The T3 designator is Heat treated then cold worked. The designation also applies to Products that are cold worked for the primary purpose of improving their strength i.e. 2024-T36 . It also applies to products in which flattening or straightening is recognized in applicable specifications such as flat sheet which is Designated 2024-T3.
The T4 designator is solution heat treated and naturally aged to a substantially stable condition. This designation applies when the product is NOT cold worked after heat treatment. The designation also applies when applicable specifications do not recognize the effects of cold work in flattening and straightening operations. The T4 designator applies when the product is heat treated by the customer.
Typical mechanical properties for 2024 : 2024-0 Ultimate 26,000 psi yield 11,000 psi . 2024 T3 ultimate 65,000 psi yield 45,000psi. 2024 T4 ultimate 64,000 psi yield 42,000psi.
So it would appear that if you can use 2024 T3 aluminum sheet you may not have to heat treat.
I'm confused by the idea of starting with T3 and ending up with T42 ... I consider them equivalent tempers.
any shape you can form in T42, you should be able to form in T3. If the drawing states T42 material, T3 should ab an acceptable substitute (so long as you can form it).
If the shape cannot be formed in T3; then HT to O, form, and HT to T42.
If the drawing states T42; then it is expecting that you'll buy T3 (or other), HT to O, form, HT to T42.
When I said ,If, you can use 2024-T3 I meant using the material as is with no forming, or very gentle forming as you would on a wing skin or flat part.
If you do have to do extreme forming, then heat treat and quench would be required then form and re- age to T4.