Advances in Welding for Sanitary Designs

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Advances in Welding forSanitary Designs

• Richard E. Avery
• Consultant to the
• Nickel Institute
• May 17, 2004

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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

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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

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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

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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

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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

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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

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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

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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

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Welding Qualifications

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

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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

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Visual Acceptance Criteria -ID OD

• Welds full penetration
• No cracks, undercut, crevices, or embedded or
  protruding material
• Offset not to exceed 10

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Visual Acceptance Criteriafor External,
Non-ProductContact Surface

• These criteria give confidence that the inside
  weld surface is acceptable without an internal
  examination

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Non-product contact surface - Maximum concavity
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Non-product contact surface- Maximum convexity
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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

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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.

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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

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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

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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)

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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

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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

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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

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Duplex Stainless Steel
Base Metal Upper Right, Weld Metal Bottom Left
SourceThe ESAB Group
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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

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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

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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

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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

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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

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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)

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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