Transcript 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
• 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 D18.1
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
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 Criteria
for 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 surfaceMaximum 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
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.
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 Steel
Base 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