1. Advances in Welding for Sanitary 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

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

9. 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

10. 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

11. 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

12. 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

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

14. Visual Acceptance Criteria for External, Non-Product Contact Surface
These criteria give confidence that the inside weld surface is acceptable without an internal examination

15. Non-product contact surface - Maximum concavity

16. Non-product contact surface- Maximum convexity

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

18. 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 ppm No ppm No ppm
No ppm No ppm No ppm No ppm
No ppm No ppm
Note: welds on type 304L SS showed no significant difference in heat tint colour from type 316L.

19. 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

20. 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

21. 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)

22. 6% Mo or Superaustenitic SS
Typically: 21 Cr, 24 Ni, 6 Mo, 0.2 N
Areas for 6% Mo not handled by high chlorides ~ over 1000 ppm - low pH environments - where better pitting, crevice and stress corrosion cracking resistance is required

23. 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

25. 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

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

28. 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 forming requires greater power - more spring-back during forming

29. 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

30. 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

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

32. 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

33. 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)

34. 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