Guide Lines For Duplex Stainless Steel Welding

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

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  • Argon-Oxygen Decarburization (AOD)What?A process for further refinement of stainless steel through reduction of carbon content. Why?The amount of carbon in stainless steel must be lower than that in carbon steel or lower alloy steel (i.e., steel with alloying element content below 5%). While electric arc furnaces (EAF) are the conventional means of melting and refining stainless steel, AOD is an economical supplement, as operating time is shorter and temperatures are lower than in EAF steelmaking. In addition, using AOD for refining stainless steel increases the availability of the EAF for melting purposes.How?Molten, unrefined steel is transferred from the EAF into a separate vessel. A mixture of argon and oxygen is blown from the bottom of the vessel through the melted steel. Cleaning agents are added to the vessel along with these gases to eliminate impurities, while the oxygen combines with carbon in the unrefined steel to reduce the carbon level. The presence of argon enhances the affinity of carbon for oxygen and thus facilitates the removal of carbon.
  • Guide Lines For Duplex Stainless Steel Welding

    1. 1. CHALLENGING PROJECTS. IT’S WHAT WE DO.Guide Lines for (Duplex) Stainless Steel Welding Zhuang Xu Welding EngieerMcDermott Wuchuan Offshore Engineer Co., Ltd
    2. 2. Presentation Overview  Duplex Stainless Steel  Alloying Properties  Piping Welding2
    3. 3. Duplex Stainless Steel  Lean duplex stainless steel  Standard duplex stainless steel  Superduplex stainless steel  Hyperduplex stainless steel3
    4. 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. 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. 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. 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. 8. Alloying Properties  Elements content of different types of duplex stainless steel  Chromium  Nickel  Molybdenum  Nitrogen  Manganese  Copper  Tungsten  Carbon  Mechancial Properties  Physical Properties8
    9. 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% 499
    10. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 20. Pipe Welding  General Welding Guidelines  Welding Procedure Qualification  Welding Methods  Post Fabrication Clean-up20
    21. 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 Balance21
    22. 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. 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. 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 & Piping24
    25. 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 34.025
    26. 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 direction26
    27. 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. 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. 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 finish29
    30. 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 Certification30
    31. 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 steel31
    32. 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. 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. 34. Tools  Stainless steel compatible files, grinding disc, wire brushes, saw blades etc.  Cold cutting & beveling machine34
    35. 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. 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. 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 Acetone37
    38. 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 8 838
    39. 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 Acetone39
    40. 40. Clean Build Philosophy  Clean with Acetone AFTER Beveling 开坡口后应用丙酮 清理 Sponge Pluge Acetone40
    41. 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 Acetone41
    42. 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. 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. 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 Acetone44
    45. 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. 46. Cutting Duplex Stainless Steel  Mechanical  Sawing  Shearing  Abrasive Wheel Cutting  Water-Jet Cutting  Thermal  Plasma cutting  Laser cutting46
    47. 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 coolant47
    48. 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 sharp48
    49. 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-heating49
    50. 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. 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 arc51
    52. 52. Joint Design52
    53. 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. 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. 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. 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. 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. 58. Welding Procedure Qualification  Requirement  Lab Testing  Tensile  Bend  Hardness  Macro-Sections  Micro-structural Examination  Impact Tests  Corrosion Testing  Ferrite Content58
    59. 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. 60. Lab Testing  Tensile60
    61. 61. Lab Testing  Tensile61
    62. 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. 63. Lab Testing  Side Bend63
    64. 64. Lab Testing  Root & Face Bend64
    65. 65. Lab Testing  Guided-Bend Test Procedure65
    66. 66. Lab Testing  Guided-Bend Test Procedure66
    67. 67. Lab Testing  Hardness (NORSOK STANDARD M-601)67
    68. 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. 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. 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.70
    71. 71. Lab Testing  Impact Test71
    72. 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/m272
    73. 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 Ferrite73
    74. 74. Welding Methods  GTAW (Gas Tungsten Arc Welding)  SMAW (Shielded Metal Arc Welding)  GMAW (Gas Metal Arc Welding)  FCAW (Flux Core Wire Arc Welding)  SAW74
    75. 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)75
    76. 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 housing76
    77. 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.77
    78. 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.78
    79. 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. 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.80
    81. 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.81
    82. 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.82
    83. 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 gas83
    84. 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. 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. 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.10-25000ppm86
    87. 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 encountered87
    88. 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. 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. 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

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