Newbie Question on Preheating

Why do you have to preheat larger diameter bars than smaller bars?
The only thing I can consider is to prevent martensite from forming.

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To get specific answers I think you're going to need to post more details, such as the material in question.
But in the mean time: Let's compare the bars to an ice cubes. A large ice cube and a small ice cube are made from the same material and both melt at the same temperature. However the large ice cube needs more heat (BTU's not degrees) to melt. Likewise "large bars" require more heat than "small bars" to heat up the center. Small bar don't require preheat, because the weld heat alone is sufficient to heat the center. So you ask "Why do you need to heat up the center?"
If the center were cold during welding then the cold center could quickly quench the weld area. This is know as "self quenching" The quenching could cause cracking, small hard grains, martensite, and distortion. In general quenching is bad, when not expected.
On the other hand plain low carbon steel is considered non-heat treatable. Therefore it does not require preheat treatment.

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Preheat reduces the rate at which the weld area cools. This allows the hydrogen trapped in the weld metal to migrate out. Richard might chime in on this subject since he has done some research on the subject. Hydrogen trapped in the structure will be a crack site in the future. A geat example I saw several years ago was a 2.5 inch plate six inches wide that was being rolled when it fractured suddenly. The material was regular old A36. Nothing could be softer than that stuff. I was told that the steel was "defective" and the supplier had been called. I took a closer look at the fracture and it appeared that someone had used the plate as a work table of some sort. There was a tack weld about a quarter inch long that had been ground flat before the plate had been cut in strips for rolling. The sudden heating in a small location and then the rapid cooling from the massive heat sink created a site for fracture. It might have been martensite or a fracture caused by entrapped hydrogen. I am not sure. I do have pictures. If enough people are interested I could post in the dropbox. As a general rule you should preheat any material over one inch thick. I have also been ordered on many occasions to tack weld at least for a length of one inch on large weldments. The weld can be very small in cross section but must be one inch long. Randy

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Randy, Post pics or send them to me on my regular (not munged) e-mail.
Randy Zimmerman wrote:

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Hi "Chopster", Randy
Will do my best...
Point which needs to be mentioned first-off - all "cold-cracking" you will encounter in commercial practice using steels will involve hydrogen in embrittling the metal. One part per million by mass of hydrogen in steel (not austenitic stainless steel - that is highly immune!) will give you a susceptibility to hydrogen cracking. By susceptibility we mean it can hydrogen crack - that doesn't necessarily mean it will if other things are OK.
The reason the section has to be big in order to have to start worrying about avoiding cold-cracking for a "weldable" steel:
* it takes time for weld hydrogen to escape the section - if it were 3mm thick it would be gone in half an hour - much reduced in concentration in minutes - too fast for cracking to have a chance. In thicker sections it can be days and weeks.
* you need static stress to burst by cold-cracking and a thin section simply cannot hold this. Thinner sections warp. Distortion control is a major issue in thinner sections. Everyone who welds has seen that! In thicker sections, you self-restrain against shrinkage stresses, often with static stresses up at the yield point (!). So then your issue becomes then this business of cold cracking.
So to a good approximation, if your component is smaller section and bending around as you weld, you are not going to be having worries about hydrogen cracking.
RS
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good info Richard, thanks
a few related question, what is considered thin and thick section? would you say anything under 1/4 " be considered thin? by allowing the welded material sit for days before stress applied, would it releave hydrogen from the metal?
will differnet welding procedure effect this hydrogen embritlement ? example TIg would have less vs. MIg (metal heated for a longer time more in TIG) same goes for HAZ?
I've heared people tlak about hydrogen embrittlemnet in stick welding, but some one told me at welding school that it is almost does not apply to MIG and TIG.is it true?
would post heating releave the chance of cold cracking? example material which is geting powder coated usually heated to 400F for at least 10 minutes?
what about parts that are getting welded on both side? they are pretty much pre heated should that rule out hyd. emb.
thanks
On 22 Feb 2005 00:53:56 -0800, snipped-for-privacy@ntlworld.com wrote:

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acrobat-ants wrote:

That's where the streetwise guys need to contribute? What do you really know to be the case? Is this guy's guess right - that regarding hydrogen-induced cold-cracking, anything 1/4inch (6mm) or less is "thin" and doesn't need preheat or other precautions?

In thick sections, the "frozen-in" stress from contraction on cooling is very very high, as I mentioned. Externally applied stress will make not-a-lot of difference. So no, you can't escape the danger of hydrogen cold-cracking that way.
About the only place where external stress does seem to make some difference is in pipelaying - if you are sort of "dangling" the pipe down into the trench, slinging off the newly attached pipe which has just been root-runned into place. The underside is going to be in a lot of tension. Seen a pick from the 1970's of this as a practice and an analysis of it.
Sound familiar to any of you practiioners?

Yes. Solid wire MIG gives very low H. I think in the USA it is used for that reason (because there isn't much of the TMCP-HSLA steel made in Germany and Japan(?)). Lots of smaller runs on big structures like say arc-furnace roof structures - more runs but no need for preheat and other expensive and troublesome procedures (do you really want to be standing in the middle of a big structure at 100C (b.pt. of water) doing a weld? Not really!) - so better-off overall.
Assume TIG gives even lower H, but it isn't used for large-scale fab. as limited in power and slow anyway compared to MIG.

welding,
Well yes, that is pretty much so, I believe. All but basic stick electrodes need some moisture in them, giving a steam blanket over the weld. Air getting to the weld pool would give an irredemably rubbish weld. With hydrogen/steam blanket, need to take anti-H-cracking precautions in thicker sections, but at least the weld is good. Basic electrodes can be baked down to an H-level about 3 to 5 times higher then the H level from solid wire MIG.
But flux can be very useful in controlling the weld. In flux-cored-wire welding (like MIG but has a core of flux in hte wire), the shielding gas does all the shielding, so don't need moisture so is baked down to very low H level in manufacture. And being inside the wire, does not reabsorb moisture. FCW down at H level of well-baked basic stick.

Post-heating is good, but usually expensive.
But do you really need it on your components? Are they thick and hardenable?

pretty
Preheat is reasonably cheap and good - heat with blowlamps or electric blankets to a specified temperature, then you take away heat and do your weld immediately. Welded both sides doesn't make much difference does it - you experienced weldors out there...?

Well, hope my contribution(?) helpful.
RS
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Ok I have posted that fracture of a plate strip in the drop box. It is listed uder "fracture"
http://metalworking.com/DropBox/fracture3.jpg
As shown the tack is quite small and the metal obviously was molten and in a very short period of time quenched down to ambient temperature. If in doubt PREHEAT!!!!
Randy

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http://www.gowelding.com/weld/preheat/preheat.htm
Tells you why preheat is required and how to calculate what it should be.
Regards
John

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According to Mr Dyson's calculator:
Shows suggestion of 1/4inch (6.3mm) for largest plate thickness butt weld without needing preheat is spot-on...
Setting the "worst" conditions I could set for "normal" commercial welding - plate had 0.22%C (highest encountered for plate steel) and 1.5%Mn (also top), but unalloyed (typical for eg. pipeline) -> 0.47 Carbon Equivalent - SMA using higher-H electrode - calculator says ">15mmlH2/100gFe" for rutile and cellulosic - though cellulosics go way above that - said to be up to 100mlH2/100gFe - fast weld with low heat input 0.7kJ/mm of arc about 250mm/min weld speed at 70A arc (arc at about 40V due to high H?) -> 0.56kJ/mm heat per length going into metal of weld (rest eg. radiated away)
No preheat needed to 14mm of combined thickness - which for a butt weld means half that in plate thickness - 7mm
My Dyson migh like to correct me, but I think a cellulosic with a healthy hydrogen level is going to need a little bit more of precaution.
So the suggestion of 1/4inch (6.3mm) for largest plate thickness butt weld without needing preheat is spot-on, given the guidance of Mr Dyson's calculator.
???
Richard Smith
John Dyson wrote:

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