A comparison of material properties for "mild steel" (1015) and 4130
provides some clues. (It should be noted that the type of heat treatment has an important effect on both materials. I will assume that both were normalized at 1600-1700 deg F).
The 1015 has an ultimate tensile strength of 61,600 psi, whereas 4130 is listed at 97,200 psi. So, the 4130 is almost 58% stronger than 1015.
The 1015 has a modulus of elacticity of 29,000 ksi compared to 29,700 ksi for 4130. So, 4130 is about 2% stiffer.
At equal static strength and stiffness, a structure constructed of 4130 will be lighter than one fabricated from 1015, as John mentioned.
However, the impact properties of 1015 are substantially better than 4130. The Izod impact results for 1015 of 84.8 ft-lbs are almost 31% higher than the 4130 at 64.2 ft-lbs. So, 1015 is tougher in impact,
Furthermore, 1015 elongates to a greater extent before failure than 4130
(37% elongation at break compared to 25% for 4130).
And impact performance should be the primary consideration for a rollcage.
Finally, welding creates thermal stresses. These stresses are more easily shouldered by 1015 than 4130. Yes, it is possible to heat treat the structure after welding, but most of us don't have an oven large enough to take a rollcage or tube frame for a Bonneville race car.
Certainly, there is great wisdom behind the rulebook's recommendation that John referenced!
Good luck with your project!
