fan cooling of weldments

Date 04-13-2006 15:37

Hello quality0537, you will likely hear that it will not harm the properties of the A-36, however, you will probably also find that it could alter the condition of the actual weld metal, as well, the 1.5" thickness does call for a preheat before welding in most cases. Thus, not a very good idea. Regards, aevald

By Bill M

Date 04-13-2006 18:07

What temperature would these weldments be at when you turn the fan on? A-36 mat'l over 3/4" thru 1-1/2" requires a minimum 150 deg. f. preheat - interpass temperature. Cooling from stress relieving allows the part to be cooled below 600 deg. in "still air".

By jon20013 (*****)

Date 04-13-2006 18:28

Someone please correct me if I'm wrong, but I've always been told that its no problem for rapid cooling A-36 (and some of the other mild steel alloys) no matter the temperature (I suspect within reason). Once upon a very long time ago, my Welding Engineer even took one of my A-36 test specimens (still hot from welding) and put it in a bucket of water to cool.... no apparent concerns as it bent just fine... one might imagine my face though being a young pup of maybe 24 years old at the time!

By CHGuilford (****)

Date 04-13-2006 20:22

My understanding is that, practically speaking, A36 is not harmed by quenching in water. That was from a seminar I just attended that was conducted by a highly respected engineer with a lifetime of welding experience. I am not in a position to refute that because I would only look even more foolish than I already am. However, many welding codes prohibit the practice and maybe that's to err on the side of caution?

I just looked quickly for my info and didn't find it yet, but I do remember something from AASHTO, or an State DOT specification, or etc. When heat cambering bridge members, it has been allowable to use dry compressed air to accelarate cooling once the tempertures were 600 deg F or lower. We still use that in our cambering procedures so we could accelerate cooling if we need to. Normally we just let heated steel cool in still air but take no special precautions to shield from wind if the doors are opened.

The difference between PWHT and cambering is that PWHT is supposed to relieve stresses and uneven cooling could be a problem. Cambering is intended to put stresses into the member to gain deflection; the heating is intentionally uneven so accelerated cooling wouldn't be a problem.

Chet

By jwright650 (*****)

Date 04-13-2006 20:39

Chet,
I couldn't find it either, but somewhere in my travels, I've seen the under 600F take the gloves off, anything goes, rule. In fact I distinctly remember a picture of a bottle with water in it and it was hooked up to compressed air that blew through creating a mist to speed up the cooling after cambering with heat. Seems like there was a note stating something about the steel was to get below this 600F threshold before applying this method of rapid cooling.
John Wright

By aevald

Date 04-14-2006 00:36

Hello Gentlemen, I might make one comment, the weld metal, not the A-36 parent metal, "can" be affected by a rapid quench, maybe not air cooling, if that makes sense. Regards, aevald

By dasimonds (**)

Date 04-14-2006 00:54

I believe that the carbon plays a significant role in the final weldment. Number 1 being the composition. Number 2 being the rate of cooling. And Number 3, come on, What is the real difference between the parent metal and the weld metal itself? There being not an exact replication of the microstructure, but one close enough to be used for engineering purposes. I mean, that is what welding is all about, isn't it? The ability to form welds with all the microstructural properties inherent to a particular application?
Help me out here,
Dale Simonds

By GRoberts (***)

Date 04-14-2006 02:50

In all actuality, the weld will typically handle rapid cooling better than the base metal because that is what it is designed for. Consider welding on a 1/2" plate without preheat. The weld will cool much faster because of the heat sink provided by the 1/2" plate than it will by fan cooling. In fact, the carbon content of the weld is usually much lower than that of the base metal in anticipation of this fast cooling. While the base metal such as A36 cooled much more slowly from the rolling temperature, so it has a higher carbon content in order to get its required strength.

By Lawrence (*****)

Date 04-14-2006 04:08

I dunno about the engineering aspects.

However, when my students take a short cut and quench even limited thickness A36 test coupons between passes or post weld, they break in two pieces every time.

By ssbn727 (*****)

Date 04-14-2006 11:28

Hi Lawrence!

That has been the experience with all of my students also - including unlimited thickness test plates...

Most of the welds looked half-way decent... In other words, decent enough to pass visual but, could'nt pass bends or even come close!!!
When the students retook their tests afterwards, all of them passed with no indications!!! Go figure!!! All of these tests were independently evaluated.

Respectfully,
SSBN727
Run Silent... Run Deep!!!

By thcqci

Date 04-14-2006 15:03

A Guide to Engineering and Quality Criteria for Steel Structures, Common Questions Answer, 4th Edition, 1997, published by AISC states:

"2.3.2 Is it permissible to accelerate cooling of structural steel after the application of controlled heat?
Yes, provided heated steel for Cyclically Loaded Structures is first allowed to cool ambiently to 600°F. Because the maximum temperature permitted by AISC LFRD Specification Section M2.1 for heating operations is below any critical metallurgical temperature for material being heated, the use of compressed air, water mist, or a combination thereof should be permitted to accelerate the final cooling of the heated. For members to be used in cyclically loaded structures (i.e., where fatigue and toughness are design issues) it is recommended that such accelerated cooling not begin until the temperature has dropped below 600°F. This limitation is more historical than technical in nature. As a fair balance between the desires of the fabricator and the concerns of the owner, it provides an additional safeguard to prevent the abuse of excessive cooling and undesirable residual stresses should accepted procedures not be strictly monitored."