Localized heating of piping

By roybuck (*)

Date 01-29-2002 15:57

Sorry guys, I left out the code: B31-1.
Niekie 2, I would especially like to hear your comments on this.
Roy

By MBSims

Date 01-30-2002 04:22

This will not cause a problem on carbon steel piping (ASME P-1) even with heating temps up to 1100 to 1200 F. But you don't need 1200 F to make it work - 600 to 800 F is fine. The diamond heat method is well accepted in the power industry for field adjustment of carbon steel piping. There may be some issues to be addressed if the piping is to be used in severe service (i.e., high pressure hydrogen, aqueous hydrogen sulfide, aqueous cyanides) or notch-toughness properties are a concern.

Marty

By pipewelder_1999 (****)

Date 01-29-2002 19:10

On plain carbon steel based on what I have read, the only "damage" could be localized stress from the "bending". The "damage" in my opinion would be no differnt than adding a weld on the opposite side. Actually it could possibly be less.

As with all of these type issues, a decision must be made based on the material, service, and risks associated with failure. All of these things are decisions to be made by engineers resposible for the product.

So my above statement is only an opinion based on information learned from engineers. I too would like to see any other information related to this.

Gerald Austin

By R. Johnson (**)

Date 01-30-2002 02:59

For plain carbon steel 800 degrees F is below the lower transformation temperature and therefore you should have no metallurigical changes in the steel. Locked in stesses on the other hand could be a problem if the pipe is pulled into positioon, held and not allowed to cool with no restraint on it.

By roybuck (*)

Date 01-30-2002 11:28

I was concerned with the induced stresses, although not because of the service medium; it is cooling water. I also take issue with the quenching, but cannot articulate why I think this is a bad idea. I'm still hungry for more information.
Roy

By M-Squared (**)

Date 01-30-2002 15:15

If your concern with quenching is the formation of Martensite (making the steel harder), I would not worry about it.

First thing to remember is that in order to obtain max hardening (hardening through the entire thickness), the steel must be quenched from the austenitizing temperature at a rate greater than the ‘critical cooling rate”. Since you are talking about heating to temperatures that have been suggested here (800 F, 1100 F) you are not even close to the austenitizing temperature. For a carbon steel having a carbon content of .2% this temperature would be over 1500 F.

In my opinion I do not believe that there is a concern for your application unless we are talking about something other than a C-Mn steel.

By Seldom

Date 01-30-2002 18:20

In my opinion, M-Squared has given you the technical answer you’re looking for. In addition, he’s also given you something very important to verify on the practical side. The temperature that the fabricator is really bringing the pipe wall to just prior to quenching!

I’ve had many years of both “hands-on” and “verification” experience with this age-old procedure and while acknowledging I haven’t “seen it all”, I can say to you that I haven’t seen it performed at 800F! Generally 1200F and too many attempts made as well over 1500! Call me a skeptic, but unless I know without a doubt that the fabricator is using a temperature-measuring device, he doesn’t know at what temperature he has brought the pipe wall to before quenching.

800F shows no visible color change to the pipe wall, 1200F shows a very dark red that can be difficult to see in sunlight. 1600F is bright red/orange and believe it or not, unless the fabricator is monitoring the temperature I’ve found more shops going at least that high and without checking swear it’s only 1200F!

I suggest that if you're the Owner or represent the Owner, grab an 800F, 1200F, and a 1600F Tempstik and pay the shop a visit so you can verify their procedure. You may find out that they do follow the procedure they told you or you may be in for a surprise!

By Niekie3 (***)

Date 02-05-2002 19:26

I have been stuggling to post replies on the BBS for weeks now, so I answered directly. As I am now able to make replies, I thought that I will also place it here, just for completion sake...

In South Africa we term this method of bending material "flame shrinking". As such I do not actually know the term "diamond heating", but I understand what you mean.

This practice is also one that is much debated around our neck of the woods, and the debate is hot because there are a number of issues that need to be considdered:

1)The temperature of 800F is well below the transformation temperature for ordinary mild steel. (This starts at around 723°C.) As such, the heating and quenching as proposed will not cause the pipe to become brittle.
2)The time periods that the material will see the high temperatures will also typically be much too short to cause significant grain growth or other microstructural changes.
3)The bending is caused by "distortion" which is pretty much the same thing that happens when you weld. (Only then you usually don't want it to happen.) This bending introduces residual stresses into the material at these points, just as one would introduce residual stresses when you weld or cold bend the pipe. Under certain circumstances, this may lead to a hightened susceptibility to stress corrosion cracking. (SCC) Another possible problem is that if the pipe has a weak spot, these stresses (which approach the yield stress of the material) may cause failure of the pipe on this weak spot. The same may happen if you cold bend the pipe.
4)Control of the heating under these circumstances is almost always very dodgy. Generally the artisan emplying the technique will heat the pipe till s/he gets a certain colour on the pipe that s/he associates with this particular technique. Usually this will be substantially above the temperatures you mentioned, and may even be above the transformation temperature. When this happens, you can start getting problems if you subsequently quench it down.

My personal opinion is that if you can make sure that the temperature remains below 600°C, this technique should not cause undue problems from a metallurgical point of view. You must just be sure to control the heating, and be satisfied that SCC is not a problem in your medium.

Just to make sure that you did not get any martensite formation, you could request the contractor to perform hardness tests on the affected areas.

You mentioned that the pipe is to ASME B31.1. I do not have the code with me, so I can not check if it is mentioned / allowed in the code. I personally have never used this technique on pipes. I have however used it often on structural sections such as I beams, which distorted following repairs and / or modifications.

Hope this helps

Regards
Niekie Jooste