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Up Topic Welding Industry / ASME Codes / cooling of pipe with forced air.
- - By 70spoiler (*) Date 11-03-2012 00:12
I have a question regarding cooling a pipe with forced air. The reason is a, few of my welders are welding O-lets on 304 pipe, in this case it is 4 inch Sch 10. At the O - let we are getting a little pulling. The shop is striating the pipe with mechanical means. The reason is that the interpass temp is exceeding the 350 stated in the WPS. All welders are aware of the interpass temp, some weld the root of other weld and come back to it to run the second pass, others are using compressed air to cool. My boss says they can not not do that and have to let air cool .
    I cannot find it anywhere in any code (AWS or ASME) where it states what can be done and what cant. I know have run across this issue at my last job but that may have been in the ground combat welding code . Can anyone point me in the correct direction, This is B31.3 but any code would be helpful. thanks jackk
Parent - - By Jovi Zhu (**) Date 11-03-2012 03:24
Jack,

I'm not very experienced in welding of austenitic SS pipes but it seems to me that your question is somewhat beyond the Code requirement based on basic SAFETY consideration. Probably the answer would not in Code books.

Let's take a look at B31.3-2010, INTRODUCTION

"The Code sets forth engineering requirements deemed necessary for safe design and construction of pressure piping. While safety is the basic consideration, this factor alone will not necessarily govern the final specifications for any piping installation. The designer is cautioned that the Code is not a design handbook; it does not eliminate the need for the designer or for competent engineering judgment."

Table 330.1.1 doesn't address specific interpass temperature control for P No.8. Table 331.1.1 doesn't address PWHT for P No. 8 either. But always remember this is just for basic SAFTY. (Let's imagine the ASME Code Committee people finish the writing of the Code book and conceitedly say to themselves, "Okay, that's safe enough!:wink:")

:confused:Never treat the Code books as encyclopedia.

Beyond the basic safety, there should be engineering judgement. I was told that a good practice for welding of 300 series include but not limited to the control of heat input to minimize the sensitization which degrade the anti-corrosion ability of the material. (refer to ASME Section II, Part D, A-320 INTERGRANULAR CORROSION for some info.)
To my knowledge and experience, for certain grades of 300 series, some engineering companies even require the control of fillet weld size (to minimize the heat input) and require the PWHT (solution annealing) for the welded parts. These considerations are for the pursuit of optimal serviceability beyond the basic safety.

In theory, generally, the rapid cooling for 300 series is beneficial and no need to consider the preheat, hardenability, slow cooling rate, martensite transformation, etc. as we do for ferritic steels (The things done for ferritic steels are even harmful for 300 series in some cases).

Back to your question.
If your boss applied his knowledge for carbon or low alloy steels to 300 series, he might be wrong;
If your boss was worrying about the compressed air with uncontrolled moisture or other contaminations, he might be right.

I will be happy to learn if someone else on this forum could share more ideas.

Thanks,

Jovi
Parent - - By ssbn727 (*****) Date 11-03-2012 03:46
Well, I know of various defense contractors as well as some other companies that institute this practice...

The Graham Corporation is one in Western N.Y.

A couple of Submarine contractors who I'll defer from naming at this time...

Dresser Rand does use this practice a lot at all of their facilities where welding is performed...

There are more that use this method and they do make sure the air used is dry and void of any moisture whatsoever!:wink:

Respectfully,
Henry
Parent - - By jon20013 (*****) Date 11-03-2012 21:22
Jack, with all due respect to your Boss, unless there are Company or Client specific requirements, I am unaware of anything precluding this activity so would disagree with him from a Code position. 

Control of the interpass is VERY important for stainless steels and as I'm sure you're aware, failure to do so can have consequences.  I have also used the forced air cooling on a few former jobs, even ASME Nuclear (ASME III) work.
Parent - By Superflux (****) Date 11-04-2012 22:13
FWIW,

A hair dryer on cool/high fan.
That pesky stainless can really hold the heat for a long time, especially when "blanked off" for inert gas purge
Parent - By G.S.Crisi (****) Date 11-05-2012 15:02
Henry,
are you talking about Graham Corp. in Ithaca, NY?
They make vacuum producing equipment: ejectors, barometric condensers and the like.
Giovanni S. Crisi
Parent - - By G.S.Crisi (****) Date 11-05-2012 14:58
70 spoiler,
Let's look at your problem from different points of view.
304 is an austenitic stainless steel that won't change its structure even if cooled down with cold water. So, the possibility of changing from austenitic to ferritic or martensitic doesn't exist.
Now, Jovi is right at warning against the possibility of the compressed air being contaminated. The worst contaminant isn't water, as I said, but oil. If the air compressor is not oil free, the compressed air will be contaminated with oil, and oil is poison for a good weld. So, it's imperative to clean the zone that was air cooled before continuing welding.
A question that comes to my mind is if the welders blow the compressed air on the weld that is still hot. In this case, the chances of the hot metal to get cracked are very strong.

You say that the problem is a little pulling at the O lets. So, I suggest you to run a test using the welding sequence explained below:
Position the O let onto the pipe.
Apply a tack weld 3/4 inches long on the 12 o'clock position.
Right after, apply a tack weld 3/4 inches long on the 6 o'clock position.
Right after, apply a tack weld 3/4 inches long on the 3 o'clock position.
Right after, apply a tack weld 3/4 inches long on the 9 o'clock position.
Let the tack welds to cool down. I assume you're using TIG or MIG welding.
Once the tacks are cool, grind them slightly with a grinding wheel just to leave their surface smooth, no more than that. Attention!! DO NOT use a silicon carbide grinding wheel BUT an alumina one!!
Complete the first pass as your welders are used to. The tacks will be incorporated into the first pass.
Allow the first pass to air cool to leave you boss happy.
Complete the weld and observe carefully whether the O let has pulled.

If you decide to run this test, I'll like to know the results. Good luck;

Giovanni S. Crisi
Sao Paulo - Brazil
Parent - - By eekpod (****) Date 11-05-2012 15:18
As a non pipe person, I have seen weld O lets and understand they maintain a small gap(1.8"? ish) where the pipe fits into the fitting.
I'm curious what exactly is the problem with the weld o let pulling, what exactly does that mean?

I only ask becasue I'm not familier with that problem, just for general knowledge and I have a little extra spare time today to type this out.
Parent - - By Cumminsguy71 (*****) Date 11-05-2012 20:55
Eekpod,

I believe what they are saying is that the weld is warping the pipe in some way as the O.P. did mention mechanical straightening I believe. I've done them on steel pipe bunches and never noticed but mechanical rooms and in the dirt they don't seem to care but talking with a friend the other day who was doing some stainless pipe and he was saying it would warp pretty easy. Plus the O.P is running schedule 10, that's thinner than some of the christmas paper cardboard tubes I used to hit my sisters with!! LOL!
Parent - By ssbn727 (*****) Date 11-06-2012 00:59
Shawn,

You hit the nail right on it's head!!! Schedule 10 will warp very easily if exposed to excessive heat and yet, the weld-o-let's will not as much but, they will move off it's datum and will have to be straightened afterwards and usually with mechanical force... Along with some applied heat as well sometimes.:wink:

Yes indeed Giavonni, and they also make heat transfer equipment for energy markets and process industries worldwide.:wink:

http://www.graham-mfg.com/default.asp

Respectfully
Henry
Parent - - By CWI555 (*****) Date 11-06-2012 00:40
It will suck up around the Olet in a similar manner as hoop shrinkage will.
Parent - By ssbn727 (*****) Date 11-06-2012 01:08 Edited 11-06-2012 01:12
I have seen that happen many times as well when someone doesn't have a clue when it comes to welding stainless steel schedule 10 pipe regardless of the type of weld-o-let being used... Using forced air as Jon and others mentioned is an excellent method to use if done properly and hair dryers blowing cold air is another one as Superflux mentioned.:cool:

The industrial type of hair dryers are the preferred type to use because they do a better job of blowing more cold air than the smaller, less powerful ones being used for personal use

Respectfully,
Henry
Parent - - By ssbn727 (*****) Date 11-06-2012 01:27
Hello my friend Giavonni,

The forced air isn't blown directly on to the weld... What happens with pipe is the air is blown through the pipe so that the entire weld zone is exposed to the cold air from the inside diameter of the pipe while the welding is being performed on the outside diameter of the pipe...

So the weld zone of the pipe isn't exposed directly with the cold air with the exception of the root of the weld and heat affected zone found inside the pipe and yet, the outside of the pipe where the weld is being deposited would not be directly exposed to the cold air.

Respectfully,
Henry
Parent - - By Cumminsguy71 (*****) Date 11-06-2012 10:25
That's what I was thinking Henry, air blown thru the pipe while welding and there you have it, they already do it! Would kinda be like welding on a gas line with gas flowing, nice and cool!
Parent - - By 803056 (*****) Date 11-06-2012 15:21 Edited 11-06-2012 15:28
Controlled cooling (slow cooling) is an issue when welding carbon and low alloy steels that have the propensity to harden when the austenite decomposes to form martensite. As the carbon equivalency increases, the propensity to form martensite becomes more likely if the weld and the adjacent heat affected zone are allowed to cool rapidly (quench). Slow cooling is the mantra we live by when welding those types of steels. Another issue is the amount of diffusible hydrogen in the weld and adjacent heat affected zones. Slow cooling, high heat input, stress relieving, etc. can be used to reduce the amount of hydrogen in the weld and HAZ thus problems with delayed cracking can be minimized.

Austenitic stainless steels (high alloy steel) are a different matter. Assuming the correct filler metal is selected and we develop some ferrite, but not too much ferrite, we should have a composition that solidifies as ferrite and decomposes into austenite if we do everything we should. Delayed cracking due to diffusible hydrogen is not a concern. Austenite, a FCC crystalline structure is very tolerant of hydrogen and can hold it in solution without causing serious problems unless the stainless is cold worked and martensite is formed.

There are several things we can do that can upset the apple cart. Complications such as too much ferrite and too slow a cooling rate can cause Sigma Phase; a hard brittle intermetallic compound that is not a good thing. Too slow a cooling rate can cause sensitization. This is a time at temperature problem that is minimized by limiting the time the weld spends in the critical temperature range (roughly 1100 to 1600 degrees) by limiting the interpass temperature. Steps taken to reduce the time the weld is exposed to the critical temperature range is beneficial. Thus we control interpass temperature as a means of reducing the time the weld is exposed to elevated temperatures.

Enhanced cooling rates will also reduce the time at temperature and it will reduce the tendency to form chromium carbides (sensitization) along the grain boundaries in the HAZ. Sensitization requires diffusion which takes time, so anything that increases the cooling rate once the weld has solidified can generally be viewed as beneficial. 

Cooling the weld and adjacent base metal with dry, oil free cold air - it is a good thing.

I have pumped cooling water through a type 316L austenitic stainless steel pipe while welding on it to prevent damage to adjacent components without any apparent ill effects.

Distortion is a different matter. One means of reducing distortion is to reduce the temperature differental between the area being welded and adjacent regions. Preheat can minimize distortion when welding carbon and low alloy steels, but it works against us when welding austenitic stainless steel where sensitization and/or sigma phase is a concern. The use of strong backs and post weld straightening is about the only solutions I have used. Precambering can also be used if there is a history that can be used as a guide to how much precambering is needed to counter the weld induced distortion.

Best regards - Al
Parent - By Superflux (****) Date 11-06-2012 16:53
When installing "O"lets on any pipe, it is best to bore the holes after welding (and cooled to ambient) is complete to reduce distortion... if possible.
Parent - - By ssbn727 (*****) Date 11-07-2012 09:17
Hi Al,

As usual, you have managed again to explain this as well as I know, and have read, and experienced as well in both the field and the shop...
I couldn't have explained it better myself brother!!! Bravo!

Btw, today is my birthday and 54 and will soon get a lot more!!! Last year, I thought it was going to be my last one and then I said to myself, "What's with these negative waves???" Then I made a promise to myself that if I made it to the next birthday, I would just keep on getting better and better and it worked!!! Sorry I went off topic everybody.

Nice explanation Al!

Respectfully,
Henry
Parent - By nantong (**) Date 11-07-2012 11:27
70spoiler, what diameter of olet are u fitting onto  4" pipe? What is the throat thickness of the weld deposit and how is this measured? (Common problem is depositing too much weld metal). Have you restrained the 4" pipe with a strong back and u-bolts during welding and cooling or tried presetting the 4"pipe before welding? If you are fabricating a header with multiple olets on a run of thin wall stainless it is difficult. Of course it is always better to use tees or reducing tees.
Parent - By welderbrent (*****) Date 11-07-2012 13:20
Obviously I'm not Al, but thanks for the update Henry.  Most of us are interested in your continued health and like to hear how it is going.  You are the object of many prayers.

Sometimes we all need an attitude adjustment, and often it is our health that brings us to times of reflection and then either makes us or breaks us. 

And yes, Al has once again done a fine job of explaining a position. 

Have a Great Day,  Brent
Parent - By 803056 (*****) Date 11-07-2012 17:32
Gentlemen;

Thanks for the compliments.

Happy Birthday Henry and may there be many more to come. I believe I can speak for all of the regulars of the Forum in saying we're happy to see you are doing better. Your wisdom and counsel is enjoyed and appreciated.

Best regards - Al
Up Topic Welding Industry / ASME Codes / cooling of pipe with forced air.

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