Just to clear things up, 4130 is NOT T45. 4130 is not accepted by may sancioning bodies due to how brittle it gets in the HAZ.. The only way to solve that problem is to normalize the material. Since there is really no way to prove the joints have been normalized. You cannot use it.
T45 is Carbon Manganese Steel. You cant even buy it in the US and the only place that I have seen it being used is in FIA Homoligated roll cages. AKA Custom Cages UK. Its got around the same tensile strength as 4130 but its properties dont change with heat.. meaning you can even MIG weld it..
Save some money and make your subframes out of DOM 1020. Use 1" x .080". Design it well.
Originally Posted by bigusnickus
Easy one: 4130 aka chrome-moly aka t45. It is what all race car roll cages and subframes are made out of. It has an infinite fatigue life (unlike aluminum) if properly designed and can be made nearly as lightweight as aluminum. Look up MSI's subframes. Common diameters are .75"-1.5" and wall thicknesses of 0.049"-0.095".