Ekis, as I mentioned in private message, nothing I say can negate what the code book says, you will have to pick it up and look at it for yourself. Look to the PWHT Tables in UW and that will define for you the thickness based on material family at which PWHT becomes a REQUIREMENT. My best guess, from being around ASME for some 25 years is you'll find 19mm is the point, but again, I'm writing from home and don't have the code book, you should have it. Check it.