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Submerged Arc Weld “Hard
Spots” |
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OVERVIEW:
 “Hard
Spots” in submerged arc welds have caused major weld failures. This
problem was identified in the early 1970’s when some weldments failed within
days of being exposed to H2S.
Compared to the much smaller “Hard Spots”
recently reported as problems in published literature (Reference 1) the
“Hard Spots” encountered in the
submerged arc welds in question are much harder and much larger! The
above mentioned article expressed concern that the hard areas in these MIG
welds which caused cracking problems in a H2S
test were about as big as the hardness tester indenter. As can be seen
below,
“Hard Spots” in a failed sub arc weld were about 40 times the indenter size!
Those located at the surface are readily seen by eye! |
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EARLY 1970’s
Harry Ebert, an Exxon Welding Engineer at
the time, published a paper in 1971 discussing hard welds caused by the use
of “Active” bonded fluxes and subsequent rapid weld failures (Reference 2.)
We worked closely with Exxon, the API Vessel Committee and fabricators
defining which combination of fluxes and wires were to be used in the as
welded and stress relived condition for this service.
In1973 a definitive 22 page technical paper
by Kotecki and Howden was published regarding sub arc weld “Hard Spots”
(Reference 3.) It clearly showed failures similar to those discussed by
Ebert and defines the cause as “Hard Spots” in submerged arc welds made with
"active"
bonded fluxes, those containing Ferro Alloys. A majority of
bonded or agglomerated fluxes contain Ferro Alloys for a
number of flux manufacture and weld performance reasons. Photo directly below shows one
figure from the Kotecki/Howden report with cracked “Hard Spots.”
They found relatively large, very high Mn and Si “Hard
Spots” in sub arc welds made with bonded fluxes. These “Hard
Spots.” cracked rapidly when exposed to H2S testing.
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1980’s
My early career was in research and
development of submerged arc welding processes, fluxes and wires involved
with the manufacture of gas and oil transmission pipe. While presenting
technical papers in conferences on the subject of submerged arc welding gas and oil transmission
pipe with a system developed that welded at 3000 amps, several papers addressed steels for use in sour gas
service. However none discussed possible weld metal problems when subjected
to sour gas.
With my experience with submerged arc weld
“Hard Spots” in pressure vessel failures it was felt important
to discuss the issue with all North American mills. There were nine UOE
pipe mills in the US and Canada all of whom were using specially designed
high performance fused sub arc fluxes made especially for this service.
Fused fluxes, as found by Kotecki/Howden (Reference 3,) did not produce
welds containing “Hard
Spots.”
[ Note: fused fluxes can be fully reacted during manufacture eliminating
the production of metal particles while welding. This compares to
bonded fluxes whose ingredients are first reacted during welding.] All these
pipemills cleaned the steel surface prior to welding. A Canadian
spiral pipe mill was also manufacturing some gas and oil line pipe. This
spiral pipe mill was also using fused pipe fluxes.
Visiting most of the UOE mills several times
per year they were verbally told about the concerns of using bonded fluxes
for the manufacture of gas and oil pipe that could contain hydrogen sulfide.
The spiral mill decided to use an active bonded flux to handle the extra
mill scale on their plate. They produced a 24 inch gas pipeline that was
placed in the Grizzly Valley in Canada. It operated for a period of time
when a gathering line from a gas field with sour gas was added. The line
failed catastrophically in very long lengths at several locations within a very short time of being put in sour gas service!
Little is
written about this major failure for some interesting reasons! However in a
Corrosion Conference held in San Francisco on March 9 -13, 1987, a technical
paper was presented and proceedings published (Reference 4.)
One of their conclusions
states; "Analysis of failures revealed they (cracks) originated in the inside weld
bead of a spiral weld which contained hard areas of martensite and/or banite
with hardness values between 300 and 500 HV." They go on to say pipe
used in this gas line made at another pipe mill (My
Note: no doubt a UOE mill using fused flux)
had no such
“Hard
Spots”
and the pipe did not fail.
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| LATE 1999 thru 2000’s
In 1999, with most of the gas and oil pipeline
infrastructure in place in the United Sates, there was only one UOE US pipemill approved for
API gas and oil pipe production (now also closed and dismantled.) They stopped
using their plate surface shot blaster because of high maintenance cost.
Therefore the fused flux they had been using was replaced with a bonded flux
to handle the plate mill scale. The flux manufacturer designated the flux as
being designed for pipe welding. It was an active bonded flux. We
were asked to examine a burst pipe weld which was made with this flux and
had failed during hydro testing. The pipe had burst along the full 40 foot
seam. We observed several large sections with the crack moving along the
weld and in and out of the weld and HAZ. The pipe mill metallurgist found no
issues with the steel different than the remaining steel used for the line
being produced.
A detailed examination of a small section of
pipe we were given showed many “Hard Spots” on
the surface (about 20 per inch.) An EDAX trace showed them to be very high
in Mn and Si. The hardness of these obvious martensitic areas was 445 VHN.
As seen in the photomicrographs below they were very large, 3 or 4 times
larger than those observed by Kotecki/Howden (Reference 3.)
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| HARD SPOT TEST DEFINED
Recently a test for submerged arc weld “Hard
Spots” was developed for a major oil company. They will use
it in specifications for pipe exposed in Sour Gas Service. |
| BOTTOM LINE
Having extensive experience related to the manufacture of gas and oil transmission pipe, as well as research experience
and patents for fluxes and wires used for submerged are welding, I am in a
unique position to define why these “Hard Spots”
exist. I can discuss issues such as flux raw ingredients, raw material sizing, mixing
etc that determine their size and frequency. I can help define
appropriate tests for these “Hard Spots.” Other welding processes
and consumables could possibly cause similar issues.
Email to discuss consulting on the subject. |
REFERENCES:
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“Effect
of Welding Parameters and H2S
Partial Pressure on the Susceptibility of Welded HSLA Steels to Sulfide
Stress Cracking; “by G.M. Omweg, G.S. Frankel, W.A. Bruce, J.E. Ramirez
and G. Koch; AWS Welding Journal, June 2003
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“Hard Welds -Their Causes and
Prevention,” H. W. Ebert, AWS Welding Journal, September 1971
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“Submerged
Arc Weld Hardness and Cracking in Wet Sulfide Service,” by D.J.
Kotecki and D.G. Howden, WRC (Welding Research Council) Bulletin Number No.184 (1973)
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“Pipeline
Failures in the Grizzly Valley Sour Gas Pipeline,” by V.B. Lawson, C.
Duncan, R.S. Treseder; CORROSION/87, paper no. 52, (Houston TX; NACE,
1987)
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