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