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 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

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