I was wondering about the merits of using Cro-Moly tube (reynolds 501 / 531 and the like). If anyone has had any experience of using it or indeed knows anything about using it, any help with the following questions, and other titbits of useful information would be of great help...

The welding would be done by a pro but is this the best way to join?

Does the completed structure have to be heat treated to stress-relieve it, if so, just the completed structure or at various stages in the build? Would this apply to a brazed frame too?

Is joining mild steel plate to Cro-Moly acceptable, ie: for things like engine mounts, and mounts for anything really?

Is gussetting with plate recommended in certain areas, as done at the head tube /top tube / down tube join of some mountain bikes?

Any other things to consider too?

Cheers..

Cro-Moly is an absolute bastard to work with, AFAIK it should be bronze welded and back gassed, you can braize it, but unless your quick and good, you will certainly suffer from distortion. It may be possible to use TIG welding, but the welds are likely to crack, especially if you use a bike engine, high frequency vibes are the killer. Gussets, brackets etc should be made from aircraft quality steel (sorry can't remember the code number, but it's got a high carbon content).
The other problems are sourcing the stuff and price.
Nice idea, but IMO not worth the hassle, unless you have access to the stock and equipment to fab and weld it for free.

Al



)i]Originally posted by Afro
I was wondering about the merits of using Cro-Moly tube (reynolds 501 / 531 and the like). If anyone has had any experience of using it or indeed knows anything about using it, any help with the following questions, and other titbits of useful information would be of great help...

The welding would be done by a pro but is this the best way to join?

Does the completed structure have to be heat treated to stress-relieve it, if so, just the completed structure or at various stages in the build? Would this apply to a brazed frame too?

Is joining mild steel plate to Cro-Moly acceptable, ie: for things like engine mounts, and mounts for anything really?

Is gussetting with plate recommended in certain areas, as done at the head tube /top tube / down tube join of some mountain bikes?

Any other things to consider too?

Cheers..

Yeah, its just an idea, mines a book mild steel job but my build partner has expressed an interest in learning a bit more about cro-mo for potentially using on his.

We have really good anytime access to an engineering workshop with tools and equipment galore. Theres a few very good welders who would happily weld it on a saturday for a few quid and / or pints at the pub if I wasnt quite up to it.. (theyve already got interested in mine sitting in the corner ).

Cost is an issue although we get trade price plus discount from various stockists. Im concerned at how technically involved it may get as we've not had much experience at work with it.. the vibrational thing you mentioned is definately something to look into.. cheers for that.. the prospect is fairly exiting but its pure homework time first I think

If you've seen any of the "Chopper is born" programs on Discovery then you may have seen the program that showed them welding up the airframe. They use Cro-Mo for that so I can't think that vibration is a big worry if it's welded correctly. After all a helicopter must vibrate more than a car even a BEC. I can't remember "how" they weld it so maybe you could search the Discovery website for the name/make of the chopper and see if the manufacturer (in the USA) has that info on their website. I'd imagine it'd be TIG welded and to a very high standard.

HTH

[Edited on 15/8/2003 by andyd]

Not to mention the fact you'd have to redesign the frame......I'm certain there is no (or little) square tube availability...

4130 type Cro-Moly is used pretty exclusively by top end race car teams for making chassis/roll cages. This has to be TIG welded with the correct rod. It isn't harder to weld than normal steel. However, you need to make sure that the weld doesn't cool too quickly otherwise a brittle joint results. Some people normalize (heat to specific temp and then cool slowly) the welds afterwards but opinion on this varies. For thin wall section tubing it probably isn't necessary. I am not sure if 4130 is the same as reynolds 501 / 531. If it is you cannot braze it. Like Alan said it is normally only available in round section tubing.

Food for thought:
CroMo welding article.
Need some CroMo tube?

Cheers for the links Rorty.

Project LMP, are you sure that 4130 cannot be brazed? Its just that I had road bike frames made of 501, 531 and 653 reynolds tubing which were all brazed, as was 753.

If I remember right 4130 (it could have been 4120 but it slips my mind as I never had one) was the stuff they made the cheaper frames out of which were also brazed..

quote:

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Originally posted by andyd
If you've seen any of the "Chopper is born" programs on Discovery then you may have seen the program that showed them welding up the airframe. They use Cro-Mo for that so I can't think that vibration is a big worry if it's welded correctly. After all a helicopter must vibrate more than a car even a BEC. I can't remember "how" they weld it so maybe you could search the Discovery website for the name/make of the chopper and see if the manufacturer (in the USA) has that info on their website. I'd imagine it'd be TIG welded and to a very high standard.

HTH

[Edited on 15/8/2003 by andyd]

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CroMo can be silver soldered, MIG, TIG, and bronze welded, but a lot depends on its use and purpose.
To be perfectly honest, it would be total overkill for a Locust. As Alan said, if you were to use CroMo, you'd want to sit down and re-design the whole structure to take advantage of all the benefits and drawbacks.

I just finished reading "Performance Welding" by Richard Finch. This book is all about racecar and aircraft welding. This guy hates MIG and swears TIG is the only way to weld. The book is very good and shows some basic fabrication techniques, but preaches TIG, TIG, TIG.

TIG welding CroMo is de rigeur these days, but CroMo was invented for building aircraft frames, about 1930, and the associated method of welding was brazing.
I've welded CroMo with TIG, MIG and bronze, it all depends on it's purpose.
If you really want to build a chassis out of CroMo, square section tube is available (as are other odd shapes, including streamline), I've even got some off cuts left over, though I can't remember what I used it for.
Just forget you ever heard of the stuff.

You can TIG weld 4130 tubing easily with the proper techniques identified in this procedure. I will answer the top ten most frequently asked questions of TIG welding 4130 Chrome-Moly. These procedures are intended for sporting applications such as experimental airplanes, racing car frames, roll cages, go-carts, bicycles, and motorcycle frames.

Can I weld 4130 using the TIG process?
Yes, 4130 Chrome-Moly has been TIG welded in the aerospace and aircraft industries for years.

Do I need to pre-heat?
Tubing applications do not require the normal 300ºF to 400ºF pre-heat to obtain acceptable strength. However, it is recommended that pre-heat of 100ºF to 125ºF be used to remove parent material moisture.

What filler material do I use?
Although there are several good filler materials, my recommendation would be ER80S-D2. This filler material will meet the strength and elongation requirements for experimental planes, racing car frames, roll cages, motorcycles and bicycle frames. Typically, you would obtain 20% elongation from this filler material after welding.

Why do I not use 4130 filler?
4130 filler typically is used for high strength and rigid applications. Due to its hardness or lack of elongation (flexibility), it is not recommended for sporting applications such as experimental airplanes, race car frames, roll cages, etc.

When I use ER80S-D2 filler material, do I give up strength for elongation?
Yes, the parent material will provide a tensile strength of approximately 95 Ksi. The filler material, when diluted with the parent material, will provide approximately 70 Ksi. However, with the proper joint design (such as cluster or gusset), the cross-sectional area and linear inches of weld will more than compensate for the strength.

Do I heat treat 4130 after welding?
NO! The term "heat treat" has been generically misused in the welding field, as has "normalizing." Heat treat and normalizing operations are extremely sensitive to heat control (preferably oven-controlled) and do metallurgically change the strength value of the parent material. Stress-relieving is the recommended practice and 1,100ºF is the optimum temperature for tubing applications. Use temperature crayon 900ºF and mark approximately 1" away from weld areas. Use Oxy/Acetylene torch with neutral flame and oscillate to avoid hot spots.
NOTE: Excessive stresses are induced by too much heat input, poor fit-up, or both, therefore stress relieving can be avoided if all fit-ups are precision (using tools like the "'Ol Joint Jigger" and applying minimum heat input, i.e. smaller welds. TIG welding should not require a weaving pattern in the welding technique.

Do I have to pre-clean 4130 materials?
Yes, remove surface scale and oils with mild abrasives and acetone. Wipe to remove all oils and lubricants. All burrs need to be removed with a hand scraper or burring tool.

Do I need to back-purge 4130 material?
Whenever possible, it is recommended to back-purge all 4130 welds using Argon gas. However, many welds are closure welds. and back purging is impractical.

Do I need 4130 plate for support gussets?
No, mild steel will provide adequate stress and strain distribution. Most gussets are designed with more than adequate linear inches of weld.

Should I cool the metal after I finish welding?
ABSOLUTELY NOT! Rapid quenching of the metal will create problems such as cracking and lamellar tearing. Always allow the weld to cool naturally.

Example of Welding 4130 Thin Wall Tubing

WELDING SPECIFICATION: Aircraft and Motor Sports
GENERAL INFORMATION:
Remove all oxides and burrs within ¼" of weld area.
Acetone-wipe to remove all cutting oils.
Assemble and TIG tack joint in a minimum of four (4) places.
Preheat to 100ºF to 125ºF to remove moisture from parent material.
TIG weld per parameters specified using Lincoln Square Wave TIG 175
WELD SCHEDULE INFORMATIONPARENT MATERIAL: 4130MATERIAL THICKNESS: .035" Wall Thickness JOINT TYPE: (Cluster) Fillet JOINT GAP: .000-.010 AMPERAGE: 0-40 Amps TORCH TYPE: TIG-9 or TIG-20 CUP TYPE: Ceramic TUNGSTEN SIZE:1/16" Diameter TORCH GAS: Argon BACK-UP GAS: Argon TACKING SEQUENCE: 4-PLCS (min.) MATERIAL CONDITION: Condition (N)FILLER MATERIAL: ER80S-D2 .035" Dia. JOINT PREP: Abrasive Clean/Acetone Wipe CURRENT TYPE: D.C.E.N. VOLTAGE:9-12 Volts CUP SIZE: Gas Lens 7/16" Orifice TUNGSTEN TYPE:2% Thoriated TUNGSTEN SHAPE: Pointed FLOW RATE: 15-25 C.F.H. FLOW RATE: 5-10 C.F.H.
NOTE: Other filler materials recommended for:
NHRA and/or Hi-Performance Dragsters
Best: ER80S-D2 Next: ER70S-2 OK: ER70S-6 Elongation: 20%-30%

Indy cars, CART cars, Semi-Rigid applications
Best: ER80S-D2 Next: ER70S-2 OK: ER70S-6 Elongation: 17%-22%

Experimental Airplanes, NASCAR, Sprints, Midgets, Go-Carts, Motorcycles, Bicycles
Best: ER80S-D2 Next: ER70S-2 OK: ER70S-6 Elongation: 17%-22%

FOOTNOTE: Welding properties change from operator to operator. Techniques such as travel speed, filler type, filler deposition rates, amperage, gas shielding, and arc voltage (distance between tungsten and weld puddle) all have an effect towards heat input, weld strength, and elongation.

James V Belfiore
RaceCar Design