Please accept this clarification; ASME Secion IX does NOT specify the thickness or materials that MUST be heat treated, this is a function of your constructing code, i.e., B31.X, ASME VIII, etc. I am unaware of any changes to the welding process parameters which would negate PWHT.
Based on my familiarity with the ASME Codes, if in fact your constructing code DOES require PWHT, the only method I know of to avoid PWHT would be to qualify a welding procedure for using the temper bead method. If the welding is already done however AND your constructing code REQUIRES PWHT, you have no option.
I´d add the following opinion to Jon´s reply:
ASME VIII rules the design and construction of pressure vessels.
ASME B.31 rules the design and construction of pressure piping.
The fact that you´re using API 5LX70 would suggest that you´re constructing a pipeline, either for oil or gas. For oil, the applicable code is B31.4; for gas is B31.8.
If by a chance you´re constructing a chemical plant or an oil refinery, then it is B31.3. Which is your case?
According to it, you should do as Jon says, i.e., consult the applicable code and follow it.
Giovanni S. Crisi
Sao Paulo - Brazil