"4130 Chromoly or ( Chromium Molybdenum ) has a 0.30% carbon content... thus you will have a higher tenancy for stress cracking or metal fatigue, specailly around the welds and transition where stress loads are high. Chromoly still is considered technically a "low carbon alloy steel." But 1020 steel ( Mild Steel ) is also a "low carbon steel" but it has a content that is 0.20%. The carbon makes the metal easier to weld, but the more carbon content makes it more brittle and thus lower the tensile strength of the metal. However, these metals both have a very high tensile strength."
Grabbed that from some random search result. It gives you the basic idea. A race car gets ran hard occasionally and retired after 2 years. The loads it has to handle are much higher than normal driving but the constant vibration and use of a street car will make it fatigue just as quick as racing frames. Unless you plan on replacing it every few years you will start seeing stress cracks show up all over it and after 5 years of use it will be so weak that any impact at all will probably rip the car apart.
The only bad part is you are probably going to double the weight of the frame you build by going to a structural steel frame. The good part is the frame will outlast the car. I guess everything is a tradeoff so it all comes down to what you want out of the car and how you plan on driving it. I am just trying to make sure you get all the information so you can make good decisions when building your ultimate car. It is a lot easier to build a car than most people think, it just takes a huge amount of time. Hopefully you will have lots of fun doing it
