We have been seeing a trend to requiring narrower bead widths for the last 8 years or so, under all common codes. Quite a change from 10 years ago. A big bore downhill pipe weld that used to get a puddle cap, now gets a 3 bead cap, and they want you to split the weld earlier. Structural welds in seismic areas are requiring narrower passes. I found it annoying at first, but we have "learned to love it".
Not that this has any bearing on the original question, but the trend is definitly toward narrowness.
JTMcC.
I have a question then since the concerns raised by everyone above are certainly valid:
If changing from stringer to weave technique is that important, why is changing this technique considered a non-essential variable in ASME section 9 for procedure qualification?
Joint design is also a non-essential variable but one wouldn't change from a butt weld to a fillet weld. Same holds true with root opening. We have to keep in mind also that ASME IX does NOT contain all variables that might otherwise be imposed by the construction/fabrication codes.
Wow, you guys have been busy. Lots of discussion on this one. I'll have to print this one and read it when I get more time. Interesting points so far.
John Wright
Like you, I am jumping into this late.
One possible answer could be that the difference between stringer and weave, primarily boils down to heat input.
When heat input is an engineering concern (generally linked to notch toughness) other supplemental essential variables that must be adhered to, take care of that issue. Heat input is addressed specifically.
Charles Hall
what a long thread !
I always thought "splitting a weld" was done to reduce/prevent the likelyhood of segregations forming repeatedly and which accumulate down through the center line of the weld joint, like piping in a casting. These segregations consist of the lower melting temperature "impurities" that freeze last and can cause centerline type cracks. Splitting kind of side steps or staggers this phenomena and the likelyhood of this type of discontinuity.
time to start a new forum topic item- takes too long to read this one in it's entirety
One justification used for splitting into two passes comes directly from Lincoln, they say "In critical applications for low temperature service, it may be necessary to maximize the impact toughness properties of a weld. In this case one technique is to split the layers into two passes as soon as possible. The objective is to align fine-grained reheated zones along the centerline of the weld."
JTMcC.
By -
Date 03-23-2005 23:27
Bill,
Your question concerning Sec. IX not making the weave/bead process an essential variable is certainly warranted. Most PQR's, included the AWS suggested format (QW-483) has a section of the PQR entitled, Technique (QW-410) that states whether the weld will be done using a bead or weave. Whatever is stated in there determines what technique is used. Also, Sec. IX does not differentiate between stainless steel essential variables and carbon steel variables. Even though it is not an essential variable according to Sec. IX, it becomes an essential variable if it is documented on the PQR and WPS and a weave or bead technique specified. If you have a WPS that the Welding Engineer has written and he specifies that only a bead technique be used, then, that essentially is an "essential" variable when welding stainless steel with that particular PQR and WPS.
CM
As a Welding Engineer for one of the world's largest producer of stainless steel pipe, plate, bar, billet, and stainless steel consumables, let me respectfully give my opinion on the original forum question. First of, it is recommended that stainless steel be welded using the "bead" technique. Therefore, "weave" beads can lead to higher heat inputs which can be detrimetal to the integrity of some stainless steel weld properties. Regarding carbide precipitation and sensitization in low carbon grades of stainless steels....Sensitization occurs when unstabilizes stainless steel weldments are exposed, or slowly cooled in the temperature range of 750 to 1550 deg. F., form chromium rich carbides (M23C6)along the austenitic grain boundaries, thus making the weldment sensitive to intergranular corrosion. Generally, the level of precipitation and degree of sensitization are approximately proportional to the carbon content. If I may, I would like to address one particular item that was discussed in one of the responses regarding "color" of a weld. I, too, am a CWI and a CWE, so I find it rather disturbing that a CWI would recommend not brushing a stainless steel weld after completion, regardless of the pretty color. Yes, the pretty gold, or silver, or rainbow color indicates that the heat input was correct, the travel speed was good, and the shieldiing was acceptable. But, the absolute bottom line is that regardless, that color is a "chromium depeted oxide layer". Stainless steel is protected by an impervious, or invisible, layer of chrome for corrosion protection. With the high heat input of the weld process, that chromium is burned off, leaving the oxide layer. That is the color you are seeing. Removing that color will allow the oxidation in the air to let the stainless steel restore the chromium layer that is necessary for corrosion protection. Thank you all ofr allowing me to speak my piece.
CM
CM, where could I find this information about "color" needing to be brushed after welding? Also as a welding engineer what information do have on dry-washing a 304L or 316L weld for the purpose of alignment. I would love to speak with you off the forum, could you e-mail me back at jimhughes71@hotmail.com so I can get your e-mail address.
Thanks
Jim
By -
Date 03-23-2005 21:01
Hi Jim,
I sent you my e-mail address so we can discuss the very big misconception of not needing to remove the "color" of a weld. As far as dry washing a weld to gain proper alignment, well the main thing wrong with that is that autogenous welding directly affects the properties and corrosion resistance of the stainless weld by not adding alloys back into the weld metal. This is mainly true due to the loss of chromium in the weld arc, which directly affects the ferrite number. As we know, chromium is a ferrite former and nickel is an austenite former. Dry washing usually applies to washing just part of the weld so that it will pull in one direction or the other. This can be a localized thermal cycle that can definitely affect the integrity of that particular weld. We can discuss this in more detail when I hear form you. Thanks...
CM
Jim,
Sorry for the mis-typed words. Sometime I type faster than my brain works. <smile>
CM
Interesting discussion. I haven't seen one run this long in awhile. But getting back to the original question - why run 2 "stringers" instead of one weave? Our specifications for pressure piping always require a minimum of 2 weld layers (not necessarily stringers) and I wonder if this is what the fitter was concerned about. I have welded many socket welds with a root pass and a wide 2-to-1 fillet on top of it, but they all had 2 weld layers.
We normally don't have maximum weave width in our WPSs. In cases where heat input is a concern (e.g. notch toughness or sensitization), we specify a maximum heat input value and the travel speed is measured in the direction of weld progression (side-to-side travel speed not included). If the welder can weave and still meet the minimum travel speed needed to stay within the heat input limits, that would be acceptable. We have welded quite a few procedure qualifications with acceptable notch toughness values with wide weaves. I agree with the earlier statement that wide weaves will result in lower notch toughness values than stringers, however the values can still be acceptable. I've also seen machine GTAW welds made with 0.035 in. wire that were weaved up to 1 in. wide (28x weave!), but still passed the A262 sensitization tests. So, there are exceptions to weave limits based on electrode/wire diameter that produce sound welds.
related info ....?
I seem to recollect that B31.1 requires a 2 pass minimum, along with a minimum 1/16 inch witdrawal on socket-type welds.
I have read this thread and its long .
My take is , by limiting how much may be deposited in one pass and then limiting how wide the welder can weave effectively puts a lower limit on travel speed , and is a way of controlling the heat input .
So when you use a given rod size and you may only weave so wide , if you do not move forward fast enough it will build up beyond the allowable limit . Some times this is about the only real restriction in the wps's .
Look at them ,they are mostly written to where , if you could possibly go outside the given parameters , you could not make the weld , especially , amperage .
Pre-heat , interpass heat and things that can be measured are easy to controll with specific directives .
However things like heat input are another ballgame as you are dealing with humans , and sometimes a bankshot is the only way.
"So" Limit the height ,limit the width , and you controll the forward speed and heat input very effectively.
Hi cm,
I hope you didn't interpret my response to mean that
I would condone leaving the color on a weld when I mentioned colors addressed on the AWS chart of 3 or better.
We use these for internal Inspection of Sanitary Tubing which has to be light straw to silver per our clients choice. This is the level required for their passivation to be effective.
" Just for the record "
Have a Great Day!
The tint of a SSt weld is required to be removed in order to obtain full corrosion resistance of the weldment material. Some corrosive media will exhibit accelerated corrosion of SSt initiated at the heat tint surface; salt water, for example.
Look up "rouging" of SSt on googoo.
By -
Date 06-22-2005 16:47
Stainless steel is protected by an invisible, inpervious layer of chrome. When welding is done, the "color" on sees is a "chromium depleted oxide layer". This is not only on the weld, but in the HAZ as well. This color should be removed not only on the surface, but immediately below the surface. This was discussed in many of the posts above.
See the Avesta Pickling Handbook "Handbook for the Pickling and Cleaning of Stainless Steel". If you would like more information on pickling, I can send you 2 papers that were submitted to NACE and AWS for publication. You can e-mail if you would like this info.
chuck.meadows@outokumpu.com
CM
By -
Date 06-29-2005 16:33
I typed in "googoo" and found it to be an on-line shopping web site. Couldn't find anything mentioning "rouging" there. Regardless, pickling is the recommended way to fully restore the corossion protection in the weldment material AND the HAZ. Maybe you meant "google"?