Is it always safe to assume that A333/LF2 can be used as a substitute for A106/A105 and is there any difference in how they are manufactured or does the manufacturer simply take a batch of A106 and charpy test it and if it passes it can now be labelled A333?
No, do not assume. You need to obtain a copy of both ASTM Specifications and compare them regarding metling practice, HT, properties. inspection and end use.
Only real difference that matters is A333/LF2 is impact test to a lower temperature.
In reality its the same material.
Never substitute without owners approval though... always saves a NCR.
At temperatures above 650F to 700F, the fine grain material has lowered creep strength and at temperatures above 750 F they are more prone to graphitization than A-106/A-105.