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Up Topic Welding Industry / Technical Discussions / Preheat temperature
- - By zambrota (**) Date 02-03-2011 04:16
Please comment,

WPS for welding low carbon mild steel requires preheat of 50°C. If water is condensed on material during night prior to welding, what would be than the preheat temperature in order to remove moisture?
Does it require the qualification of  the new welding procedure for the changed preheat temperature?

Parent - - By MBSims (****) Date 02-03-2011 05:18
Drying moisture off the surface does not take much heat.  Passing the flame over it a few times would be enough.  Preheating to 50°C (122°F) with a torch should be sufficient to dry the surface.  Most codes only require requalifying the welding procedure if the preheat temperature is "decreased" more than a certain amount (e.g. 100°F) over that qualified.  An increase would not usually require requalification, but you need to look at whatever code the procedure was qualified under to determine the requirements.
Parent - - By zambrota (**) Date 02-03-2011 08:49
Thank you MBSims,

I've got a Welding Inspector who states that all welding procedures should have a minimum preheat
of 100°C to make sure moisture is removed. According to him that is a "good practice". It was the first time I hear such statement. Has anybody else experienced similar request in any fabrication specification. Is there any reference material on this topic I can read? Come on Henry, Al ...!!!

Best regards
Parent - By Shane Feder (****) Date 02-03-2011 09:59
Ask him to show applicable clause in applicable code !
Wait, that pitter patter I can hear is the sound of the welding inspector running out the door !!!
To put it bluntly, he doesn't have a clue what he is talking about.
Table 3.2 of AWS D1.1 2004 states for material 1 1/2" to 2 1/2" (38 to 65 mm) the minimum preheat is 65 degrees celcius.
Over 2 1/2" (65 mm) the minimum preheat is 110 degrees celcius.
Parent - - By Superflux (****) Date 02-03-2011 10:43 Edited 02-03-2011 10:50
Is 100C, or 212F preheat a "good practice"? There can be intelligent arguments made, but nothing set in stone. In certain applications such as Sheet Metal, this much preheat might be undesirable and cost prohibitive due to excessive heat input (warpage?) and innerpass cooling rates.
If scientific study proved it necessary to produce sound mechanical properties by having base material be 100% moisture free prior to welding, then codes would specify a minimum preheat equivalent to SMAW Lo-Hi storage temperatures.
Sometimes QCs say stuff above and beyond their pay grade. I'm sure every Inspector has been guilty of this at least once in their career. Some make it a habit.
I think good practice should dictate all Inspectors have at minimum a CWI (or equivalent). Does this one?
Just my thoughts on the subject.
Parent - By Jim Hughes (***) Date 02-03-2011 12:32
I agree with Mr. Sims, If you are pre-heating to 50 deg. C that is more than enough to expel any moisture. The inspector is correct in one regard, and that is that obvious moisture should be removed prior to welding C.S material, but it does not take 212 deg F to do it. Another reason to pre-heat is to control warpage. Sometimes it is a judgment call for an inspector. If I see water dripping off pipe or structural steel, I will ask for it to be removed prior welding even though it might be 75 deg. F out side.

Parent - By MMyers (**) Date 02-03-2011 19:51
That's nice...

...on steel.  Now what about other materials that don't take kindly to preheat?  The last Inconel job I welded had a max interpass of 250F.  I can see it now, preheat to 212F, max interpass of 250F.  Great idea.  Put a pass in, go take a 20 minute break.  Put a pass in, go take a break.  2 passes an hour for a weld that takes 60 passes.
Parent - By jwright650 (*****) Date 02-04-2011 02:41
There is more to "Preheat" than just ridding excess surface moisture. Table 3.2 in AWS D1.1 is a good start for mild CS materials. You need to take the material thickness in account for starters when requiring preheat. The preheat also slows the cooling rate in thicker materials so that the surrounding material does not act as a huge heat sink and essentially quench the weld too quickly. On thinner material you need less preheat because heat from the welding process alone can be sufficient. I recommend using Table 3.2 in AWS D1.1 for mild CS as a guide for the amount of preheat required.
Parent - - By welderbrent (*****) Date 02-03-2011 14:22 Edited 02-03-2011 14:25
Okay, I may be asking more of a question than stating an opinion but here goes,

You are talking about degrees Celsius??  For my limited mind I had to refer to Table 3.2 of D1.1 to get a FAST comparison as I don't do conversions very quickly in my head.

This brought me to wondering the next item, which code are you dealing with, not that there is a huge difference when it comes to pre-heat of similar class materials?

So, according to Table 3.2 materials of the class you asked about up to 3/4" thick can be welded with low hydrogen processes starting at 32*F (0*C).  Materials on up to 1 1/2" are good with a pre-heat of 50*F (10*C).  And from there to 2 1/2" @ 150*F (65*C). 

Noting that at ambient temps lower than 32*F (0*C) you need to pre-heat to 70*F (20*C) I had always figured that to be a reasonable place to start. 

Now, PERSONALLY I like to see material to be welded at about 50*F and on cool mornings with much moisture in the air I like to take a rosebud or propane 'bertha' and warm till the moisture is gone.  Especially if the sun is not up yet to do some natural heating and moisture removal.  But this PERSONAL PREFERENCE does not REQUIRE a pre-heat of a couple hundred degrees!!

If I have read, and you posted, your original query correctly that you are pre-heating to 50*C then I see no reason to have to go beyond that except on heavier materials.  You should be getting most all the moisture out at that temp. Especially if you keep it there and warm more than just the edge of the parts.

But, bottom line, no, I don't see where a new WPS would be required.

Have a Great Day,  Brent
Parent - - By zambrota (**) Date 02-03-2011 21:40
I showed these comments to my inspector and guess what? His answer was that you should know the temperature of boiling water is 100°C. So, his conclusion is that heating to 50°C is not enough for moisture to evaporate completely and hydrogen can develop in weld area? No comment from my side!!!

Parent - - By MBSims (****) Date 02-04-2011 00:25
Time to let this inspector go...boiling water is not what is needed here.
Parent - By welderbrent (*****) Date 02-04-2011 04:14
I agree.  This 'inspector' is not doing a proper job.  His job is to make sure the work is being done to Engineer approved WPS's that are in compliance with the code being used.  As long as the pre-heat temps are within code specifications and were followed on the job he can't simply press his own agenda or make demands outside of code that will cause undue contractor time and/or expense in completing the job.

Sounds like one of those times when the contractor needs to get clarification from the Engineer. 

Have a Great Day,  Brent
Parent - By Jim Hughes (***) Date 02-04-2011 13:51
10-4. The inspector is not trying to part of the solution but has made himself part of the problem. Time to get someone else to play this role.

Parent - - By 3.2 Inspector (***) Date 02-04-2011 11:36
HAHAHAHAHAHA, what a moron.
Besides, water can boil at temperatures below 100 C

Parent - - By jwright650 (*****) Date 02-04-2011 12:07
Sure water can boil at less than 212°F.....if you're welding in a vacuum.
Parent - By 3.2 Inspector (***) Date 02-04-2011 15:05
Ask people of Tibet at what temp their water boils.

Parent - - By G.S.Crisi (****) Date 02-04-2011 19:12
In the city of La Paz, Bolivia, located at 3,400 meters above sea level, eggs don't get hard when put into boiling water, even if you leave them for an hour. Boiling water temperature isn't high enough for eggs getting hard. I've been there and seen it happen.
Giovanni S. Crisi
Sao Paulo - Brazil
Parent - - By DaveBoyer (*****) Date 02-05-2011 02:02
How do they make a hard boiled egg, in a pressure cooker?
Parent - - By 803056 (*****) Date 02-05-2011 14:13 Edited 02-05-2011 14:16
The preheat must be sufficient to evaporate the surface moisture.

Sure, the water will eventually evaporate at 0 degrees F through sublimation, but do you really want to wait that long?

I've encountered problems on numerous jobs as a welder when there was surface moisture on the steel. Any welder that works in the field has encountered the problem at one time or another. The problem will occur anytime the temperature dips below the dew point, so it can  happen in the spring, summer, fall, or winter. The problem is even more acute when the joint involves lapped members that were assembled and left in place overnight. The moisture wicks up between the two members by capillary action. The daytime temperature can easily be higher than the minimum preheat temperature, so the question is, "How hot do I heat the carbon or high strength low alloy steel to completely evaporate the moisture?"

The preheat listed in Table 3.2 of D1.1 are the minimum preheat temperatures. The emphasis is on minimum. The next key words are carbon and high strength low alloy steels. You need to bracket the discussion if you want to maintain a realistic conversation. Every family of base metals responds to the affect of heat differently. What works with carbon and HSLA steels may not be considered good welding practice for other alloy systems such as austenitic stainless steel subject to sensitization for example.

Merely heating the joint with lapped members may not be sufficient to evaporate the moisture between the members in a timely manner. The question is, “How long do you want to delay the initiation of welding operations waiting for the moisture to evaporate?”

As a welder, I’ve had many situations where the welding foreman (he was foreman because he couldn’t pass the test) moaned and groaned that I was wasting time preheating the steel on a warm humid summer morning. I am so easy to get along with, I asked him if I should dispense with the preheat?

“You don’t need to preheat, it’s plenty warm enough. It is above 50 degrees and the plate is only 1 inch thick!” he said.

Eager to comply, I said,  "Just hang around for a couple of minutes, I want you to check my parameters and the weld before you leave.”

After completing the weld, I lifted the hood and asked him to check my weld. “While you’re at it, you might want to go fetch the carbon arc so I can gouge this Sxxx out and weld it right!”

He never did question me about “preheat” again. Next job, next idiot, repeat lesson as needed.

You can wait several months if you want to wait for sublimation, wait an hour at 60 degrees F, or heat the joint and surrounding area to a sufficiently high temperature to reduce the time needed for complete evaporation to something reasonable. The other consideration is that the area heated must be sufficient to ensure any remaining moisture will not evaporate and migrate toward the weld zone during the welding operation. Any moisture migrating toward the weld can easily manifest itself as porosity or potentially, as previously suggested, provide a source of hydrogen as the moisture decomposes into atomic hydrogen and oxygen in the presence of the welding arc.

Is there any one temperature that is best? I would hesitate to say "yes" to any one temperature. I would offer that if is dependent on the job conditions. An open groove weld may only require the application of sufficient heat to warm the surfaces above the dew point and several minutes to allow for evaporation. However, a thicker joint with lapped members, think of paddle plate semi rigid moment connections, may require higher localized preheat to ensure the evaporation is complete and doesn’t necessitate delaying operations for a half hour or more while the moisture evaporates from between the lapped members.

While the inspector was trying to be helpful, his suggestions were not welcome. In situation such as that described, the inspector could easily keep his opinions to himself and let the contractor learn the lessons the hard way by removing the resulting porosity, excavating the cracked joints, while the inspector simply collects the overtime checks as compensation for the time spent observing the repairs and reinspection.  Life as an inspector can be difficult or it can be easy. I prefer the easy life, I love hard work. I can sit around and watch the contractor struggle all day without ever having to wipe sweat from my brow.

Best regards – Al
Parent - By MBSims (****) Date 02-05-2011 14:43
Good advice Al.  This brought back a memory of a discussion we had one time on a job about "how wet is wet".  AWS D1.1 contains a requirement that welding shall not be done "when surfaces are wet or exposed to rain, snow,..."  The discussion was whether the light condensation film that forms from flame heating, or dampness from fog constituted being "wet".  Just passing a torch flame over the surface is usually sufficient to evaporate surface moisture, even without any significant heating of the base metal.  A minimum base metal temperature does not need to be specified to accomplish this.
Parent - By 803056 (*****) Date 02-05-2011 16:21
Boiled in oil perhaps?

Parent - By G.S.Crisi (****) Date 02-07-2011 17:42
Yes, Dave. It's impossible to cook it at local atmospheric pressure.
Up Topic Welding Industry / Technical Discussions / Preheat temperature

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