Heat cambering rolled W shapes

Date 01-05-2007 14:18

Good question!

But when you are heat cambering, you are shrinking the web and the flange at all of your heat spots and or heat wedges depending on your cambering procedure. And then there will be a point where heat spots or heat wedges will not move the piece any further and you will need some mechanical assistance.

Will be watching this one to see how others will chime in and comment this.
Very Good Question....

By aevald

Date 01-05-2007 15:42

Hello John, I believe that you are correct in your comment regarding the amount of cambering that can be achieved through the heat shrinking method. Certainly a point will be reached where you will no longer achieve movement. One other thought here though, If the beam material is anything other than A-36 grade, a person might be doing some serious harm to the mechanical make-up. A cold-working process would be the preferred method for these other types of materials. Just my $.02, Regards, Allan

By Joseph P. Kane (****)

Date 01-05-2007 16:48

My experience indicates that the thickness of the web is what seems to control the warpage. If a web is less than 9/16", even 60 MM camber in 20 Meters will result in web distortion that is excessive per the NYSSCM. What is more, this will happen when you heat the first heat triangle. (If heating is performed in accordance with the NYSSCM.)

There are three very good examples in one of your current projects being fabricated in NH.

By CHGuilford (****)

Date 01-08-2007 18:06

It is difficult to eliminate web distortion, or "oil-canning", when heat-cambering beams. I have found that we get better results when we use 2 torches to vee heat for camber - one on each side of the web- for beams with webs 3/8" and thicker. The oil-canning is definitely less than using one torch. Normally there is no problem in meeting the D1.5 requirements. It also works better to take enough time for the heat to "soak" through the thickness- that includes the "k" section (which is often not heated enough to get the best results).

A very un-official rule of thumb that I use is- 1/8" change in camber for each vee heat made. This is for beams 27 to 36 inches in depth, about 50 feet long. Sometimes we get more reaction for each heat, sometimes less, but usually it works out to be close. The vees are a minimum of 75% section depth. Heating is done with smaller oxy-propane rose bud torches set to be slightly fuel rich (feed just enough oxygen so that the primary flames are bright blue with feathery tips- but no white streaks- and the flame is clean) for a slightly lower flame temperture but higher BTUs. This has the appearance of being slower to do but overall, the beam is finished sooner and with fewer vee heats.

I believe it was John Wright who posted that re-heating steel, that was heated and allowed to cool, has no additional effect. I agree with that. Once you have heated the entire flange that's about all you are going to get without going to drastic measures.

Thermal pre-loading is effective in gaining more reaction - simply preheat the flange opposite from the one you will heat (the vee points to it) to 250 degrees for a length 4 X what the vee width will be on the other flange. Then make your vee heat.

If anyone wishes to preload beams with hydraulic jacks before making their vee heat, I have 2 cautions. 1st- this usually requires an engineering evaluation to make sure the beam is not crippled. 2nd- I know of 2 guys who were killed when the jacks kicked out and released the beams like a bow and arrow. Sadly, they were sitting on buckets in between.
Also use caution if preloading by piling weight on top. Again, an engineering evaluation needs to determine how much weight can be used. Heated beams move around and weight can destabilize and fall if precautions are not taken.

By thcqci

Date 01-12-2007 14:30

I was in the welding industry back in the late 80's and early 90s. Was in a couple of bridge shops and a few structural shops where heat cambering/straightening was performed. Back then A36 was more common. I remember V shaped wedges, ¾ of depth or better being used everywhere (as described above). Was almost out of welding industry for most of the 90s but jumped back in here in 2001. We have a cambering frame so rarely have to heat camber anything and have not for a couple of years or more. I have seen these guys struggle, using ½ circle shaped patterns that only go about ¼ depth of section. They do this because they claim that A992 won't move like A36 did, which makes some sense that it may not, and so they can avoid heating the biggest part of the web so it won't oilcan. They say if they use the deep V pattern, as I suggest, they will oilcan the web so refuse to try it again (could not be the technique they use to heat it either could it?). Of course they work 10X as hard trying to move the section than if they would follow any of the suggestions I have given. Guys here have been doing this 25, 30, 35 years so they know better than the stupid QA/QC guy. Had real battles at first keeping them at <1200°F instead of just heating to bright red steel (did you know they make tempstix?). "We have to heat A992 over 1200° just to get it to move". Not on my watch you don't! And DUH! If you don't heat the web very deep, it won't move very far! And no, you can't use water afterwards until it cools to <600°. I think we need to let AISC and AWS know that A992 is special so they can change their heating limits just for us!

Have any of you ever seen shallow, ½ circle shaped heat patterns before? Do you know where there is an article about cambering or an industry standard procedure covering the cambering of A992 that I can present to these guys? I have a couple of other company written procedures, but they seem to discount them. Any help would be welcome.

By Lawrence (*****)

Date 01-12-2007 16:15

http://www.aisc.org/MSCTemplate.cfm?Section=Back_Issues1&template=/ContentManagement/ContentDisplay.cfm&ContentID=26794

Not exactly specific to your steel but it is mentioned and the temp limitations are also emphasized

By jwright650 (*****)

Date 01-12-2007 16:45

Thanks for that link, Lawrence....I just printed it out for our folks to read....just to reinforce my instructions to our shop personnel. They are under the impression that people make up rules to run companies out of business.

"C'mon, I just don't go around making this stuff up. Guys, there are reasons for using our procedures...."
- quoting myself

By thcqci

Date 01-12-2007 21:45

Thanks Lawrence. That is an article that I have in my electronic library that I had forgotten was there when I asked my question. I have printed the full article version and given to our shop personnel, but evidently AISC's advise does not pull enough weigh with them. We will see next time the issue comes up. I can enforce the not going over 1200° limit, but I can not make them heat in the wedge shape instead of ½ circle shapes. Their loss.

By the way John, I have never heard myself say that around here.

By jwright650 (*****)

Date 01-13-2007 11:55

Doug,
Don't forget about the book by John P. Stewart....Flame Straightening Technology for welders. Lots of good applications in there, but the copy that I have deals mainly with light weight materials, frames, plates...but the principles are the same. Check your email...I have something for you..... ;-)

By CHGuilford (****)

Date 01-15-2007 17:45

I have heated a lot of A992 and not had any difficulty in getting the results I wanted. I believe getting the heat all the way through the steel thickness has more influence on good results than higher yield strengths has, when cambering beams.

One caution on heating bridge projects- whether you use line heats, vee heats, or semi-circular heats, most DOT's require you to submit a heating procedure.

By HgTX

Date 01-29-2007 21:50

Haven't seen it, and it doesn't make sense. If you only heat part of the web, then (a) the unheated part will be restraining the heated part from shrinking (b) you're only shrinking part of it and you're leaving the rest unshrunk, and it seems to me that if you try to just pucker one edge of the web instead of taking out a wedge, you'd get more distortion rather than less.

In the bridge girder shops I've been in they typically use 50-ksi material (more like A 572 than A 992, but not a big difference), and they use deep V-heats to correct camber in the built-up shapes. I haven't made a statistical study of it, but it didn't look to me like they had more web distortion problems in those webs than in the ones they didn't have to correct; I've seen plenty of webs that needed heat-correction that hadn't had any camber correction done to them.

Hg

By HgTX

Date 02-01-2008 16:31

http://tinyurl.com/2pgm64

Federal Highway manual for a course they did a few years ago (good lord, almost 10 years ago; time flies) on heat-straightening. A lot of the same principles apply to heat-cambering and heat-curving. Fairly academic; not so much a "how to" for the guy on the floor, but I think you can extract some practical advice from it.

And...what they're now calling A 992 is very likely the same stuff they've been using for probably 15 years.

Hg

By hogan

Date 11-20-2008 16:54

HgTX,
Thanks for the link "http://tinyurl.com/2pgm64"

By shortterm40 Date 02-01-2008 14:02

We have a cambering machine but are unable to camber W 30x391x50' to 58' long beams. It will push them about 3/8" but we are looking for 5/8, 3/4, 1, and 2".
With heat we are getting up to 1" camber quite easily. But for the 2" I think we may have to heat and push it with the cambering machine.
Has anyone tried this? I was thinking of heating just outside of the rams, push it then let it cool and heat the center. I would heat 6" wide all around the beam.
Any advise would be appreciated.

By jwright650 (*****)

Date 02-01-2008 15:28

I wouldn't do both...the results may prove to be unpredictable. I would stick to heat cambering. Are your spots of heat concentrated and centered on the flange to where it soaks into the web, and then are you also heating up into the web with wedge shaped patterns? You will need to start at quarter points along the bottom flange and then if that does produce the amount fo required camber move into to eighth points, and then divide that up again....eventually you will get that 2" of camber...make sure you have the beam standing on the top flange and sitting on blocks at each end so it can hang in the middle, the weight of itself helps you get that amount of camber. We do this all the time, so I know 2" is easily achievable in a 50'+ piece. Be sure to monitor your heat with 1100F tempil sticks, you don't want to ruin your beam by over cooking it.

By CHGuilford (****)

Date 02-01-2008 18:07

You could use the machine to get whatever cold camber you can, then finish the cambering with heat. But I would be very careful if you intend to use the cambering machine to preload the beam while you heat it, or even jacking while the beam is hot. You could easily cripple the beam if you go overboard with the rams but I think you could control that sufficiently. The question will be: Is the steel that yielded under high temperature to achieve the camber still sound?

By eekpod

Date 07-17-2008 11:19

Our heat cambering procedure is the same as the info. posted here; multiple Vee's, 75% depth of the web, standing up on flange on blocks out at the ends, don't exceed 1100 degrees.

My question is, is there a certain temp to heat the material up to that works best or should they just go for the hottest they can without exceeding 1100?

Will 600-800 work versus 900-1000? I imagine thickness of material comes into play.

Now the materail they are working on is a 60' pc of A992 W40x 431, it needs 1 1/2" of camber.
Chris

By jwright650 (*****)

Date 07-17-2008 12:39

Chris in my experience, you will most likely see the most movement around 1000 - 1050F.

By HgTX

Date 07-17-2008 15:46

In theory, the hotter you go, the lower the yield strength is, and the more it'll move under whatever preload (including gravity) you may have set up. I've seen a graph showing that yield strength for typical structural steels doesn't drop much till around 900F.

On the other hand, recent research shows that the steel will be in better shape later on, from a ductility standpoint, the less heat you put into it--at least when you're using force to bend actively, rather than just a heat-shrink method. Probably not a concern for most non-bridge applications though.

Hg

By eekpod

Date 07-17-2008 16:37

Thanks guys, Chris

By jwright650 (*****)

Date 07-17-2008 16:47

[60' pc of A992 W40x 431, it needs 1 1/2" of camber]-quote

piece of cake...it probably has half of that or more already in it from the mill.

By eekpod

Date 07-17-2008 17:52

We checked before hand hoping the same thing. I only had a 1/4". This was most likely the straitest piece to come out of the mill, just my luck.

By jwright650 (*****)

Date 07-17-2008 18:04

LOL....never will be that straight unless you don't need for it to be.....

By sunn625 (*)

Date 07-18-2008 02:22

maybe for the dimension precision requirement of the camber or ring, you can adopt VSR system. not to heat or cold. my Email:sunnn@semboo.com

By swnorris (****)

Date 07-18-2008 10:26

John,

Aint that the truth!!

By 803056

Date 07-20-2008 22:29

You can start to see some permanent deformation at about a delta T of 250 F.

The trick is to maximize the difference in temperature between the area being heated and the adjacent areas. When possible, I clamp an angle or a bar to "outline" the area being heated. I use water soaked rags on absorb the heat that defuses beyond the bar (or angle) and to maximize the delta T.

Heating starts in the most restrained location, at the apex of the triangle as described by eekpod, and I move the torch toward the free edge using a zig-zag pattern. Do not heat the entire area uniformly. Heat the area directly under the torch until it is to temperature and move on along the zig-zag trace. In the case of the beam, the flange is the free edge, and it too has to be heated, but not until the web has been heated and cooled. You can air cool it or you can use water soaked rags to hasten the cooling as long as you didn't heat it above the lower temperature of transformation which is 1330 F for carbon steel. There is no reason to heat it that hot, good results can be achieved at temperatures of about 800 F without damaging the steel member. In the case of the flange, move the torch across the width of the flange starting at one edge and moving steadily toward the opposite edge. Move an inch or two down the length of the flange and heat again. The bigger the torch, the better. Heat the area quickly as possible with the torch and then allow it to cool.

Now for the reality, if you are working with a plain carbon steel (ASTM A36), even if you heat it above 1330F and cool it with wet rags or an air spray and water, there is too little carbon to cause problems with excessive hardness and loss of ductility. The same can't be said as the carbon content increases or the base metal is quench and tempered. In the case of quench and tempered base metal, you don't want to exceed the tempering temperature. The trouble is, most welders do not know the carbon content of the base metal they are working with. If they don't know the carbon content and the other alloying elements, they'll not be able to calculate carbon equivalency. So, to be safety the "rule" is to keep the temperature below the lower transformation temperature and the operation shouldn't cause any unacceptable microstructure if the base metal is a plain carbon steel or a high strength low alloy steel.

Best regards - Al

Attachment: CamberingbyHeating20-July-08.dwg (28k)

Attachment: CamberingbyHeating20-July-08.wmf (5k)

By HgTX

Date 07-21-2008 15:54

My statement about nothing much happening till around 900F had more to do with bending by force, not purely by heat upset. Sorry to confuse topics.

You say you don't heat the flange till after the web has been heated *and cooled*? How can the web shrink if the flange is still there unheated, holding it apart?

Hg

By CHGuilford (****)

Date 07-22-2008 02:23

That's how I do it too - start at the apex and move out to the flange.
I believe that is part of the reason why you get some "oil-canning" in the webs - the flange continues to shrink a bit after the web has stopped.

We have also found that you don't seem to get the full reaction from a heat until the steel has cooled almost enough to touch it. And once an area has been heated and cooled, you don't get any practical gains if you try heating the same location again (matter-of-fact, that's a trick we have used to reduce distortion on certain weldments; heat the steel to 1100F in the weld zone and let it cool; then fit and weld it).

I won't pretend to know why any of that works, I just know that's what we have seen over a long period of time.

By 803056

Date 07-22-2008 02:41

The basis of flame straightening or cambering is to cause a portion of the material to expand, exert a force on the surrounding material, cause yielding to occur (plastic flow), and then upon cooling exert a tensile force equal to the yield strength of the material.

As it turns out, a delta T of about 220 degrees is all that's needed to reach the yield point of a number of base metals.

The web is exerting a tensile force on the surrounding metal after being heated and cooled. As soon as the flange is heated, it starts to yield because of the surrounding material still at "ambient" and the adjacent web which is in tension.

As for only heating and cooling once, I agree if you heat the base metal up to a very high temperature, but you can have better control and you can repeat the cycle a couple of times (with less reaction each time) if the temperature is limited to about 500 degree. It depends on where the emphasis is, close control or speed.

Best regards - Al

By hogan

Date 11-20-2008 16:55

803056,
Thanks for your insight

By 803056

Date 11-21-2008 04:10

It looks like you are doing a little research on flame straightening or cambering. This is a pretty old thread.

I find that heating plain carbon steel and low alloy steel in the area of interest to temperatures between 600 and 800 degrees F works very nicely without any danger of increasing the hardness. Just the opposite, as long as the temperature is below the T1, the material will be softened slightly by tempering.

Best regards - Al

By hogan

Date 11-21-2008 14:04

803056,
This technique works very nicely. Thanks again

The trick is to maximize the difference in temperature between the area being heated and the adjacent areas. When possible, I clamp an angle or a bar to "outline" the area being heated. I use water soaked rags on absorb the heat that defuses beyond the bar (or angle) and to maximize the delta T.

By 803056

Date 11-21-2008 19:04

Happy to hear you it worked for you.

Best regards - Al

By Metarinka (****)

Date 07-22-2008 13:25

I've never heard of the term "oil-canning" anyone care to explain

By CHGuilford (****)

Date 07-23-2008 01:33

The bottom of old style oil cans were made convex and you would press your thumb on the bottom to make it "snap" so the oil would spurt out. The bottom would snap back when you let it off. Distorted webs of beams that have been heated in a vee pattern often resemble the bottom of an oil can. Some folks noted that if you whacked the distortion with a hammer it would simply snap to the other side. It doesn't always happen that way but when it does, you have the oil can effect.
The term was used years ago but an similar term would be panel flatness.