welding chromoly to mild.....any issues?

[manta22]

Gas welding works just fine on chrome- moly tubing but the tubing has to be "Condition N"-- normallized. As others have already said, mild steel is a better choice for most applications; its cheaper, easier to weld, and it absorbs energy in a crash by deforming plastically. Cr-Mo is more expensive, stronger, easier to find in very thin wall sizes, but it does not absorb as much crash energy as mild steel.

[saltfever]

Excellent thread and many thanks to Willie for starting it and sharing the pics.  It should be noted the strength of the weld is also dependent on the thickness, or the amount of material of the fillet. The filler material and tubing both have a specific tensile strength. However, the thicker they are the more force it takes to deform or break. On many weld symbols the fillet thicknes is called out to guarantee enought strength is put into the weld joint. All of the fab procedures mentioned are critically important but the amount of filler is equally important.

[saltfever]

When NHRA mandated a CM cage and TIG; I thought it was strictly for safety. Obviously CM is lighter but they make rules changes like that mostly for safety reasons no? There is no question 4130 is cleaner (electric arc melted, vacuum degassed) and it is much easier to weld with any method. Mild steel has inclusions and any manner of garbage in it. Who knows where it is comming from? I have found great variability in mild steel. 4130 is certified tubing. It is wonderful stuff to work with because it is predictable and always good. That is why I thought NHRA made the requirement. I don't have the NHRA book but does it define the process? I.e. post weld annealing?  That is the only way to get a strong 4130 weld. After the discussion here I am lead to believe that mild steel is the most forgiving of process, fab technique, craftmanship or skill and therefore safer. Willie's testing seems to support that so I am in a quandary as to why NHRA made the rules change?  This is a big disparity I wish I knew NHRA's technical or engineering reasons.

Wether a cage should be rigid and strong or yield in a crash is a continuing discussion in all racing venues. But almost everyone agrees the force must be attenuated in some manner and not imparted to the driver. The speeds in NHRA are equal to many LSR classes. Yet mild steel seems to be the choice in LSR. Is it because there is a wide range of fabrication skill allowed in LSR and due to this unpredictability, mild steel is safer?  I know the NHRA cage is certified. Does that mean the welder is certified also? so many questions . . .
 

[willieworld]

saltfever---you know its funny that you mention a certified welder --i do and have done many nhra cars that require tig and CM --so i thought that i would go down to the local welding school and take the test --when i got there and talked to the owner he explained that the test was a pass or fail (he had been in my shop and seen my welds and he said i would have no trouble passing the test )the test was very expensive and only covered thin wall round tubing of minimum and maximum size--anyway he explained the test would be a butt weld on a piece of thinwall tubing--NHRA does not allow a butt weld on CM except under very limited situations then the weld has to have a sleeve inside that goes into each tube 3 inches 6 inches total with rosette welds --any way i didnt take the test and saved myself a lot of money  willie buchta -------- you dont have to be a certified welder to weld on a nhra car  or a lsr car or any other kind of race car that i know of i think if it was airframe you might have to be ---------

[manta22]

You don't need a certificate to weld on an airframe if it is classified "Experimental".

[Ratliff]

When NHRA mandated a CM cage and TIG; I thought it was strictly for safety. Obviously CM is lighter but they make rules changes like that mostly for safety reasons no? There is no question 4130 is cleaner (electric arc melted, vacuum degassed) and it is much easier to weld with any method. Mild steel has inclusions and any manner of garbage in it. Who knows where it is comming from? I have found great variability in mild steel. 4130 is certified tubing. It is wonderful stuff to work with because it is predictable and always good. That is why I thought NHRA made the requirement. I don't have the NHRA book but does it define the process? I.e. post weld annealing?  That is the only way to get a strong 4130 weld. After the discussion here I am lead to believe that mild steel is the most forgiving of process, fab technique, craftmanship or skill and therefore safer. Willie's testing seems to support that so I am in a quandary as to why NHRA made the rules change?  This is a big disparity I wish I knew NHRA's technical or engineering reasons.

Wether a cage should be rigid and strong or yield in a crash is a continuing discussion in all racing venues. But almost everyone agrees the force must be attenuated in some manner and not imparted to the driver. The speeds in NHRA are equal to many LSR classes. Yet mild steel seems to be the choice in LSR. Is it because there is a wide range of fabrication skill allowed in LSR and due to this unpredictability, mild steel is safer?  I know the NHRA cage is certified. Does that mean the welder is certified also? so many questions . . .
 

The preferred material for World of Outlaws sprint cars and Super Mods is chromemoly.

Whether mild steel or chromemoly is better at dissipating energy is a moot point because effective impact attenuation requires crumple zones that don't exist in many land speed classes.

[manta22]

The chassis & roll cage are the "crumple zones". Plastic deformation of the tubing structure absorbs energy.

[Sumner]

The chassis & roll cage are the "crumple zones". Plastic deformation of the tubing structure absorbs energy.

I would hope my chassis and body would "crumple", but not sure if I want the roll cage to "crumple" much.

c ya,

Sum

[Peter Jack]

The reason W of O and modifieds use moly is because they can use thinner therefore lighter tubing. Weight or lack thereof is everything.

Pete

[manta22]

Sum;

I don't want my roll cage to crumple either but, in a hard crash, it probably- will but only a little. Every little bit of deformation absorbs energy that would otherwise be transferred to the driver. We don't want the cage to collapse but it will deform some in a hard enough crash. Much of the energy absorption takes place as the car's body is crunched, the suspension is torn off, and various bits & pieces are shed during the event. All that kinetic energy has to be dissipated somehow.

[interested bystander]

A few posts up NHRA was referred to as mandating 4140 tubing . To be exactly correct NHRA relies on an SFI spec, just like safety belts, bell housing shields, rollbar padding, ad infinitem.

The SFI program was formerly the SEMA Chassis Builder program and the whole now giant SPECIALTY EQUIPMENT MARKETING ASSOCIATION- the guys with the second biggest trade show in 'Vegas -actually started with need for a chassis spec for slingshot dragsters. SFI divorced from SEMA some years back and, although not part of NHRA, has always been closely affliated. 

Because of recent fatalities/crashes the SFI program has come under a lot of flack. Part of it deals with summarily dropping chassis comitteee members because they didn't agee with the SFI head and his close friend, a prominent midwest builder who IMHOP legislated rule changes without proper engineering diagnoses that were prone to causing catastrophic failures-and DID!

Oxweld 65, an old Linde number was the rod of choice for years but I UNDERSTAND it's no longer made- if anyone knows different, please post.

You'll be safe in TIG welding 4130N with ER70-S2 rod- commonly available- don't be mislead by chromoly rod -unless the whole assembly is undergoing normalizing after weldment. 

[willieworld]

that might of been me  --what i was refering to was a sfi chassis --faster than 7.50  --the chassis has to be  CM and tig welded --nhra has changed some rules lately  and i havent done a nhra car sence i got cancer 4 years ago but below 7.50 the chassis can be mild steel ew-dom or seamless with a minumim of .118 wall thickness --tubing is checked with a sonic checker  which i still have if anyone ever needs to use it --i will say this for the NHRA rules they are very precise and easy to understand --no wiggle room your either legal or your not --no in between ---if anyone is thinking about building a chassis and cant weld do yourself a favor and get someone to do it that knows how its not that expensive compared to the alternative    willie buchta

[Peter Jack]

Interested:

I think you'll find that Oxweld 65 and ER70-S2 are almost identical in specs. I think probably Oxweld 65 met the ER70-S2 spec before the spec was made. I know I burned a lot of it. You'll definitely know if you don't have the right rod and try to tig weld with a steel rod designed for oxy/acetylene.

Pete

[saltfever]

. . . Whether mild steel or chromemoly is better at dissipating energy is a moot point because effective impact attenuation requires crumple zones that don't exist in many land speed classes.

But if one was designing-in a crumble zone, the selection (or performance) of the material is critical. If using mild steel the geometry “might” be triangulated spaces? But if 4130 it might be trapezoidal spaces? These may be bad examples but I think the kind of material dictates the shape for a crumple zone. Geometry works hand-in-hand with tensile strength and stress/strain. Agree, that LSR past-practice negated or minimized the use of crumple zones. In fact, until the Dale Earnhardt accident very little focus was given to attenuating forces on the driver. NASCAR spent millions after that. Nowadays all the major racing venues spend major dollars on design and research to lessen impact force to a driver. And that is my quandary. I constantly heard conflicting opinions, or anecdotal examples about what is the best material for a cage in LSR. That was the same scenario in NHRA for the past 50 years! Do you make it Light(4130) and strong, or heavy(1030) and ductile?  Finally, 2 years ago, NHRA ended the debate and required TIG and CM. I don’t know the data they based that change on. I am sure they had very good engineering data and excellent accident analysis to come to that decision. I just wish we had a way to access that information. All the major racing venues are entertainment corporations. They face the same constraints on their business as any corporation. They must weigh, risk, and liability but at the same time keep their product attractive to their audiance. They don't make changes capriciously. They spend serious money on good engineering and careful anaylsis making safety rules decisions. Knowing that, maybe the NHRA spec is a safer way to go?

Willie, do you have the SFI spec number for a CM cage?   

[willieworld]

saltfever--there is no sfi number its just a spec sheet that you have to go by--the nhra cars and lsr cars have not much in common other than the speed --in lsr there are no other cars to run into there  are no guard rails -grandstands-steel poles and cables-broadcasting stands-or cement walls at the end of the track--i think that engine explosions are the biggest problem for the fast guys and they dont care if they explode the engine as long as they make the next round--they just shortened the track from 1320 ft to 1000 ft i dont think that will solve any thing--if they would quit blowing up the motors i think most of there problems would be solved in that area ------- i would never build a car or bike for myself of CM , i have always  thought MS was the way to go and still do --just remember before you go trying to change the rules (LOL) that the rule books are minimums and if you arnt comfortable with that then add tubes and gussets and crumple zones and even air bags if you like   just some thoughts  willie buchta

starting a new topic on this subject