Welding Chromoly Frame With Flux Core

No flames intended, but let me ask why are you wanting to use chrome moly?

It's harder to weld, it's no stiffer than mild steel, and no lighter than mild steel. If you simply substitute chrome moly for mild steel on your frame you will end up with a frame that's harder to fabricate but no stiffer and no lighter. It won't dent quite as easily though.

It sounds like it's your first frame. Go with mild steel, chromoly is wasted on the average builder. If you don't have a very specific reason for using it, don't.

If it were ME, I'd go with mild steel and a oxy/acetylene torch with bronze filler. Flux core is for field fixes when you can't get out of the wind IMHO...

A bit of a hijack here, let me know if this is bad behavior (a term my granddaughter uses). I digress, I would like to make a similar attempt with frames and my gut says brazing would be the way to work with older bicycle frames but I like the idea of TIG too. I suspect the black frame pictured is not chromoly and I wonder if the rear dropouts are brazed and crimped or just crimped in place.

Hopeful to post this in the builds section one day, as a "Mini Me TRex" with knobbies of a "Hat in the Ring Top Hat" with street tires. (Those forks don't fit any other frame I tried them in, possible configuration with this frame)

Flying Merlino Brother said:

A bit of a hijack here, let me know if this is bad behavior (a term my granddaughter uses). I digress, I would like to make a similar attempt with frames and my gut says brazing would be the way to work with older bicycle frames but I like the idea of TIG too. I suspect the black frame pictured is not chromoly and I wonder if the rear dropouts are brazed and crimped or just crimped in place.

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No way that frame is any kind of upmarket material, the construction is so low end it would be insane to use anything other than the cheapest steel they could get.

I'd guess the dropouts were crimped and spot welded. Again, cheapest/fastest methods would have been used and there's no evidence of brazing. I'd be shocked if they were simply crimped.

You didn't ask this but I'll also mention you can get the set up to braze/gas weld things cheaper than a decent TIG set up. And once you learn how to gas weld the other forms are easier to learn. Not to mention, having a torch around is pretty handy for other things too.

I definitely disagree with the statement that chromoly is not stronger than mild steel. It is called "mild" steel for a reason
https://blog.thepipingmart.com/metals/chromoly-tubing-vs-mild-steel-whats-the-difference/

https://mooreracechassis.com/blogs/motor-plate-and-engine-placement/4130-vs-mild-steel-roll-cages

https://mrssteel.com.vn/blogs/steel...eel-the-sustainable-steel-standard-for-future

https://cyclesource.com/chromoly-vs-dom-tubing?amp=1

"4130 and 1018 weigh the same at 491 lbs. per cubic foot. The weight saving in using Chromoly to construct a frame is obtained through the use of thinner walled tubing, therefore less volume of steel. Chromoly has a significant strength advantage over mild steel in both tensile strength and yield strength. This increased strength to weight ratio in Chromoly allows for the use of thinner walled tubing to obtain the same structural support."

MattiThundrrr said:

I definitely disagree with the statement that chromoly is not stronger than mild steel. It is called "mild" steel for a reason

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Who made the statement it wasn't stronger?

"Mild" steel is called mild for a few reasons. Although it has lower tensile strength, it is also "mild" in the sense that it is more affordable, more ductile, easier to machine, and easier to weld than the fancier alloys. "Mild" isn't a bad thing.

The strength advantage doesn't exist if the material is damaged in the welding process. Who cares if the tube itself is 20 times stronger than mild steel if the joint is weaker due to improper welding? Where do most frames break?

The material is touted for being stronger, so you can use less and get a lighter part. Taking that to a limit, you can use less material until you have the same strength but less weight. Great! Except your thinner/lighter part is more flexible now. Lighter, just as strong, but more flexible. Is that a desirable trade off? It could be but it's certainly not a given.

Long winded point being, the advantages of chromo to the home builder are almost entirely theoretical. Virtually no one does the math to optimize the design. They buy whatever is available from a convenient supplier, have no idea what the final strength or stiffeness will be, build the frame, and then use it the same way they'd use it no matter what the material is. In reality, the main advantge of chromo becomes the tubing doesn't dent as easily as mild steel, and even that is only if they don't go super thin to save weight.

For someone who is just learning to weld I'll give the advantage to mild steel. In fact I'll give mild steel the advantage in 95% of homebuilt situations but that's an even longer post.

DesmoDog said:

Where do most frames break?

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I run a local bike coop where we get about 500 bikes each year. I see all kinds of damaged bike stuff.

In general bike frame don't break on their own. Most are crash damaged or abused. I've seen split open tubes from freezing with water inside the frames, broken dropouts, seat tubes split from hamming in the wrong size seat post, fork steerers split from the inside out from over tightening expanders. The most common damage is from front end impacts where you get 1: bent fork, 2: bent frame, 3 trashed wheel, in any combination of those 3. One of the three is weaker than the others but you will never know until the crash. A harder crash can damage 2 of the 3. All three is possible but not common. IN all my years of refin' bike races, I've seen it's usually the bike or the body that takes the major impact. When you see a rider walking away, he is usually carrying the bike in pieces. If the bike is untouched, the rider is usually on his way to the emergency room.

Bike warranties are intended to cover manufacturing defects. Most bikes that fail usually fail sooner than later. If they survive the first year, they will usually be okay. Most manufactures have dumped "lifetime" warranties in favor of 5 years or treks' "Useful lifespan of the product" as they see it. The vast majority of broken frames are not from defects. Customers think they should last for ever and the warranty should cover any abuse or neglect.

I have seen some bikes that didn't hold up. The 2 most common are Centurion brand road bikes from the bike boom era. Most of those were over heated during brazing and that weakened the down tube resulting in fatigue cracks near the shift levers. The other batch were welded aluminum frames sold under many names such as Mongoose, Rock Machines, GT, Raleigh, etc. I suspect there was 1 asian factory that wasn't up to par around 1998 to 2000. Those develop cracks in the head tube from the top down or bottom up. If you have a 20+ year old welded aluminum road or mountain bike, inspect the head tube for cracks. Those should be scraped out immediately.

4130 chromoly = same strength for less weight. Which includes stiffness and other factors.

Tubes of equal dimension - chromoly is stronger.
That’s why it’s used in higher end steel bikes. And race car roll cages.

23 lb or less finished bike with thin wall 4130 vs. 35 lbs or more with thicker, to make the same strength, mild steel.

Yes, harder to weld due to the thin wall, probably why some see the easy burn through. You really do need to TIG it so you have better haet contol. And you need better skill. But at equal wall thickness, say 0.065” or more, they weld about the same.

I would argue 4130 is easier even then. But that’s subjective and probably has to do with different formulations of mild steel or quality control standards.

But for the beginner, with MIG, there’s nothing wrong with mild steel and thicker walled tubing unless your goal is weight savings. Also learn that there are different formulations of steel that are termed “mild,” and not the same.

Watch some vids, read a lot, get some practice with 0.065” wall tubes and go for it.

DesmoDog said:

Who made the statement it wasn't stronger?

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DesmoDog said:

it's no stiffer than mild steel, and no lighter than mild steel. If you simply substitute chrome moly for mild steel on your frame you will end up with a frame that's harder to fabricate but no stiffer and no lighter. It won't dent quite as easily though

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This seems to imply less strength. I don't want to get in a big argument over this, I just put forth the facts regarding the material. Chromoly gets used in spite of its extra expense because it is stronger. I definitely agree with the point that mild steel is easier for a novice builder. The last article I linked even addresses the challenges of welding with chromo:
"The most common source of argument surrounding the construction of a Chromoly frame is the welding process. 4130 contains a higher amount of carbon than mild steel and is classified as a heat treatable steel. As a consequence, large increases in heat from welding can require that a weldment be normalized or (drawn out) after welding to avoid embrittlement of the material. However, in the case of thin walled Chromoly tubing – (.120” or less) it has been determined and published by the AWS that there is no need for post heat treatment of a weldment if the weld is made using the GTAW (TIG) process, a mild steel filler rod is used, and proper welding procedure is adhered to."