Author Topic: A question about welding (Read 2891 times)

MillCreek · « **on:** August 25, 2006, 07:17:48 PM »

As is so often the case, when I am ignorant on a given subject, I like to turn to my colleagues here for guidance. I have been reading some threads about bicycle frame welding over on Bike Forums. There is much debate over the smoothness and finish of the welds of different frame manufacturers. I know nothing about welding. Does the smoothness, eveness or the final finish (after grinding and sanding in some cases) correlate with the strength of the weld? Once there are no skips or voids in a weld, is the smoothness of the final join merely esthetics? I have seen some bicycle frames in which the welds look like a bad caulking job by someone who was hungover, while some other welds look like the final joints are all one piece. Is one truly better than the other from a practical standpoint, or is it just looks?

As always, please edumacate me.

Headless Thompson Gunner · « **Reply #1 on:** August 25, 2006, 07:44:04 PM »

It's mostly cosmetics. A smoother and more even weld, regardless of its size, tends to indicate a higher quality of craftsmanship. Thus the better it looks the more desirable it is. But as long as you stick to mainstream, reputable bike/frame manufacturers you shouldn't come accross any inadequate welding jobs.

Those big welds, the ones that look like caulking, are usually an issue of marketing. Those big, beefy welds look stronger and more manly. Like great big knobby tires, the goal is to increase the bike's showroom appeal by making it look more aggressive. They aren't necessarily any better in terms of function or utility. Note that you never see those bulky welds on road bikes, only on mountain bikes.

At least, that's what I've always thought. I could be completely wrong here. I trust that our resident bike gurus will correct me if I've given bad info.

-HTG, whose half-built Surly is currently spread out all accross the living room floor, leaving greasy spots on the carpet. Tongue

wingnutx · « **Reply #2 on:** August 25, 2006, 11:25:03 PM »

Excessive spatter probably indicates that you have porousity in the weld, which is bad for strength. Stumpy, narrow beads mean you are running cold, and probably don't have good penetration.

Looks aren't everything, but a good-looking weld and a structurally sound weld usually go hand in hand.

Grinding smooth doesn't mean much if it was a sound weld to start with, although you need to do it in order to do a lot of structural testing. On bikes it's probably just cosmetic. Personally I like to leave a good-looking bead visible.

<- former navy nuke welder, current Seabee steelworker

280plus · « **Reply #3 on:** August 26, 2006, 02:33:55 AM »

Yea grinding off a bead only weakens it.

mtnbkr · « **Reply #4 on:** August 26, 2006, 04:22:14 AM »

HTG pretty much nailed it with regards to the bike industry.

That said, builders such as Klein and Cannondale worked out procedures where they could have a good weld and grind it smooth. On those bikes, the tubes almost meld together visually as one unit.

http://www.kleinbikes.com/us/fit_and_ride/construction.html#void

On older, filet brazed (not welded) bikes, the joints are nice and smooth because the builder would file them smooth after brazing. Tom Ritchey used to make some beautiful frames this way. I think he TIG welds now, but I could be wrong (assuming he's actually building anything these days).

The big chunky weld fad has really taken off with the freeride/all mountain/hucking fad. You can't have nice clean welds on your human powered motorcycle, now can you?

Chris

DJJ · « **Reply #5 on:** August 26, 2006, 10:07:16 AM »

All else equal, smoother is better, in that irregularities and discontinuities will cause stress concentrations. Combine that with, uh, "cyclic" loading (like bouncing), and you'll have fatigue cracks and failure. Beat that pun, Preacherman!

mtnbkr · « **Reply #6 on:** August 26, 2006, 10:38:37 AM »

Quote from: DJJ

All else equal, smoother is better, in that irregularities and discontinuities will cause stress concentrations. Combine that with, uh, "cyclic" loading (like bouncing), and you'll have fatigue cracks and failure. Beat that pun, Preacherman!

I've been a mountain biker for just over 20 years and am on my third bike. I can't say I've seen anything to support the generality that a chunky weld is less durable than a smooth one. However, I'm talking about properly formed welds in either case. What might appear "chunky" to me may very well be a perfect weld. I think the skill and care of the welder will dictate how many cycles a weld will take before failure. A friend of mine wrapped a TIG welded Trek around a tree at high speed 14 years ago. The frame was bent out of alignment, both wheels tacoed, he was battered and bruised, and the tree scarred (could see the damage a couple years later), yet none of the less than perfectly smooth welds failed.

Chris

280plus · « **Reply #7 on:** August 26, 2006, 10:42:23 AM »

Quote

both wheels tacoed,

So THAT'S what you call that. First time I ever heard taco used as a verb Cheesy

K Frame · « **Reply #8 on:** August 26, 2006, 10:44:55 AM »

I've always heard that the rougher a weld is the more likely it is to have voids and inclusions, effectively weakening it.

K Frame · « **Reply #9 on:** August 26, 2006, 10:45:55 AM »

"So THAT'S what you call that. First time I ever heard taco used as a verb."

But they are a BITCH to keep lettuce in...

280plus · « **Reply #10 on:** August 26, 2006, 10:55:08 AM »

Yea, it's those damn spokes! Tongue

I can stick two pieces of metal together but I don't call myself a welder. Anyhoo we're talking about different things maybe? A good solid weld can be either smoooth and flat or meaty and raised. If the weld looks like what we called "bubblegum" THEN you're talking a weld with inclusions and slag pits. I have just always been averse to grinding down a meaty weld if it's not necessary. I'll take strength over cosmetics every day of the week. I'd also venture to say that your alloy frame bikes were most likely all TIG welded which IIRC involves no slag and therefore no slag imperfections.

mtnbkr · « **Reply #11 on:** August 26, 2006, 11:00:36 AM »

Quote from: 280plus

Yea, it's those damn spokes! Tongue

I can stick two pieces of metal together but I don't call myself a welder. Anyhoo we're talking about different things maybe? A good solid weld can be either smoooth and flat or meaty and raised. If the weld looks like what we called "bubblegum" THEN you're talking a weld with inclusions and slag pits. I have just always been averse to grinding down a meaty weld if it's not necessary. I'll take strength over cosmetics every day of the week. I'd also venture to say that your alloy frame bikes were most likely all TIG welded which IIRC involves no slag and therefore no slag imperfections.

Yes, I think we're talking about two different things. Yes, most bikes are TIG welded these days, though you can still find a few that are filet brazed (not really welding). Also, I'm not referring to welds that looked like a monkey with an arc welder created them, but those that are just chunkier than a filed and smoothed weld.

Chris

280plus · « **Reply #12 on:** August 26, 2006, 11:19:09 AM »

I'm not familiar with this filet braze. I know to lay one weld bead on top of another is to filet or fillet. I always thought it was 2 "L"s. Off to the Webster's... Cheesy

K Frame · « **Reply #13 on:** August 26, 2006, 11:24:53 AM »

"Also, I'm not referring to welds that looked like a monkey with an arc welder created them..."

I thought you sold your arc welder...

280plus · « **Reply #14 on:** August 26, 2006, 11:25:50 AM »

Here we go...

I HAVE to believe a TIG welded frame is much stronger than a brazed one.

http://www.bohemianbicycles.com/fillet_brazing.htm

280plus · « **Reply #15 on:** August 26, 2006, 11:32:22 AM »

Then again maybe not...

http://www.gtgtandems.com/tech/brazing.html

mfree · « **Reply #16 on:** August 26, 2006, 11:33:27 AM »

It definitely is much stronger. The question is though, do the stresses involved in a bike frame ever exceed what a filet braze can handle.

MillCreek · « **Reply #17 on:** August 26, 2006, 12:12:14 PM »

As I look at my four bikes, the Motobecane road, Specialized hardtail MTB and KHS FS MTB all have nice enough welds, but the Indy Fab titanium Crown Jewel road bike has welds that are a thing of beauty. Probably one of the reasons that the frame for this type of Indy Fab sells for $ 3200 or so. I would suspect that the welds on any contemporary bike frame manufacturer would virtually never fail.

Having said all that, I still believe that the beautiful lugs on my very first serious road bike, a lugged Reynolds cro-mo frame that I bought in the late 70's, far exceed the esthetics of just about any of the contemporary welded frames.

mtnbkr · « **Reply #18 on:** August 26, 2006, 12:26:15 PM »

I love a nice lugged frame too.

In fact, one of these days, I'm going to get serious about bidding on and winning a Raleigh International. Beautiful lugged frame and Campagnolo components, a winning combination.

Chris

mtnbkr · « **Reply #19 on:** August 26, 2006, 12:27:48 PM »

Quote from: 280plus

I'm not familiar with this filet braze. I know to lay one weld bead on top of another is to filet or fillet. I always thought it was 2 "L"s. Off to the Webster's... Cheesy

You're right. Dunno where I got "filet" from. rolleyes

Chris

mtnbkr · « **Reply #20 on:** August 26, 2006, 12:28:57 PM »

Quote from: Mike Irwin

"Also, I'm not referring to welds that looked like a monkey with an arc welder created them..."

I thought you sold your arc welder...

I did.

I was also guilty of creating such welds. That said, none of mine ever failed. cool

Chris

280plus · « **Reply #21 on:** August 26, 2006, 12:57:21 PM »

It's used interchangeably on the sites I found so I guess it's either way. Websters has no "filet" though.

Headless Thompson Gunner · « **Reply #22 on:** August 26, 2006, 01:09:15 PM »

I always thought that lugged and brazed frames were a touch stronger than welded frames. The intense heat of welding can ruin the heat treatment of the tubes at their most critical ponts, wgere the tubes are joined together. Brazing won't harm the heat treatment.

Welded joints are indeed stronger than brazed joints. But a bike frame is a system of both joints and tubes, and this system tends to be stronger if it built by lugged/brazed construction.

That's my understanding, anyway...

MillCreek · « **Reply #23 on:** August 26, 2006, 05:11:30 PM »

I have heard that when titanium is being welded, ideally, it is done so in an inert gas atmosphere to prevent oxygen embrittlement or some such that will weaken the weld. I have seen pictures of titanium bike frames being welded, and it is clearly being done in the open atmosphere. How is inert gas welding done out in the open?

PS: I had also read elsewhere that the Soviets, as a test, built up to six Alfa-class submarines with a titanium hull. Apparently, they constructed an airtight building, pumped out the atmosphere, flooded the building with argon, and had welders with oxygen masks weld up the hull.

caseydog · « **Reply #24 on:** August 26, 2006, 05:27:39 PM »

Quote from: MillCreek

I have heard that when titanium is being welded, ideally, it is done so in an inert gas atmosphere to prevent oxygen embrittlement or some such that will weaken the weld. I have seen pictures of titanium bike frames being welded, and it is clearly being done in the open atmosphere. How is inert gas welding done out in the open?

The inert atmosphere is only critical at the point of molten metal, hence the Tig torch flows gas around the tungsten electrode , also when welding tubing in a closed joint like a bikeframe it is common to "backpurge" the tube while welding , that involves injecting a flow of inert gas inside the tube while welding the outside.

Another variant calls for an aftershield which involves another attachment on the torch to keep the last 2 or 3 inches just welded behind the torch in a gas flow until it cools further (usually argon , sometimes helium or multi gas blends) depends on the engineers spec.

With steel, particularly moly steel tube , hydrogen is the atmospheric gas that causes the most trouble , hydrogen embrittlement , backpurging is a necessity.

Ray

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Author Topic: A question about welding (Read 2891 times)