Welding Chromoly: Please Help!

[JonCT]

Hello,

I have a couple of questions. I am working on restoring a vintage motocross bike and I noticed some cracks in the frame. I mig welded the cracks and another crack appeared adjacent to the HAZ. When doing more research I discovered I have a cromoly frame. Did I do any permanent damage to the frame? I believe now I need to "temper" the steel so that it will not be brittle. What is the best way to do this? I will probably have someone TIG weld it for me...should I temper it before or after TIG welding it?

Any info would be appreciated.

Thanks,

Jon

[engnerdan]

I "think" (and there is some of the problem all of the time) you might want to anneal it before welding and again after. I worked in an aircraft welding shop for a summer and all the engine mounts were 4130, all tig welded. After all welding was done in an area they would heat it red hot with a rosebud and let it cool to anneal it. I do not know what they used for filler but it seemed to be an acceptable practice for the aircraft engine mounts.

-Dan

[Rojodiablo]

If it's a vintage era bike, then it was likely O/A welded. The bonus of oxy/ acet. welding is the natural preheat and postheat of the weld areas. Sometimes, chromo will take a mig or tig weld just fine, no preheat, nothing. But if it is thin wall, or it is heat treated after welding, then you may need to preheat/ postheat the weld areas. Also, the grade of chromo will have a lot to do with the cracking. 4340 is usually for parts, not frames. It usually calls for heat treatment after the weld. 4130 N is the most common type of chromo tube, it is normalized, or basically has the baseline heat treat from the factory. I forget the marking for annealed chromo, maybe type A. Annealed chromo is a bit softer, it has been specifically annealed for use. This is usually tube or aheet that will have lots of bending/ welding done to it, and then after fabrication, it will get heat treated in an oven. For filler, usually we use er70 series mild filler on 4130. For 4340 parts, we use 4340 filler. You can use 4130 filler on the 4130, but cracking can be a problem. I don't think you want that!!!

Maybe post up the type and year bike, someone here may know where to look to find out what material and process were actually used on the bike.

[JonCT]

Thanks for the replys! The bike is a 1973 Montesa Cappra VR, made in Spain. It looks like the welds are O/A welded from the factory. It is cracked at the swingarm mounting point and looks like it is most likely from someone MIG welding it at some point. Would it be an acceptable method to preheat, MIG weld it and anneal the tubing? I could do that myself. Or is it better to have a professional TIG weld it.

Jon

[Donald Branscom]

A lot of confusion.

I would not MIG weld. It needs to be TIG welded.
The problem with the MIG is that it has a very high heat input and that just degrades the HAZ even more. TIG has the lowest heat input and good penetration. Make sure the filler wire is correct. IF it is cracking try using 309 instead of ER70s-2 . But if the metal had a large gap originally, it might just keep cracking.

When a weldment is heated up and then allowd to cool that is NOT annealing. That is just stress relieving BUT it needs to be done evenly in a controlled way.. It is usually done in a more with a time schedule involved.

Stress relief with a vibratory process will work also.OR ultrasound.
An old motorcyle frame would not need that. A brand new frame just completed would be a better application.

When welding chromoly it needs some preheat on the joint prior to welding. just use a temp crayon rated at 250º. You can double check that temp online if you want.
Even though the metal may look dry when you pass a torch over the tubing you will see moisture develope and then dissapear. It is then ready to weld.

When you are welding that old frame clean up the weld area then when you strike a TIG torch on the area look for cracks. Just make your weld and let it cool down on its own.

Annealing is when metal is heated then cooled with water. There are other methods, but the main purpose is to make the metal soft so it can be formed then it is heat treated after forming. Try it with aluminum, Heat it up and quench with water and it will be soft as butter.

[JonCT]

Thanks for the info, I will have it TIG welded then.

[Rojodiablo]

Since you point out the crack is in the general area of the swingarm pivot, I have a couple questions, and some advice.....#1. Does the swingarm pivot through the frame?? If it does, the frame junction is supported partially by the motor, and that is a good thing, so long as the motor does not vibrate too much when running. #2. Is the motor solid mount, or rubber mount? Certain bikes just vibrate a LOT- Big singles like a Matchless, Moto-Guzzi, Harleys all vibrate, and all in different ways. Now, if the swingarm does not mount through the motor, I would suggest looking into possible gusseting of the frame junction at the swingarm pivot. Where an old frame has cracked, there is always a reason. And.... that reason is usually still on the bike somewhere. Gussets can help dramatically by spreading the stress out further from the load center at the swingarm. This will give more surface area "Attached" to the pivot, which can help to reduce or cure flex, spread nasty vibration out to a greater area, lessening its' ill effects on the bike. And, for sure on an old mx bike, the pivot has seen some serious abuse over the last 30 years.

[JonCT]

#1 Yes the swing arm pivot goes through the frame and is also supported by the rear motor mount.

#2 The motor mounts are solid. It is a 250 two stroke, it will vibrate some but doesn't have the mass that a big 4 stroke single would.

#3 There are a couple of reasons why it could have cracked and I am just putting all of the puzzle pieces together. First off they moved the rear shock mounts to provide a "laid-down" suspension angle for more travel, when doing this they put forces into the frame where they weren't designed to be. Also the right side swing arm pivot was re-welded, when this was done it was MIG welded causing hairline cracks along the HAZ. This I know, because that is what happened when I welded it Then as the bike was ridden the bottom cradle was only supported by the left side which eventually cracked as well. Now only supported by the rear engine mount the forces sheared the ears of of the mount, causing the down tube to pull away from the steering head. That is how I think the sequence of events happened, and the reason I am here.

#4 I have thought about gussets and they pose a problem as the rear brake assembly conflicts with the only possible gusset area.

Everyone tells me they have never seen a Montesa frame cracked...guess I'm the exception to the rule.

[hotrodder]

Originally Posted by Donald Branscom

A lot of confusion..

Yes!

Originally Posted by Donald Branscom

I would not MIG weld. It needs to be TIG welded.
The problem with the MIG is that it has a very high heat input and that just degrades the HAZ even more. TIG has the lowest heat input and good penetration. Make sure the filler wire is correct. IF it is cracking try using 309 instead of ER70s-2 . But if the metal had a large gap originally, it might just keep cracking.

Welding 4130 (or carbon steel) with austentic stainless filler can result in a brittle joint. 309 is designed for joining stainless to steel- it's alloyed specifically for the dilution that occurs with disimliar joints to avoid brittleness

Originally Posted by Donald Branscom

Annealing is when metal is heated then cooled with water. There are other methods, but the main purpose is to make the metal soft so it can be formed then it is heat treated after forming. Try it with aluminum, Heat it up and quench with water and it will be soft as butter.

That's quenching, it is NOT how carbon steels are annealed- it makes them HARD and BRITTLE

There's far more detail here

[Supe]

Depending on how much MIG welding was done and the extent of the pre-existing cracks, you may want to consider having those areas cut out and replaced, or sleeved if possible. Just welding up over the cracks on thin wall material isn't going to get you far before the crack shows up again.

[MoonRise]

Originally Posted by Donald Branscom
Annealing is when metal is heated then cooled with water.

Nope.

Annealing STEEL is done by controlled heating to a certain temperature at a certain rate and for a certain time, and then cooling it at a certain SLOW rate. That will soften the steel.

If you typically heat steel (heat-treatable steel anyway) to a certain temperature (which varies depending on the specific steel alloy in question and the desired results) and then RAPIDLY cool it (via water or oil or brine or air, depending on the specific steel alloy) you have QUENCHED and hardened the steel.

If you then reheat the quenched and hardened steel to another lower temperature, you have slightly softened the steel but usually made it 'tougher' and not as brittle in a process called TEMPERING.

This is all for 'typical' heat-treatable steel alloys. Not all steel is hardenable via heat-treatment. Although various -other- heat-treatments may be applied to steels that are not appreciably hardeneable via heat-treatment in order to do things like stress-relieve the steel and such.

Other metal alloys may behave in a similar or a completely different manner from steel alloys in regard to heat treatment and the effect on the metal. For instance, brass is annealed (softened) by heating it up and then rapidly cooling it (which is the opposite of what you usually do to steel).

Jon,

If the (original) steel has been 'monkeyed' with via incorrect welding/filler/heating/etc, you may have a real mess going on. Which may take re-heat-treating things and grinding/cutting out questionable areas and then trying to re-fix things from there.

Because there may be questionable microzones (or macrozones!) of various undesirable localized alloy formations or crystalline formations in the steel from not-quite-right fillers or welding parameters (which include proper preheat and postheat and possible heat-treatments AFTER welding) being used. Think of a HAZ, and also throw in possible alloy micro-changes in the HAZ, and what could have gone wrong to the internal metal crystals from all that monkeying around. Ugggg.

[Yamahammer]

I am a vintage mxer myself and I am always looking for parts. I came accross a '75 VR frame on E-Bay.

You could probably get this one as cheap as you could have yours welded.

http://cgi.ebay.com/ebaymotors/MONTE...mZ170263837667

[mcgirrt]

It's amazing anyone remembers those old machines. I TIG welding for a few MX pros in the mid seventies. Sooner or later they all break. I've welded Ossa, Bultaco, Rickman (Matisse Frame), Elsinores and TMs. I made money when the lay down shock kits came out. Anyway, TIG welding with 70S-2 or S-6 is the way to go. Many of the frames that came from Europe were brass soldered, if you can believe it.

[JonCT]

Thanks everyone for replying.

Yamahammer: Yeah, I saw that too. Unfortunately the 73 frame is very different from the V75. I think I'll have this one TIG'd and inspect the frame after every moto. In the mean time I'll look for a replacement.

Jon

[Donald Branscom]

Originally Posted by MoonRise

Originally Posted by Donald Branscom
Annealing is when metal is heated then cooled with water.

Nope.

You are right MOONRISE. I just did not want to go into it completely and that is why I said there are other methods. But dipping a hot piece of aluminim into water will make it very soft because it is rapid cooling. Some metals are air hardened ,some are water hardened and some are oil hardened. It is a huge subject.
From now on i will just say look up annealing by Googleing it. Let the viewer read all about the process.

BTW I used to have a 250 OSSA TT Special with a Dick Mann designed frame. I won over 60 races.
The handling was rock solid. It had a lot of power above 9,000 RPM.
It had factory r/h shift. That is why i could beat tall those Bultaco's

[Donald Branscom]

I think this whole thread has gotten completely over the top as far as tech information.

All that frame need was to be cleaned and TIG welded. Thats it. A simple repair job.

I know there are all kinds of other considerations but geeeesh!

The people on this forum know a lot, I will give you that.
Even i got sucked into some kind of one-upmanship i think.