Chrome Moly Question

[src5]

I have a question concerning MIG welding 4130 Chrome Moly Tubing. Scenario: Track Promoter/Owner has legalized the use of 1.50" x .083" chrome moly tubing for the roll cage of an asphalt stock car in place of the 1.75" x .095" DOM mild steel tubing currently legal at all other race tracks. I am concerned about the safety of lowering both the tube size and wall thickness as well as weldability & would like some feedback.

[Paul Seaman]

The 1.75x.095 would be stronger simply due to more material per length. The 1.5x.083 is still a strong material being CrMo. I would still prefer to weld CrMo with tig to reduce the HAZ{heat affected zone**. I have migged CrMo and it will work the trick comes in using E80s2 or E80s6 and c25 gas. Keep heat input to a minamum and weld partials on several tubes and going back several times to make partials for the finished weld. This is a chassis version of backstepping used on welding sheetmetal in larger panels to avoid distortion.

I would do several test pieces on some scrap and do distructive testing before building a full chassis and finding a flaw with a finished chassis.

Good luck,

[dyn88]

the strength of the CrMo is virtually the same at the smaller dia. as the larger dom steel. The reason for this is the much higher tensile strength of the CrMo at around 80 -90 thousand pounds as apposed to steel around 55-65 thousand pounds. pj is right to use er80 filler on the CrMo. the trick about using CrMo is that you use less weight per foot of construction while maintaining the same strength= lighter cage= faster car, or the use of ballast at more benificial locations(like to better balance the F-R weight balance or change the center of gravity on the vehicle,etc)

[Paul Seaman]

My appologies I misread the post I though you were comparing moly to moly of different diameters, Sorry about that.

Dyn88-Great catch, thanks.

I've made 2 in 2 days, time for and optical visit.

[Portable Welder]

I have mig welded up chromoly for test purpose only using ER70-S6 verses Tig welding with ER70-S6 to show a friend of mine the strength of doing it both ways.
His tig weld broke first, he diddnt use enouph filler metal because he wanted the cool looking small tig weld bead.
We had welded short stub pcs. to a bench and beat them silly with a sledge hammer, Both pipes bent before they broke after several hits.
My reccomendation is to do some testing. It does'nt cost much but it will sure give you pc. of mind.