Topic: My frame has a crack in it !

[kev93]

 I bought a 1972 Cb750 and I noticed there is a small crack in the frame right in front the the kickstand on the very bottom of the frame. I suppose water sat in there and froze at one time. Causing the crack, Anyone seen this happen before???

[camelman]

Very common on the 350F and 400F frames, so I expect it shows up on most of the other CBs.  I've seen a few of them now.  I believe the design of the kickstand puts undue forces on the frame, especially if the rider repeatedly sets it down heavily.  The freezing water concept is a new one to me.  It doesn't seem very likely, but anything is possible.

You could have it welded up, but the metal MUST be heat treated afterwards or it will be even more prone to cracking.  Surface treatment of the weldment and heat affected zone with a flapper wheel or shot blasting might suffice too.  The best bet would be to weld a small plate over the affected region.

Camelman

[754]

Why heat treat it? Its mild steel..

 Yes they do crack there at times.

[kev93]

It looks like it was shot from the inside, meaning it isn't flat with a crack in in. it protrudes out where the crack is. Like I would have to hammer it down to get it flat.

[camelman]

It might have been freezing water then.  Strange way to break.  Isn't there a drain hole around there?  Pounding it in and welding it would be a decent option.

The heat treat or surface treatment is necessary if there is welding done.  The weldment solidifies quickly, so the grains of steel do not have time to grow.  Smaller grains are harder than larger grains, so you end up with a very hard and stiff material zone right at the weldment.  This creates a condition where the surrounding material is subjected to high stress because it is more flexible than the weld.

It is a lot like patching a hole in jeans.  If you patch a hole with one of those iron on patches, then you have created an area on your jeans that is stiffer than before (the patch), but the rest of the jeans material is still soft and flexible.  Whenever the jeans are flexed (like when bending your knee) the area around the patch stretches the most because the patch is too stiff to stretch much.  If you have ever ironed on a jeans material patch to an old pair of jeans, you have likely seen that the jeans fail pretty quickly again, and they fail around the edges of the patch.

That's the concept at play with the weldment.  Heat treating the area, or surface treating the area, will take care of the issue.  The heat treat gets all of the material back to the same stiffness.  The surface treat mechanism is different, in that it will create compressive forces on the surface that resist cracking.

Looks up "martensite" and "austenite" if you want to learn about it.  Material Science is pretty neat.  I think there's a Material Science PhD grad from Berkely who just bought a bike and joined this forum.  He/she could probably explain it better.

Camelman

[Gnat]

Wow martensitic and austenitic metals...haven't heard those terms since the materials course I took in Dental School. And yes you are absolutely right understanding what you work with puts you way down the line towards fixing them.

Gnat

[camelman]

Yeah, it had been a while for me too.  I had to think about it before I wrote it.

One other thought... to answer a question before it is asked:
I doubt the rest of the frame was heat treated after it was welded up at the factory, and those welds evidently have not cracked.  However, considering the fact that the side stand area fails on these frames, and the fact that the metal has been worked so much, I think it is necessary to at least surface treat the material to keep it from cracking again.

C-man

[kev93]

here is a dumb question. How do You heat treat it?

[sangyo soichiro]

To heat-threat it, can you just heat up the area with a torch (cherry red) and somehow cool it very slowly (as slowly as possible)?  Kind of like annealing it?  That's what comes to my mind, for a DIY-type solution anyways.  I'd hate to have to pay a heat-treat place to do the whole frame...


Edit:  But I never really thought about having to heat-treat cold-rolled steel.  In fact, in order to heat-treat it to make it harder, don't you have to introduce carbon somehow (like when they case-harden it)?  Cold-rolled steel is just one of those steels that doesn't get that hard as far as I know.  When I did tool and die, it was pretty much the easiest steel to work with, whether welding or machining.  The tool steels were trickier.

[camelman]

This is a mild steel and needs to stay that way.  If you heat it up with a torch to red hot (just starting to glow red) and let it air cool, then you will have treated it sufficiently without changing the material surrounding it.  Let it cool down, clean off any cruddy paint, prep, paint and ride.

Don't let the torch sit in one spot either, just pass it back and forth.  If you let it sit in one spot, then you will overtemp it.

Don't quench it either.  Just let it air cool.

Good luck, and let us know how it goes.


Camelman

[wdhewson]

Hi:

I'll have to check out my 1972 CB350F for a frame crack in this area.

My heat treating of steel is a bit rusty, having peaked about 10 years ago.  Let's give it a shot and see if we can dig out a real expert.  

I do believe that welding induces heat related stresses, but these may not be the austenitic and martensitic crystallographic transitions normally associated with true heat treating.  For true heat treating one must carefully follow the temperature/time/transformation diagrams availale for heat treatable alloys.  And those alloying elements that make steel heat treatable are probably not present in the mild steel frame tubing.  One can carburize or nitride mild steel but these are surface treatments and take a long time for the carbon or nitrogen to diffuse into the metal, so many hours are required at high temperatures in carbon rich or nitrogen rich atmospheres.

So the bottom line is heat treatable steels are expensive, and not likely used for frames.

This begs the question, what does the factory do?  Are frames "stress relieved" after welding them?  My guess is not.  And think about all the other welding on cars, buildings, bridges, etc that are not subsequently stress relieved or otherwise tempered.

If my CB350F frame tube is cracked, I'll have a TIG guy stitch it up or maybe even braze, which is pretty darn strong too.

Feel free to take me to task and offer corrections.  I won't be insulted

[scottly]

I've seen that type of crack several times: the first time was on a pick-up truck lumber rack. The rectangular 1x2 vertical tube was swollen and rounded in the area; took a few minutes for a few of us to figure out that frozen water had done the damage.
If the crack is length-wise with the tube, just weld it. If the crack is around the pipe, weld it, then grind it flat and fab a "doubler" patch, with tapered ends, on top, to spread the stress over a larger area. No need to heat-treat, or "normalize" for this type of repair. Just allow to air cool...

[Old Scrambler]

That's why this forum is so useful...............When we can find expert opinion it helps to keep these bikes on the road and on the race track.   I wonder how many of the race-frames are properly welded?  Apparently the frames are prone to stress cracks below the front neck caused by the forces of hard braking with double-disks and modern tires.

[scottly]

  Apparently the frames are prone to stress cracks below the front neck caused by the forces of hard braking with double-disks and modern tires.

I'm not an expert; but I do have some experience. Got any photos of the stress cracks?

[kev93]

Scary thing is with stock exhaust on I couldn't see the crack. I looked at another early 750 and it had been repaired in the same spot.

[Don R]

My K0 is split from the stand to the motor mount. I cut a section from another frame and plan to tig it in when the bike comes apart for paint and restoration. Tig it and forget it.

[scottly]

Tig, mig, stick, doesn't matter as much as the type of joint: if you are talking about a butt weld in a tube, I would try to make it away from the original stress point, and then I would still try to use a doubler as insurance. It really depends on how and where the crack formed..

[camelman]

Hi:

I'll have to check out my 1972 CB350F for a frame crack in this area.

My heat treating of steel is a bit rusty, having peaked about 10 years ago.  Let's give it a shot and see if we can dig out a real expert.  

I do believe that welding induces heat related stresses, but these may not be the austenitic and martensitic crystallographic transitions normally associated with true heat treating.  For true heat treating one must carefully follow the temperature/time/transformation diagrams availale for heat treatable alloys.  And those alloying elements that make steel heat treatable are probably not present in the mild steel frame tubing.  One can carburize or nitride mild steel but these are surface treatments and take a long time for the carbon or nitrogen to diffuse into the metal, so many hours are required at high temperatures in carbon rich or nitrogen rich atmospheres.

So the bottom line is heat treatable steels are expensive, and not likely used for frames.

This begs the question, what does the factory do?  Are frames "stress relieved" after welding them?  My guess is not.  And think about all the other welding on cars, buildings, bridges, etc that are not subsequently stress relieved or otherwise tempered.

If my CB350F frame tube is cracked, I'll have a TIG guy stitch it up or maybe even braze, which is pretty darn strong too.

Feel free to take me to task and offer corrections.  I won't be insulted

Wdhewson,

I like how you are thinking this through, but I think you are a little off track.  All metals are heat treatable.  Also, welding isn't really a heat treat, although it does create martensite because the base material is heated well above the austenitization temp then cools quickly, even in air, to produce martensite.  Even if it doesn't, it will result in stiffer local material around the weldment and into the heat affected area.

Heat treating will make the material less strong and more flexible (both of which are required after welding).  It doesn't have to be perfect science, but heating up the material red hot (below austenitic temp), and then letting it air cool is definitely going to help.  I'm not going to get into an analysis, but it ain't gonna hurt.

I first ran across this welding issue back in college.  One of my professors was called in to a factory to help them redesign a lifting structure.  It had failed, was welded, then failed again repeatedly.  They kept building up the area with more weld, but the material neighboring the weld kept cracking.  They thought they needed a whole new structure.  He had them cut the old crap out, weld in new structure, locally heat treat it, then surface treat it.  It was still fine eight years later when I was in school.
These frames don't need heat treating for the most part, but I do recommend it if welding up an area that has cracked.  Something caused it to crack, so unless you want to keep welding it, you should treat it.

Camelman

[754]

It cracked from water, that is why they crack...

 I think you are calling what some call post heating, heat treating.
...................................................................................
 "Heat treating will make the material less strong and more flexible".. quote from Camelman.

 That is not true with all materials, most steel that is heat treated is higher carbon, and end result is usually  higher strength. Tempering reduces hardness and increases toughness. However not all materials  react the same to heat treating.

 Factory frames, I believe, are simply wire welded, without additional process.. except if out of tolerance they would go to post weld straightening..

 I talked to a structural steel fabricator yesterdauy, his opinon on a double was  weld along the sides but not across it..

[camelman]

754,

You're right.  I got tunnel vision on that one.  I didn't mean to make a blanket statement about all heat treating, I meant to say annealing (with respect to mild steel and the heat treat method I mentioned).

C-man

[wdhewson]

Hi Camelman.

Two things to add.  One might be easy to understand, and the second might be beyond my ability to write about clearly.

The simple one first.  "All metals can be heat treated."  That's a broad statement.  What about gold?  Maybe.  I don't know.

The complex one second.  When I mentioned the temperature/time/transformation diagrams for heat treatable steels, I perhaps should have underlined time.  The formation of austenite takes hours of time, because the carbon is slow to dissolve and diffuse in the steel even at red heat.  A commercial heat treater will austinitize for hours, and a few moments at cherry heat is simply insufficient time for the formation of any significant amount of austenite.  And without the austinitic transformation, one cannot then enter the martensitic phase.  There just ain't no thermodynamic pathway.

However, the weld will leave residual stresses that have more to do with the shifting of intergranular boundaries during heating and cooling, and little to do with intragranular crystallographic forms or interstitial carbon.

I know there is a lot of high falutin' vocabulary in the above, but I think if you look up the words it might make sense.

About 12 years ago I had the hard job of understanding the research done by the big US Steel laboratories in Pittsburgh in the early 20th century.  I think that that is where Mr Martin (Martensite) and Mr Austen (Austenite) might have worked.  But I never boast about having a good memory.  (Just checked, Mr Austen was in London) Thanks, Don

[scottly]

 I talked to a structural steel fabricator yesterdauy, his opinon on a double was  weld along the sides but not across it..

What I like to do is drill holes in the doubler, and then welding through the hole, filling it up. ("Rosette weld")

[754]

When you reach the point of recalesence, all hell breaks loose...

 oddly enough I forget the temp for that but a magnet should fall off....................................................

[BigBoi]

Had this same issue a while back:

http://forums.sohc4.net/index.php?topic=61417

Had it welded and no worries.

[camelman]

Good lord, this heat treat/anneal discussion has gotten way off track.  Heat treating to the point of stress relieving, which is all that needs to be done after welding this frame, only requires heating this particular steel to red hot, and letting it air cool.  There are a million permutations for different metals and different required results.  I think we are all getting too many terms caught up in this discussion.

Back to the original question:
If you are concerned about the crack reappearing (you should be if you have to bang the metal back into place and then weld it), then heat the sucker up until it is red hot and let it air cool.  You can also check it with a magnet to know when it is hot enough.  As soon as it looses its magnetism, it is hot enough.

Any welding shop would do this for you.  I only mention it because you are doing it on your own.

Camelman