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SEMI F20-0305 © SEMI 1995, 2005 8 APPENDIX 2 THE EFFECTS OF COPPER AS A TRACE ELEMENT ON GTA WELDING OF 316L STAINLESS STEEL NOTICE : The material in this appendix is an official pa rt of SEM I F20 and was approved by fu…

SEMI F20-0305 © SEMI 1995, 2005 7

A1-2.1.6 These effects lead to the following recommendations to ensure optimum weld quality when welding

stainless steel:

A1-2.1.6.3 Check compositions of pieces to be welded. Match Sulfur contents to within ± 0.007 percentage points.

A1-2.1.6.3 Welding of pieces with very low Sulfur (< 0.003 % approximately) may require additional attention to

the establishment of appropriate weld settings to achieve full, uniform penetration. It is recommended that product

with wall thickness > 0.100 inch approximately be restricted to > 0.003 % Sulfur.

A1-2.1.6.3 Establish weld settings for piece lots to be welded; recheck whenever changing lots.

SEMI F20-0305 © SEMI 1995, 2005 8

APPENDIX 2

THE EFFECTS OF COPPER AS A TRACE ELEMENT ON GTA

WELDING OF 316L STAINLESS STEEL

NOTICE: The material in this appendix is an official part of SEMI F20 and was approved by full letter ballot

procedures.

A2-1 Copper in Stainless Steel

A2-1.1 Copper is a residual or unspecified element in AISI 316L. It is not an intentional addition, but is present in

any heat of material as a result of the composition of the furnace charge materials used, which are typically

primarily scrap with elemental and master alloy additions to achieve the composition desired. Levels of unspecified

elements such as Copper are permitted up to 0.50% maximum by AMS 2248E

. Copper content can only be

reduced in the melt by dilution. Over time, as more scrap material is recycled, the residual levels of copper in heats

of AISI 316L is anticipated to increase.

A2-1.2 Copper is an austenite phase stabilizer. It also has been shown to improve the resistance of austenitic

stainless steels to stress corrosion cracking in chloride solutions.

A2-2 Effects of Copper on Welding

A2-2.1 In welding, copper is a surface-active element (like S, O, and Se) and has been shown to contribute

significantly to weld bead meander. Like manganese, it will volatilize during welding and redeposit downstream of

the weld, providing a site for the initiation of corrosion in some conditions. Some end users may restrict copper as a

potential source of weld contamination or corrosion.

A2-2.2 A correlation was noted between increased copper content and weld bead meander in a wafer fab project

Five heats of 0.25 in. tubing were evaluated. Copper was very prominent in the HAZ of one heat (0.47% Cu and

0.011% S) which had a bead meander problem when welded. Another heat with the same sulfur content and a lower

copper content (0.03% Cu) did not exhibit bead meander. Another heat (0.31% Cu and 0.010% S) showed oxidation

after welding. All welds were performed in argon.

A2-2.3 In a study of 28 heats of tubing from 0.25 to 2.0 inch in diameter, copper ranged from 0.05 to 0.41%

. Of

the 28 heats tested, 10 had copper contents above 0.25%, and among the nine heats of 2.0 in. diameter tubing, all

had copper levels above 0.22%, with four of the heats above 0.25%. Cosmetically unacceptable welds (slag,

discoloration and/or bead meander) occurred on heats with: 0.26% Cu, 0.005% S; 0.28% Cu, 0.009% S; and 0.41%

Cu, 0.005% S. Conversely, cosmetically acceptable welds were performed on a heat with 0.3% Cu and 0.011% S.

All welds were performed using argon.

A2-2.4 Data was presented at a SEMI Workshop and repeated here concerning the effect of copper content and

shield gas interactions on weldability

. Two examples of heats of AISI 316L tubing, 2.5 in. in diameter, with copper

contents above 0.3%, demonstrated that adding hydrogen to the argon shield gas adversely affected the weldability

of the tubing (see Figure A2-1). This same effect was not noted in heats of tubing with copper levels below 0.3%. In

addition, the weld electrodes undergo severe degradation when welding tubing with copper contents above 0.3% and

using Ar/H

shield gas (see Figure A2-2).

A2-2.5 Based on the above information, the following conclusions can be made:

A2-2.5.1 The copper restriction in SEMI F20 currently stands at 0.30%. This restriction is due to weldability

effects.

4 Aerospace Materials Specification (AMS) 2248E, "Chemical Check Analysis Limits: Corrosion and Heat Resisting Steels and Alloys,

Maraging and Other Highly Alloyed Steels, and Iron Alloys," SAE International, Warrendale, PA; issued 15 January 1960, Revised 1 October

1993, 6 pp

5 R. M. Cohen, Identifying Trace Elements Affecting the Weldability of 316L Stainless Steel Tubing, Future Fab International 1997, pp. 271-281

6 Sunniva Collins and Pete Williams, Weldability and Corrosion Studies of AISI 316L Electropolished Tubing, Proc. Interphex 2000.

7 Sunniva Collins, The Effect of Copper on GTA Welding of 316L Stainless Steel Tubing, Stainless Steel and Surface Analysis Workshop,

SEMICON West Standards Meeting, July 14, 2003

A2-2.5.2 There appears to be a Cu/S interaction; the copper effect on welding is more pronounced at sulfur levels

under 0.010%.

A2-2.5.3 Commercially available tubing of 2.0 in. diameter and above is currently being offered with residual

copper levels above 0.2%.

A2-2.5.4 Cohen’s data shows that oxidation after welding will occur at 0.31% Cu, 0.010% S in argon; Collins’ data

shows an acceptable weld at 0.30% Cu, 0.011% S in argon.

A2-2.5.5 A cosmetically unacceptable weld, with bead meander and Cu redeposited in the HAZ, was found at

0.41% and 0.47% Cu.

A2-2.5.6 Heats above 0.30% Cu may be welded successfully in argon, but may exhibit bead meander and electrode

attack in Ar/H

(a) 2.5 in. tubing, Cu content 0.37%. Bead

meander condition became evident with addition

of H2 to shield gas.

(b) 2.5 in. tubing, Cu content 0.34%. Bead

meander condition became evident with addition

of H

to shield gas, worsened with each weld.

Figure A2-1

Weldability: Copper Content and Shield Gas Interactions

A2-2.5.7 Scanning electron micrographs of welding electrodes after welding 2.5 inch tubing with Cu content

0.34%; Shield gas Ar/H