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Advances in Welding for Sanitary Designs

Advances in Welding for Sanitary Designs. Richard E. Avery Consultant to the Nickel Institute May 17, 2004. Possible Materials. 304L& 316L – used for vast majority of applications 6% Mo or super-austenitic SS Duplex stainless steels Ni-Cr-Mo nickel alloys

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Advances in Welding for Sanitary Designs

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  1. Advances in Welding forSanitary Designs Richard E. Avery Consultant to the Nickel Institute May 17, 2004

  2. Possible Materials • 304L& 316L – used for vast majority of applications • 6% Mo or super-austenitic SS • Duplex stainless steels • Ni-Cr-Mo nickel alloys • Commercially pure titanium

  3. Service Considerations • 304L vs 316L – Mo (2-3%) in 316L improves pitting & crevice cor. resist. • Both sensitive to stress cor. cracking over about 150oF • Duplex SS good resist. to SCC • Higher chlorides, low pH may require 6% Mo SS or Ni-Cr-Mo or titanium

  4. Welding Processes Used • GTAW or TIG- manual- orbital tube welding or automatic sheet • GMAW – MIG, pulsed arc mode • SMAW or covered electrode • Laser welding for manu. of welded tubing

  5. Typical Sanitary Piping Systems • Welded by gas tungsten arc welding (TIG) • Lines designed for CIP • Inside of tube welds often not accessible for grinding or inspection

  6. Manual vs Automatic Orbital Tube Welding • Short projects may favor manual welding • Manual welders better able to accommodate poorer fit-up conditions • Orbital welds have more consistent root weld beads and practically free from heat tint

  7. In response to 3-A Request • AWS D18.1Specification for Welding Austenitic Stainless Steel Tubing Systems in Sanitary (Hygienic) Applications • AWS D18.2Guide to Weld Discoloration Levels on Inside of Austenitic Stainless Steel Tube

  8. Goals of D18.1 & D18.2 • Guidance of judging root welds of tubes from OD appearance • Guides for Procedure & Performance Qualification, Preconstruction Weld Samples • Weld visual acceptance criteria • Illustration of weld discoloration levels

  9. AWS D18 Committee Work • Members – equipment producers, users & general interest groups • 36 weld samples, many with ID defects – examined on OD & ID by 3-A inspectors • Tube with varying levels of weld discoloration • Tube with varying discoloration levels

  10. Welding Qualifications • Welding Procedure Specification (WPS) - for each type of weld • Performance Qualification - to test welder’s ability • Preconstruction Weld Samples (PWS) - 3 welds made by each welder to aid in evaluating production welds

  11. Visual Examination Requirements • OD of welds examined by welder & inspector, to be consistent with WPS • Welds not meeting OD standards examined by borescope or other suitable means

  12. Visual Acceptance Criteria -ID & OD • Welds full penetration • No cracks, undercut, crevices, or embedded or protruding material • Offset not to exceed 10 %

  13. Visual Acceptance Criteriafor External, Non-ProductContact Surface These criteria give confidence that the inside weld surface is acceptable without an internal examination

  14. Non-product contact surface - Maximum concavity

  15. Non-product contact surface- Maximum convexity

  16. Visual Acceptance Criteria for Internal, Product Contact Surface • Max. concavity 0.012 in. • Max. convexity 0.012 in. • Oxide islands (slag spots), not greater that 1/16 in. in diameter & 4 per weld • No excessive heat-tint oxide

  17. AWS D18.2 (1999): Heat Tint Levels on the Inside of Welded 316L Austenitic Stainless Steel Tube The Sample Numbers refer to the amount of oxygen in the purging gas: No.1- 10ppm No.2 - 25ppm No.3 - 50ppm No.4 - 100ppm No.5 - 200ppm No.6 - 500ppm No. 7 - 1000ppm No.8 - 5000ppm No.9 -12500ppm No.10 -. 25000ppm Note: welds on type 304L SS showed no significant difference in heat tint colour from type 316L.

  18. Heat Tint - Acceptance Limits • Acceptable limits could vary with end application service, D18.1 or D18.2 • Typically 5 and greater is unacceptable • An acceptance level should be identified by number rather than ppm of oxygen or by workmanship standards for particular contract

  19. Factors Influencing Heat Tint • Oxygen in backing gas increases HT • Moisture in backing gas increases HT • Contaminants such as hydrocarbons increase discoloration • Hydrogen in backing gas decreases HT • Metal surface finish can affect appearance

  20. AWS D18.3 (Pending)Specification for Welding Tanks, Vessels, and Other Equipment in Sanitary (Hygienic) Applications • Welding Procedure & Performance Qual. • Visual Examination Acceptance Criteria:- reject defects; cracks, lack of penetration etc- acceptable & unacceptable weld profiles prior to weld finishing- annex – Weld & Adjacent Zone Finishes – WF-1 (as-welded) ~ WF-8 (ground flush & electropolished)

  21. 6% Mo or Superaustenitic SS • Typically: 21 Cr, 24 Ni, 6 Mo, 0.2 N • Areas for 6% Mo not handled by 316- high chlorides ~ over 1000 ppm- low pH environments- where better pitting, crevice and stress corrosion cracking resistance is required

  22. Welding 6% Mo SS • Use over-alloyed filler metal – minimum of 9% Mo Ni-Cr-Mo alloy • GTAW welding procedures similar to that for 304/316 except:- preferably avoid autogenous welds to avoid lower corrosion resistance - somewhat lower heat input and interpass temperature

  23. What are Duplex Stainless Steel? Low-carbon stainless steels containing approx. equal parts of ferrite and austenite from a balance of ferrite formers (Cr,Mo) with austenite formers (Ni,N) and heat treatment

  24. Duplex Stainless Steel Base Metal Upper Right, Weld Metal Bottom Left Source:The ESAB Group

  25. Duplex SS – alloy 2205 • Typically: 22 Cr, 5 Ni, 3 Mo, O.15 N • Structure is austenite islands in ferritic matrix ~ 50/50 is ideal • Higher strength – YS 2 to 3 times 316- forming requires greater power- more spring-back during forming

  26. Duplex SS – (cont.) • Stress corrosion cracking resistance substantially better than 304/316 • Pitting & crevice cor. Resistance equal or better than 316 in many media • Good resistance to erosion & abrasion

  27. DSS Welding - General Requirements • No preheat – 300F interpass typical • Heat input 15 to 65 kJ/in. • To avoid high ferrite in welds, filler metals with higher nickel used ~ 2209 with 9% nickel • Avoidance of arc strikes, oxidation, grinding out of craters

  28. GTAW Process - DSS • Used for root passes and orbital welds • Filler essential for ferrite-austenite balance • Ar + 20-40% He + up to 2.5% N2 to counter N loss from weld - no hydrogen • Backing gas to maintain weld N content

  29. Duplex SS - Welding • To avoid high ferrite in welds, filler metals with higher nickel used ~ 2209 with 9% Ni • Avoid loosing N in weld – N backing common • Heat input 15 to 65 kJ/in • Interpass temperature 300F typical

  30. Nickel Alloys & Titanium • Selectively used for their high corrosion resistant properties • Ni-Cr-Mo alloys – weldability comparable to austenitic SS • Commercially pure titanium – readily welded- extra care to prevent contamination from atmosphere (oxygen, nitrogen)

  31. Summary – Welding for Food Industry • Technology well established for making structurally sound welds • Greatest challenge is hygienic surface considerations, i.e.- welds free from surface defects- surface finishes comparable to base metal- control weld discoloration to levels acceptable for end application

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