51093707-welding-1
TRANSCRIPT
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Advances in Welding forSanitary Designs
Richard E. AveryConsultant 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 welder’s 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-Product
Contact 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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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.
AWS D18.2 (1999): Heat Tint Levels on the Inside of Welded 316L Austenitic Stainless Steel Tube
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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 SteelBase Metal Upper Right, Weld Metal Bottom Left
Source:The 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