I am not sure about it becoming common in the structural world, what I believe is that someone is making you weld structural to a subsea pipeline spec. Unless an ECA is being developed for your welds and you will have a different acceptance criteria.

Actually, though no code that I know allows it as a replacement for PWHT, except, as I discovered, the standard stated above, there is a metallurgical justification. PWHT not only reduces stress and increases ductility, it does other things. It precipitates carbides, segregates elements, creates carbon denuded zones, varies creep rupture properties, to list but a few, and also increases, with most ferritic alloys, impact toughness. The very property CTOD's are designed to measure.
And toughness would be, in cold subsea environments, a major issue. And since CTOD's are considered a real material property with a mathematical relationship I can see where it could be used to index the acceptability of other porperties.

Hi Toddler

Post is a little late, but only just returned to AWS forum after an absence of a couple of years! Hope you are still interested in an answer:

CTOD testing does not replace PWHT, the intention is to perform a test to show that the weld will be able to perfom its intended function without requiring PWHT. While thicker materials can obviously have a greater "quenching" effect, resulting in hardened microstructures, this can be dealt with in a number of ways, such as high pre-heats and welding with high heat inputs. Also, multi pass welds (and here I am looking at normal C/Mn steel) tend to have the passes deposited first, "tempered" by the subsequent passes, so can actually have quite a ductile and impact resistant structure. The main reason that thick materials (especially structural steel) require PWHT is that the thicker the material, the more restraint there is to the joint experiencing plastic deformation, which can result in even a ductile material acting in a brittle manner. (Often called the triaxiality effect.) This means that the high residual stresses built up in these welds (due to the welding) could potentially not "relax" due to plastic flow when placed under high loads, but will just fracture in a brittle manner, as the local stresses exceed the UTS. By reducing the residual welding stresses (through PWHT) the welded structure is then able to carry its intended design load without fracturing, as the local stresses will only be due to the design loads (well almost) and not due to a combination of design loads and residual stresses.

If it is possible to prove that the weld will not act in a brittle fashion, without the PWHT, then in effect you have proven that the PWHT is not necessary. This is where the CTOD test comes in. While CTOD and CVN both measure the material's "fracture resistance", the CVN impact test is performed on standardised test specimens, (typically 10mm x 10mm cross section) so they will give the same answer whether the material welded is 12mm thick or 120mm thick. (Assuming that the material microstructure is the same, that is.) They can not take any joint geometry issues into account. The CTOD test on the other hand is performed on specimens that are essentially as thick as the weld, and have a reasonable correlation to the residual stress state of the weld. If the CTOD test shows that the weld does not fracture in a brittle manner, then we can have a high certainty that the weld will perform well in service without the PWHT.

Hope this answers the question.

Regards
Niekie