Please consult any certified consultant, they will help in this issue.

The questions are too complex to be answered on thi sforum

Applications? Pressure ? Temperature ? Stress levels? ...

If you want the ASTM std for plate materials, how can you expect us to provide that ? that is the work of the designer who has designed the structure

Foe weldment, if it is an ASME pressure vessel then use the corresp ASME code. Else you can go ahead with the AWS or API (depending on the applicability)

Check this list (took less than a minute to find from google it was in first page, )

http://www.ndt-ed.org/GeneralResources/Standards/ASTMStandards.htm

If you follow the ASME, AWS(D1.1) or the API then most of your queries wiil be answered.

The ASTM E164 will be sufficient for your application, get the copy, and the procedure, probes, the frequency(MHz), dB wts all will be covered.

But not the acceptance levels, that is your job to get.

BTW 6mm root looks to be a bit high for the welding

Depending on the base material the friction stir welding process may be your best bet.