
specification (eg., ASTM, ISO UNS) and project specifications for duplex stainless steel
Manufacturer Approval Materials supplied in accordance to this specification shall only be supplied by company
approved manufacturers
Specification ASTM A789/ A789M ASTM A790/ A790M
Grade(s) UNS S31803 UNS S31803
Manufacturing Process Electric arc or electric furnace and refined by AOD or equivalent process
Heat Treatments All material shall be delivered in the solution annealed (followed by water quench) condition
Pitting Resistance Equivalent Pitting Resistance Equivalent (PRE)=Cr+3.3Mo+16N. The PRE shall be greater than or
equal to 34.0
25

26. Selection of Base Metal
 Purchasing Guidelines for Duplex 22% Cr Stainless Steel Process
Tubing & Piping
Product Form Duplex 22% Cr Stainless Steel Duplex 22% Cr Stainless Steel
Process Tubing Seamless Pipe
Per ASTM standard. If no harness required by ASTM standard, maximum shall be HRC28,
Hardness HB271, or HV290
- Charpy V-notch (ASTM A370) - Charpy V-notch (ASTM A370)
Impact Testing -Not applicable when the maximum -Not applicable when the maximum
obtainable charpy specimen has a width obtainable charpy specimen has a width
along the notch of less than 2.5mm along the notch of less than 2.5mm
-Absorbed energy shall be 48J average and -Absorbed energy shall be 48J average and
36J single value minimum 36J single value minimum
-Test temperature shall be minus 46 deg. -Test temperature shall be minus 46 deg.
-Specimens shall be oriented transverse to
the rolling direction
26

27. Selection of Base Metal
 Purchasing Guidelines for Duplex 22% Cr Stainless Steel Process
Tubing & Piping
Product Form Duplex 22% Cr Stainless Steel Duplex 22% Cr Stainless Steel
Process Tubing Seamless Pipe
ASTM E562 ASTM E562
Ferrite Content -Ferrite content shall be determined oon a -Ferrite content shall be determined oon a
full cross section near the OD and ID full cross section near the OD and ID
surfaces and at mid-wall location surfaces and at mid-wall location
-Samples shall be electrolytically etched in -Samples shall be electrolytically etched in
either NaOH or KOH, and in such a manner either NaOH or KOH, and in such a manner
as to provide optimum contrast for austenite as to provide optimum contrast for austenite
and ferrite phase discrimination and ferrite phase discrimination
-Point cont shall be conducted at minimum -Point cont shall be conducted at minimum
of 500X magnification of 500X magnification
-A minimum of 30 fields and 16 points per -A minimum of 30 fields and 16 points per
field shall be used field shall be used
-Ferrite content shall be between 35%- 55% -Ferrite content shall be between 35%- 55%
-Ferrite content of the seam weld shall be
25%-60%
27

28. Selection of Base Metal
 Purchasing Guidelines for Duplex 22% Cr Stainless Steel Process
Tubing & Piping
Product Form Duplex 22% Cr Stainless Steel Duplex 22% Cr Stainless Steel
Process Tubing Seamless Pipe
-Samples shall be etched using ASTM E407 etchant number 98 (K3Fe(CN)4 with KOH or
Metallographic NaOH)
Examination -Sample cross section shall be examined at OD, ID and Mid-wall locations
-Examination shall be conducted at a minimum of 500X magnification.
-Intermetallic phases or precipitates are allowed up to a max. of 0.05 percent.
-All testing shall be conducted on a lot basis. - All testing shall be conducted on a lot
Extent of Testing A lot is defined as the same tubing diameter, basis. A lot is defined as a maximum of 60
thickness, heat and heat treatment charge, meters of pipe of the same diameter,
up to a maximum of 125 tubes. thickness, heat and heat treatment charge.
28

29. Selection of Base Metal
 Purchasing Guidelines for Duplex 22% Cr Stainless Steel Process
Tubing & Piping
Product Form Duplex 22% Cr Stainless Steel Duplex 22% Cr Stainless Steel
Process Tubing Seamless Pipe
-10% of all tubes shall have ferrite content determined by fischer ferrite scope
Non-destructive -Measurement technique shall be in accordance with company approved procedures
Ferrite -Ferrite content shall be between 35% - 55%
Measurement
-In accordance with ASTM A789/ A789M - In accordance with ASTM A790/A790M
Hydrostatic Test and ASTM A450/ 450M
-Weld repair of defects is not allowed
Repair of Defects
-White Pickled
Surface finish
29

30. Selection of Base Metal
 Purchasing Guidelines for Duplex 22% Cr Stainless Steel Process
Tubing & Piping
Product Form Duplex 22% Cr Stainless Steel Duplex 22% Cr Stainless Steel
Process Tubing Seamless Pipe
-Product shall be handled, shipped and stored in such a manner as to prevent or minimize
Handling, Shipment, and the possibility of free iron contamination
Storage -Product shall not be handled with bare steel hooks, chains or lifting forks without the use of
protective insulating material
-Only stainless steel wire brushes, designated for use only on stainless steel products, may
be used for brushing and descaling
-Suspected free iron contamination, such as evidenced by unusual stains or discoloration,
shall be verified by ferroxyl testing in accordance with the procedure outlined in ASTM
A380
-Free iron contaminated areas shall be cleaned and passivated, at the manufacturer’s
expense, using the procedures outlined in ASTM A380
-EN10204 – 3.1B
Certification
30

31. Material Receving
 Material certificate review
 Physical check including magnetic check or PMI as
required.
 Application of Material Traceability Number (MTN)
 To use chloride-free pen/marker suitable for stainless
steel
31

32. Handling & Storage
 All pipe/tube material will be store in a covered
warehouse
 Separate area for storage equipped with sign board
 To use wood, rubber vinyl or cardboard for protection
from handing devices
 To use only web sling for lifting
 Placed upon non-carbon steel surface
 Plastic end caps on pipe/tube
 Plastic cover on pipe/tube surfaces.
32

33. Facilities
 Clean area with metallic iron dust free environment
 Non-carbon steel covering for all work surfaces
 To put neoprene rubber cover on flange end
 To put plastic end cap(pipe), prior to leave work.
33

34. Tools
 Stainless steel compatible files, grinding disc, wire
brushes, saw blades etc.
 Cold cutting & beveling machine
34

35. Clean Build Philosophy
 Prevention is the Key 预防是关键
 Professionalism is the Perception 专业技巧是方法
 Product Quality is the Result 产品质量是结果
35

36. Clean Build Philosophy
 Pre-fabrication Storage 不锈钢管焊接组装前的储存
Polythene or Tarpaulin Sheeting聚乙烯薄膜或防水帆布覆盖
Woods/ Rubber Racks木质或橡胶支垫
All Pipes/ Tubes to be Fitted with End Caps所有管子都需装有合适的端盖
36

37. Clean Build Philosophy
 After Cut Before Beveling切割之后开坡口之前
Clean Cloth Dampened with Acetone
用丙酮浸湿的无纺布清洁管子内部
Pull Through to Remove Dust avoid contamination
String绳子 “Pull Through” After Cut and Before Beveling
Acetone 丙酮
Acetone
37

38. Clean Build Philosophy
 “Sponge Plug” Insertion before Beveling 开坡口前插入海
绵塞子
 Constructed from pre-cut foam which is wrapped in Lint
Free Cloth 海绵塞子由无棉绒的泡沫构成
 Each Sponge Plug will have a unique ident and shall be
accounted for at the end of each shift 每一个海绵塞子都应
有一个独立的编号，以防止替换时混淆
 Plug must be removed before fit-up and returned to control
point 定位焊之前，海绵塞子应被移除，完整后重新返回
Sponge Plug海绵塞子
Prevent Dust / Contamination from Entering 8
8
38

39. Clean Build Philosophy
 Clean with Acetone BEFORE Beveling and Polishing 开
坡口和打磨之前应用丙酮清理
Min 25mm
Sponge Pluge
Acetone
39

40. Clean Build Philosophy
 Clean with Acetone AFTER Beveling 开坡口后应用丙酮
清理
Sponge Pluge
Acetone
40

41. Clean Build Philosophy
 Ploshing 打磨
 Remove all surface oxide films from both internal and
external surfaces to a minimum distance of 25mm using
the correct and autorized equipment 用允许的工具将管子
内外径距离坡口位置至少25mm内地氧化物移除
 Sponge plug MUST still be in place 海绵塞子必须保持在原
位置 Min 25mm
Acetone
41

42. Clean Build Philosophy
 Fit-Up 组对（定位焊）
 After sponge plug removal and final clean with Acetone
dampened cloth, commence Fit-Up and Tack in accordance with
the approval WPS 在最后清洁完成并移除海绵塞子后，开始根据
相关的焊接工艺规程进行组对
 Sponge Plug MUST be returned to control point after removal
from pipe or fitting. 组对完成之后，海绵塞子必须返回原位置
 Every registered “Sponge Plug” shall be accountable at the end
of each shift. 每个带有编号的海绵塞子在下一次使用前是可控的
42

43. Clean Build Philosophy
 Post Weld Clean 焊后清理
 Clean Using dedicated wire brush during and immediately after
welding 利用合适的刷子在焊接过程中或焊接完成后进行清理
43

44. Clean Build Philosophy
 Clean Using Acetone Dampened Cloth 利用丙酮浸湿的
的无纺布清理管径内部
Acetone
44

45. Clean Build Philosophy
 Protect the weld & HAZ by wrapping cellophane 利用包
裹的玻璃纸来保护焊接区和热影响区
Wrapping Cellophane包裹的玻璃纸
45

46. Cutting Duplex Stainless Steel
 Mechanical
 Sawing
 Shearing
 Abrasive Wheel Cutting
 Water-Jet Cutting
 Thermal
 Plasma cutting
 Laser cutting
46

47. Sawing
 Similar with austenitic stainless steel
 Powerful machine
 Proper blade alignment
 Coarse toothed blade
 Slow to moderate cutting speed
 Heavy feed
 Generous flow of coolant
47

48. Shearing
 More force and heavier equipment will be required to
shear stainless steel compared to carbon steel
 Carbon steel – ½” thickness shear limit
 Austenitic stainless steel – ¼” max thickness shear limit
 Duplex stainless steel 3/16” max.
 A general clearance guide is to use a clearance of 5% of
the metal thickness between shear knives
 To counter the shearing force required for duplex
stainless steel, the hold down pressure on the clamps
may have to be increased
 Blades must be sharp
48

49. Abrasive Cutting
 Abrasive wheels, rotating at high
speed can be used for straight line
cutting of sheet and thin gauge
plate and for cut-off operations on
relative small sections.
 Thick section cut-off operations are
usually done wet
 Use uncontaminated vitrified or
resin-bonded wheels
 Do not induce over-heating
49

50. Plasma and Laser
 Same equipment as for 304/316
 Optimum parameter vary slightly
 Two types of plasma cutting machine
 Mainly advantages of second one:
 The nozzle can be recessed within a ceramic shield gas,
thereby protecting the nozzle from double arcing, if no
shield gas were present, the ceramic shield gas cup could
be deteriorated because of the high radiative heat
produced by the plasma jet.
 It can protect the cutting surface from oxidation caused by
oxygen.
50

51. Two types of plasma cutting
a. Dual flow plasma cutting power source b. Dual flow plasma cutting machine
c. Cutting surface without oxidation d. Cutting gas
d. Secondary shielding gas e. The sketh of dual flow plasma arc
51

52. Joint Design
52

53. Preheating
 Preheating may be only beneficial when used to
eliminate moisture from the steel as may occur in cold
ambient conditions or from overnight condensation.
 When preheating to deal with moisture, the steel should
be heated to about 100 deg. Uniformly and only after the
weld preparation has been cleaned.
53

54. HI and Interpass Temperature
 Compare with ASS
 DSS can tolerate relatively high HI
 DSS is resistant to hot cracking
 DSS is with higher thermal conductivity and lower
coefficient of thermal expansion
 Exceedingly Low Heat Input
 May result in fusion zones and HAZ which are excessively
ferritic with corresponding loss of toughness and corrosion
resistance.
54

55. HI and Interpass Temperature
 Exceedingly High Heat Input
 Increasing the danger of forming intermetallic phases.
 150 deg of maximum interpass temperature for lean and
standard DSS, 100 deg for SDSS
 To avoid problems in the HAZ, the weld procedure should
allow rapid cooling of this region after welding.
 The temperature of the work piece is important because it
provides the largest effect on cooling of the HAZ.
55

56. PWHT
 PWHT is not need
 It is likely to be harmful because heat treatment may
precipitate intermetallic phases or alpha prime
embrittlement casuing a loss of toughness and corrosion
resistance.
 PWHT temperature in excess of 315 deg can adversely
affect the toughness and corrosion resistance of DSS.
 ANY PWTH should include full solution annealing followed
by water quenching.
56

57. Desired Phase Balance
 Phase Balcance for Ferrite and Austenite
 It generally agreed that the characteristic benefits of DSS
are achieved when there is at least 25% ferrite with the
balance austenite.
 Normally the pahse balance has been adjusted toward
more austenite to provide improved tougness, offsetting
the loss of toughness associated with oxygen pickup from
the flux.
 The phase balance in the HAZ, being the original wrought
plate or pipe plus an additional thermal cycle, is usually
slightly more ferritic than the original material.
57

58. Welding Procedure Qualification
 Requirement
 Lab Testing
 Tensile
 Bend
 Hardness
 Macro-Sections
 Micro-structural Examination
 Impact Tests
 Corrosion Testing
 Ferrite Content
58

59. Welding Procedure Qualification
 Requirement
 Because of the need to limit the total time at temperature
for the HAZ, the properties of duplex grades will be
sensitive to section thickenss and details of actual welding
practice. Therefore, “qualification” must be considered in a
broader sense, that is a demonstration that the welding
procedure that will be applied during fabrication will not
produce an unacceptable loss of engineering properties,
especially toughness and corrosion resistance.
 It would be conservative to qualify the welding procedure
at every thickness and geometry of welding because the
minor differences in setup may be significant in the results
achieved in production.
59

60. Lab Testing
 Tensile
60

61. Lab Testing
 Tensile
61

62. Lab Testing
 Tensile
 For pipe having an outside diameter of 3in. (75mm) or
less, reduced-section specimens conforming to the
requirment given in below figure.
62

63. Lab Testing
 Side Bend
63

64. Lab Testing
 Root & Face Bend
64

65. Lab Testing
 Guided-Bend Test Procedure
65

66. Lab Testing
 Guided-Bend Test Procedure
66

67. Lab Testing
 Hardness (NORSOK STANDARD M-601)
67

68. Lab Testing
 Macro-Examination (ASME IX - 2010)
 The examination of the cross sections shall include only
one side of the test specimen at the area where the plate
or pipe is divided into sections, adjacent faces at the cut
shall not be used.
 Acceptance creteria
 Visual examination of the cross sections of the weld metal
and heat-affected zone shall show complete fusion and
freedom from cracks
 There shall be not more than 1/8in. (3mm) difference in the
length of the legs of the fillet.
68

69. Lab Testing
 Micro-structural Examination
 Acceptance Criteria
 The micro-structure shall be suitably etched and examined at
400 X magnification and shall have grain boundary with no
continuous precipitations and the inter-metallic phases,
nitrides and carbides shall not in total exceed 0.5%
69

70. Lab Testing
 Impact Test
 Requirement
 Impact testing of welds shall be according to following table,
full size specimens shall be applied where possible.
 If two types of materials are welded together, each side of
the weld shall be impact tested and fulfill the requirement for
the actual materal.
 The weld metal shall fulfil the requirement for the least
stringent of the two.
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71. Lab Testing
 Impact Test
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72. Lab Testing
 Corrosion Testing
 The test specimen shall have a dimension of full wall
thickness by 25mm along the weld and 50mm across the
weld. The test shall expose the external and internal
surface and a cross section surface including the weld
zone in full wall thickness. Cut edges shall be prepared
according to ASTM G48. The specimen shall be pickled
(20%HNO3 + 5% HF, 60 deg., 5min). The exposure time
shall be 24 hours.
 The test temperature shall be 40 deg.
 The acceptance criteria
 No pitting at 20X magnification
 Weight loss shall not exceed 4.0 g/m2
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73. Lab Testing
 Ferrite Content
 Acceptance Criteria
 For the stainless steel Type 22 and 25 Cr duplex the ferrite
content in the weld metal root and in the last bead of the weld
cap shall be determined in accordance with ASTM E562 and
shall be in the range of 30% to 70%.
Austenite
Ferrite
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74. Welding Methods
 GTAW (Gas Tungsten Arc Welding)
 SMAW (Shielded Metal Arc Welding)
 GMAW (Gas Metal Arc Welding)
 FCAW (Flux Core Wire Arc Welding)
 SAW
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75. GTAW (Gas Tungsten Arc Welding)
 Equipment
 1. Power source. Transformer/Rectifier
(Constant Amperage type)
 2. Inverter Power Source. (More
compact and portable)
 3. Power Control Pannel (Amp. AC/DC,
gas delay, slop in/out, pulse, etc)
 4. Power cable hose (of a suitable
amperage rating)
 5. Gas flow-meter (correct for gas type
and flow rates)
 6. Tungsten electrodes. (of a suitable
amperage rating)
 7. Torch assemblies. (of a suitable
amperage rating)
 8. Power reture cable. (of a suitable
amperage rating)
 9. Welding Visor (With correct filter glass
rating)
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76. GTAW (Gas Tungsten Arc Welding)
 Torch Head Assembly
 1. Tungsten electrodes
 2. Spare Ceramic Shield
 3. Gas lens
 4. Torch Body
 5. Gas Diffuser
 6.Split copper collett (For
securing the tungsten
electrode)
 7. On/off or latching switch
 8. Tungsten housing
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77. GTAW (Gas Tungsten Arc Welding)
 Types of Arc Start
 Scratch Start
 Easily cause contamination of the tungsten
 High Frequency
 Can avoid the contamination of the tungsten
 Cause interference with hi-tech electrical equipment and
computer systems.
 Lift arc
 Has been developed where the electrode is touched onto the
plate and is withdrawn slightly.
 An arc is produced with very low amperage, which is increased to
full amperage as the electrode is extended to the normal arc
length.
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78. GTAW (Gas Tungsten Arc Welding)
 Slope in and Slope out
 During welding it is used to control the rise and delay of the
current at the start and end of a weld as shown below
 This is very beneficial in avoiding crater pipes at the end of
weld runs.
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79. GTAW (Gas Tungsten Arc Welding)
 Gas cut off delay
 The gas cut off delay control delays the gas solenoid shut off
time at the end of the weld and is used to give continued
shielding of the solidifying and cooling weld metal at the end of
a run.
 It is often used when welding materials that oxidise at high
temperatures such as stainless and titanium alloys.
79

80. GTAW (Gas Tungsten Arc Welding)
 Electrode
 The most common types of tungsten used are thoriated or
ceriated for DC and zironiated with AC (Aluminium alloys).
 The vertex angle of the tungsten is often a procedural
parameter and therefore gringding needs to be very controlled
activity that should be carried out on a dedicated grinding
wheel.
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81. GTAW (Gas Tungsten Arc Welding)
 Filler Metal
 Matching with 2 – 4% more nickel than in the wrought product.
 The nitrogen content is typically slightly lower in the filler metal
than in the base metal.
 More highly alloyed DSS fillers are suitable for welding the
lower alloyed DSS.
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82. GTAW (Gas Tungsten Arc Welding)
 GTAW (Gas Tungsten Arc Welding)
 Shielding
 The purity of dry welding grade of inert gas, argon, should
equal or better than 99.95%.
 Gas flow should be initiated several seconds ahead of
striking the arc, and it should be maintained for several
seconds after the arc is extingushed, ideally long enough for
the weld and HAZ to cool below the oxidation range of the
SS.
 For electrode coverage, suggested flow rates are 12 – 18
l/min (0.4 – 0.6 cfm) when using a normal gas diffuser screen
(gas lens), and with half that rate required for a normal gas
nozzle.
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83. GTAW (Gas Tungsten Arc Welding)
 GTAW (Gas Tungsten Arc Welding)
 Shielding
 Purging gas, because argon is heavier than air, the feed
should be from the bottom to the top of the enclosed
volume, with purging by a minimum of seven times the
volume.
 Additions of up to 3% dry nitrogen will aid in retention of
nitrogen in the weld metal, particularly of the more highly
alloyed duplex stainelss steel. While the nitrogen adddition
has been found to increase electrode wear, the addition of
helium partially offsets this effect.
 Additions of oxygen and carbon dioxide to the shielding gas
should be avoided because they will reduce the corrosion
resistance of the weld.
 Hydrogen should not be used in the shielding or backing gas
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84. GTAW (Gas Tungsten Arc Welding)
 Technique and Parameters
 Any arc strikes outside of the welding zone will creat local
points of autogenous welding with very high quench rates,
ruslting in locally high ferrite content and possible loss of
corrosion resistance at those points.
 Tacking welds should be made with full gas shielding.
There should be no tack weld at the starting point of the
root pass.
 The work piece should be allowed to cool below 150 deg
for standard duplex stainless steels and below 100 deg for
superduplex stainless steels between welding passes to
provide for adequate cooling of the HAZ in subsequent.
84

85. GTAW (Gas Tungsten Arc Welding)
 Technique and Parameters
 The heat input is typically in the range of 0.5 – 2.5 kj/mm
(15 to 65 kj/inch).
 General heat input recommendations:
 2204 or lean duplex 0.5 – 2.0 KJ/mm (15 – 50 KJ/in)
 2205 0.5 – 2.5 KJ/mm (15 – 65 KJ/in)
 2507 0.3 – 1.5 KJ/mm (8 – 38 KJ/in)
85

86. GTAW (Gas Tungsten Arc Welding)
 Weld Discoloration Levels on Inside of ASS
 AWS D18.2 – 1999 Guide to Weld Piscoloration Levels on
Inside of Austenitic Stainless Steel Tubes
 No. 1- 10 ppm No. 2-25 ppm No. 3-50 ppm
 No. 4-100 ppm No. 5-200ppm No. 6- 500ppm
 No. 7- 1000ppm No. 8-5000ppm No. 9-12500ppm
 No.10-25000ppm
86

87. Post Fabrication Clean-up
 Crayon marks, paint, dirt, oil
 Embedeed iron (ferrous contamination)
 Weld spatter, weld discoloration, flux, slag, arc strikes
Typical fabrication defects or surface conditions which may be encountered
87

88. Crayon marks, paint, dirt, oil
 All these surface contaminants can act as crevices and
can be initiation sites for pitting or crevice corrosion of a
stainless steel.
 These contamination should be removed with solvents.
88

89. Embedded Iron (Ferrous Contamination)
 The iron rusts in a moist or humid environment and can
initiate corrosion on the stainless steel surface.
 One approach is to avoid all contact between stainless
steel and carbon steel. Only stainless steel
tools, stainless steel wire burshes, stainless steel
clamps, and new, uncontaminated grinding wheels
should be used on stainless.
 Often the tools are color coded in the shop.
89

90. Weld Spatter, weld
discoloration, flux, slag, arc strikes
 All these defects may occur during welding. They can act
as crevices and initiate crevice corrosion in chloride-
containing environments.
 Welding spatter
 Weld spatter can be avoided during fabrication by using an
anti-spatter compound
 Weld discoloration
 Weld discoloration causes a loss of corrosion resistance
due to the destruction of the passive layer.
 Heavy weld discoloration or heat tint should be avoided by
inert gas shielding and by pruging the back side of welds
with an inert gas.
 Heat tint cannot be totally avoided and must be removed
during postweld clean-up.
90