By just going through the definitions it appears that 4130 in it's normalized state is an ideal material for these structures. But as stated earlier, stronger is often considered better. But in reading all of the definitions, it may appear that toughness is just as important. Well, actually it is more important in this application as we shall see. But first, let's see where one would use a high tensile strength material. Piano wire, spring steel, drawn steel wire for suspension bridges, submarine hulls, etc are all applications that benefit from higher tensile strength at the expense of toughness. But in structures that have to be damage tolerant, toughness, with an adequate yield strength, are desirable properties. The energy needed to fracture a mild steel part is very large compared to that of high carbon heat treated part. Look at the area under the stress-strain curve in the following diagram. Even though the tensile yield is higher, the Fracture Toughness is lower. Notice also the fact that heat treatment does not affect the modulus of the material. Heat treat that part all you want, the flexability remains the same. It is just the yield point that changes.
There we have it. If you want a structure that can take the beating and not fall apart, it's mild steel and not high tensile steel.
But we must ask why 4130N. Well, 4130N has properties that maximize both strength AND toughness. This relationship is well proportioned for use in vibrating, crash tolerant structures. It also has the added benefit of slow crack propagation which is important in aircraft welded steel tube structures. Other very important benefits are price, weldability, ease of forming, etc.
But it is possible that a material with a lower yield and higher toughness than 4130N be better suited for this application? Possibly yes. But with 4130N being a very common controlled material because of its applications in aerospace, it may just be the ideal material. Although generally not done or required, slight pre-heating of 4130N might be prudent, especially if welding in a cold environment
For some very interesting information on welding 4130, and why mild steel filler material is used such as ER70S-2, visit these interesting web sites.
http://www.netwelding.com/4130%20Tech%20Details.htm
http://www.lincolnelectric.com/knowledge/articles/content/chrome-moly.asp
Here is an interesting web site on the use of Mild Steel for chassis material. It has a lower yield but higher toughness. This single article presents very compelling reasons to not use high yield / low toughness material in the chassis.
http://www.netwelding.com/Welding%20Race%20Cars.htm
While on the subject, we should briefly touch on design. Many have commented on the past and recent catastrophic failures of Top Fuel Dragsters as if the "tremendous loads" were to blame. Also mentioned was that the chassis is "designed to break apart in a crash". This of course is all non-sense. The loads on a Top Fuel Dragster chassis, while higher than an average race car, are by no means tremendous. And in a crash, containment is always preferred to non-containment. The reasons for many of the failures are due to poor, or lack of, proper structural design. Steel structures, if designed so the maximum stress is less than .5 of the tensile yield of the material used, should not only take the load easily, but have an infinite fatigue life. But another issue which is apparent on Top Fuel Dragsters is the long unsupported lengths of tubing on the lower compression rail. This is a buckling situation in the waiting. Any damage, bending, or misalignment due to clamping on brackets, will greatly increase the likelyhood of buckling.The reason the sport has been getting away with a poorly designed chassis, is due to the outstanding properties of 4130N. Deviations
References: Wikipedia Lincoln technical references John Schwaner, Sky Ranch Engineering Manual J.E. Gordon, Structures, or why things don't fall down Marks' Standard Handbook for Mechanical Engineers |
I am by no means an expert in this field of metallurgy or structures, but as one can see by searching around and finding the correct information and presenting it in proper form, the reason for using 4130N becomes clear.
Joe LaCour, started 6-19-2007, finished 9-25-2007 & added to as needed.
To read Keith Burgan's Counterpoint to this, click here.