Airburst

Cromoly steel BMX frames, as well as other welded BMX parts, are often advertised as "fully post-weld heat-treated". Just what does this mean? Is it simply a normalisation process to relieve any stresses caused by deformation during welding? If not, what does it do?

How hot do they have to get the frame, and for how long? Is there some form of quenching involved?

I'm just curious here, it's not like I'm going to try it, but I figured this would be the best place to ask.

Thanks in advance

Falanx

DMR claim to already perform a similar treatment for one of their AM/dirt jump frames. They're retailing it as the '898' and that figure is allegedly the stress-relief temperature they use, although in 4130 that seems just a little high. You wouldn't need to quench from there to harden, let alone stress relieve.

Aaaaaaaaaaanyways.

There is little point to stress-relief on its own in a welded, tubular structure designed for high strength. Certainly the soask to allow relief and the gentle cool will reduce any mechanical stress residula in joints or tube walls from assembly, but to normalise a steel tube or anneal it and then just leave it would reduce strength noticeably. The tube as supplied and prior to welding will almost certainly have a degreee of cold work applied and therefore some work hardening.

The cycle is most likely (not knowing the companies individually); stress relieve - quench - temper - (small degree of straightening, much less than the stress from welding)

For a quench in 4130, you don't need to go much above 850, and quench into oil. In pre-heatreated, thinner tubes, you can get away with an air quench to values of Vickers below 400, which equate to UTS of over 1100MPa. Tempering below 400 degrees would barely remove any strength, either.

NoReg

Are we talking 850 Centigrade? Just to be clear.

In welding aircraft in 4130 they often do post weld heat treat, which involves nothing much more than a gentle playing of the flame over the joint area, sorta an extended cool down. This is done even though the preferred filler is a mild steel. Obviously if one wanted to use a 4130 rod there would be even greater concern about hardness in the weld area. This process is done only to the weld area not the whole frame, though one could heat the whole frame to some mild heat, just as part of the process if one preferred, the heat isn't enough to hurt anything.

One of the things that interests me in the metals area are the different working myths. So when it comes to whether you can harden metals with 40 points of carbon, at all significantly, whether using a lye quench or super quench, there are some who regard this as a ridiculous claim. Yet in the welding end of things where cool down is always gradual, there is great concern about air hardening. One could sorta split the difference and realize that there isn't that much in play in either direction.

Falanx

Yes, sorry. We haven't used farenheit in the UK for a while, and seeing as the OP is from this sceptred isle and all that...

Now, I was unaware that anyone welding 4130 in the aerospace industry used anything but VIM melted 4130 filler. The aerostructures companies my employer works with would kick you out of the office if you suggested anything else.

One of the biggest misconceptions, and I've said it here before, is the difference between hardness and hardening. A 0.3% carbon steel will harden, and to an appreciable degree with just 0.20% molybdenum in it. As supplied tube is drawn and normalised, unless it's already heat-treated and i assure you that 4130 tube for bike frames doesn't require a 'superquench' at all. Its section is far to small to require it. Near a weld fillet, perhaps, but then you have a nice cold tube sucking heat out of the fillet far quicker than any fluid medium will. So it has sufficient hardenability. What it won't have is high hardness. But that's not a problem, indeed, it's beneficial.




This all comes down to my previous, (and previous-previous ad nauseum)statement about the confusion of hardenability and hardness. These stories of superquenching come from people attempting to through-harden a piece of 4130 a quarter inch thick, which is silly. Nowhere structural on a bike, save heavy-duty dropouts, is anywhere near that thick. Or amateur engineers attempting to take really cheap stock and make it into a shear-blade. You'll easily get 4130 in bike-section thicknesses to over 40HRc /390Hv which is about 1300MPa, or 180ksi in old money, with an oil quench which is a good level of strength for a basically workhorse steel in a structural application.

And please, stop with the 'points of carbon'? For my sanity, pretty please? It's just a darned percentage, after all... :-)

NoReg

"Now, I was unaware that anyone welding 4130 in the aerospace industry used anything but VIM melted 4130 filler. The aerostructures companies my employer works with would kick you out of the office if you suggested anything else."

yeah, aerospace is a different category. Aircraft spruce sells the mild rod, EAA teaches it, and it's been done for 100 000s of airframes. Actually it's interesting to hear aerospace is still making airframes out of 4130 tube. That may explain all the secrecy around the F-35.

"One of the biggest misconceptions, and I've said it here before, is the difference between hardness and hardening. A 0.3% carbon steel will harden, and to an appreciable degree with just 0.20% molybdenum in it. ... i assure you that 4130 tube for bike frames doesn't require a 'superquench' at all. I... What it won't have is high hardness. But that's not a problem, indeed, it's beneficial."


When you put it that way, I can see the confusion. Unless the harden-ability allows material levels of hardness who cares whether you heat treat it. It's all good. If your hammers don't cost 300 dollars, and you don't FEA every minor design change (or anything in the whole industry), then as a boatbuilding giant once said to me about epoxy admixtures, "I just don't think you need to take it that far".

I am certainly not suggesting anyone superquench, or anything-quench, a bicycle frame.

"And please, stop with the 'points of carbon'? For my sanity, pretty please? It's just a darned percentage, after all... :-)"

100 points equals 1 percentage point, though possibly not in engineerese. My recollection is that there was a 100% failure rate of engineering students at my University, 10 000 points to you, as far as passing the entry English exam is concerned.

Airburst

Just out of interest, exactly how much of a difference does all that make on a bike? Is it just a manufacturer's gimmick as far as bicycle frames are concerned, or is there an appreciable benefit?

Falanx

That one's all in Ti. *Taps his nose*

You quench a 6000 series frame, remember? ;-) Just find a company that has a nice big sealed quench, or better still a total vacuum system, and do it that way :-)

The one thing it will do, if done correctly, is increase the frame's longevity. Fatigue limit on steels rises with yield strength, all other things being equal.

ftwelder

As with any bicycle industry claim, carefully read the copy. It really just means that is was" heated" unless they also claim some sort of tangible benefit.