LF2 has a richer chemistry that will impart better through thickness properties in a Q&T product.
A key factor is the steel making process for each. If Charpies are a key factor, you want to specify a low sulfur steel (calcium treated for sulfide shape control). Sulfides are a notable bad actor.
Depending on the forging schedule, you can work the product more uniformly resulting in reduced material property anisotropy - a good thing.
The SA 350 is a specification for forgings versus SA 516, which is for plate. Most tubesheets for feedwater heaters, high pressure, call out SA 350. Regarding mechanical properties, you need to compare directly and thickness becomes important in relation to these mechanical properties (especially for plate).
In H2S service, the SA-350 is less likely to suffer from hydrogen induced cracking (HIC) or stress oriented hydrogen induced cracking (SOHIC). Why not give us the specific application to enable a better evaluation of the question?
