annealing with a torch

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Hey everyone,   As a self-taught weekend welder for many years (oxy/acet & mig), I thought I knew the basic rules to getting a clean, penetrating and strong weld.     Anyway, I've run into a problem welding a bicycle frame that started getting stress cracks. I ground the splits to clean shiny metal and used my torch to stitch them up with what I thought were good repairs.      As you may have guessed, each one of them reopened after some heavy use, cracking through my filler metal like it was plastic.      The frame is American, early fifties. I can't exactly vouch for the base metal, but it behaves like mostly mild steel. The problem seems to be with the welds themselves, and I think it may be that I'm just letting the repairs air cool in my (heated) shop -too fast- and the repair work comes out brittle. Does this make sense?      I understand the concept of tempering vs. annealing, but have no guidelines on the subject. My new idea is to re weld, then bring the heat down slowly - but how slowly? I know there are ovens for this, but that's not an option here. My question, I guess, is whether I can make a difference by just backing off on the heat with the torch as soon as I'm done with one spot - and if so, for how long at each color?   Also, what's up with "peening"?thanks for reading,saul
Reply:What make and type of bicycle is it, and where were the cracks first showing up?
Reply:Firestone "supercruiser", early fifties, USA (I think). The cracks are showing up at the head tube where the forks bear against the cylinder wall and, to a lesser extent, around the bottom cylinder that houses the crank bearings. .
Reply:You might say it's a cyclically loaded structure.  I am guessing that you are not getting full penetration, which is a very delicate balancing act regardless of the process when you have thin wall tube.City of L.A. Structural; Manual & Semi-Automatic;"Surely there is a mine for silver, and a place where gold is refined. Iron is taken from the earth, and copper is smelted from ore."Job 28:1,2Lincoln, Miller, Victor & ISV BibleDanny
Reply:Have you considered brazing instead of welding?
Reply:"...brazing instead of welding?"     No, it didn't occur to me because it was a crack that needed filling. As far as the penetration goes, it was a delicate operation, yes. I used a "0" tip and went slowly, though, and I got a wet "crescent" that spanned the gap all the way on the first pass, and then I was able to beef it up on a second pass. It bubbled through on the inside some, but since I knocked off the slag with a rasp it won't interfere with the forks.     ...But I'm curious, folks. Does anybody have an opinion about my annealing questions? I want to know  for this project, and for general information too. Any basic tips, or is this something that I don't need to consider?
Reply:Originally Posted by saulgood"...brazing instead of welding?"     No, it didn't occur to me because it was a crack that needed filling. As far as the penetration goes, it was a delicate operation, yes. I used a "0" tip and went slowly, though, and I got a wet "crescent" that spanned the gap all the way on the first pass, and then I was able to beef it up on a second pass. It bubbled through on the inside some, but since I knocked off the slag with a rasp it won't interfere with the forks.     ...But I'm curious, folks. Does anybody have an opinion about my annealing questions? I want to know  for this project, and for general information too. Any basic tips, or is this something that I don't need to consider?
Reply:burying it in sand can help slow the cool down.Like someone else said, braze it.200amp Air Liquide MIG, Hypertherm Plasma, Harris torches, Optrel helmet, Makita angle grinders, Pre-China Delta chop saw and belt sander, Miller leathers, shop made jigs etc, North- welders backpack.
Reply:gas welding, leaves metal in a near annealed condition. The heat is so diffuse in gas welding that by the time your to welding temperature the wide HAZ has annealed the surrounding metal. Mind you a 0 tip is pretty small, but in general no additional post heat is needed for gas welding.Futhermore there's a difference between annealing (bringing the base metal back to it's softest most stress free condition) and post weld heat treating (keeping the weld at temperature after welding)    If you strictly want to slow the cooling rate, you can keep throw on a welding blanket, or use a rosebud to keep the part around 500-600 degrees for a few minutes then throw on the weld blanket. that will slow the cooling rate to what would anneal most common steels.Second thing would be to reevaluate the original diagnosis of stress cracks. If it truly is a mild steel bike frame I doubt it would get work hardened enough to be stress cracking. Which means either the weld is bad in some other regard, OR there is a design flaw on the frame and it's undergoing more force than it could withstand.  Make sure your bead shape is correct, no severe concavity or convex bead, preferably flat, also make sure the metal is 100% clean of foreign contaminants. Low melt point inclusions in cheap steels like zinc, lead, manganese etc will put a lot of internal stress on the metal right at the center of the weld bead. As will bad bead geometry. Also here's where chemical composition could hit you.  If I got a "mystery" metal I start with a high nickel filler, or stainless steel like 308 or 309. The higher nickel content and grain structure helps them to "accept" more impurities in the steel.I've found a lot on cyclically loaded structures, such as motor mounts there's a serious design flaw and the vibration will crack the welds no matter the strength. In these circumstances an elastic material such as bronze will do much better.   IF all else fails, I would suggest bronze brazing, and since you're having trouble with the blowing through anyways, it would be a good chance to kill 2 birds with one stone. I've fixed a few minibike motor mounts using silicone bronze. Works wonders.Welding EngineerCertified Scrap Producer
Reply:Originally Posted by Metarinkagas welding, leaves metal in a near annealed condition. (snip)Second thing would be to reevaluate the original diagnosis of stress cracks. If it truly is a mild steel bike frame I doubt it would get work hardened enough to be stress cracking. Which means either the weld is bad in some other regard, OR there is a design flaw on the frame and it's undergoing more force than it could withstand.  (snip)I've found a lot on cyclically loaded structures, such as motor mounts there's a serious design flaw and the vibration will crack the welds no matter the strength. In these circumstances an elastic material such as bronze will do much better.   IF all else fails, I would suggest bronze brazing, and since you're having trouble with the blowing through anyways, it would be a good chance to kill 2 birds with one stone. I've fixed a few minibike motor mounts using silicone bronze. Works wonders.
Reply:Are these parts mitered into each other at the factory and fused so you can barely make out the process used to connect them?Do you have any pictures?What may seem like a possiblity is that they were brazed or more likely tinned and soldered at the factory, then brushed clean, the bike sat for awhile and began to corrode so you thought you could save so you ground the corrosion and weaked the joint and the filler may not have enough to grab onto for a good strong joint.It is hard to understand if it is before my time how the bike was made.
Reply:Here is what I am wondering. From the time period you are talking, brazed lug was real common in frame construction. Im wondering if you are trying to weld over a brazed joint, are overheating (reheating) the braze material and causing grain permeation which would easily lead to a continued failure. Any chance you can verify the joining method?-Aaron
Reply:pistolnoon, makoman,    I think you both hit it on the head, more or less. Original joint was a clean, tight fit with the transition line actually visible beneath the paint - not a seamless transition like on an old Schwinn. No lugs were used. The cracking on the head post(?) actually occurred on either side of the brazed joint, but the joint was still fine.   Matt McGuire and  Metarinka, thanks for the information re:annealing - -that's exactly what I was looking for.   Metarinka, What is "silicone bronze" - is that what bronze filler rods are, or is it something more? Do they use it to make breast implants for robots ?
Reply:Do they use it to make breast implants for robots ?