MARK J WEATHERS writes:
> low carbon steel and mild steel and 1020 etc are the same basic thing. I
> have no idea what chrome-moly steel is either but it sounds cool when you
> say it.
Sounds like a need for a terminology post. You're correct that low-carbon
steel, mild steel and 1020 are roughly the same thing. But, 1020 is a
specific grade of mild steel, just as 1010 and 1012 are. The 1000 series is
composed of all low-carbon, malleable steels, with slightly varying amounts
of carbon and alloying elements.
"Chrome-moly" steel is usually the term for AISI 4130, although it can also
mean 4140. Ask for chrome-moly tubing and you'll likely get 4130, since 4140
is normally used not for tubing (doesn't draw quite as well as 4130) but is
used in billet form for cranks, etc.
ERW stands for electric resistance welded. This grade is probably
permissible under some rules for roll bars, but will probably require a
thicker wall to meet the rules than seamless. Seamless, of course, is
considerable more expensive. If you can't afford the weight, check the rules
and see if a lighter wall seamless is equivalent to a heavier wall ERW. ERW
is rolled and the seam electrically welded--therefore, it's not quite as
strong as seamless, if all other specifications are the same.
I may have mentioned previously that while the rules may allow 4130 for roll
bars, it's not recommended, for a couple of reasons. Because of the
extremely low elongation of the material, the difference between the points
of yield and failure is small--that means that if it fails, it doesn't bend
much--it snaps. Pieces of it can break loose and become spears. Not
something I'd like in a roll cage. The other detriment in fabrication is
that the material work-hardens with welding, and requires rather uniform and
precise annealing for at least a foot each direction from each weld.
Difficult to do with a large piece such as a roll bar or cage, and
especially so if some of the welding has to occur in the car.
Mild steel, because of its malleability, yields much more easily, but it
doesn't fail catastrophically--it gives, bends, bends some more, all the
while absorbing energy. It welds easily and doesn't require extensive
annealing because it doesn't have enough carbon in it to noticeably
work-harden.
Cheers.
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