[chicane]

On another note... this is just one of the reasons I went DOM, vice E4130 or ERW.

One misconception that a lot of people have is that different alloys of steel have different stiffness. This is not true. All steels have almost the same stiffness (modulus of elasticity) within 5%, (stainless is about 10% less). If you want a different modulus of elasticity then you need to use a different material ie: aluminum, titanium ect. The modulus of elasticity determines how far the material will flex under a given load (ie: rubber low modulus, glass high modulus). The modulus for steel is 30,000,000 PSI

Different alloys of steel in different conditions of heat tread and work hardening do however have different strengths. There are to values for strength that we need to look at, Ultimate strength and yield strength. The yield strength tells how much stress the material can handle before it permanently deforms. This is the spring back or memory that the material has. Ultimate strength is the stress level that the material can take before it finally fails, at this point however the part has already bent and is ruined. The other material property that we should look at is Elongation, it is described as a percent and is the amount of permanent deformation that the material will have before failure (ie: silly putty very high elongation, Glass almost zero elongation). More elongation will allow the part to get a pretzeled up and not fracture. On our tubing bender we have had some problems in the past with braking the martial during the bending process, the solution was finding a metal distributor that had the same alloy of material but with a higher elongation value.

The strength of steels is largely dependent on the heat treat. The difference between the alloys is how heat treatable they are. Lower alloys can’t attain as high a level of heat treat.

As far as cromo vs. mild steel here are some specs to look at.

1020 DOM Yield 70 Ksi Ultimate 80 Ksi Elongation 15%
4130 NORM Yield 70 Ksi Ultimate 90 Ksi Elongation 20%
4130 HT@800F Yield 173 Ksi Ultimate 186 Ksi Elongation 13%
ERW tube Yield 25 Ksi Ultimate 45 Ksi Elongation 35%

As you can see 4130 is only slightly better than DOM, also note that it has a higher elongation value. This means that everyone’s conception that 4130 will splinter on impact is false.

As far a weld zone effects there are a lot of conflicting theories regarding this, filler rod type, normalizing stress reliving, ect. One welding practice that is recommended for 4130 and rarely done is preheating. 1020 will be more forgiving to non professional or professionally funded welds.

I would put more energy in to the design rather than the material, avoid single sheer mounting and structures in bending. Don’t use ERW tube, use 4130 for bragging rights or smaller part that you can send to heat treat, other wise use 1020 DOM.