brazing 4130?

I read somewhere that you should not braze 4130 chrome-moly steel as it's "grain" cracks as it cools. Any truth to that ?

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"Stupendous Man" wrote: I read somewhere that you should not braze 4130 chrome-moly steel as it's "grain" cracks as it cools. Any truth to that ? ^^^^^^^^^^^^^^^^ Never heard that one, but it makes me ask: How did the steel get cool after it was rolled initially without cracking?
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There is a whole time/temperature schedule that minimizes grain growth while giving the desired hardness/ductility in the final product. Rolling or drawing refines the grain, holding at a high temp encourages grain growth. A weld or braze joint will give a HAZ that is quite different. There are a lot of opinions on O/A vs brazing vs TIG on 4130. Some of those are hoary old tales, some have some fact. Sorting among them is a challenge.
Leo Lichtman wrote:

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I used to work for a major aircraft manufacturing co. and they had several 4130 assemblies that were brazed together somewhat out of necessity as the part would be tough to weld , never heard of a problem with these units , many years ago an EAA member got the OK from the FAA to braze his entire four place airplane together with Eutectic FC 16 Braze filler , never heard of any problem with this aircraft , I brazed some .035 wall 4130 specimens with this material and couldn't break any brazed fills , just my thoughts , Phil Lohiser

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The combined experience of the hundreds, if not thousands, of people who have brass brazed bicycle frames out of 4130 can't be ignored. Brazing 4130 works great.
However, because it is *remotely* possible for an aircraft grade 4130 tubes to be damaged by using brass, at near welding temps, it is (or at least was) not allowed on airframes. If a plane's frame breaks, the occupants have a lot further to fall than a bicyclist. So the FAA (Federal Aviation Administration) won't take any chances, however remote. If a person's experience is welding, or working with thicker metals, and he has a #5 filter in his goggles, then the chance of cooking some brass into the steel may not be all that slim.
There is a warning in the EEA welding manual and another welding book simply states "Because brazed joints are not as strong as welded joints, brazing is not used for structural repairs on aircraft."
In the case of airframes this may be true. But bike builders are pushing the envelope for thickness/diameter ratios. It may be that bikes (and similar structures) are the only case where braze welding (AKA fillet brazing) and fusion welding strengths are comparable. Airframes (and just about everything else) are made of smaller diameter, thicker walled tubing than bicycles. While brass/bronze isn't as strong as steel, on a bicycle the cross section area of even a small fillet is many times the cross sectional area of the tube. A thicker, smaller tube of the same cross section wouldn't give you as much periphery for a fillet.
Another reason I've been told to avoid brass was that the heavier fillet could accentuate vibrations in the airframe. Some of those multi tube cluster joints in an airframe could be massive if braze welded.
BTW Brazing was allowed on military airframes during World War II.
I queried the bike framebuilders list a few years ago and, beside myself, there was only one person who would admit to having damaged a 4130 tube by brazing. In both cases a poorly designed joint and sloppy work were involved. In my case I was trying to braze a thick steel fitting inside a thin tube. However very little of the fitting was exposed, so nearly all of the heat was applied to the tube.
I belong to a group of HPV builders who's members have built over 300 recumbents. There are some seriously underskilled brazers in this group, their fillets looking like brass plated bubble gum and their flux charred and blackened glass. Yet I haven't seen any such failures here.

Interesting, were these units made from tubing?
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wrote:

I've both welded and brazed (actually sil-phos) 4130 tubing with no problems. Welding 4130 is a common practice in the aircraft industry and a motorcycle frame maker in England was using some sort of brazing back in the 70's and were bragging that their joints were actually more robust then a welded joint.
Bruce-in-Bangkok (Note:displayed e-mail address is a spam trap)
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I have brazed plenty of 4130 with no problems, but what i read in 2 sources said not to. One said the brass filled the grain and then the steel contracted while cooling and "you could sometimes see the cracks form". I just brazed a couple of oil pickup tubes for dry-sump engines and am worried about future leaks. I should have some mild steel tubing in next week to make the rest of them.
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Would that have been Rickman? They, and most other British framebuilders (pedal or motor bike) used Reynolds 531 chrome - manganese tubing instead of the chrome molybdenum steels, such as 4130, used by most of the rest of the world.
Fatigue testing in the bicycle industry has shown that braze welding (AKA Fillet Brazing) is superior in fatigue to TIG welding on chrome molybdenum steels. And that low temp (<1200 F) silver brazing with lugs is better yet.
However the # of cycles suggests that, for road use, a TIG welded bike should last a lifetime and a lugged frame will last two.
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On Sat, 20 Oct 2007 19:09:27 -0700, snipped-for-privacy@gmail.com wrote:

Yes, that was Rickman but it was a long time ago (and I ain't so young either) so I'll claim that as an excuse for mixing up metals.
HOWEVER, I also remember an article about the "feather bed Norton" which claimed that the brazed joints actually made the frame stronger because there was a little "give" in the joint whereas a welded joint wouldn't "give" and therefore would crack....
Bruce-in-Bangkok (Note:displayed e-mail address is a spam trap)
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The whole fillet itself isn't going to flex. But properly done, the edge of a bronze fillet is relatively thin, and slightly flexible. Less likely to create a stress riser than a weld.
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Brazing is the prefered method of constructing hand-made chrome-moly steel bicycle frames, I have service frames that are at least 40 years old and showing no signs of cracking despite being used nearly heavily since purchase. One of them is (was) a light-weight racing frame too.
Cheers
Joel
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Same with vintage British race cars. Lotus frames and suspension parts are "bronze welded". Here is a new/old-stock mild steel Lotus 7 chassis with lots of 4130 reinforcements brazed in. I will assemble it after the two class-B "formula ford" cars are done.
http://s221.photobucket.com/albums/dd236/arborigine/?action=view&current=MVC-011S.jpg
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