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SEMI F78-0304 © SEMI 2003, 2004 2 A450 (Section 25) — General Requirements for Carbon, Ferritic Alloy, and Austenitic Steel Tubes A632 — Specification for Seamless and Welded Austenitic Stainless Steel Tubing (Small Diam…
SEMI F78-0304 © SEMI 2003, 2004 1
SEMI F78-0304
PRACTICE FOR GAS TUNGSTEN ARC (GTA) WELDING OF FLUID
DISTRIBUTION SYSTEMS IN SEMICONDUCTOR MANUFACTURING
APPLICATIONS
This practice was technically approved by the Global Gases Committee and is the direct responsibility of the
North American Gases Committee. Current edition approved by the North American Regional Standards
Committee on October 16, 2003. Initially available at www.semi.org February 2004; to be published March
2004. Originally published July 2003.
1 Purpose
1.1 The purpose of this practice is to provide
procedures for welding stainless steels and other
corrosion resistant metals and alloys (CRAs) for fluid
(liquid or gas) distribution systems in semiconductor
manufacturing applications. Welds performed
following these procedures are of sufficient quality to
provide the required system purity, weld integrity, and
weld strength for use in semiconductor manufacturing
applications.
2 Scope
2.1 This practice provides procedures for gas tungsten
arc (GTA) autogenous butt joint welds of stainless steel
and other CRAs in fluid distribution systems. The fluid
distribution system includes tubing, pipe, fittings,
valves, subassemblies and components that contain and
distribute fluid.
NOTICE: This practice does not purport to address
safety issues, if any, associated with its use. It is the
responsibility of the users of this standard to establish
appropriate safety health practices and determine the
applicability of regulatory or other limitations prior to
use.
3 Limitations
3.1 The stainless steels covered by this practice are
limited to the austenitic and superaustenitic grades of
stainless steel.
3.2 Corrosion resistant metals and alloys covered by
this practice are limited to solid solution grades of
nickel alloys and solid solution grades of titanium
alloys.
3.3 This practice applies only to autogenous GTA
circumferential butt joint welds performed on fluid
distribution system components 6 inches or less in
diameter.
3.4 This practice applies only to automatic,
mechanized, or machine GTA welding processes.
3.5 This practice applies only to welds performed with
no fillers and no fluxes.
3.6 This practice does not apply to pressure vessel or
process chamber welds.
4 Referenced Standards
NOTE 1: The following documents become part of the
practice to the extent that they are included herein.
4.1 SEMI Standards
SEMI F20 — Specification for 316L Stainless Steel
Bar, Extruded Shapes, Plate, and Investment Castings
for Components Used in High Purity Semiconductor
Manufacturing Applications
SEMI F81 — Specification for Visual Inspection and
Acceptance of Gas Tungsten Arc (GTA) Welds in Fluid
Distribution Systems in Semiconductor Applications
4.2 ANSI/ASME Standards
1
ANSI/ASC Z49.1 — Safety in Welding, Cutting, and
Allied Processes
BPE — Bioprocessing Equipment Standard
B16.25 — Butt Welding Ends
B31.3 — Process Piping
Boiler and Pressure Vessel Code — Section IX,
Qualification Standard for Welding and Brazing
Procedures, Welders, Brazers, and Welding and
Brazing Operators
4.3 ASTM Standards
2
A269 — Specification for Seamless and Welded
Austenitic Stainless Steel Tubing for General Service
1 American National Standards Institute, New York Office: 11 West
42nd Street, New York, NY 10036, USA. Telephone: 212.642.4900;
Fax: 212.398.0023 Website: www.ansi.org
2 American Society for Testing and Materials, 100 Barr Harbor
Drive, West Conshohocken, Pennsylvania 19428-2959, USA.
Telephone: 610.832.9585, Fax: 610.832.9555, Website:
www.astm.org
SEMI F78-0304 © SEMI 2003, 2004 2
A450 (Section 25) — General Requirements for
Carbon, Ferritic Alloy, and Austenitic Steel Tubes
A632 — Specification for Seamless and Welded
Austenitic Stainless Steel Tubing (Small Diameter) for
General Service
4.4 AWS Standards
3
AWS A3.0 — Standard Welding Terms and Definitions
AWS B2.1 — Specification and Qualification of
Welding Procedures and Welders for Piping and
Tubing
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
5 Terminology
4
5.1 Definitions
5.1.1 angular misalignment — the condition that exists
when the tubing angle is unintentionally changed at the
weld.
5.1.2 autogenous weld (2) — a fusion weld made
without filler metal.
5.1.3 automatic arc welding downslope — the time
during which the welding current is reduced
continuously from the final level until the arc is
extinguished.
5.1.4 axial misalignment — the offset caused by tubing
being in line but not centered at the weld.
5.1.5 backing gas — an inert gas (or gas mixture) on
the interior of the weld joint used to prevent or reduce
formation of oxides and other detrimental surface
substances during welding, and to provide pressure for
weld profile.
5.1.6 bead (2) — non-standard term for weld bead.
5.1.7 bead overlap — in a pulsed weld the amount of
coverage of a weld pulse of the previous weld pulse,
usually measured in percentage of the diameter of the
pulse.
3 American Welding Society, 550 NW LeJeune Road, P.O. Box
351040, Miami, Florida 33135, USA. Telephone: 800.443.9353,
Website: www.aws.org
4 The terminology has been derived from the following sources:
(1) Webster’s New World College Dictionary Fourth Edition
(2) ANSI/AWS A3.0 Standard Welding Terms and
Definitions
(3) ASME BPE Bioprocessing Equipment Standard
5.1.8 bead variation — the amount of change of ID
bead width from one area to another.
5.1.9 bead width — the width of the weld bead on the
ID, normally measured in units of T, where T is the
nominal tube wall thickness.
5.1.10 center line shrinkage — a profile-reducing
defect or discontinuity normally formed by shrinkage
during solidification.
5.1.11 color — the darkness of the oxidation of the
weld or adjacent surfaces. Non-standard term for
discoloration.
5.1.12 color line — acceptance criteria of the
maximum amount of discoloration allowed on the weld
or adjacent surfaces.
5.1.13 concavity (3) — a condition in which the
surface of a weld is depressed relative to the surface of
the tube or pipe. Concavity is measured as a maximum
distance from the outside or inside diameter surface of a
weld along a line perpendicular to a line joining the
weld toes.
5.1.14 convexity (3) — a condition in which the surface
of a weld is extended relative to the surface of the tube
or pipe. Convexity is measured as a maximum distance
from the outside or inside diameter surface of a weld
along a line perpendicular to a line joining the weld
toes.
5.1.15 coupon — weld sample which is opened for
inspection to insure that the weld meets specifications.
5.1.16 coupon-in — first coupon prior to production
welding of butt weld joint.
5.1.17 coupon-out — last coupon after production
welding of butt weld joint ends.
5.1.18 discoloration (3) — any change in surface color
from that of the base metal. Usually associated with
oxidation occuring on the weld and heat affected zone
on the outside and inside diameter of the weld joint as a
result of heating the metal during welding. Colors may
range from pale bluish-gray to deep blue, and from pale
straw color to a black crusty coating.
5.1.19 downslope — See automatic arc welding
downslope.
5.1.20 dross (2) — non-standard term for slag.
5.1.21 electrode (2) — non-standard term for tungsten
electrode.
5.1.22 enclosed weld head — weld head in which the
weld joint is held and welded within a closed chamber
containing a shielding purge gas.
SEMI F78-0304 © SEMI 2003, 2004 3
5.1.23 encroachment — non-standard term for ID
convexity.
5.1.24 examiner — a person who performs
examination of a particular object, or evaluates an
operation, for compliance to a given standard. The
examiner performs quality control for the manufacturer,
fabricator, or erector.
5.1.25 fluid (1) — liquid or gas.
5.1.26 gas (1) — the fluid form of a substance in which
it can expand indefinitely and completely fill its
container; form that is neither liquid or solid.
5.1.27 gas tungsten arc welding (GTAW) (3) — an arc
welding process that uses an arc between a tungsten
electrode (nonconsumable) and the weld pool. The
process is used with a shielding gas.
5.1.28 halo — non-standard term for discoloration
resulting from welding procedure.
5.1.29 haze — non-standard term for discoloration
resulting from welding procedure.
5.1.30 heat-affected zone (HAZ) (2) — the portion of
the base metal whose mechanical properties or
microstructure have been altered by the heat of
welding.
5.1.31 heat tint/color — non-standard term for
discoloration resulting from welding procedure.
5.1.32 inclusion (2) — entrapped foreign solid
material, such as slag, flux, tungsten, or oxide.
5.1.33 inert gas — a gas that normally does not
combine chemically with materials. A protective
atmosphere.
5.1.34 inspector — a person who verifies that all
required examinations and testing have been completed,
and who inspects the assembly to the extent necessary
to be satisfied that it conforms to all applicable
examination requirements. The inspector performs
quality assurance for the owner. The inspector is
designated by the owner and shall be the owner, an
employee of the owner, an employee of an engineering
or scientific organization, or of a recognized insurance
or inspection company acting as the owner’s agent.
5.1.35 lathe welding — automatic or machine welding
of tubes or pipes in which the electrode is stationary
and the weld joint rotates. Lathe welding as defined
here is a fusion process without the addition of filler.
5.1.36 liquid (1) — having its molecules moving freely
with respect to each other so as to flow readily, unlike a
solid, but because of cohesive forces not expanding
infinitely like a gas.
5.1.37 liquid cylinder — often referred to as a dewar,
an insulated and pressure controlled metal cylinder used
to store fluids in their liquid form.
5.1.38 meandering (3) — of or pertaining to a weld
bead that deviates from side to side across the weld
joint rather than tracking the joint precisely.
5.1.39 orbital welding (3) — automatic or machine
welding of tubes or pipes in-place with the electrode
rotating (or orbiting) around the work. Orbital welding,
as it applies to this standard, is a fusion process without
the addition of filler.
5.1.40 oxidation (3) — the formation of an oxide layer
on a metal surface. When excessive oxidation occurs as
a result of welding, it is visible as discoloration.
5.1.41 oxide island — non-standard term for slag.
5.1.42 pressure cylinder — a metal cylinder used to
store gases under pressure.
5.1.43 profile defect — any defect or discontinuity that
reduces the wall thickness below that of the parent
metal.
5.1.44 pulsed gas tungsten arc welding — a gas
tungsten arc welding process variation in which the
current is varied in regular intervals.
5.1.45 purge — the application of an inert gas (or gas
mixture) to the OD or ID surface of the weld joint to
displace non-inert atmospheric gases. A block purge is
a non-flowing purge with positive pressure.
5.1.46 purge gas — an inert gas (or gas mixture) used
to displace the ambient atmosphere from the inside (ID)
of the weld joint.
5.1.47 root — non-standard term for root surface.
5.1.48 root surface (2) — the exposed surface of a
weld opposite the side from which the welding was
done.
5.1.49 rotation delay — time delay between when the
arc is initiated and the rotor begins to turn.
5.1.50 shield gas — inert gas (or gas mixture) that
protects the electrode and molten puddle from
atmosphere and provides the required arc
characteristics.
5.1.51 slag (2) — a non-metallic product resulting
from the mutual dissolution of non-metallic impurities
in some welding processes.
5.1.52 tack weld (2) — a weld made to hold the parts
of a weldment in proper alignment until the final welds
are made.
5.1.53 tail-out (2) — non-standard term for automatic
arc welding downslope.