Advances in Welding for Sanitary Designs Richard E. Avery Consultant to the Nickel Institute May 17, 2004
Aug 10, 2015
Advances in Welding forSanitary Designs
Richard E. AveryConsultant 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• Commercially pure titanium
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
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
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
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
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
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
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
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
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
Visual Acceptance Criteria -ID & OD
• Welds full penetration• No cracks, undercut, crevices, or
embedded or protruding material• Offset not to exceed 10 %
Visual Acceptance Criteriafor External, Non-Product
Contact Surface
These criteria give confidence that the inside weld surface is acceptable without an internal
examination
Non-product contact surface - Maximum concavity
Non-product contact surface- Maximum convexity
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
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
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
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
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)
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
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
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
Duplex Stainless SteelBase Metal Upper Right, Weld Metal Bottom Left
Source:The ESAB Group
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
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
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
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
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
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)
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