Page 52 - Vol.01

NEW FAB Te ch notes
TECHNOLOGY
J OURNAL 技術專文

Multi-pass welding
FOR TSMC FAB14 PHASE4 1990’s it was determined that narrow welding of stainless steel delivery sys- when exposed to certain gases and
In the late 1980’s through the early
ANALYSIS OF internal weld beads were desirable the early to mid 1990’ s. During these revise his initial recommendations and
compounds. This lead Dr. Ohmi to
tems for UHP gas delivery systems in
revert to recommending single pass
studies Dr. Ohmi noted that multi-pass
when joining 316L stainless steel
MULTI-PASS WELDING pipe/tube and fittings for semiconduc- welding using a pulsed welding system welds for all thin walled stainless
produced a weld with a very narrow
steel tube/pipe distribution systems.
tor manufacturing facilities. This
internal bead width. This lead to
determination was due to assumption
Single Pass vs. Multi-Pass Welding of Stainless that as the weld area did not meet the multi-pass welds being incorporated
surface roughness standards of ≤
into the welding procedures in many
10µinch Ra, a narrow weld bead would
places around the world.
Steel Piping Distribution Systems for disturb a smaller area of the internal But Dr. Ohmi soon realized that
Semiconductor Manufacturing Plants surface of the tube/pipe. As the sur- multi-pass welding imparted much
more heat into the weld area. This
face roughness was considered to
effect the generation of particles, the affected the Heat Affected Zone
less disturbed surface area would be (HAZ), the area just adjacent to and on
半導體不鏽鋼管材自動焊接之 more acceptable. both sides of the weld, sensitizing the
Dr. T. Ohmi of Tokyo University material in the HAZ, making these
一圈與重覆焊接差異性分析 made extensive studies into orbital areas more susceptible to corrosion

External view of a single pass weld on
There has been much discussion lately as to the relevance of multi-pass welds on UHP gas supply systems. 150A BA 316L S/S Pipe.
Much of the confusion as to the acceptance of multi-pass welds on small bore piping, ½” and smaller in
diameter stems from the practice being used in Taiwan for the past 20 years. Even though this may have been
the standard for the industry in the late 1980’s through the early 1990’s, much has changed due to studies on
corrosion processes in the HAZ (Heat Affected Zone) and on the welds themselves.
By_Robert (Bob) Bearden (RTL, Resplendence Technology Limited)

Internal view of two pass weld on
External view of two pass weld on 150A BA 316L S/S Pipe. Note portion
150A BA 316L S/S Pipe. Note portion of weld where “suck back” has Internal view of single a pass weld on
of initial weld that was not consumed occurred due to reheating of the weld 150A BA 316L S/S Pipe.
but second pass.
area during second pass.

Weld Mechanics

As a result of the welding process tible to corrosion when exposed to area that is welded more than once;
Fe (iron), the major constituent of ce rt ai n s u bs t an ce s . Thi s there is a small area where the end of
316L and 304L stainless steel, is re-distribution of iron can be demon- the weld overlaps the start of the
re-deposited on the surface of the strated by holding a small magnet to weld. This overlap of the weld is done
weld joint and in the HAZ. This is the surface of the parent material well to assure that the weld has been com-
caused by a number of processes such beyond the weld areaand the HAZ and pleted around the entire joint. It has
as convection and electro/magnetic noting that the magnet is not attracted been noted during corrosion experi-
currents inside the weld puddle and to the parent material. When the same ments conducted on 316L stainless
vaporization of the parent material small magnet is held near the weld steel piping, that these areas of over-
during the welding process. This itself the magnet will be attracted to lap of the weld show more corrosion
redistribution of Fe causes the areas the weld surface. than do the other portions of the weld
of re-distribution to be more suscep- Even single pass welds have an area. This would indicate that the

050 051
NFPED JANUARY 2011